DEPARTMENT OF THE INTERIOR MONOGRAPHS OF THE UNITED STATES GEOLOGICAL SURVEY NOs URE DOGG el WASHINGTON GOVERNMENT PRINTING OFFICE 1899 “cpa ent Ries eb Vi3ZG UNITED STATES GEOLOGICAL SURVEY CHARLES D. WALCOTT, DIRECTOR AD Jeb ie CRYSTAL FALLS IRON-BEARING DISTRICT OF MICHIGAN J. MORGAN CLEMENTS ann HENRY LLOYD SMYTH WITH A CHAPTER ON THE STURGEON RIVER TONGUE WILLIAM SHIRLEY BAYLEY AN INTRODUCTION CHARLES RICHARD VAN HISE WASHINGTON GOVERNMENT PRINTING OFFICE 1899 COIN WT ea TS) LETTER OF TRANSMITTAL INTRODUCTION, BY CHARLES RICHARD VAN HISBE..-------------------------~----- 222-702-077 OUTLINE OF MONOGRAPH Parr I.—THE WESTERN PART OF THE CRYSTAL FALLS DISTRICT, BY J. MORGAN CLEMENTS. CHAPTER I.—INTRODUCTION Previous work in the district Mode of work Magnetic observations CHAPTER II.—GHOGRAPHICAL LIMITS, STRUCTURE AND STRATIGRAPHY, AND PHYSIOGRAPHY ---- Geographical limits Structure and stratigraphy Physiography Topography Timber and soil CHaPtTeR II].—THE ARCHEAN Distribution, exposures, and topography Relations to overlying formations Petrographical characters Biotite-granite (granitite) Gneissoid biotite-granite, border facies of granite Acid dikes in Archean Basic dikes in Archean Schistose dikes Massive dikes CHarrER [V.—THE LOWER HURONIAN SERIES Section 1.—The Randville dolomite Distribution, exposures, and topography Petrographical characters Relations to underlying and overlying formations.-- - Section 2.—The Mansfield slate......-.-------------------------- Distribution, exposures, and topography Possible continuation of the Mansfield slate Petrographical characters Clay slate and phyllite Origin of clay slate and phyllite Present composition necessarily different from that of rock from which derived.... Analysis of Mansfield slate Comments on analysis Comparison of analysis of Mansfield clay slate with analyses of clays Comparison of analysis of Mansfield clay slate with analyses of other clay slates. -- Siderite-slate, chert, ferruginous chert, and iron ores Relations of siderite-slate, ferruginous chert, and ore bodies to clay slate Relations of Mansfield slate to adjacent formations Relations to intrusives Relations to voleanics 59 59 61 62 Wil CONTENTS. Cuaprer I1V.—THe Lower HuURONIAN SERIES—Continued. Section 2.—The Mansfield slate—Continued. Page. Simei) Oye WA WE GIG MEE SSeS cSeisos oo oes 5 osbems soos Soodso coco beceesesnese cece cece 64 IMAI) an oe see coo ses OpoboS SeRg OnEeSo ceeds oats comers Cros enon cose sooe Toone soodases 64 (OG eles ie Seine ees eee DeeRSe Oona ces bade caomed soc sabe oman Ssagsctens ssessse7sdsoare2 65 General description of Mansfield mine deposit. .-...----...-...---.-------------------- 66 ERG TAG VONIS HONS Te OUT HAT ELS et re ee 68 (Ohoyeay so} Oe OW) 6 Bosens Soa aso coooes ceo sce o esoce Hero Some Seco nOoose sean eobeeeeee 68 Microscopical character of the ores and associated chert bands -....----.-----.-------- 69 Oxieinloh Geroreld epost ya oe eee ote ya are eee te ee eee 70 Conditions favorable for ore concentrabion~ ~~ oes oe oe ee ne ee wen ene wee 72 BGO OPINDM S55 hosc on cesdescocb oaceca acon nsosae poeSononndos SEOSog 2S oro aSscee seen see 73 SO CULOM see EMO TIN CSSA 15 100 AE OTe ee eta 73 Distribution, exposures, and! topooraphiy-easesss-eee ease sas see ee eee eee ee eeee eee eee 73 HUMOLSNASS cede concdc coda seas ncassadanesoa8 164 GWmIGIENNOM cs 5520 cane sete cescse oscses ones mesaceade Pecan cata won oes pada SoCneroSEoeEse 164 Petrographical characters -...-...------------ -----+ -------- +--+ +--+ += 2-5 225 2222-222 =--- 165 Serableaverneye7 WOON cecns e5oces Soo 6a5 peebsn SeGogS soseds HSeeHs cees8505 SS0es CSeere Bgeoee 165 Microscopical description of certain of the sedimentaries.----..------------------- 169 Ieneous rocks ..-----.----------- --0--+ -22 2 022222 eee ee ee en ne eee 174 @re deposits s- cass = = =e amas oma ase amie a ale ea ie nie elm eal 175 History of opening of the district.-....---..----..----.------------------------------- 175 ID SMI NUNO M5 bods bosees ceo ercoss ooo abe odecos quannseose ceoden Seed odisendse tooo SoO8bs 175 Wyemucnan Ine? OF see, Bui, MS AGING, ib BB WW cose ossce0s0s00 co0ces Stee sonsseeaSangenes 176 Reo, As 0S AB INI IRs OS MVscesconscsce gens obasce.csoe soscos ecaoesoenenoses Secesceaos 176 Ning aR) BARE hnoe cod cas epou subdan laos eaonnecoeeapSebeu0 sass spas a4eses 6Sq5 aa05 177 Wine Chaysiell PAM CREP oes cacood ofess0o5e505 socces Sonecs sagoSesSees6 saesc5 soeS 500s 178 CIMARACIER GE WN® ORO .sondc coheed condor Sceebaooosaé cocres Seed esse csadesesocasesssds 550° 180 Relations to adjacent rocks.---..-.---. ---- <2. <2 = 222 os ae ns ee w= === = 182 (ie 3-2 6e6 Seen See saaoes Saacieces ues Suse ES Abr eas pocesh sSbes05-o08caGcd0 cosSaes6 obec 183 SAS OF WHE GRE WOES sco sca seas adas cond ede sod ecoeadons céosco cacecososscsansasnses 184 MIGMOOGIS GP WHINE oo 5 Se aeds codec coo deorossecoseons Sona $s Seen e csc sad ooso sa50 6cs5 184 TPS} DEOMI Bo 8 Ba oe ae coos bean ene sod SoeEey sede bor aae 5025 b226es S5o5n0 55665 sbaco0 sooceS 185 Production of cre from the Crystal Falls area... ..---...----------------------------- 186 Creuse WL Saiete II PORUISIOVAYS jocse5 eons: oneee4 soe eos 555525 cossse yaeeas 235822 cose sascnse 187 Ontlor Gi? TORTIE Na 6 sesacosece coceso ckee Su AGEs Baedes Hanae base 5505 5905 Uae sua suas baDoES 188 Age of FINO TELUS Ia eee Oars te seer erste ee Mey 144 asc cree) See erate ee TR Eocene 188 Relations of folding and the distribution of the intrusives........-.-...---.--------------- 189 Sectionele—Ummnrelahe dain bros ives eeeeee tere eee ee eee este eee ee eee eee eee ser erent te 190 CHAS SITOAITOM Goce so case code cenbac ceeded Bassas BAbesdoe soene= sadeo cod Roe subd eonoeesodeeces 190 ANONG THMPPTSINCIs asecad coon ondooe eacccege sosSs= cece geeros COdconbeoses os5S eoSmosOReS Seen dan 190 Geographical distribution and exposures of granites -.-.---.---.---------------------- 190 TORIG-ORINIID 5 = 2500 snes $55600 seonns SRSRe a RaaS S00 9o06 coceos consaa seas seco egseus seme 191 INT CRODOGMMANTIOS 4 c2cc05 anceos sada ono Hoogds edo0 sSad sade ors sSe soso saSs0 easogasoad 192 NITRO OKATIEEI IO MM IEHEANANIOS cos coasogeqss sesees cose esas oeScessccses cess gacegsQsde dame 193 Relations of granites to other intrusives .......-.------------------------------------- 194 Dynamic action in stanites:-------- ---- = 2 << = soe eee a ne en 194 Contact of granites and sedimentaries -.-...---.---.---------------------+------------- 194 DRAGGED OF UMAPUSIOD 52250 soaaseosns sQsoo> SoSess Ssn0 spb soo seas Saas seHasee= esse" 195 TRARHE THAIREUSINTOS s525 so5200 segSag ceo ons Saen Sone seeded Conse 465500 coeSs50cce senane ceaccssonc 198 WGI Grate 5 sas os bocad hone aaa aSaa ese eoHeaeaSs seb545 650099055605 Uae SeH os cSco Bees Gece 199 Geoonaphicalidisinibubione sss] peer esse ornate eee ee ae eee alee eae ee 199 Petrographical characters ..--..------------------=- ------ ---- ---- =------- ---- --«= 199 WIE CUONCOMICA « 3c aosona conaeebaSees CoLecs Sooo was oSessse5o0 Sans Hon Sees eons 199 INV@nOE@O NICH cosces soscad names soo sos 7 essen sse5 Soesns SoSTosonnE oSsaTaERcaos 200 Vill CONTENTS. CHAPTER VI.—TuHE INTRUSIVES—Continued. Section I.—Unrelated intrusives—Continued. Basic intrusives—Continued. Metadolerite—Continued. TRGIEN Tay bake) EER MO 2s aor co coeo sas Srescoses coosee eae cose Seep oss cedoososs Relations to Lower Huronian Mansfield slates -...--..-.-.---.--.--- eee Relations to Lower Huronian Hemlock voleanics.........----.---------------- Riel ations) to) Upper, TA e Onn aa ete eyelet eet eel le eee aera Relations tolotherwntrusiv es: a= see e lee e eee ei ato eee Contact metamorphism of Mansfield slates by the dolerite ........-.-.-.-.---.--.- SIOWOSUES SSno60 dseaco sS0550 sSda56 ss esos Soedogcoss asas ss9o0csssDas cosSEUeDEIDS JEN AES OL SOMOS - 55 pose o2cese sa noosoo cee Wess soe ase cose Soon osaese MesmOsites sc ase sere eee ce eee eee eer cee erate et nme Renee (career ate EGU CSREES BER ease Oa ROSS or Soca bion add os becddoummoucticds peboos coco eada see cud] INTE O18 HOWINO AS. 55 se ccot es9e 96 650s Shoes she On SoRs Sedece sacosc S506 Comparison of analyses of normal Mansfield clay slates and the contact prod- No endomorphie effects of dolerite intrusion..-...--.-.---...---.-------------- WIG EISEN. 25 Space cboq soo5ae oso soe ocss SoaSsesoseed SoensdasceesssouSsssSoosSstosassass OTR STG IN TMEIOVES oo bse coodes cee Coon oso Soo egucou cand. coss5e eoeSae SoeaSe s2enes shosce Picrite-porphyry (porphyritic limburgite) --......-.-.-....----.-----.---------------- Geographical distribution and exposures.....---------------------.--------------- Petrosraphical/ characters. —- oe a eee ee = a ele eae tei Gray tremolitized picrite-porphyry -..---.-----.-.-----.---------------------- Dark serpentinized picrite-porphyry -.---.-------.-.---.--------------------- CIES ETORI TION s sass peessoesaced tooodd cosaeehsases Aapasosoemoo assem dasooo assess ssadodes Section II.—A study of a rock series ranging from rocks of intermediate acidity through those of basic composition to ultrabasic kinds.......--...---.--.-------.----- IGG) ch ssad Stes cooegebeceds deeaboddss sabads Sco pod cons esapIsdos Sena saosopen scéobseN Seac ING AY GhINKe) yors oaneee Bedeoo chod sone Hero Sood Samo eaed saeasoocos Codods cacsoo Gaccecc dans IDM o COIN EHAG! GRqAMEWERN. oe soocoes soso seeese cnes sees coSeoS cocoa pas saeRSsSac cassse IPE nye VCC CMMEMPNCIDES 3 ooo soo ares soos cosa cee S89 SoeseS Shen Shoo Sees ooeeeceSesce Description of interesting variations ----------.----.------------- ---- ------ ------2- =e Stare 1G No EP N[og JED BUL WW onos cosa ssc esos ache Soosscot Woe saeisco GososécassoneenonEes PACLOSSMGUVOL bn OT Cr yi tied] eS eee Smee Ot Chaysiel MS o5 aoe cee cee tose cote socace cesses ese rsse sees tend Soenes INTEND RYT OW GUONANG) 955 coe oSeo Soo Sooo nnoeOs seSSo See esas coos VSS sse0 See ancsonsoss (GeO GANG NONIS) | soodo cocsissoneced sosd cons seas nee coRoodae oSas scideso snos cosas sogsasaS Petrographical characters ..---. ---. <<< <= 2+ 12 -- 22 on ene nn ns ww ew we nnn Description of interesting kinds of gabbro...-........-------..----------------------- Hornblende-gabbro in sec. 15, T. 42 N., R.31 W ...-...----.---..------------------- Stace), 16s, 22} PS) ehavel 28), ANG) Wig Ti Bl WW) mone soos sponse onameosoore osese0 dese 9059 Istana oleyaGloa sey Nee) GChURG\~ 5s soe oo on soos cneems Sosa oesso0 vaceasodosas saeaso ore IBAA IRON) CWO) = aeca seaaocaees oneces SeSSosaSseSp ened Ofeg05 COs cave conansesco Se ZB) 15 42) Wig, THE Nifong TOAD) IN 200) VAY = eos on oeno ossoce coouem aso pseauasee cess IDaemaeeMlhy aMlrenteel A VO) <3 56 sea Cees ose ono sooo uae aan soos Tooene sagede cers sosenS Relative ages of gabbros -.--- 2. 1. - =. 25s are aw en ww ws a aw ee TREATIES soos coon soooob Asan Hbob ebenes Lees conc Coes coon ecde panos rue msde bees osSesoseRHSS Distribution, exposures, and relations 2225-1 eee ema a= mise ee eee ai Petropraphical characters). <= - << 2 2 =< oem ei rata ie ala = nla mee lll TRE Genii WHOIS) Se aador eames sosoace cherie aoccon sseess00dons Goebedso coos easoSocous 2 YG Oh co Scbrea ses UBSEeRBABoce LosaaS cecince concn quasaS datodgeboues sbocasonakss AN oni yo) Bee IS prc eee sone oncens poo cd osests soeedo oeneooaSsoos sSbso5 oSDaSS Gradations of amphibole-peridotite to wehrlite and olivine-gabbro .....--.-.-.---. Processloferystallizationy: -25-or eee e ee R eee eee sacs meee eee eee Bec Esme NV OVE TOA MNCY Fegeso Genees cocked OMG - 22252 cso cooces cosas onesss ones Sass gn0NS2 O2Sag9 co sees sons asSsSe oe SoSs seeeeoses 183 Sizohofatheroresbodiessemen eee aces aetieeo-e sense eee eee eee eee eee ee eee eee 184 WIGHNOGS OF WNMITNE 555 osseace seo cose consoo cass esse cesses ensoe So SoSs CoOoSE Sseeeyeses 184 JERS) EXC Gace Gago Bonus sse eae bE BE Seo BEE Ed don auabonoibcaeas ones. cous Soaseeeeasoese 185 Production of ore from the Crystal Falls area .-.--------.-------------.----.-----.---- 186 CHAPTERGY I. — INH h MIN TRUSIVIES sects nai- b's ctes/aajn se 4/-se Soe Seneca aero eine ee sec ie seis elec 187 Orderfofstreatmenttee reas eee s= S eae cereal eee eee eee eee eee eee rere ese res 188 JNG@ OP WH TMTRUSINCSocs5c4 osce Sse Snes Sos spsu cdesco case eSesea nd5cc0 pS sb an DER ee CR ae 188 Relations of folding and the distribution of the intrusives ....-....--...---.--...----.---- 189 Section. —Unrelatedeimtrusives)ssee-r tcc = =u ase ieee cise ete eee elec essere aaa see 190 C@lassiticatloniecne seer iase aes = mets a sce 2-5 soit eae eee eee ene Meine emcee ae 190 = ANOIG) MUU OE5 5056 sdebsad Sone eee BODS CUCU SEAS Seco DUSOHC Seis Saq dkaoboSs Sond aane ceaSeeaBaS 190 Geographical distribution and exposures of granites..............----..----.----.----- 190 ISTO NIGEE AEN) Base oconss BAe eEO SSB Se oO Crem rs bosboo Sas000 baes Sacned Sdeunce sabe HaSosees 191 WINGO OCHO) casos coosos ees seob onSede EOS baC oc onts coseeenseses CoseedbaaRES Hoss 192 WINE CONE DOMINIO s 56 oeeae peo soo cao bos nS SKet Seon soSood sae ceadeaceneseecosee 193 Relations of granites to other intrusives .-.--.....-....--.-----------.------2--------- 194 Dynamic action in granites .......--...--.-.---.--.---.----- Mee ees Memes sms oeeisere 194 Contact of granites and sedimentaries .-...-....---.----....--...----.---------------- 194 Evidence of intrusion 6 CONTENTS. CHAPTER VI.—TuHE INTRUSIVES—Continued. Section I.—Unrelated intrusives—Continued. IB IEKO Tal A EVNOS| Sooo Ges ooS SaeE SOR ODOSO ESE Teo coonSo Oto SSH cigs dooses Sago SoS Sooeesess sessss Met RCOLGLT LG 5 cca am ale mmc a ala mim 5 om mm me ele eel Geographical distribution: ---- ------ .---- 5-2-2 2 n e n n= Petrographical characters .----.------------------------------------- Weeeeacieaeie ss Macroscopical ..-.-. ---.-----------------. ------ Soscind ones lonepsoeseo sane Seisc WAKO) COEMORM, Seon Sees eons cone cose seobersa mene One sees os eScatosesceerscsecss TRENTON) Ue) EXCHENGE NA RO KE) SS Sa 5 Somechnqecos dosed sasSons neosod so SScnoS che ooSAse Relations to Lower Huronian Mansfield slates-.-.-.....--..--------------.---- Relations to Lower Huronian Hemlock voleanics......-.--...----------------- Relations to) Upper, buono nian sae eteeetaaaty eee eee ae ee Relations tolothenintrusiviesseece- ese ee nes eee ees eee eee eee eee Contact metamorphism of Mansfield slates by the dolerite. ...-.....-...----..----- SMIDSHIGE Good oomoss cass ss ress snseaoses soo sons soSsSS Dano Sono seScoU CoosO Lee Amnally SS Oss PLLOSUtCS eee t el te ele TDRSS NE mec cso ooe mages Sebo Ecos ecehas chocos cossor aggsde sede sess pSooKoee Adinoles Secon nee eres ee ee eee Sannin © eeibelcea steele ae ate ee eee Analy sestot 2d 1nolesrescestse= ae eee ae eee een ene ee eee Comparison of analyses of normal Mansfield clay slates and the contact prod- No endomorphie effects of dolerite intrusion ..---....-...-.--.-----------.---- INIGRH PSU son6 2595 sooesa ose cess onsoe SoS pees Cog saad cooy OsenOSES Sess SSoN 550 9205 S605 UWltrabasichintrusives te: << oss se see eee sae ales aoe Mee ate ele mien eta yaa Iocan Pierite-porphyry (porphyritic limburgite) ..--...-..- ep oeenintel wares ialsiniasis tian cee eee Geographical distribution and exposures Petrographical characters .-.--....- isy Neoncd GocoSS DaCNSaooRESTocsSsosSSneg Sone Sees Gray tremolitized picrite-porphyry ...-...--..---------.--2..-----------.----- . Dark serpentinized picrite-porphyry.----.---.-----. ---5-- ---- 5 --- e--4 eee == CHEESHHCRIIOMN -cosceckbocusdes sousobebcdee sotese Sem ned cbored Bac cae soe Ses Sco Seed esse Section IJ.—A study of a rock series ranging from rocks of intermediate acidity through those of basic composition to ultrabasic kinds IDNOW. soc6 oso o co Shot sabe Seesoocose dousos casSOSseSeos ctieRes esseesoesoos soceme scores SstS INO AG EERE) 2 Sete Cote oaoOdoS Sono Some Shes S505 Seon odecas somOed Gace Sede GuOwosseaa Sass Distribution and exposures Betrosrap bicalichanacters sre eee te eee eee eet eter ote eater ete Description of interesting variations..---.-.---------------.-----. 2222 2222-25-52 Seesaw 425N eR. oliWe ae eseee eee sooo a5s0 Sassoo esos 2S aoce sess Hoeseesosess NOHO phere theyeN (Cray sueMl ENN) £555 555525 csoc cod6 Sosese Seon eese sees BEAR ae Southeast of Crystal Falls Analysis of diorite ...-.--.- Gabbrovand Mm Onto me eee= tena see este ee Ree ee er ee eer aaa tee eee eee Petropraphicalicharacters)-—-- eee ree eet eee reer rere ee eee aaa eee ees eee Description of interesting kinds of gabbro-.....---.-.-.-.-.-.---.---------------.---- Hornblende-gabbro in sec. 15, T. 42 N., R. 31 W Sees. 15, 22, 28, and 29, T. 42 N., R. 31 W Hornblende-gabbro dikes BLO ZEOLITE) CIC ane ene arene ee eee eee eet eee te ee eee Secn2o Da 2INe hil Wie, 200) Ni 200 Wik ene eee te ee tema ea : Dynamically altered gabbro Relative ages of gabbros TEGO 2265 pesca. secon SUSE SSSESO Eon GAd6 2S Soacibasties Or osossased men sackescosgeseses Distribution, exposures, and relations Petrographical characters CONTENTS. CuHaArtTEeR VI.—THE INTRUSIVES—Continued. Section II.—A study of a rock series, etc.—Continued. Peridotite—Continued. Beridobibewarietlesic ssc ties. ose s ene ee cee eaee Seen oen sane eiasetree ee cele somis setesene AWWiehh Tite senses ce eve cele be myak iS eee Sie ee Se oe Se eee ee I ee Re ep re cee eels Amphibole-peridotite =e 2 -)--eest <= o2ee ac se so see aaa ee ce cele moe oe eee es Sane Gradations of amphibole-peridotite to wehrlite and olivine-gabbro ..-....-.--.---- IPTOCESSTOfcLyStallizablonee nese ee Meee eee eeeeeeee eee SUNS eee yeast eines cee ANalySisiofperid oblbel jasc! seas le oe clones pee eoee eee aise seen ee eee ie Peridotite from sec. 22, T. 42 N., R.31 W., 1990 N., 150 W ....---..-------------.----- Relationsjofeperidotitestolouler LOCKS) seen eee eee eaiseeeieen cere eee ree aes eee perofperrd Obvites: 35%,- j= ccioe neinePicioac = sce eue tone eee ease ee et ee eenee ee Secise be General observations on the above series --2-=- -252-- 22 ss-2 eee eeees sees eee a aa neces ee exturalicharacters) of the)series=-=-)--)-2 ee eee alee erence eee eee se eee Chemicalicompositiontofatheseriess ease se eee ee sea eeee tee eee eee eee eee eeeee Relativerasesotrocks\of he|series|s--2 -senes se eee eee ee eee tear e eee reacee =I AUS RAD OSs Page. PiatE I. Colored map showing the distribution of pre-Cambrian and other rocks in the Lake Superior region, and the geographical relations of the Crystal Falls district of Michigan to the adjoining Marquette and Menominee districts of Michigan - .----- 11 Il. Topographical map of the Crystal Falls district of Michigan, including a portion of the Marquette district of Michigan-----.-..---.---------------,-------------- In pocket. III. Geological map of the Crystal Falls district cf Michigan, including a portion of the Marquette district of Michigan. .-<— ~~~ - === on ee nnn In pocket. IY. Portion of a geological map of the Menominee iron region, by T. B. Brooks and C. E. AVG 25606 gasses cgoses pokes CosSs a uboHss asunaD sn sopepacSag coco eassas cose sass 18 V. Generalized sections to illustrate the stratigraphy and structure of the northwestern , part of the Crystal Falls district of Michigan ..---.-..-...----..----------------- 28 VI. Generalized sections to illustrate the stratigraphy and structure of the southern part of the Crystal Falls district of Michigan .-..-.......---.-----.------.------------- 28 Wil. Generalized) columman, SeCtlOn) s- jena m 2 me a ia ee ee Ren le 30 VIII. Map of a portion of the Crystal Falls district, showing in detail the glacial topog- raphy and illustrating the development of the Deer River --.-.-..---.------------ 32 IX. Sketch of the Mansfield mine as it was before it caved in, in 1893 ..-...------------- 66 X. 4, Reproduction of the weathered surface of a variolite; B, Reproduction of the polished surface of a variolite.-.--.----.--.----..--------------------------------- 110 XI. Colored reproduction of an ellipsoid, with matrix, from an ellipsoidal basalt --.- --.- 116 XII. Mount Giorgios, viewed from its west flank, in April, 1866, illustrating the charac- teristic block lavas, from Fouqué’s Santorin et ses Eruptions, Pl. VIII------------- 120 XIII. Reproduction in colors of a basalt tuff... .--..------------------------------------ 140 XIV. Idealized structural map and detail geological map, with sections, to show the dis- tribution and structure of the Huronian rocks in the vicinity of Crystal Falls, WINGS Ges Saodes - oes cose cose cass shes Sas aesoed Sees cons Seno thee Goce Eso SHE SbyS 160 XY. Portion of Brooks’s Pl. IX, Vol. III, Wisconsin Geological Survey -.-...-.-.-.------- 172 _XVI. Detail geological map of the vicinity of Amasa, Michigan.----..-.--.--..---------- 176 XVII. Detail geological map of the vicinity of Crystal Falls and Mansfield, Sheet I----. ---- 178 XVIII. Detail geological map of the vicinity of Crystal Falls and Mansfield, Sheet Il. .-----. 178 XIX. 4, Inclusions in a fractured quartz phenocryst; B, Quartz phenocryst with rhombo- Ine! jenn Oe =o oe Soe pe soso seiaag Ge Sams So Seco e sobs SosSccs nesses come ooad eons 268 XX. A, Micropoikilitie rhyolite-porphyry; B, Micropoikilitic quartz-porphyry----------. 270 XXI. 4, Very fine-grained micropoikilitic ryholite-porphyry viewed without analyzer; B, Very fine-grained micropoikilitic rhyolite-porphyry viewed with analyzer---. ---- 272 XXII. A, Perlitic parting in aporhyolite; B, Perlitic parting in aporhyolite------.---.-... 274 XXIII. A, Schistose rhyolite-porphyry; B, Aporhyolite breccia. ...--.-.----.---------------- 276 XXIV. 4A, Schistose rhyolite-porphyry; B, The same viewed between crossed nicols- --.-----. 278 XXY. 4, Amygdaloidal texture of basalt; B, Amygdaloidal vitreous basalt-..----.-.-.----- 280 XXVI. 4, Amygdaloidal vitreous basalt; B, Amygdaloidal vitreous basalt showing sheaf-like ageregates of feldspar_..---.-----------------+---------------------------------- 282 XXVII. 4, Reproduction in colors of amygdaloidal basalt; B, Pseudo-amygdaloidal matrix of ellipsoidal basalt; C, Water-deposited pyroclastic. --.--.-..-.--.------------------ 284 XXVIII. 4, Fine-grained basalt with well-developed igneous texture: B, Illustration of the obliteration of the igneous texture of a basalt by secondary products when viewed POG ORoLICGl MME). csocec sees chee colese S2ssee oseso sass cosa esse sessco ese sess 286 10 ILLUSTRATIONS. PLaTE XXIX. 4, Basalt showing characteristic texture in ordinary light; 6, Basalt showing obliteration of texture between crossed nicols .....-...----..----.-----.------ XXX. 4, Basalt showing in ordinary light a distinctly amygdaloidal texture; B, The same basalt with its amygdaloidal texture obliterated when viewed between Chossed ini Cols seme mies ee ne ae eee ee eee ee ae eee a XXXI. 4, Basalt affected by calcification process; B, Basalt affected by calcification process viewed between crossed nicols..-.-....---...----------.--------------- XXXII. 4, Illustration of perlitic parting in a fragment from a basaltic tuff; B, Sickle- Shaped bodies mks vole syn ct itt gee eaten eee eee XXXIII. 4, Water-deposited sand; B, Gradation in water-deposited volcanic sediment - -- XXXIV. 4, Contact product of granite; B, Brecciated matrix between ellipsoids. .----. -- XXXY. 4, Contact between granite and a metamorphosed sedimentary; B, Contact between granite and a metamorphosed sedimentary viewed between crossed NYKO pees co nooe HooKS0 abe Saclosesehce oo cebaooon easolobesacasopaseadeseetos San6 XXXVI. 4, A variety of spilosite with white spots; B, A variety of spilosite with white spots viewed between crossed nicols.......-..-..----.-----.-----------.-+----- XXXVII. 4, Normal spilosite or spotted contact product; 5b, Normal spilosite of some- WI NAD ChE G GMARAOUETE 5 oS song Se pon eencseoeo sonccb oSoaso See cee osSeac ceeees XXXVIII. d, Passage of spilosite into demosite; B, Occurrence and alteration of bronzite TMD CVALO NO) Dae oe Ree es COSe ES ERE bom GSE SoU SS Sac SoSIA Bam oooaaEaoseacoo XXXIX. d, Biotite-granite viewed between crossed nicols; B, Mica-diorite viewed between OROSEEGL MCD Ks +5 cocdespseccocbsoopbosbes sanded Socuos Pesose csoacd noadee aceese XL. 4, Quartz-mica-diorite-porphyry; 6, Quartz-mica-diorite-porphyry viewed be- tween crossed nicols -....---..----- ddosopissasos Cabs obaooon Saneoucaones SHweLe XLI. A, Porphyritic poikilitic hornblende gabbro; B, Poikilitic hornblende gabbro-. - XLII. A, Moderately fine-grained hornblende gabbro showing parallel texture; B, Mod- erately fine-grained hornblende gabbro showing parallel texture viewed Ibe twieentcrosse dni Osi peee ee eeatarar ate se ee ate aaa eee eee te XLIII. A, Normal granular hornblende gabbro; 6, Schistose hornblende gabbro viewed between crossed micol si. sees ree ee ese setalse ee ete ales eee see ets XLIV. A, Moderately fine-grained hornblende gabbro; 6, bronzite-norite XLV. 4, Bronzite-norite-porphyry; 6, Feldspathic webrlite ...-...........--.-..----- XLVI. 4, Feldspathic wehrlite viewed between crossed nicols; 6, Feldspathic wehrlite. - Fic.1. Reproduction of a portion of the geological map of the Upper Peninsula of Michigan, by) William yA: Burt 846e oso see sees esee eee enleeenisa sets sean eee eee meee eee es . Enlarged reproduction of a portion of a map of the Lake Superior land district, by Hostexiand) Whitney. = -sasesaa eeeeteee eeiertereareecee eee alee eee tenee Serene eeee 3. Enlarged reproduction of a portion of a geological map of the Upper Peninsula of Michi- gan, by Rominger, Brooks, and Pumpelly, 1873 ....--..----- eae ste te ea ey 4, Granite-porphyry with inclusions of gneissoid granite .-.......-...--.------.---------- 5. Illustration of the effect on the topography of the differential erosion of basic dikes and to 6. Concentric cracks formed by the caving in of the Mansfield mine...--......-..-----.--- 7. Sketch of the surface of the outcrop of an ellipsoidal basalt, showing the general char- ACTED ODE WEEMS ELC MMR HABE. coor ecesend coe nes Hosdee donate booams Sor eee asec cece 8. Sketch showing the concentration of the amygdaloidal cavities on one side of an ellip- soid, this side probably representing the side nearest the surface of the flow 9. Ellipsoids with sets of parallel lines cutting each other at an angle 10. Reproduction of illustration of aa lava, after Dana ...--....-..---..-------.----------- 11. Profile section illustrating results of diamond-drill work 12. Sketch illustrating contortion of Upper Huronian strata. ....-.........-.---.---------- 13. Sketch showing change of strike of Upper Huronian beds, due to the folds 14. Sketch to illustrate the occurrence of ore bodies Page. 288 290 292 294 296 298 300 302 304 306 308 310 312 314 316 318 320 322 U.S. GEOLOGICAL SURVEY S 9p Jana % J Apas JULIUS BIEN &CO. LITH. NY. GEOLOGIC MAP OF PART OF Compiled from Official maps of1 MONOGRAPH XXXVI PL | i Watianaan 1a u CUS The Original Huronian The Or Tee Sertes, SS Ah3 The pare. Falls Iron-Bearing Series. Ah4 The Menomince ERIE NE) series. 5 The Wisconsin Valley Slate: Ah HURONTIAN % Ahé he PenokeeIron-Bearing cee 4 LAKH Seon REGION Ane The Quppewa Valley Quarta Lack: iste esota, and Canadian Surveys Ani Te a Kee cin bearing ia Ahi Abtl The Hit Scests. ror Canadas 100 STAT. M1, O JUUUS BIEN@CO LITH N.Y, " KEWEENAWAy POST-ALGONKIAN _ GEOLOGIC MAP OF PART OF HE LAKE SUPERIOR REGION Sfinne: F Compiled from Oficial mapsofe™” SN*S0U.and Canadian Surveys ee STAT M1. na =e Be MONOGRAPH XXXVI PL } Mi¢hipicoten. The Original Huronian re bow (La Dron-Bearin LeTeee Penokee IroneBearing Servex i J eS t ; , ‘ ae \ ' : : ie etn uBR 5 ‘ . ‘ THE Cla ioledb FALLS IRON-BEARING DISTRICT OF MICHIGAN. PART I. THE WESTERN PART OF THE DISTRICT. By J. MorGAN CLEMENTS. CHAPTER I. INTRODUCTION. The present report is an account of a portion of the Crystal Falls dis- trict of Michigan, so called from the most important town, Crystal Falls, the county seat of Iron County. The iron-bearing district along the Paint River, near the site of the town of Crystal Falls, was first called in literature the Paint River district by Brooks.t As soon as the town was begun, about 1880, the name of the town was applied to the district.” It is situated on the Upper Peninsula of Michigan, adjoining the northeastern border of Wisconsin, and serves as a link connecting the two well-known iron-ore- producing districts of Michigan, the Marquette, and the Menominee. The Crystal Falls district is of itself of considerable economic importance, as will be seen, though not deserving to be ranked with either of the two above-mentioned iron districts. Since the geological relations of the rocks ' The iron-bearing rocks (economic), by T. B. Brooks: Geol. Survey of Michigan, Vol.I, Part I, 1873, p. 182. 2 Rept. Com. Min. Statistics Mich. for 1881, p. 222. 11 12 THE CRYSTAL FALLS IRON-BEARING DISTRICT. of the Marquette district have now been ascertained, it is hoped, by means of the determination of the succession in the intermediate Crystal Falls district, that the Menominee rocks may be closely correlated with those of the Marquette district. The accurate delimitation of the iron-bearing or coal-bearing forma- tions, or any other formations containing valuable mineral products, is of inestimable value to miners and investors. In the iron districts of Michigan alone innumerable test pits have been sunk in areas of solid granite, and at - great distances outside of the possible iron formations, thus wasting large sums of money. Although the investigations carried on in the Crystal Falls district, the results of which are here recorded, do not enable us to point out definitely the places where the prospector will find iron deposits, they have enabled us to delimit in a broad way the various formations, and warrant the statement that iron deposits may occur in certain areas and that the prospector will assuredly not find iron deposits in certain others. The opportunity of studying the Crystal Falls district was given me through Prof. C. R. Van Hise. In the prosecution of the field studies and in the preparation of the report I have availed myself of his advice and suggestions, which have been generously offered and which have been _ found of greatest value. To him I am most deeply indebted. The report is based not only on my own field work, but also on the field work done by a number of other geologists, whose notebooks have been placed at my disposal. The names of these geologists may be found on ~ page 22. Among them, the notes of Mr. W. N. Merriam and Dr. W. S. Bayley have been found especially valuable. Mr. Merriam, assisted by Dr. Bayley, spent a season in doing very detailed work on the area shown on the sketch map at the bottom of Pl. III, between Crystal Falls and Mansfield, and from this point northwest to some distance beyond Amasa. The mag- netic lines represented in this part were traced by Mr. Merriam, and the geology in general is the same that he outlined on his final field map I wish to thank Mr. C. K. Leith, who has been of the greatest clerical assistance, and Mr. EK. C. Bebb, by whom the maps were drawn; also Mr. ‘The Marquette iron-bearing district of Michigan (preliminary), by C. R. Van Hise and W. 8. Bayley; with a chapter on the Republic Trough, by H. L. Smyth: Fifteenth Ann. Rept. U. S. Geol. Survey, 1895, pp. 477-650. Ibid. (final), Mon. U. S. Geol. Survey, Vol. XXVIII, 1897. PREVIOUS WORK. 13 J. L. Ridgway, by whom the colored plates of natural size specimens were prepared. : PREVIOUS WORK IN THE DISTRICT. On account of its comparatively slight economic importance, and also on account of its isolation, very little work of which the results have been published was done in this district prior to that on which this monograph is based. As a rule, the earlier observers began the season’s work either in the Marquette or in the Menominee range, and working westward the Crystal Falls district was reached only as the season neared its close, or as the appropriation was nearly exhausted. The published work upon this district is given below in chronological order. 1850. Burt, Wm. A. Report of linear surveys with reference to mines and minerals, in the Northern Peninsula of Michigan in the years 1845 and 1846. Dated March 20, 1847. Thirty-first Congress, first session, 1850; Senate documents, Vol. III, No. 1, pp. 42-882, with map. During the year 1846 a lear survey was made of that part of the Upper Peninsula of Michigan described as being bounded on the north by the fifth correction line, on the south by the fourth correction line and the Brule River, on the east by ranges 23 and 26 W., and on the west by range 87 W. This includes in its limits the district under discussion. In the course of the survey, geological observations were made by William A. - Burt, the deputy surveyor in charge of the work. The report and accom- panying geological map embodying the results of these observations are concealed among the Senate documents of the Thirty-first Congress. The followmg quotations from this report give all the observations on the part of the territory surveyed in which we are at present interested: Topography.— West of range 31 west, and north of the Brule River to the fifth correction line, is a tract of about 43 townships in which the rock is mostly greenstone and hornblende slates. This part of the surveyed district is less broken than that above described, and a large proportion of it may be denominated rolling lands. There are, however, many ridges and conical hills of various heights upon this part of the survey, and also deep valleys of streams, many of which have ledges upon their sides. These general characteristics are often changed for cedar, spruce, or tamarack Swamps, which are most numerous in townships 46, 47, and 48 N. [This includes the part of the district supposed to be the continuation of the northern Wisconsin peneplain (p. 31).| 14 THE CRYSTAL FALLS IRON-BEARING DISTRICT. Granite (and syenite)—These rocks occupy an area of about 22 townships on the northeast part of the survey, between the fifth correction line and the south boundary of township 45 N., and east of range 32 W., in a series of irregular uplifts, frequently forming high cliffs and sloping ledges on the most elevated portion of this district. [This covers a part of the Archean granite oval of the Crystal Falls district, as well » as the large Archean areas northeast of it. | Argillaceous slates—The argillaceous Slates alluded to in townships 42 and 43 N. are generally overlaid by deep drift; their boundaries, therefore, could not be satisfactorily defined. West of the Peshakumme River these slates appeared to have undergone _ considerable change by igneous action, and were often associated with an oxide of iron; but east of the Peshakumme no change by igneous action in the slates was observed, and on this part they have generally a reddish color. : They dip variously at a high angle, and are supposed to conform to ‘dhe greenstone on the north and west, and to overlie or pass into the mica-slates on the south; and in their middle portion they dip about 90°, with strike nearly east and west. [These slates correspond to our least metamorphosed phases of the Upper Huronian.| Greenstone and hornblende slate—These rocks occupy a larger area in the district surveyed than any other class of rocks. They extend from the granitic and other- rocks east of them westward beyond the survey. [See their outline on map, fig. 1.] The greenstone and hornblende slates form a less broken surface than the eranitic range; and next to it is the most elevated range in this district, having an estimated altitude, in many places, of from 1,000 to 1,100 feet above Lake Superior. These rocks are frequently seen in the beds and banks of streams and in ridges and conical hills of various heights, often forming precipitous ledges upon their sides. c The greenstone of this region is generally more or less granular and syenitic, with a dark-green color when moist; its composition is hornblende, feldspar, and quartz—the former mineral greatly predominating. In some places the feldspar and quartz are nearly or quite wanting, leaving a granulated hornblende rock. Another variety of this rock was frequently seen which was composed of the same ingredients but very fine grained and compact and having frequently a laminated or slaty” structure, the cleavages of which generally dip from the granitic rocks at a very high angle. Some of “nage hornblende slates have in their seams and cleavages a silky luster, from the presence of mica or tale in very fine grains. All of these rocks are traversed by many quartz sain, from a line 40 4 feet or more in width, and with still larger veins and dikes of more recent trap rock. This range is smmapasedl to have become blended with the trap range of Keweenaw point as it passes under the red sandstone lying between them, and probably farther west the two are united in one range. [These are the altered and more or less schistose basalts and accompanying fragmentals which are comprised in the Hemlock formation. | Mica-slates—These slates stretch along the southerly side of the argillaceous slates on the south part of the survey. They extend from the Brule River on a course east- northeast for about 22 miles, in townships 41 and 42 N., ranges 29, 31, and 32 W., and have an average breadth of about 4 miles. The mica-slates are supposed to dip northerly idler the argillaceous slates at a high angle, varying at the surface from 45° to 80°. PREVIOUS WORK. 15 This rock is composed of mica, quartz, and feldspar. Its lamin are undulating or waved, but its cleavages, on a large scale, are even and regular. These mica-slates are best developed on the south boundary of ‘vopanishita 42N., ranges 31 and 32 W., in the beds and banks of the Peshakumme and Mesqua-cum- Me t Scale of miles A) ° 5S 10 is — —= —s ts 4 Fic. 1._Reproduction of a portion of the geological map of the Upper Peninsula of Michigan by Wm. A. Burt, 1846. cum-sepe, and at the falls near the junction of the latter stream with the Brula River. [These, according to our observations, are the most altered phases of the Upper Huronian. | That part of the eeological map accompanying the above report which corresponds to the Crystal Falls district is reproduced in fig. 1. 16 THE CRYSTAL FALLS [RON-BEARING DISTRICT. 1851. FostEr, J. W., and Wurtney, J. D. Report on the geology of the Lake Supe- rior land district. Part Il. The iron region, together with the general geology. Thirty-second Congress, special session, 1851; Senate documents, Vol. III, No. 4, pp. 406, with maps and section. In 1851 there was published a report by Foster and Whitney on the iron regions of the Lake Superior land district, together with the general geology. This gives the first connected account of the results obtained by the various surveyors who had been engaged on the Government survey of the Upper Peninsula of Michigan. Accompanying this report there are two colored maps anda section. The subdivisions of the rocks as made by Burt in the Crystal Falls district are not retained in this report by Foster and Whitney. The map is generalized, and the hornblende-slates, ete., of , Burt are included under the general term “crystalline schists,” and are placed by the authors in the Azoic system. There are represented here and there throughout this Azoic area a trappean knob and bed of marble. The granite area shown on Burt’s map is very much reduced in size, and no longer connected with the large granite areas to the east. The granite on the lower reaches of the Michigamme, in T. 42 N., R. 31 W., is here indicated for the first time. In these respects only does this portion of the map show a decided advance in knowledge of the distribution of the rocks. A copy of the map, showing the distribution of the rocks by symbols instead of colors, is reproduced as fig. 2. 1873. Brooks, T. B. The iron-bearing rocks (economic). Geol. Survey of Michigan, Vol. I, Part I, 1873, pp. 319. With Atlas Plate 1V and general map, by Rominger, Brooks, and Pumpelly. ; The next mention of the district that I have been able to find was made in 1873 by Maj. T. B. Brooks, in his report on the iron-bearing rocks of Michigan. However, this report seems to show a decided decrease in knowledge from that possessed by Burt concerning the geology of this district. It is true that indications of iron had been seen, but the observations made were so meager that nothing could be done toward determining the relations of the rocks or unraveling the structure of the area. Upon the map accompanying the report (Geol. Survey of Michigan, PREVIOUS WORK. 17 1873), a portion of which is reproduced in fig. 3, Brooks has failed to outline the granitic areas known to the previous explorers. Except in a Scale of miles s} Fic. 2. Enlarged reproduction of a portion of a map of the Lake Superior land district, by Foster and Whitney. G=granite; T=trappean rocks; f =iron; (EB — beds of marble. Igneo 1s formation. ——S — eee eal (Associated with the Azoic.) Trappean rocks. Granite. Metamorphic formation. Azoic system. ——— Cm a eae a | Quartz. Crystalline schists. few places, which have been left uncolored, the district is covered with the -color representing the Huronian. MON XXXVI 2 18 THE CRYSTAL FALLS [IRON-BEARING DISTRICT. Brooks refers the ore-bearing rocks to the Huronian in the following words: Too little is known about the remote Paint River district, in townships 42 and 43, ranges 32 and 33, to enable me to give anything of interest regarding its geological Scale of miles 10 1S Fie. 3.—Enlarged reproduction of portion of a geological map of the Upper Peninsula of Michigan, by C. Rominger, T. B. Brooks, and R Pumpelly, 1873. structure. The Huronian rocks are extensively developed there, and contain deposits of hard hematite ore. I had the opportunity to examine only two localities at the Paint River Falls, sec. 20, T. 43, R. 32, and sec. 13, T. 42, R. 33, (p. 182). dew paiojoo jeuisiio yo Buimeip aul ul Uononposday “LHDIYM "3D GNV SHOOUS ‘gL Ag ‘SNOIDAY NOY! SANINONSW SHL 4O dVW 1VOIDO10439 ¥ 43O NOILYOd Sa | gos 4SIHOS Amoudy 48 NMOHS did ‘LNG YO GFLYOLNOD ONIGO39 SIHOS VOIN = LSIHOS SNOS9VqNONY LSIHOS LSIHOS 4sIHoS BLUYAHAYOd SLINVYO SSIAN9O snouasiLiounvis Vv 1 BEES (GS AHUVSNEO AVDIELE NIG LG Sa UG OTE oO DIN = GNV 31V1IS AV190 91114019 3NOLSN3349 AGNSISNHOH Ziuwnd P % ZZ + S904 40 INIGGIG JO dIG GNY FHIALS CAA SS NMONUHNN JIG ‘SHD04 JO INIGOIG FO FWIHLS Z WAX Poo ab” Al “Id IAXXX HdVHSONOW ABAYNS 1V9ID01039 “Ss “N i . in My ve re - id ’ 1 a i , " 4 : " i 7. i i i i V i i ig ” is i i f .f v y i” 1 Lee, iy i ad ) be ays ‘yy \ As tes ' 1 : ae i : , ' ‘ 5 Y PREVIOUS WORK. 19 He also gives his analysis (No. 68, p. 302 of Brooks’s report) of an ore sample from the district, and calls attention to its abnormally high water content, freedom from silica, and richness in iron as compared to those of the more eastern mines in the Menominee region. isso. Brooks, T. B. Geology of the Menominee region. Geol. Survey of Wisconsin, Vol. III, Part V, 1880, pp. 430-655. Atlas folio. Pls. XXVIII and XXIX, and PI. XXX, by C. HE. Wright. In an article on the geology of the Menominee region, which was written for the geological survey of Wisconsin, the same author briefly touched on that part of the adjoining Michigan territory which is included in the district under consideration. His observations were thus confined to a few exposures in a limited portion of the area. He attempts to correlate ‘certain beds by means of their lithological character with those with which he was familiar im the Marquette district and refers them uniformly to the higher members of the Huronian. They are also so referred on the map which accompanies the report, though this is dated a year earlier than the date of publication of the report. That portion of the map covering a small part of the Crystal Falls region is reproduced on Pl. 1V. The present survey enables us to add very little to this, and these additions are chiefly of a petrological character. 1881. ROMINGER, CARL. Geology of the Menominee iron region. Geol. Survey of Michigan, Vol. IV, 1881. In 1880 Dr. Carl Rominger, at that time State geologist of Michigan, spent a season in the Menominee district, and in his report gives detailed descriptions of a few occurrences in the Crystal Falls district, to wnich I shall refer later on. He considers the rocks in general to belong to the Huronian, and distributes the beds among his diorite group, iron-ore group, and arenaceous-slate group, as given and defined in the previous report on the Marquette district No attempt at more definite correlation was made. 1890. VAN Hisz, C. R. An attempt to harmonize some apparently conflicting views of Lake Superior stratigraphy. Am. Jour. of Sci., 3d series, vol. 41, 1891, p. 133. On December 30, 1890, Prof. C. R. Van Hise read a paper on Lake Superior stratigraphy before the Wisconsin Academy of Sciences, Arts, and 20 THE CRYSTAL FALLS IRON-BEARING DISTRICT. Letters, the same article being published the following year in the American Journal. The iron-bearing series of this district was in this article referred to the Upper Marquette (Upper Huronian). 1893. Wricut, C. E. Report of State geologist from May 1, 1885, to June 1, 1888, in Rept. of the State Board of Geol. Survey of Michigan, 1893, pp. 33-44. State geologist, Charles KE. Wright, in a report for the seasons from 1885 to 1888, inclusive, merely mentions the general strike of the rocks of the district, and makes no attempt to determine their age nor to unravel the structure. WapswortH, M. E. Sketch of the geology of the iron, gold, and copper dis- tricts of Michigan. In Rept. of State Board of Geol. Survey for years 1891-92, 1893, pp. 75-186. Dr. M. E. Wadsworth, who on Mr. Wright’s death succeeded him as State geologist of Michigan, mentions the occurrence of carbonaceous slates, of granite and melaphyre, and of conglomerate near Crystal Falls, but does not enter into a discussion of the relations of any of these rocks. Dr. Wads- worth agrees with the correlation of Professor Van Hise, and places the Crystal Falls ore deposits in the Upper Marquette series (Wadsworth’s Holyoke formation) (pp. 117, 182), but the evidence for so doing is not given in the report. He also is the first to recognize the volcanic nature of the rocks in the vicinity of Crystal Falls (p. 134). 1895. RominGER, C. Geol. rept.on the Upper Peninsula of Michigan. Geol. Survey of Michigan, Vol. V, Part. I, 1895, pp. 1-164. In his report of work done on the Upper Peninsula of Michigan from 1881 to 1884, published in 1895, he follows the same plan, referring the various rocks exposed by mining operations to his different groups.’ CLEMENTS, J. MorGAN. The volcanics of the Michigamme district of Michigan. Jour. of Geol., Vol. IIL, 1895, pp. 801-822.” In a preliminary article on this district, by the writer, published in 1895, the volcanic character of the rocks which cover a large area of the Crystal Falls district was emphasized, and in a sketch map in the same ' Geol. Survey of Michigan, Vol. IV, 1881, p.8. 2 After the publication of this article, the name Michigamme having been applied to a formation, it was deemed advisable, in order to avoid confusion, to change the naine of the district to the Crystal Falls district. PREVIOUS WORK. 21 article was given an outline of the distribution of the various rocks for a portion of the district, with their stratigraphical succession (p. 803), the dis- cussion of the structure and correlation being left for the present report. The above-mentioned sketch map, with the maps by Burt, Foster and Whitney, Brooks, Brooks and Wright, the section by Foster and Whitney, and the section by Brooks, along the Paint and Michigamme rivers, are the only maps or sections which, so far as can be learned, have been published of that part of the Crystal Falls district under discussion. MISCELLANEOUS REFERENCES. JULIEN, ALEXIS A. Appendix A. Lithology. Geol. of Michigan, Vol. II, 1873, pp. 1-185. WICHMANN, ARTHUR. Microscopical observations on the iron-bearing rocks from the region south of Lake Superior. Brooks’s Geol. of the Menominee Iron Region, 1880, Chap. V, pp. 600-655. WRIGHT, CHARLES EH. Geology of Menominee Iron Region. Geol. of Wis- consin, Vol. III, Part 8, 1880, pp. 665-741. LANE, A. O. In sketch of the geology of the iron, gold, and copper deposits of Michigan. Rept. of State Board of Geol. Survey for 1891-92, 1893, p. 182. Parton, H. 8. Microscopic study of some Michigan rocks.. Rept. of State Board Geol. Survey for 1891-92, 1893, p. 186. During the progress of the Michigan and Wisconsin State surveys specimens from outcrops were collected, and descriptions of these discon- nected specimens are found in the State reports. References to the pages on which the individual descriptions may be found will be given under the petrographical discussion of similar rocks here described. UNPUBLISHED WORK. In 1891 a survey was organized by a private corporation, and put in charge of Prof. C. R. Van Hise. He consented to take charge of this work on the conditions that all maps and notes should be available for this report and that no other compensation was to be made by the company. The object of this survey, known as the Lake Superior survey, was to study that part of Michigan of which Crystal Falls is the center, in order to determine the feasibility and advisability of openmg up the mines of that district. This survey was vigorously prosecuted, and an excellent topographic map made of an area 32 miles north and south and 42 miles east and west, cover- ing a large part of four 15-minute atlas sheets of the United States Geological 2D? THE CRYSTAL FALLS IRON-BEARING DISTRICT. Survey. At the same time, in connection with the topographic work, a reconnaissance geological survey was made. The following is a list of those who took geological notes for this survey: Andrews Allen, A. H. Brooks, W. 8. Bayley, J. P. Channing, E. T. Eriksen, J. R. Finlay, F. J. Harriman, F. T. Kelly, E. B. Matthews, E. R. Maurer, J. A. McKim, F. W. McNair, W. N. Merriam, and H. F. Phillips. The following season was devoted to a detail study of the iron-bearing belts which had been outlined by the reconnaissance. This detail work in the western part of the district was prosecuted by parties in charge of W. N. Merriam, and in the eastern part of the district by parties in charge of H. L. Smyth. When they ceased work, the two areas mapped were sepa- rated in the north by about 12 miles, and a narrow belt separated the mapped areas to the south. During the season of 1894, under the direc- tion of Professor Van Hise and assisted by G. E. Culver, and during part of the season by 8. Weidman, I was engaged in completing this unfinished work for the United States Geological Survey, preparatory to connecting this district with the Menominee iron-bearing district to the southeast. This work was carried on in 1895 by Dr. W. 8. Bayley, 5. Weidman, and myself, and the mapping of the district extended as far as the Menominee district. Mr. H. L. Smyth has written Part II of the present report, covering the portion of the district which was worked by his party. My description of the part of the district worked by me is based largely on my own obser- vations. Many of the facts of field occurrence, however, mentioned in the following paper were observed and recorded by the several men mentioned above, and were subsequently verified by my own observations in portions of the area surveyed by myself, and by visits to localities in other portions. The topography of the greater portion of the district was taken by the members of the Lake Superior Survey. ‘The remainder we owe to the topographical division of the United States Geological Survey. The areas covered by the respective organizations are shown on the sketch map below the topographical map (PI. I). MODE OF WORK. As explanatory of the locations given in the paper, it is perhaps not out of place to give a brief description of the plan of work followed by the Lake Superior Division of the United States Geological Survey in this as ee ee ee MODE OF WORK. 23 well as in the other Lake Superior iron-bearing districts which have been previously surveyed. | The Upper Peninsula of Michigan affords an excellent example of the excellence which can be obtained in the rectangular land survey, when properly carried out by the Government. The section corner posts originally established are in many cases still to be seen, and of course the bearing trees are even more common. Since the original survey: the timber value has increased so much that in certain forested areas the section lines have been resurveyed. Not uncommonly trails follow the section lines for long distances. Moreover, the roads are frequently laid out along the section lines, thus giving permanent land boundaries. The section corners con- sequently offer the most reliable points from which to make locations. Traverses are made across each section, either frcm east to west or from north to south, and at varying intervals, according to the discretion of the geologist and the exigencies of the case. Hach geologist is accompanied by a compassman, whose duty it is to determine the course of the traverses by means of a dial compass, and the distance traveled by pacing at the rate of 2,000 steps to the mile. Corrections are made at the corner and quarter posts. The compassmen employed are Michigan woodsmen, land lookers or cruisers as they are frequently called, and it is remarkable with what accuracy they will pace mile after mile through swamp and over rough hills, windfalls, ete. The geologist explores the territory on both sides of the line followed by the compassman. Ledges are located by the geologist pacing to the compassman as he comes opposite him in a due east-west or north-south direction. With two coordinates thus determined, the ledges are located with reference to the starting point. For uniformity and to facilitate ref- erence and cataloguing, it is customary to give the location with reference tothe southeast corner of the section. Thus, 1,000 N., 1,000 W., SE. cor. sec. 5, T. 42 N., R. 383 W., gives the location of the outcrop at the center of the section, and affords a means of finding that ledge which could not be so accurately and concisely stated by the use of any ordinary land- marks. Moreover, easily recognized landmarks, such as houses, quarries, etc., are few, and exceedingly great changes may occur very rapidly, such, for instance, as those caused by widespread forest fires, so that such a method of location is practically valueless. 24 THE ORYSTAL FALLS IRON-BEARING DISTRICT. MAGNETIC OBSERVATIONS. It has long been known that many rocks are possessed of decidedly magnetic properties, due to the presence in them of varying quantities of magnetic iron ore. By the mining engineers and prospectors this property has been turned to a practical use in aiding in the location of iron mines where the ore is of a magnetic kind. It is only in the past three decades that this property has been used to any extent by geologists as an aid in the interpretation of the structure of a region. So far as I can learn, the best published account of its use thus is in Brooks’s report on the iron- bearing regions of Michigan.t Conelusive proof of its geological value was given in the mapping of the Penokee area, in 1876, by R. D. Irving of the Wisconsin survey.” That area extends for about 60 miles northeast- southwest, and is on the average about 4 miles wide. For the eastern part of the Wisconsin area the outcrops are few, and Irving located the iron formation by magnetic work. Along that belt have been sunk shafts belonging to various mines which have raised quantities of ore, and m no case has a shaft sunk outside of the limit indicated by Irving come upon paying ore. By means of the dip needle and solar compass, observations were taken which enabled us to trace a curving magnetic formation and connect the outcrops, which were separated by about 16 miles. The same bed was further delimited, and the direction partly checked, by the occurrence, at varying distances along this course, of outerops of rocks of the underlying formation. Since the second part of this report contains an exhaustive article on the methods and use of the magnetic needle,’ the subject is not further treated here. The lines of maximum magnetic disturbance—or briefly, the magnetic lines—are represented on the accompanying general map, Pl. II, by blue lines marked with letters D and HE. 1 Magnetism of rocks and the use of the magnetic needle in exploring for ore, by T. B. Brooks. Geol. Survey of Michigan, Vol. J, Part I, 1873, pp. 205-243. 2Geol. of the eastern Lake Superior district, by R. D. Irving. Geol. of Wisconsin, Vol. III,. 1880, pp. 53-238. Atlas sheets, XI-XXVI. 3 See Part If, Chapter II, by H. L. Smyth, pp. 336-378. CoP AG RA yell GEOGRAPHICAL LIMITS, STRUCTURE AND STRATIGRAPHY, AND PHYSIOGRAPHY. GEOGRAPHICAL LIMITS. The portion of the district here described extends from the north line of T. 47 N. to the south line of T. 42 N., and from the center of R. 31 W. to the west line of R. 33 W., and contains approximately 540 square miles. Upon the small sketch map at bottom of Pl. III is outlined the por- tions of the district which have been studied and described by the different authors. The detail character of the formations is unknown for parts of the area under discussion. This is especially true of the north, west, and southwest parts, where, owing to the readily decomposable nature of the rocks, as determined by the few ledges observed, and to the drift mantle, very few outcrops are to be found. STRUCTURE AND STRATIGRAPHY. The Crystal Falls district is not sharply defined petrographically, but is continuous with the Marquette district on the northeast and the Menomi- nee district on the southeast (Pl. I). It is, however, remarkable for the vast accumulation of voleanic rocks, which, while by no means absent from the adjoining districts, do not there play so conspicuous a réle. Structurally this district can hardly be better separated from the Menominee and Marquette districts than it can be petrographically. The important sedimentary troughs of the two adjacent districts are separated by an average width of 40 miles. The area between the districts on a direct course is occupied chiefly by Archean rocks, with narrow infolded troughs of Huronian rocks playing a very subordinate réle. At the east 25 26 THE CRYSTAL FALLS IRON-BEARING DISTRICT, the Archean is overlain by the sedimentaries of the Paleozoic, the Cam- brian, and the Silurian. The connecting Crystal Falls rocks are west of this Archean dome. In the Marquette district the essential structural features have been shown! to be a great east-west synclinorium, upon which more open north- south folds are superimposed. At the western end? of the district the superimposed north-south folds become close, and the Republic trough is a close fold with an axis in an intermediate position. In the adjoining Crystal Falls district there are also two sets of folds with their axes approxi- mately at right angles to each other. The closer folds are represented by the great anticline in the central part of the district. This anticline has its axial plane trending west of north and south of east, and the axis plunges down both at the north and south ends. The more open set of folds at right angles to the above set, is repre- sented by the Crystal Falls syneline, with its axis striking to the south of west, and plunging west. Farther south the axes of the folds become much closer and more nearly east and west, thus nearly according in direction with the close folds of the Menominee district. Thus the structural features of the Crystal Falls district merge into those of the Menominee district, which joins the Crystal Falls district on the southeast, where the great structural feature is a synclinorium similar to that of the Marquette, but with its axis trending north of west and south of east. ; A glance at Pl. III will show the presence in the eastern part of the northern half of the district of an oval-shaped mass of Archean, and, nearly surrounding this, a number of rock belts. The Archean ellipse is 11 miles long and 3 miles wide on the average. The rocks are mainly granite and gneiss. They are cut by rather infre- quent acid and basic dikes. Immediately surrounding the Archean is a quartzose magnesian lime- stone formation, to which the name Randyille dolomite has been given.’ In the eastern half of the district described by Smyth, where more numerous exposures are found than occur in the western half, the formation has an estimated thickness of about 1,500 feet.* Not only are the exposures ‘Mon. XXVIII, cit., p. 566 et seq. 2 Loe. cit., p. 570. ’See Part II, Chapter IV, by H. L. Smyth, p. 431. 4See Part II, Chapter IV, See. III, p. 433. STRUCTURE AND STRATIGRAPHY. Dall more numerous, but owing to the fact that the strata stand on edge, due to the closer folding of the rock series here, a more accurate estimate of their thickness can be made. According to Smyth, this limestone formation, in the southeastern end of the ellipse, at its upper horizon becomes mixed with slates, and these increase in quantity until the formation passes above into a slate formation, called the Mansfield slate." This slate formation is found overlying the limestone to the west of the central ellipse likewise, but as few outcrops have been found, it is not positively known to exist as a continuous zone encircling the northwestern end. In a direct line with its probable continua- tion to the north, a graywacke was found at one place, sec. 19, T. 46, R. 32. This single outcrop is insufficient evidence to warrant the introduction of a graywacke formation as the northern equivalent of a part of the Mansfield slates, and it is probably but a phase of that formation. The only mine of this district producing Bessemer ore is in a deposit in the Mansfield slate. The close of the Mansfield Slate time was marked by the extrusion of a great series of volcanics, which constitute the next formation in the succession. This volcanic formation has its best and most typical develop- ment west of the western Archean ellipse. Because the Hemlock River and its tributaries have exposed good sections in the voleanics, and because this river drains a great portion of the volcanic area, the name ‘‘ Hemlock formation” is applied to the voleanics. The dip of the flows and of the tuff beds wherever observed is about 75° west. The maximum breadth is about 5 miles. Deducting 15° for initial dip, this would give the enormous maxi- mum thickness of 23,000 feet to the volcanics, upon the supposition that no minor folds occur. These voleanic rocks have associated with them rocks of unquestionably sedimentary origin, as is shown by their well-bedded condition and the rounding of the fragments. The subaqueous rocks are, however, composed of little-altered volcanic materials, and evidently point to oscillations of the crust during the time of voleanic activity—such oscillations as have long been known to be common in volcanic regions. Following the voleanics, and overlying them, probably unconformably, comes a series of sedimentary rocks, believed to belong to the Upper Huronian. These comprise chloritic, ferruginous, and carbonaceous slates, 28 THE CRYSTAL FALLS IRON-BEARING DISTRICT. associated with quartzites, graywackes, and small amounts of carbonate: beds. The general character of the series is what one would expect in rocks the detritus of which was from the Hemlock volcanics. It is in this slate series that, with the exception of the Mansfield mine, the ore deposits of the Crystal Falls district are found. The sedimentaries extend west from the Hemlock voleanics to the limits of the district, underlying thus a very broad expanse of country. Where exposed, they show frequent changes of character. This prevents the identification of individual beds for any considerable distance. Owing to the imperfect exposures of the beds and their close folding, it has been found impossible to subdivide this 5) series of rocks into distinct formations. The series has in places been highly metamorphosed, resulting in the production of gneisses and mica-schists, in places garnetiferous and stauro- litic. The series corresponds in a broad way stratigraphically and litho- logically to the Michigamme formation of the Marquette district.’ Since, however, it has been found impossible to subdivide this series, and because it may possibly include more than the Michigamme formation of the Mar- quette district, it is considered advisable to speak of it simply as the Upper Huronian series. The generalized sections through the western half of the Crystal Falls district, which are given on Pls. V and VI, will aid in the comprehension of the structural and stratigraphical features thus briefly outlined. Here and there in the Crystal Falls district isolated patches of Upper Cambrian Lake Superior (Potsdam) sandstone are found. This occurs in beds which are either horizontal or only a few degrees inclined from the horizontal. They overlie unconformably the steeply inclined Huronian strata. The great lapse of time represented by this unconformity is indi- cated by the deposits of the Keweenawan and Lower and Middle Cambrian time, found elsewhere. The Lake Superior sandstone grades from the very coarse basal conglomerate below into a moderately coarse sandstone above. The sandstone is of a reddish brown to gray color, and is not well indurated as a rule, but is loosely cemented with ferruginous and in places calcareous. material. As a result of this imperfect induration, the sandstone is not very resistent to the agents of disintegration. Hence it is that only remnants. have been found, but enough is present to indicate that the greater part, 1 Fifteenth Annual, cit., p.598, and Mon. XXVIII, cit., p. 444. -. ee reigns ties = Daas U.S. GEOLOGICAL SURVEY x LNINEX \ es LISINTSING XK x ’ oY OEM 0. Eee BISNIS OES ZO IK OOOT at eet Nag tty / aut at MAGA A Oh Ty tty LN gt CAG, Suey XOXO Ala Aln_ SS Aer GENERALIZED SECTIONS THROUGH NORTH 3 HORIZONTAL SCALE, | INCH =1 MILE. ELEVATION OF BAS! NOTE: Formations are brought tothe surfa ARCHEAN (sa E Sturgeon and Ajibik Kono andRandvyille Man Granite quartzite | dolomite Sia 4) yi ‘ Uh vie nga Sips MONOGRAPH XXXVI PL.V Sketch showing location of sections on General Map. ae vs OOO P NLKY Ala Aln Au Atn Ala i STERN PART OF CRYSTAL FALLS DISTRICT VERTICAL SCALE, 1 INCH=1320 FEET. ES 1000 FEET. where exposures have been observed. \GONKIAN UPPER HURONIAN lo” nd Hemlock formation Groveland &Negaunee Undivided te formation MONOGRAPH XXXVI PL.v U.S GEOLOGICAL SURVEY Sketch showing location of sections on General Map DeerR. Alh Aln Ala Ala Aln Au Aln Ala NE Alin Als Ala sH NORTH ESTERN PART OF CRYSTAL FALLS DISTRICT : ; TIONS THROUGH NO GENERALIZED SECT * oe ae HORIZONTAL SCALE, ELEVATION OF = 1000 FEET. NOTE: Formations are brought © the ALGONKIAN. have been observed _ARCHEAN PONIAN eS ot ee | an Undivided las dville ‘GgrSlate Groveland &Negaunee i 5 ii Kono and Ra d ; Granite Sturgeon es viol dg a earnith formation Cp eew ; I (i Ay Tae or leprae Po \ ha ‘ ; y * ‘ ‘ mi a ‘ , U.S. GEOLOGICAL SURVEY wee OO ORS oe fos x MOOS a Ae a we SOs Pee Oe 1 RO Oe oS TR: WwW. Mi ichigamme renee Coen Acne JULIUS BIEN &CO.LITH.N.Y. GENERALIZED SECTIONS THROUGH 50} HORIZONTAL SCALE, 1 INCH=1 MILE. ELEVATION OF ARCHEAN Were een Granite Sturgeon and. Ajibik Kono andRandville Mansfield and Hemlock formatiol quartzite dolomite Siamo slate q MONOGRAPH XXXVI PL. VI SSS aa a aaa a ana aA | Ah teehee te Ad = POSIT IRY. a Alh =e fe ee Al ie) ee a : LR X HOON OO: Sketch showing location of sections on General Map. Alr ap st sc i} ey 7 ena aN PART OF CRYSTAL FALLS DISTRICT VERTICAL SCALE, 1 INCH=1320 FEET. S 1000 FEET. jyhere exposures have been observed. PLEISTOCENE a UPPER HURONIAN INTRUSIVE UNDIVIDED a SY and &Negaunee Undivided. ; Dolerite Gabbro ormation MONOGRAPH xxxyI PL. VI U.S. GEOLOGICAL SURVEY WI BN “7 LI ISIN MOK EOIN ARM XIN IY LO% ERROR OSS ONEREREALLA Mansfield | E : Ah Mi amme R, ee — <> Alh__Aker5¢ Ado W. Alh Au OE i VORA OO RU, VIA ote “ z ol Moda arn nesters A — = x SY DUS AY _ XOX NOC iM ON pine nboneoeuie eno Db Sketch showing location of sections on General Map NLE. ASb Michigamme River — = <7, NW. AN SSS ANNS ty antes wa SAA SASSY NN Ale Als py TO RTT GENERALIZED SECTIONS THROUGH SU@ERN PART oF CRYSTAL FALLS DISTRICT VERTICAL, E. SCALE, 1 INCH=1320 FEET. S , 1 INCH=1 MIL < LIN : i HORIZONTAL SCALE LEVATION OF BE we 1000 FEET. 13) wo 'Yy Where €Xposure : to the > s have been ob a. ARCHEAN NOTE: Formations are brought ‘AL GOSLAN observe PLEISTOCENE é UPPER HURONIAN INTRUSIVE DADIVIDED LOWER HURONIAN a pe BURONIA? ‘ a oy) AE tte 5, eS ee : mat ®gaune ne Z Granite — Sturgeonand Ajibik Kono andRandyille Mansfield and Hemlock ation | ‘-UNdividea . Dolerite Cappie quartzite dolomite Siamo slate STRUCTURE AND STRATIGRAPHY. 29 and probably the entire Crystal Falls district, was covered by Cambrian deposits. The thickness of the Cambrian deposits can not be determined. The next higher portion of the geological time scale represented in the district is that part of the Pleistocene period which in this part of the United States is characterized by the past existence of great ice-sheets. The evidences of the existence of the ice are everywhere present, either in the rounding and polishing and scoring on the surfaces of the rocks exposed or in the character of the drift deposits. The direction of the ice movement was clearly from the northeast to the southwest, as is shown by the trend of the striz, which were observed upon the rounded rock out- crops in various places. The thickness of the drift deposit varies very materially. In places it has been almost entirely removed by denudation, if in such places it ever formed anything more than a thin veneer upon the surface. In other places it reaches a very considerable thickness, as is shown by the glacial topography characteristically developed in T. 45 N., Rea. Ve As the present report is confined to the pre-Paleozoic rocks, no detail description will be given of these Cambrian and Glacial deposits, nor are they represented on the map, except in those places where it has been found impossible to map the underlying rocks. The generalized columnar section on Pl. VII gives in condensed form our knowledge concerning the formations mentioned. PHYSIOGRAPHY. TOPOGRAPHY. The topography in its large features is pre-Glacial, and in some cases this older topography is rather distinct. For imstance, in the case of the Deer River Valley, drift covers the gentle slopes and bottom, but is not sufficiently deep to completely hide the pre-Glacial Deer River Valley. In the southwestern part of the district west of Crystal Falls, or, more generally, west of the Paint River, pre-Glacial topography is seen in places. Here we find the drift as a veneer and only partly hiding the bed-rock topography, which depends mainly on the strikes, dips, and varying charac- ters of the rocks. It is so well known that this part of the country was at one time covered by ice, that it is useless to cite such proof as the rounding and 30 THE CRYSTAL FALLS [IRON-BEARING DISTRICT. scoring of the rocks and the character of the drift material, a good portion of which can be readily seen to have been brought from some other region, no such rocks as those forming it existing where the bowlders now lie. The ice-sheet left a deposit of drift, and we find the pre-Glacial topography essen- tially modified by it. As a result of this, the prevailing and most noticea- ble topography of the western half of the Crystal Falls district is that of the drift, and is characterized by short ridges and broken chains of hills, usually oval, though at times of very irregular outline, between which are lakes and swamps. The swamps are even occasionally found on rather steep slopes, where a thick spongy carpet of moss (sphagnum) retains sufficient moisture for cedars and other trees and shrubs characteristic of the Michigan swamps to grow. The swamps follow the carpet of moss up the hills to the spring line. The Glacial drift topography is especially marked where the drift was of considerable depth. These conditions are well exhibited in parts of T. 45 N., Rs. 31, 32 W., shown on the large-scale map, Pl. VIII. Here, even though the ground is very heavily timbered, one may easily trace out the sinuous course of the eskers. When traversing the country, one is constantly descending into pot-holes or is climbing ridges, some of them 75 to 100 feet high, often with a crest only a few feet, in some places not more than 4 feet, wide. Where the drift mantle has been removed, the rounded character of the rock exposures is usually shown. This holds good especially for the more resistant rocks, such as the granites and massive greenstones. Slates and tuffs, weathering more readily, have in numerous cases had time since the ice retreated to be weathered into rough broken ledges, some of which show perpendicular cliffs. The elevations range usually from 1,400 to 1,600 feet above sea-level. The hills rarely rise more than 200 feet above the low ground at their bases. The extremes of height noted in the district are from 1,250 to 1,900 feet above sea-level, corresponding, respectively, to the valley of the Michigamme on the south and the watershed between Lake Superior and Lake Michigan on the north. Between these two extremes there is a strip of territory, 25 miles across from north to south, in which the variations in height are within the limits of 200 feet. A consideration of the slight difference of level which prevails over U. S. GEOLOGICAL SURVEY MONOGRAPH XXXVI PL. VII PERIOD. FORMATION NAME. FORMATION SYMBOL CoLum- NAR SEC- TION. | THICKNESS, IN FEET. CHARACTER CF ROCKS. PLEIS- TOCENE. Glacial drift. CAMBRIAN. Potsdam sandstone. €p. ALGONKIAN. UPPER HURONIAN Undivided. Au LOWER HURONIAN. Hemlock | Alh. Mansfieid siate. | Alm. Usual characters. Thickness unknown Yellowish to reddish brown sandstone, not thoroughly cemented, therefore disintegrates readily, Found in patches in many places, and always lying either in beds which are horizontal or else possess slight dip to the south This may represent the initial dip with which the beds were deposited. A series of very great but unknown thickness. It consists of alternat- ing beds of slates, zraywackes, siderite, and chert. With these, espe- cially associated with the last two, are found hematite and limonite ore bodies of variable size and of great economic importance. From this series is derived nearly all the ore supplied by the Crystal Falls district. In the southern part of the district, especially well exposed in the vicinity of the Paint and Michigamme rivers, the slates and graywackes have been metamorphosed into schists and gneisses. This series is cut by dikes of rock ranging from acid to ultrabasic, which have, in places, metamorphosed the sediments. The thickness of this vast pile 0° volcanic ejectamenta can not be estimated with any degree of accuracy. It consists chiefly of interbed- ded acid and basic lavas and associated tuff deposits, and the water- deposited materi-Is derived from them. Near the top of the volcanics | a lenticular aiea of norma! sediments, siates with lenses of limestone, is found. This formation is cut by acid and basic dikes. Estimated to be about 1,500 feetthick. It consists of interbedded frag- mentals,slates,and graywackes and, associated with these, feriuginous chert andcarbonate. From these last has been derived the ore found associated with them The Mansfield mine, by which is exploited the only ore body in the Mansfield formation, supplies the only Bessemer ore of the Crystal Falls district These slates are cut and metamorphosed by basic dikes. Randville dolomite. | Alr. ARCHEAN. Gran te. N I> tbs ZAUSSae GENERALIZED The thickness is that estimated for this formation in the eastern part | of the district by Smyth. The prevailing rock is quartzose dolomite, of a very friable character. it shows ihe usual characters of granite. It is schistose on flanks of massif, and is cut by acid and basic dikes, which are mass ve and schistose. COLUMNAR SECTION. s ial ig tale Wyld : pr imonet ig an 2 2 yt - aaa ideal sid PHYSIOGRAPHY. 31 the greater part of the Crystal Falls district has led Smyth to the conclusion that this portion of Michigan had before Glacial times been reduced to the condition of an approximate peneplain. (See Part II, Chapter I.) This peneplain is a continuation of the peneplain of northern Wisconsin, and lies between the northern Michigan base-level on the north and the central Wisconsin baselevel on the south, to both of which attention has recently been called by Van Hise." DRAINAGE. The greater heights in the Michigamme district are i the northern part, where some few of the hills rise to a height of 1,800 feet, and one to a maximum of 1,900 feet above sea-level; but the majority do not rise above 1,600 feet. The belt including these higher elevations extends about NE-SW. This belt represents the crest of the watershed, from which all streams on the northern side flow to Lake Superior, and on the southeastern side all flow to Green Bay of Lake Michigan. _ A part of this watershed is undivided, and it is not uncommon to find extensive swamps in which streams flowing fo opposite sides of the watershed take their origin. The portion of the Crystal Falls district which is tributary to Lake Superior is so small that it will be totally neglected in the further discussion of the drainage. The topographical map, Pl. II shows the general slope and drainage of the district to be SSE. The eastern part of the district is drained by the Michigamme’ River with its tributaries, the Fence (Mitchi- gan), and the Deer, while the Paint (Mequacumecum) River, with its main tributaries, the Hemlock and the Net, drains the west and northwestern por- tions. The Brule (Wesacota) flows along the southern part of the district, being for the most part just below the southern limits of the present map. It forms throughout its course the boundary line between Michigan and Wisconsin. The Paint flows into the Brule in sec. 12, T. 41 N., R. 32 W., and the Brule and the Michigamme unite in sec. 16, T. 41 N., R. 31 W., to 1A central Wisconsin base-level, by C. R. Van Hise: Science, new ser., Vol. IV, 1896, pp. 57-59, 219. A northern Michigan base-level: ibid., pp. 217-220. 2 The Indian names which the streams and lakes of this district formerly bore have either been dropped or else, in a few cases, have been replaced by translations, though most commonly they have been replaced by English names, which are altogether new. Those names which have been retained receive various spellings at the hands of different authors, and even at the hands of the same writer. The Michigamme River, for example, is frequently spelled by Burt in the same article Peshakumme and Pesh-a-kem-e. The name Michigamme is also spelled on various maps Machigamig and Michigamig. Whereas the Paint we find spelled Mequacumecum, as above most commonly, though Burt spells it Mesquacumecum and also Jesqua-cum-a-cum. 32 THE CRYSTAL FALLS IRON-BEARING DISTRICT. form the Menominee River. This last flows southeast through the adjoin- ine Menominee district, and is the boundary line between Michigan and Wisconsin from its source to its mouth. A glance at the map, PI. I, will show the presence, especially in the northern half of the district, of a great number of lakes of varying sizes. These lakes of clear water, with bottoms of gravel, or most commonly of a thick deposit of decayed vegetable matter, are a very characteristic feature of the landscape. Many are in the midst of swamps, surrounded on ali sides by a quaking bog, which prevents one from approaching very closely ; others are surrounded by steep but low drift hills. The lakes may or may not have a visible inlet and outlet. In all cases the present water levels are considerably below the original water levels. In many cases the lakes are but remnants of much larger bodies of water. They are gradually filling up with silt and vegetable growth. These lakes, covered with float- ing lily pads and surrounded by more or less extensive hay marshes, are favorite places for the deer, which in many parts of the district are still fairly numerous. The numerous lakes indicate the youthful character of the drainage. Many of the streams head in thelakes. In other cases they flow through them, connecting them in chains. This indicates the mode or origin of the most of the streams of the area. The youthful character of the drainage is still further shown by the fact that with but few excep- tions the rivers have not reached rock. They are still cutting in drift. In the case of the Deer River this gradual development from the original condition of a chain of lakes to the present condition of a river im which the lakes play very subordinate parts is well shown. Moreover, its development illustrates very well several of the stages passed through by rivers in general, and for these reasons it may be well to describe it in detail. The life history of the Deer River,’ as it is to-day, began with the deposit of the drift, which destroyed the former streams of the district and concealed their records. It appears probable from the topography that the river occupies the same, or approximately the same, bed in which its pre-Glacial forerunner moved. The noticeable valley occupied by the stream is at a maximum about 3 miles broad, though its drainage area is a strip averaging 'The substance of the following was presented to the Wisconsin Academy of Sciences, Arts, and Letters at the annual meeting, September 27, 1895, in a paper entitled ‘‘Some stages in the development of rivers, as illustrated by the Deer River of Michigan.” An abstract of the paper was published in Amer, Geol., Vol. XVII, 1896, p. 126. . Soe — a ine ; ») me Gries Wo \ \ GNA y \ } » mn 6 | MAP OF PART OF CRYSTAL FALLS DISTRICT SHOWING GLACIAL TOPOGRAPHY AND ILLUSTRATING DEVELOPMENT o DEER RIVER 1 Mile Scale _____! eae mere ia ee. tae age ok Coal ee ri , cea . i s PHYSIOGRAPHY. 33 5 or 6 miles in breadth. The hills between which the stream flows are not very high above the river bed, the maximum elevation being 175 feet. ‘The few rock outcrops are in all cases found on the tops and flanks of these hills, where they have been exposed by denudation. At one point only has rock been found in situ near the river bed, and that is toward the mouth of the river. The conclusion is natural, since the river is 175 feet below these exposed rocks and has not reached rock, that it must be flowing through a preexisting depression or valley partly filled by the drift of the Glacial epoch. The partial fillmg of this valley at the time of the retreat of the ice to the northeast was accompanied by the fillme of the depressions in the drift by the water flowing from the front of the melting glacier. After the depressions were filled, the overflowing water naturally followed the general southeastern slope, which exists throughout the area and is shown by the topographical maps and by the flow of the rivers. The immediate course of the water was determined by the former valley, which was not completely obliterated by the drift deposit. Drift barriers across the valley separating the ponded water, or lakes, from one another were cut through, the material eroded being spread over the bottoms of the lakes below. Thus was formed a chain of lakes, connected usually by narrow streams; the processes by which the channels were cut out and the lakes drained and filled up with the débris were gomg on at the same time. The result has been to obliterate the lakes to a great extent and to accentuate the char- acter of the stream. ; The final effect of the processes, briefly outlined, would be to destroy the lakes entirely and produce a stream. By following on Pl. VIII the Deer River from its mouth to its source, we may see the several stages in its development, which are also typical for other streams of the glaciated portions of the world. The river is about 20 miles long and has a width near where it enters into the Michigamme of 20 to 30 yards. Near its mouth it is a slow-flowing, sluggish stream, which has nearly reached its base-level of erosion, and like many of the older streams of the Coastal Plain region of the United States is gradually fillmg portions of its channel with the silt and vegetable matter brought down from above. A short distance from its mouth it resembles such streams also in the MON XXXVI-—3 34 THE CRYSTAL FALLS JRON-BEARING DISTRICT. meandering character of its channel. This resemblance is still further enhanced by the presence of a remnant of a crescent-shaped cut-off, so_ characteristic of the old age of rivers. Just opposite this cut-off is a lake, which is of interest on account of its possessing two outlets, both leading into the river. Unfortunately this fact was observed on the topographical sheet too late to permit of a return to the field for the purpose of determining the cause of the presence of the two outlets. Passing up the stream we soon reach the lakes, which farther on become more numerous. The life history of these lakes is inseparably connected with that of the river. They reached maturity during or at the close of the Glacial epoch, and since that time their history is that of decline. This part of the history of these lakes may be briefly stated as follows: As the erosion continues, the areas of water are reduced and the surrounding swamp areas are correspondingly increased. If a lake were large and considerable inequalities existed in its bottom, two or more small lakes connected by the stream flowing through them may be formed. The final stage is a swamp, traversed by the slow-flowing river. The various stages in the history of the lakes are well illustrated on the accompanying map, Pl. VIII, by the following series of lakes. In Nos. 1 and 2 the general character of such bodies of water, which may be con- sidered essentially as mere expansions of streams, is seen. No.3, and Deer Lake, have long since reached maturity and are advancing rapidly to the point where they will each be separated into two bodies of water. No. 4 has already reached this stage, and in the swamp marked A we have the last stage, the swamp, with the stream flowing between peaty banks. On Light and Liver lakes, in the lower part of the Deer River, we may see all but the last of these stages illustrated. The lakes are attached to the main river by very short streams. The main river after leaving the rapids above, where it accumulates considerable detritus, enters a flat por- tion of its course partly occupied by the two lakes in question. Here, its rapidity beg diminished, the stream deposits the detritus. Thus it has eradually built a delta, now for the most part covered by swamp growth. This tends to advance the shore line, and thus diminish the water area. The rapid cutting down of the barrier immediately below the lakes by the swiftly-flowing stream tends to lower the lakes and thus diminish their surface area still more. PHYSIOGRAPHY. 35 The combined effect of the draining and filling has been to separate what was formerly a long narrow lake trending NE-SW. into three rounded bodies of water, two of which are connected with each other, the larger of these two and the third lake being connected with the main stream by very short necks. An artificial dam has been built across the narrow channel below the lakes, and the effect has been to flood the delta and unite the lakes into one large body of water, occupying, approximately, the area covered by the glacial lake, thus restoring the conditions which existed before the natural barrier had been trenched. In the remainder of the course of the Deer River the tendency of other artificial dams to restore the river to its original condition, that of a series of connecting lakes, is well shown. These dams were built by lumbermen at the foot of the lakes or swamps when it was desired to retain a large body of water at these places. When, on the other hand, the desire was to enable the logs to pass rapids, a dam GQnarked B on the map) was built near the head of the rapids. The back water would bring the logs to the dam, and on opening the gates the flood would carry them over the rapids into the deeper water beyond. The Deer River thus, after having reached a somewhat advanced stage, has been rejuvenated by the Michigan lumbermen. A study of the small tributaries shows the same condition of things, although not on so large a scale nor so perfectly as in the main stream. The source of the Deer River is in the copious springs which rise out of a spongy, marshy piece of ground less than 125 yards distant from Bone Lake, and about 20 feet below the usual water level of Bone Lake, and are really fed by the lake water percolating through the drift and appearing at this point. From the springs there is a depression which leads up to the lake. The highest point of this depression was about 3 feet above the normal water level of the lake. The outlet of Bone Lake is the Fence River. The river leaves the lake at a point three-quarters of a mile distant from the head of the Deer River Valley. In order to obtain a supply of water for driving the Fence River, Bone Lake has been converted into a reservoir. A dam was built at the outlet which raised the water about 4 feet, and the result was to turn some of the water of the lake into the Deer River, necessitating also a dam across this small valley near the lake shore. At present only a few strokes of the shovel would be necessary in order to turn the water of the flooded 36 THE CRYSTAL FALLS IRON-BEARING DISTRICT. lake from the Fence into the Deer River, thus gaining for it a drainage area extending 7 miles farther north and including three large lakes, the main sources of the water supply of the western branch of the Fence. I have no data which would enable me to show that the valley at the head of Deer River was ever a channel for the waters of Bone Lake. I am inclined to believe that such was not the case. For had it existed with the present slope, 20 feet in 875 feet, or even a much lower one, the water would have had a marked erosive power, and it would have cut back its channel much more rapidly than the Fence, which for a mile below the lake is a comparatively sluggish stream, and would have eventually captured Bone Lake and its feeders. The Deer River is still continuing the process of lengthening its chan- nel, and the springs which give it birth are gradually undermining the barrier at its head, so that it is possible that it will, unless artificially restrained, obtain much more water from Bone Lake than it does at present. A change in atmospheric and other conditions, which would insure a state of equilibrium between the incoming and outgoimg waters, thus preserving the waters of Bone Lake at their present level, would be favorable for the final successful robbery of the upper Fence River system by the Deer River. - This favorable condition, as may be readily seen, would be greatly increased in proportion as the increase of inflowing over outflowing water raised the level of the lake. TIMBER AND SOIL. The district was at one time very heavily timbered, with hard wood and pine, the former predominating on the whole. Along the flood plains of the large streams one finds sandy pine barrens where once there were heavy pie forests. On the headwaters the pine are found scattered through the hard wood. Individually these trees are very much larger and better than the thick and therefore smaller growth of the plains. Lumber- ing, which had been confined for years to the main drainage channels of the district, has of late been rapidly extended, following all the ramifica- tions of the tributary streams, until at present there remains in this district only a few years’ cut of pine at the very headwaters of the rivers. Following the lumbermen comes the forest fire, which finds its most nourish- ing food in the dry resinous pine tops left by them. The fires, once started, PHYSIOGRAPHY. BU are not confined, however, to the cut pine, but spread to the adjacent standing pine and even into the hard-wood forests, carrying destruction with them, and leaving but the gaunt, bare, and blackened trunks to mark the sites of what were formerly thick forests. The .pine-covered areas have a thin soil and are poorly adapted to agriculture. The areas covered with hard wood have, on the contrary, soil well adapted to the crops of the latitude. The advance of the lumberman has necessitated the damming and clearing of streams and the blasting of channels to permit the floating of the Jogs, and this has driven the fish, especially the speckled trout, which formerly crowded all the streams, into the smallest and most inaccessible ones. Ruffed grouse, Bonasa umbellus, and deer are still rather plentiful in certain portions of the area, although the pot-hunter with set guns, spring nooses, and pitfalls is rapidly exterminating them. The deadly character of such appliances is brought vividly to mind, when, as happened in my own case, one is suddenly arrested, while following a deer trail through the underbrush, by a hay wire noose around his neck, and he may be thankful if the bent sapling, having been bent so long as to lose its elasticity, fails to spring up and render the device effective. Oral ei live Ieee THE ARCHEAN. DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY. The granite described in this chapter belongs to the oldest system in the district, and forms the western elliptical core designated on PI. III as Archean. It is surrounded by sedimentary strata, which have a quaqua- yersal dip away from the granite as a center. The portion of the Crystal Falls district, in which the granite outcrops, is about 13 miles long by 3 miles wide, its longest axis extending in a NW. and SE. direction and covering parts of Ts. 44, 45, and 46 N., Rs. 31 and 32 W. The exposures of granite are especially numerous in the southeast part of the oval area, where, owing to the proximity of large streams, the #ence and Deer rivers, and the consequent increased erosion, the drift has to some extent been removed. In the northwest part of the area, with rare excep- tions, all the rocks are deeply covered with drift. In general the topography of the area is that of the drift, but in the southern part it is seen to have been considerably influenced by the char- acter of the underlying rocks. The granite usually outcrops in small, rounded, and isolated knobs, whose relations to one another can. only be conjectured. Where an occasional knob is composed of massive granite and more or less gneissoid granite, the exposed surface is so small as to prevent the observer from determining the relations between the two. Cutting the massive and schistose granite are certain long narrow masses of dark-colored rocks of rather fine grain, and, with few exceptions, very schistose. From their geological occurrence it was concluded, in spite of their appearance, that they are dike rocks cutting the granite. The follow- ing paragraph, quoted from the manuscript notes of G. O. Smith, describes very clearly their field occurrence: The gaps in this granite ridge seem to indicate greenstone dikes, as here the granite usually has a facing of the greenstone more or less extensive, and often in the center of the gap there are several small areas of greenstone. In all cases the 38 RELATIONS OF THE ARCHEAN. 39 greenstone is markedly more affected by weathering than is the granite. A study of the relations at the few points of contact did not yield much more than negative results, but these pointed to the intrusive character of the greenstones. RELATIONS TO OVERLYING FORMATIONS. The relations of the granite to the sedimentary rocks might be explained in two ways; the former may serve as the base of the latter rocks, or it may penetrate them. The occurrence of the granite in an elliptical shape, with sediments surrounding it showing quaquaversal dips, might be regarded as evidence of its intrusion in the Huronian sediments, and on this theory it would follow that the granite is of Huronian or post-Huronian age. If intrusive, it should be found to penetrate and metamorphose those sedi- ments. Against the intrusive character of the granite, and in favor of its pre-Huronian age, are the following facts: (1) There is a total absence in the surrounding sedimentary strata of any dikes which are related to the granite. (2) There is a total absence of any metamorphic action, so far as observed, in the sedimentaries. (3) On the east flank of the granite core, on the west bank of the west branch of the Fence River in the SW. corner sec. 1, T. 45 N., R. 32 W., is a recomposed granite, which passes up into a fine sericitic quartzite, with false bedding. These rocks evidently derived their material from the granite, and hence mark the beginning of sedimenta- tion in this area. Thus the positive evidence confirms the negative, and since the granite underlies the oldest sedimentary rocks, whose age has been determined to be Huronian, the former is classified as Archean, that term being used here to designate those rocks of undoubted igneous character which form the foundation upon which rest the oldest determinable sedimentary rocks. It is not the province of this paper to enter into a speculative discussion of the origin of the Archean rocks of the district. For such a discussion the reader is referred to Professor Van Hise’s exhaustive disquisition on the Principles of North American pre-Cambrian Geology,’ where the conclusion is reached that ‘the Archean is igneous and represents a part of the original crust of the earth, or its downward crystallization.”? The Archean has gradually reached the surface by the removal by erosion of the superjacent rocks. 12 ‘Sixteenth Ann. Rept. U. S. Geol. Survey, Part I, 1896, pp. 571-874. 2 Loe. cit., p. 752. 40 THE CRYSTAL FALLS [RON-BEARING DISTRICT, PETROGRAPHICAL CHARACTERS. The rocks of the Archean comprise biotite-granite, gneissoid biotite- granite, and acid and basic dikes. BIOTITE-GRANITE (GRANITITE). The rock occupying the main and central part of the Archean area is a biotite-granite. This rock is also found to some extent on the border of the area. The rocks of this kind vary in color from light-gray rocks to those having various tints of red, depending usually upon the degree of alteration. They vary also from medium to coarse grain. Some varieties show a decided porphyritic texture, and in some cases also an approach to a laminated structure. The porphyritic character is due to the presence of large crystals of feldspar, which stand out from the surrounding granitic groundmass, thus producing a typical granite-porphyry. The feldspar phe- nocrysts lie with their longer axes parallel, and thus help to produce an imperfect laminated structure. This parallel structure in the granite- porphyry is apparently analogous to the flow structure of the volcanic rocks, and probably was produced by movements in the magma before it had reached even a viscous state, as we find that the phenocrysts give no evidence of having undergone excessive mashing or torsion. The different textural varieties grade into one another in such a way as to indicate that they are merely modifications of the same magma. In addition to these textural varieties, which are original, we find in certain places a passage from massive to schistose rocks, in which the schistosity is of dynamic origin, i. e., of secondary nature. In the thin sections these rocks show the normal granitic texture and the usual mineral constituents which characterize biotite-granites. The chief minerals are orthoclase, microcline, plagioclase, quartz, and. biotite. Zircon and apatite are the accessory minerals present, and the secondary minerals include epidote-zoisite, chlorite, muscovite, rutile, and iron pyrites. Quartz occurs in grains forming the cement and molding around the other minerals. In one of the granites it has a peculiar saccharoidal char- acter macroscopically, and under the microscope such portions are resolved into very fine aggregates of quartz grains. The quartz is also frequently found in round blebs of varying size included in the best crystallized feldspar crystals. Thus the crystallization PETROGRAPHICAL CHARACTERS OF ARCHEAN. 4] of the quartz, unless such quartz represents the ‘‘quartz de corrosion” of the French authors, continued through the entire time occupied by the crys- tallization of the feldspars, since it is included in the oldest feldspar of the rocks, and also forms the matrix in which lie the youngest feldspars. Undu- latory extinction, so general in the quartzes, shows that the rocks have been subjected to pressure, and in some cases it has been sufficient to produce the extreme cataclastic structure of very greatly mashed rocks. The quartz includes numerous gas and fluid inclusions, the latter frequently with dancing bubbles and forming negative crystals, by means of which it is easy to orient the irregular grains. The quartz of one of the specimens was found to contain liquid inclusions, each of which, besides the usual bubble, held a small rectangular crystal. These crystals are trans- parent, with a light greenish tinge. A crystal similar in appearance found in the same quartz individual is partly inclosed by a large U-shaped bub- ble, and gave inclined extinction, though no further optical tests could be made upon it. Three kinds of feldspar are present: (1) A finely striated plagioclase ; (2) a feldspar, unstriated, or at most showing Carlsbad twins, and presumed to be orthoclase; and (3) microcline, these last two being frequently inter- grown after the manner of perthite. The plagioclase was the first feldspar to crystallize. It is invariably so altered that the twinning lamine are nearly obliterated, thus preventing accurate measurements. It is probably oligoclase; and if so, it is highly probable that much of the white mica produced by its alteration is paragonite instead of muscovite, a fact not determinable microscopically. The phenocrysts are orthoclase, usually in Carlsbad twins, and thus at first sight appear to have been the first feld- spar to crystallize; but I find that these phenocrysts not uncommonly inclose small rectangular, more or less automorphic,’ crystals of plagioclase, which is in reality the oldest feldspar. Hence these. orthoclases, notwith- standing their porphyritic character, are later than a part of the plagioclases. One phenocryst with Carlsbad twinning was observed in which one part of 1 Automorph, Xenomorph; Uber die Eruptivgesteine im Gebiete der Schlesisch-Maehrischen Kreideformation, by Carl E. M. Rohrbach: Tsch. Min. Pet. Mit., Vol. VII, 1886, p. 18. Idiomorph, Allotriomorph; Rosenbusch: Mik. Phys., Vol. II, 1887, 2d ed., p. 11. L. VY. Pirsson has recently proposed in a paper, read before the Geological Society of America, on A Needed Term in Petrology, the term anhedra for minerals which do not possess crystallographic outlines and are xenomorphic, in contradistinction to those which we properly call erystals and which are automorphic: Geol. Soc. Am. , Vol. VII, 1896, p. 492, and Am. Jour. Sci., 4th series, Vol. II, 1896, p. 150. 42 THE CRYSTAL FALLS IRON-BEARING DISTRICT. the individual shows microclinie striations. The other part was untwinned, and near the center of the phenocryst, bisected by the Carlsbad twinning plane, was found a rectangular plagioclase crystal. The microcline is usually the best crystallized feldspar in the ground- mass, and also by far the freshest. In the few cases in which it was observed in contact with plagioclase, the latter molded it, and is therefore older than the microcline, which in its turn is older than the orthoclase. In one case a microcline individual showing the lattice structure over a portion of its surface possesses no twinning lamellee in another portion, the twinning lamella fading until they totally disappear. Thus no sharp delimitation is apparent between the twinned and untwinned portions of the individual. In most slides all the feldspars are much altered, but even in those in which the microcline is fresh the plagioclase and orthoclase always show alterations, the plagioclase altering most easily and usually being so changed that itis with difficulty that one can recognize the twinning lamelle. Hence some of them may have been taken for the nonstriated orthoclase. In an early stage of the alteration of the feldspars minute dark ferrite particles which impregnate them are hydrated, and this gives the feldspars a more or less distinctly red tinge. In a more advanced stage of alteration, muscovite and a little epidote-zoisite are produced. Another alteration of the feldspar is always associated with marked pressure phenomena, and hence is pre- sumed to be the result, partially at least, of dynamic action. This is the partial or complete granulation of the feldspar and the production from that mineral, with the addition from other sources of the iron and magnesia necessary, of secondary white mica and quartz, and some biotite. It is highly possible that some of the small limpid grains considered to be secondary quartz are really an acid feldspar. Orogenic movements are also indicated by the bending of twinning lamellee, and were probably the partial cause of the twinning. : Biotite occurs in plates, and as a rule shows better-developed crystals than does the feldspar, though it frequently occurs in decidedly ragged flakes. It is strongly pleochroic, showing absorption in the followimg colors: Pale straw yellow to yellowish brown, for rays vibrating perpendicular to cleavage, to very dark chocolate brown and greenish brown for those par- allel to cleavage. In the case of the biotite showing a greenish color this PETROGRAPHICAL CHARACTERS OF ARCHEAN. 43 seems to be the result of incipient alteration, since the edges of the flakes are ragged, and in many cases almost the entire biotite of the section is altered to a chlorite, which shows ordinary white to light greenish pleoch- roism, with the simultaneous production of epidote and bundles of needles with high single and double refraction, having yellowish or brownish color. These needles are taken for rutile. The biotite is found usually lying between the feldspar and quartz grains almost as though it had been the last product of crystallization. It has suffered crushing with the other minerals. Apatite and zircon were observed in a few crystals. No original iron ore was seen. As intimated above, by the use of the term ‘epidote-zoisite” the exact character of this secondary material is not always determinable. In some instances: parts of an epidote crystal show the deep blue inter- ference color of zoisite, apparently indicating a mixture of the zoisite and epidote molecules, the latter predominating in the crystals." The remaining secondary minerals mentioned as occurring in the granite show their usual characters. GNEISSOID BIOTITE-GRANITE, BORDER FACIES OF GRANITE. About the central area of biotite-granite just described, and in part forming the border of the Archean area, are rocks having a gneissic structure. With these are associated the biotite-granites. The eneissoid rocks in general are markedly darker in color than the granites, showing normally a rather dark gray. They vary little from one another in texture and are much finer grained than the granites. The fine-grained condition of these schistose and banded rocks has perhaps a great deal to do with their dark color, though this is primarily owing te the amount of biotite present. In some of the specimens the bands can be readily distinguished under the microscope, and are seen to contain a white mica and a much smaller amount of biotite. These two minerals are present in fine films between the crushed quartz and feldspar grains, giving to the rocks a very decided schistose character. These mica folia are much more numerous in certain areas than in others, thus producing a more or less perfect banding. The mica plates are not all regularly parallel, although ordinarily having a 1On some granites from British Columbia and the adjacent parts of Alaska and the Yukon district, by F.D. Adams: Canadian Record Sci., Sept., 1891, p. 346. 44 THE CRYSTAL FALLS IRON-BEARING DISTRICT. tendency to this arrangement, and are usually parallel to the banding. The most perfect schistosity is thus developed parallel to the micaceous bands. The banding and the schistose structure are plainly of secondary origin, the result of dynamic action. Others of the gnessoid granites, however, when examined under the microscope, are decidedly massive, and it is only on a large scale that the banding shows distinctly. In such cases the cause of the bandmg could not be determined, and might by some be ascribed to differentiation, though, from the association of these gneissoid granites with those just described, it is assumed that the banded stricture is due to dynamic action. If this be the case, however, a complete recrystallization has taken place, and slight dynamic effects are now shown. The strike of the banding, wherever it was taken, was uniform, varying from N.-S. to nearly N. 45° W., agreeing, on the whole, with the trend of the Archean oval area. The microscope shows that the constituent minerals of the gneissoid granites are the same as those which compose the granites just described. These show also the same relations to one another and the same general char- acters as in the granites, except where mashing has completely obliterated the original texture, and hence no further description of them is necessary. The crushing to which the gneissoid granites have been subjected is very clearly shown in the present cataclastic condition of the quartz and feldspars. As stated above, both the gneissoid granite and the granite proper are found in the border area of the Archean. In those rocks in which the con- tact shows a gradual transition from the banded rock to the unbanded, the micaceous bands are clearly secondary, and are the result of the crushing of the original granite, these lines representing macroscopic and microscopic shearing planes along which the feldspar and quartz have been thoroughly eranulated, and sericite and some biotite produced, as was found to be the case also in some of the granites. These rocks thus agree in their dynamic origin with a similar but apparently more extensive and better developed eneissoid border facies in the Morbihan (Brittany) granites, which have been described, and whose origin has been so clearly demonstrated by Barrois.’ Numerous other similar cases have been described recently from the Canadian granite massifs and from Sweden and other districts. ‘Ann. Soe. Géol. du Nord., 1887, p. 40. ACID DIKES IN ARCHEAN. 45 ACID DIKES IN ARCHEAN. Observations upon dikes of acid rocks cutting the Archean granite are very few, and we may suppose this to be partly due to their occurrence in isolated knobs, which prevented the determination of the relations of adjacent exposures of rocks of slightly different character. Some few dikes were, nevertheless, observed, and are granites varying from medium to coarse grained, granolitic’ to porphyritic rocks. The porphyritic facies is the most common. The dikes do not show differences from the main mass of the Archean granite sufficient to warrant detailed petrographical description. The following description of one mass of granite-porphyry is given, as it offers good proof of its relation to the schistose border facies of the granite. In this case the gneissoid rock is found as inclusions in the granite-porphyry, as is illustrated in the accompanying diagrammatical sketch, fig. 4, taken from « ledge in the field. In this sketch the DAG Sand yt a i sharply outlined angular and lenticular areas represent the eneiss included in the granite-porphyry. The phenocrysts of this granite-porphyry have a_ parallel arrangement, the long direction of the phenocrysts agreeing also with the trend Mee Eee ee a of gneis- of the longer axes of the inclusions. The banding and foliation in the inclusions strike at a right angle to the flow- age structure of the granite. The lines of separation between the areas of gneiss and the granite, as shown in this outcrop, are sharp, and point to their nature as inclusions, and such is accepted as the true explanation of their angular character and sharp outlines. As this porphyritic granite was intruded through the border facies of the Archean granite, these frag- ments were taken up, and were so arranged as to agree with the direction of movement in the intruding mass. This occurrence shows this granite- porphyry to be younger than the great mass of Archean granite, whether we consider the inclusions to be a border facies of the Archean granite, derived from it by dynamic action, and thus of secondary origin, or to have resulted from differentiation of the molten magma. ‘This term has been proposed by a committee on nomenclature for the geologic folios of the United States Geological Survey, for use in place of ‘‘ granitic.” 46 THE CRYSTAL FALLS IRON-BEARING DISTRICT. BASIC DIKES IN THE ARCHEAN. The influence of the dikes on the character of the topography has already been mentioned. They occur im long narrow bands of varying widths, and with one exception are markedly schistose. Considering the granite on a large scale as an approximately homogeneous mass, we would expect to find lines of weakness, which might be indicated by the arrange- ment of the dikes. No such defimite arrangement can be seen, however, as the dikes are found to extend in all directions. A good example of their mode of occurrence may be seen in fig. 5, which also illustrates very clearly their influence on the topography. A small valley, in sec. 1, T. 44, R. 32, through which a brook flows, is occupied by the main dike, from which diverge the smaller ones, [Ne ficannonamoventescnists penetrating the granite on SSSRlar ne (CATACLNST IC) : (Gees both sides. These, not having been much more deeply eroded than the granite, do not form chan- nels deep enough to be shown on a map with a 10-foot contour interval. It is without doubt owing Scale ofmiles Cy vo % Ye 1G. 5.—Iustration of the effect on the topography of the differential erosion tO the fact that the dikes oS a aa weather so much more readily than does the granite that we may partly explain the comparative scarcity of exposures. The depressions separating the granite knobs are believed to indicate in many cases the position of dikes, but bemg now filled with glacial deposits, the underlying dike rocks, if such are present, are covered. Thus we find them exposed only where erosion has cleared this débris away, or where portions of the dike still border the steep sides of the granite on the sides of depressions. The dikes may be classified as (1) earlier dikes, showing a schistose structure, and with no trace of igneous textures, and (2) later massive dikes, showing original igneous textures. (1) SCHISTOSE DIKES. The general character of these rocks occurring as dikes may be briefly mentioned. They are schistose, for the most part fine grained, and black BASIC DIKES IN ARCHEAN. 47 in color. The constituents of the schistose eruptives, arranged according to their relative importance, are biotite, hornblende, chlorite, quartz, feld- spar (2), calcite, epidote, iron oxide, sphene, and muscovite. The clear limpid grains which form the groundmass are undoubtedly for the most part quartz. No satisfactory results were obtained in the tests for feldspar, but it is highly probable that some is associated with the quartz. Dark chocolate-brown to light-brown biotite is almost an invari- able constituent. In some cases it is accompanied by a little chlorite, which appears not to have been derived from the biotite. In a few rare instances biotite is absent altogether, chlorite taking its place. The biotite and chlo- rite are usually found between the quartz grains. They have a parallel arrangement, and this gives the rock its schistosity. Biotite and epidote are found included in the grains of quartz of the groundmass. Muscovite is rarely present, but when found is in medium-sized automorphic plates. Ragged pieces of ore, either ilmenite or titaniferous magnetite, and sphene, secondary to these, are found in almost all specimens, and in a few instances iron pyrites was observed. Calcite is invariably present in irregular, fairly large grains, almost equaling the quartz in quantity. Epidote is found in large quantity, both in crystals and in irregular grains, the crystals occurring among the bunches of biotite and included in the grains of quartz. The large amount of epidote in association with the calcite seems to point to the very basic character of the feldspar of the original rock. A bluish-green hornblende is rather frequently associated with the mica. In rocks in which the hornblende predominates mica is always pres- ent, but the reverse is not true, the most micaceous rocks being entirely free from the hornblendic component. The hornblende is found in large prismatic individuals without terminal faces. This mineral contains some of the other constituents of the rock in which it is found, such as quartz, epidote, and more rarely iron oxides. The interspaces between the hornblende crystals are filled with irregular biotite flakes and with grains of quartz, epidote, and iron oxide. This hornblende is apparently one of the last, if not the last, mineral to develop. The hornblendic rocks are not nearly so schistose as the micaceous ones. The secondary origin of the hornblende is clearly shown in one of the sections which is traversed by a fissure; the hornblende can be seen extend- ing into, and in places crossing, this fissure. The other minerals are 48 THE ORYSTAL FALLS IRON-BEARING DISTRICT, presumed to be secondary, but this can not be proved for them. The schistose character of the rocks is evidence of dynamic action. The pres- ence of undulatory extinction was noticed in the quartz of some specimens, but.its absence is the rule. However, from the absence of great pressure phenomena, and the remarkably fresh condition of the minerals composing the basic rocks, which contrasts strongly with the generally altered condition of the minerals of the more refractory acid rocks including them, it would appear that complete recrystallization has occurred.’ The schistose structure can undoubtedly be referred to the dynamic action which resulted in the upturning of the sedimentaries and caused the ° development of schistosity in certain portions of the border of the granite. This dynamic action was in all probability also the chief force in the pro- duction of the secondary minerals. The schistosity of the dikes does not agree in direction with the gen- eral strike of the schistosity throughout the entire district, but is always nearly parallel to the long extension of the dikes. These dikes represent belts of weakness, and it is therefore natural that the movements should occur along these belts rather than across them. This schistosity of the dikes also furnishes a slight clue as to their age. Younger than the granites they cut, they must have occupied their present position at the time the dynamic revolution took place which resulted in the development of schistosity in the granite, as well as in the sedimentaries, It is impossible to bring: the date of their intrusion within narrow limits. It seems very probable, however, that they were formed at the time of the extrusion of the basic Hemlock voleanics, though it is impossible to prove their connection with them. (2) MASSIVE DIKES. The only dike rock which retains to some extent its original texture is a much-altered medium-grained dolerite (diabase). The alterations it has undergone are those usual for such basic types of rock, and this one exhibits nothing peculiar or of special interest. An ophitic texture, while still recog- nizable, is more or less obscured by the uralite which has developed out of the pyroxene. The remnants of the original plagioclase feldspar present show exceedingly slight pressure effects. The alteration processes would ooo ——— —— =< ‘Principles of North American pre-Cambrian Geology, cit., pp. 706-707. RESUME OF ARCHEAN. 49 therefore seem to have been due to the action of percolating water, without special mechanical influence. Hence we may date the intrusion of this particular dike after the orogenic movements which affected the granite core, rendering portions of it schistose, and crushing all of it to a greater or less extent. ‘These movements are presumed to have taken place just prior to or during Keweenawan time; and therefore the age of this dike is Keweenawan or post-Keweenawan.' RESUME. Tn the above-described granite massif we have a rock of pre-Huronian age, as shown by its relations to the overlying sedimentaries. It possesses in general a coarse granular, and in places porphyritic texture. Along its border it contains portions which are much finer grained, darker than the rest of the mass, and very well banded. The boundaries between the banded rock and the granite at times are sharp, but frequently are very indefinite. This banded schistose portion is found to be due to pressure, causing the gradual passage from the granular granite to the gneissoid, schistose granite. One instance of undoubted inclusion of gneissoid granite by a true granite was observed. If the gneissoid granite was derived by pressure from the Archean granite, then the particular granite dike which includes the fragments must be of later age than the great mass of granite of the Archean area. The Archean is cut by basic dikes of two ages. The earlier ones were rendered schistose, and the production of this secondary structure was accompanied by a total obliteration of the primary igneous texture and the production of a large amount of mica and hornblende. All the dikes were probably injected at the time of the volcanic activity when the vol- canics of the higher series were ejected, but no proof of their connection ean be produced. They were, however, mjected before the folding of the area took place, as shown by their having been rendered schistose by it. A single dike belonging to the later series was studied. It is massive, and therefore was irrupted after the foldmg which produced the schistosity in the earlier series of dikes. It belongs probably to a Keweenawan or post- Keweenawan period of eruption. 'For a discussion of the orogenic movements which aftected the Crystal Falls district, the reader is referred to p. 158 et seq. MON XXXVI 4 ChE AG MU aBiekveg ave THE LOWER HURONIAN SERIES. This series is represented in the Crystal Falls district by the following formations, given in order from the base upward: The Randville dolomite, the Mansfield slate, and the Hemlock formation. At the beginning of the deposition of the Lower Huronian series the entire district was covered by the pre-Cambrian sea, with the possible exception of a small island in the Archean area. SECTION I.—THE RANDVILLE DOLOMITE. The best exposures of this dolomite are found near the center of the district east of the western ellipse and in the extreme southeastern part of the district in the Felch Mountain range. Both areas are described by Smyth, to whom we owe the name, and the reader is referred to his descrip- tion on p. 406 and p. 431 for the detail characterization of the formation. It will suffice for our purpose to state that it is a medium-grained crystalline dolomite. The few outcrops which I shall mention are important as showing the relations of the formation to the underlying rock, but are, petrographically considered, rather exceptional phases of the formation. Hence my descrip- tion will be brief. DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY. The area in which the Randville dolomite immediately underlies the drift is a continuous zone adjacent to and surrounding the Archean core. The belt varies slightly in width along the sides of the ellipse. At the ends it is two or three times the width at the sides. This is due to the lower dip of the beds at the ends. Exposures are found in the area studied by me only on the northeast and southwest flanks of the granite core. The west branch of the Fence River follows the limestone area for a short distance in the northeastern part of the district, skirting the Archean 50 PETROGRAPHICAL CHARACTERS OF RANDVILLE DOLOMITE. 51 granite. On the whole, however, the Randville dolomite has had no marked effect on the topography or the drainage. PETROGRAPHICAL CHARACTERS. In general the Randville dolomite consists petrographically of a fine- grained dolomite, with some quartz. This grades down through a calca- reous quartzite by increase of quartz into a true quartzite. The nearer the granite, the more quartzitic is the formation. At the southeast corner of sec. 2, T. 45 N., R. 32 W., on the west bank of the west branch of the Fence River, is a very good exposure of the quartzite. Its derivation from the underlying granite is here shown. The rock is a very fine-grained, almost novaculitic, quartzite. It shows current bedding in some places, though no true bedding was observed. Immediately below this quartzite is a very schistose rock, in which one can readily distinguish macroscopic- ally rounded to lenticular quartz areas, with masses of sericite flakes between them. The contact between the quartzite and the schistose rock seems very sharp when viewed from a short distance, but is found to be indefinite when closely examined. A close search was made along a con- tact for pebbles from the granite, but such were not found. However, small rounded pieces of vein quartz, most probably derived from the granite, were observed. The schistose rock in its turn grades down into a grayish granite, which is also more or less schistose. We have here evidently a transition from the granite, through the intermediate schistose recomposed granite, to the true sedimentary rock above. The meaning of this transi- tion is considered below. Under the microscope the cause of the schistosity of the rock inter- mediate between the granite and the quartzite is plain. Quartz and sericite, with some feldspar, are alone present in it. The quartz is grayish and granulated, and mashed out into oval areas representing original quartz grains. Various fragments constituting the areas are, however, angular and more or less equidimensional, and when not so never have a definite orientation of their longer axes. Between these large areas, but not between the individual small fragments constituting the areas, sericite is abundant. When the sericite is not predominant, the flakes lie in a fine mass of quartz grains, each of which agrees in long direction with the mica plates and large oval quartz areas. The sericite flakes are both included in this quartz, 52 THE CRYSTAL FALLS IRON-BEARING DISTRICT. and also lie between the grains. In one instance fragments of the original feldspar were found in the midst of such an area. These quartz-sericite areas are unquestionably of secondary origin, and the minerals have devel- oped in connection with pressure. They were probably produced from feldspar which existed in the original granite. Whether this schistose rock was formed from a weathered but not transported granite, from an arkose or feldspathic sandstone, or from the solid granite, it is impossible to say. A similar sericite-schist which developed from recomposed granites has been described by Van Hise as occurring at several localities in the Marquette district. In these cases at places the fragmental characters are still sufficiently clear to admit of the statement that the rocks are sedimentary. In the Crystal Falls rock mashing has destroyed all original characters. The rock occupies an intermediate posi- tion between a metamorphosed sedimentary and a metamorphosed eruptive, and grades on the one hand into the sedimentary and on the other into the eruptive. This makes it impossible to say whether it belongs exclusively to the one or to the other, or in part to both. Similar relations in other parts of Michigan were explained by Rominger’ as cases of progressive metamorphism of sediments, the granite being supposed to be the extreme stage of alteration of the sedimentary rock. Later the finding of basal conglomerates at or near these localities has shown conclusively that this explanation is incorrect, and it has been abandoned by Rominger. The quartzite, which immediately overlies the rock of doubtful char- acter, is composed of angular grains of quartz, between which are plates of sericite which have an imperfect parallelism, thus giving a certain degree of schistosity to the quartzite, possibly enough in places to warrant its being called a quartz-schist. The rock shows no conclusive microscopical evidence of a sedimentary origin, but differs from the cherts, with which it might be confused, in the size of the grains and in the presence of sericite. This rock was originally probably chiefly composed of quartz sand, with some feldspathic material from the disintegrated granite. Coincident with the pressure which produced the striking schistosity in the underlying rock, this sand was also mashed, resulting in the production of sericite and quartz ‘Mon. U. 8. Geol. Survey Vol. XXVIII, 1897, p. 226. >The Marquette iron district, by Carl Rominger: Geol. of Michigan, Vol. IV, Part I, 1878-1880, pp. 15-52. PETROGRAPHICAL CHARACTERS OF RANDVILLE DOLOMITE. 53 from the feldspar and in the crushing of the quartz grains, thus completely destroying the rounded clastic grains and obliterating all the sedimentary character of the rock, except the macroscopic structure of current bedding. On the west side of the granite ellipse, at N. 1750, W. 1550, sec. 12, T. 44 N., R. 82 W., about 100 yards from the granite, to the north, and lower down on the slope of the same hill on which the granite is found, is found a carbonaceous quartzite or quartzose dolomite. The strike is N. 25°-35° W. ‘The surface only is seen, so that the dip could not be taken. Microscopical examination shows the rock at the eastern side of the exposure to be made up of quartz grains held together by a fine-grained carbonate cement. This grades up to the west by increase of calcite and correspond- ing diminution of quartz to a quartzose dolomite. At N. 500, W. 1550, sec. 1, T. 44 N., R. 32 W., one-fourth mile distant from the granite, is seen another outcrop of a very dense quartzose dolo- mite, appearing macroscopically almost like a vitreous quartzite, but really with just enough quartz grains in it to enable the qualifying term ‘‘quartzose” to be appropriately used. The brown ferruginous crust on the weathered surfaces point to a percentage of iron in the magnesium-calcium carbonate. The pure limestones are to be sought slightly farther away from the Archean shore, where the conditions were more favorable for the production of a pure nonclastic sediment. RELATIONS TO UNDERLYING AND OVERLYING FORMATIONS. At only the one place cited above has a contact between the granite and the Randville dolomite been found. It is probable that unconformable relations exist, even though no basal conglomerate has been discovered as evidence of wave action on the Archean coast. Relations between the Randville dolomite and the overlying forma- tions have not been observed in the part of the district studied by me. THICKNESS. Reliable data for estimating the thickness of the Randyville dolomite have only been obtained in that area surveyed by Smyth. (See p. 433.) According to his estimate, the formation possesses a maximum thickness of 1,500 feet. 54: THE CRYSTAL FALLS IRON-BEARING DISTRICT. SECTION II.—THE MANSFIELD SLATE. The formation of the Lower Huronian, which is next higher than the Randville dolomite, is composed of sedimentary beds, in which a slate pre- dominates. This formation is found in its most typical development in a narrow valley through which the Michigamme River flows, and in which the village of Mansfield and a mine of the same name are situated. The valley and the slates are well known in the Crystal Falls district on account of their eco- nomic importance. For this reason the name “Mansfield slate” is here applied to this formation. DISTRIBUTIONS, EXPOSURES, AND TOPOGRAPHY. The part of the valley occupied by the Mansfield slates begins at the northern section line of sees. 17 and 18, T. 43 N., R. 31 W., and extends due south for 8 miles to the southern section line of sec. 29 of the same township. The slate belt is widest at the north, being over one-fourth mile wide on the westren side of section 17. To the south it gradually diminishes in width, until it finally disappears in sec. 29. The strike of the sedimentary rocks is almost due north-south, except in a few places where the rocks have been gently flexed and the strike varies a few degrees. The dip is high to the west, ranging from 65° to 80°. The influence of the Mansfield slate belt upon the topography is strikingly shown by the depression in which the slates are found, and which contains the Michigamme River. The slates are surrounded on all sides by igneous rocks which form fairly high hills, those to the west being composed of rocks of volcanic origin, those to the north, east, and south being intrusive, and later than either the sedimentaries or the vol- canics. The Michigamme River flows south through sec. 1, T. 43 N., R. 32 W., and meets the east and west ridge of intrusives in the northeastern part of sec. 12 of the same township and range. It cuts through this at an oblique angle, changing its course to the southeast In see. 7, T. 43 N., R. 31 W., it leaves the intrusives and penetrates a short distance into the voleanic rocks, their contact not being able to cause a change in the course of the river, owing to the slight difference in resisting power between the intrusives and the volcanics. Still flowing to the southeast, it finds at THE MANSFIELD SLATE. 55 the Michigamme dam, on the section line between secs. 7 and 18, near the southeastern and northeastern corners, respectively, the contact between the three kinds of rock, the sedimentaries, the voleanies, and the intrusives. Where the water leaves the eruptive and enters the sedimentary area the more easily erodible nature of the rocks of the latter is well shown by the falls which have been formed, the volcanics constituting the barrier over which the water plunges into a deep basin worn from the slates. Crossing the slates in the same direction, i. e., southeast, the river strikes squarely against the intrusive dolerites and is deflected to the south, following the contact between the two rocks for a short distance, then gradually working to the west into the center of the sedimentary area, the river takes an almost directly southerly course, with only minor bends. In the slates the river has fairly low flat banks on both sides. In the southern portion of the area the valley is narrower, owing to the progressive narrowing of the sedimentary belt. As soon as the river leaves the Mansfield slate belt, it resumes the sinuous course it had before the Mansfield belt is entered, and flows between high banks through the intrusives, out through the sand plains near Lake Mary. POSSIBLE CONTINUATION OF THE MANSFIELD SLATE. In sec. 10, T. 44 N., R. 32 W., about 7 miles northwest of the extreme northern end of the Mansfield area of slate, there are one or two exposures of much crumpled interbedded brown and black slates. Their strike is about N. 16°-20° W., but owing to their plicated condition the dip varies from 55° southwest over to 85° northeast. The average dip, however, is presumed to be to the southwest, which is in accord with the general structure of the area. The slate exposures are surrounded by coarse-grained basic intrusiyes, dolerites, which outcrop within short distances on all sides. The nearest sedimentary beds are quartzose dolomite ledges which outcrop 14 miles to the east, in secs. 1 and 12, T. 44 N., R. 32 W., rather close to the Archean granite. A section across the Lower Huronian rocks at this point shows the Archean granite overlain by quartzose dolomite, which is in its turn overlain by the slates. The relations which these rocks bear to one another are those which similar ones bear to one another near Michigamme Moun- 56 THE CRYSTAL FALLS LRON-BEARING DISTRICT. tain,’ and the slates of the two areas are consid red to be of the same age. Since the slates correspond. stratigraphically to the slates of the Michigamme Mountain and to those of the Mansfield area, they have been connected on the map with the slates of Michigamme Mountain by a narrow belt included between dotted lines; but this belt is not based on any connecting exposures. These two ledges of slate are taken as the northernmost outcrops of the Mansfield slate formation, although a number of miles north and in direct continuation of them along the strike there was found a single doubtful out- crop of a graywacke, showing neither strike nor dip. Whether it represents a shallower water deposit contemporaneous with the slates it is impossible to say. However, on such slight evidence it was not deemed advisable to continue the slate belt to this point. PETROGRAPHICAL CHARACTERS. A petrographical description of the Mansfield slate belt must neces- sarily be very brief, owing to the small area and to the scarcity of the exposures. The rocks of the Mansfield slate belt are graywackes, clay slates, phyllites, siderite-slates, cherts, ferrugmous cherts, and iron ores, with the various rocks which have been derived from them by metamorphism. They vary from coarse-grained rocks to very fine grained slaty ones. The latter predominate, and for that reason this belt is called a “slate” belt. The color of the rocks varies from an olive green and purplish black to bright red for those which are very ferruginous and more or less altered. The ordinary detrital rocks may be divided into the coarser and the finer kinds. The first are the graywackes, and the second are the ordinary clay slates and phyllite. There is, however, a gradation from the one to — the other. GRAYWACKE. The graywackes consist largely of grains of quartz and feldspar of unquestionably detrital origin. Associated with these is a large amount of mica, chlorite, and actinolite, with invariably more or less rutile. This last is in minute grains as well as in crystals. Many of the crystals show fine knee twins, triplets, and more rarely, heart-shaped twins. Tourmaline ‘See Part II, Chapter IV, Sec. IV, by H. L. Smyth. PETROGRAPHICAL CHARACTERS OF MANSFIELD SLATE. ON is sometimes present. The ferro-magnesian minerals develop chiefly from the alteration of the feldspar, and from the finer detritus which is presumed to have existed between the grains. As a consequence, the secondary minerals lie between the original grains. Many of the quartz grains are enlarged, and here the secondary minerals are included in the new areas of the enlarged grains. In numerous cases the new quartz occupies about as much space as the original grains themselves. This shows very clearly the porous character of the original sandstone. All original grains of the rocks show signs of extensive mashing. Some specimens contain a large amount of tourmaline in long slender crystals, which penetrate both the feldspar and the quartz grains. The presence of tourmaline is especially interesting as indicating that these sedimentaries may have been subjected to a certain amount of fumarole action. According to the proportion in which the various minerals have developed, we obtain sericite-, actinolite-, or chlorite- schists produced from the graywackes. CLAY SLATE AND PHYLLITE. The clay slates are dull and lusterless and are black, olive green, or red in color. They are usually impregnated with more or less iron pyrites in large macroscopical crystals. One can distinguish in them quartz, white mica, a few needles of actinolite, rutile, hematite, with a small proportion of a dark ferruginous and carbonaceous interstitial material. The amount of iron which these clay slates contain varies considerably. In some, hematite is present in such quantity ‘as to cause the slates to be appropriately called hematitic slates. Such, for instance, is the one forming the foot wall of the Mansfield ore body. The iron oxide gives to the slates a very bright red color where they are weathered. These weathered hematitic slates are very commonly known in the district as red slates, or as ‘‘paint rock” or ‘‘soapstone,” though rocks of very different character are also at times designated by these names. The phyllites have a silky luster and a bluish-black color. They are composed essentially of white mica quartz, some feldspar, innumerable minute crystals of rutile and dark ferruginous specks. ‘These seem to differ from the rocks called here clay slates only in that they are more completely crystalline, the interstitial material of the slates having disappeared. THE CRYSTAL FALLS IRON-BEARING DISTRICT. or CO ORIGIN OF CLAY SLATE AND PHYLLITE. The origin of the clay slates of the Mansfield formation is probably to be looked for in the disintegration and decay of the Archean granite, and the subsequent metamorphism of the resulting clay. For between the granites and the slates no other rock masses are known to have existed from which the clay could have been derived. The phyllites are presumed to have resulted from the metamorphism of the clay slates. PRESENT COMPOSITION NECESSARILY DIFFERENT FROM THAT OF ROCK FROM WHICH DERIVED. It is a well-recognized principle of rock weathering that in the altera- tion of rocks near the surface of the earth there is a relatively rapid diminution in the quantity of the more soluble constituents. Hence a clay shows a lower percentage of alkalies and alkaline earths than is found in the-parent rock, with an increase in the percentage especially of alumina and water. This relation is made clear by Adams in a statement of the comparison of the composition of certain slates and granites:’ “On com- paring the analyses of a series of granites with those of a series of slates, as, for instance, those given in Roth’s ‘Gesteins Analyzen,’ the latter are seen to be on an average considerably higher in alumina and much lower in alkalies, while at the same time they are lower in silica, which has been separated both as sand and in combination with the alkalies which have gone into solution, and in most cases contain more magnesia than lime instead of more lime than magnesia, as is usual in granites.” Adams con- cludes further, after a comparison of the alkalies in the slates and granites, that ‘The slates thus contain on an average about two-thirds of the amount of alkali present in the average granite.”* An examination of series of — analyses of granites shows that while the percentages of soda and potassa vary considerably, now the one being predominant, now the other, on the whole in the typical granites the potassa is higher than the soda.? This is the relation which we would expect in the case of an ideally pure granite, ‘A further contribution to our knowledge of the Laurentian, by F. D. Adams: Am. Jour. Sci., 3d ser., Vol. L, 1895, p. 65. 2 Loe. cit., p. 65. \ . 3 Zirkel states that in the weathering of granites the soda is much more readily removed than is the potassa: Lehrbuch der Petrographie, Vol. II, 1894, p. 32. PETROGRAPHICAL CHARACTERS OF MANSFIELD SLATE. 59 in which no anorthoclase replaces the orthoclase. As a consequence of the easier solubility of the soda, this relation between the two alkalies, soda and potassa, is maintained, and is often made more striking in the clay slates. An average of 31 analyses of clay slates taken from various sources shows two and one-half times as much potassa as soda. In the case of the Mansfield slate this difference has been increased, so that there is ten times as much potassa as soda present. ANALYSIS OF MANSFIELD SLATE. Mr. George Steiger, of the United States Geological Survey, has prepared a complete analysis (No. 1 in the following table) of a typical specimen of the Mansfield clay slate. Analyses Nos. 2 and 3 were pre- pared by W. Maynard Hutchings," and numbered by him Nos. 2 and 5, respectively. Analysis of the Mansfield clay slate. Constituent. il 2 3. BiOZo0008e 560000 cade00. 0005 60. 28 59. 28 53. 57 IWiOn canbs cose cosnDee65a05 6915. {Ee sroctetetsere|leiereeeisieiets INN Os dasaconeabaa nese asas 22. 61 21.85 24. 53 l@AO5. caosoncdbe onde coneoEs 2. pe \ = 80 6.51 IRXO mnoase cgasdeoceuse caes 45 |) WOO) Scud socaoacEcoecusen Trace:,: \|-ckareccaltetseaise = OCHO pcocssbeecescreeceae .13 45 76 IO pepcuanocsasoanene pees PAO) Eel ayia Sollesomacianca MO) SooooouononsceoooesE 1h a5) 1. 24 1. 81 IiG{O) See acapaeods qoseeecerc 5.73 4,13 4,34 Ney Oseadacesasonoosooeane 54 1.18 97 H20 at 100° Se SuULGGRLT a . 60 | oe Wes H20 above 100°........-.- 3.62 |) IPOs ooaaaoneageoo coon ouoG RBM es asa Gono ssoosKGEse OO) bosc50 oscGcn cnoonabede Nones isso ceeoon Boose sen ChSepeetrnacclec seeeesieie iets Sifts sears Raed llaeeeacoonn MO talltnemaecrciee oerl= 99. 57 100, 18 100. 12 COMMENTS ON ANALYSIS. That which is the most striking about the analysis is the relative pro- portion of the alkaline earths, lime, and magnesia, the latter being present ‘Notes on the composition of clays, slates, ete., and on some points in their contact metamor- phism: Geol. Mag., Vol. I, 1894, p. 38. 60 THE CRYSTAL FALLS IRON-BEARING DISTRIOT. in the greater quantity. As a rule, in all of the igneous rocks (and to the igneous rocks all clay slates owe their ultimate origin), except in the nonfeldspathic ultrabasic ones, the reverse condition exists, namely, the magnesia subordinate in quantity to the lime. The difference in amount of soda and potassa is very striking and shouid be noticed, in view of cer- tain points to which attention will be called in subsequent pages. The percentage of alumina is higher than is usual in the clay slates. It will be noticed that considerable water is present, but in consideration of the char- acter of the rock this is to be expected. If anything, the value is rather lower than would be expected, indicating a possible loss of water due to the rock haying already undergone some dynamic action. The carbon present is considered as offering trustworthy evidence of the presence of organic life at the time of the deposit of the slates, though no more satisfactory evidence of the existence of life has been found. COMPARISON OF ANALYSIS OF MANSFIELD CLAY SLATE WITH ANALYSES OF CLAYS, During the last few years there have appeared in the Geological Magazine, from the pen of Mr. W. Maynard Hutchings, some very elaborate and suggestive articles upon the composition of clays, shales, and slates, | and from one of these! I have taken two analyses of Carboniferous clays for comparison with the Mansfield clay slate. These two analyses, Nos. 2 and 3, p. 59, are from the very fine grained clays, in which the quartz was not distinguishable with the microscope, and are the analyses showing the highest and lowest percentages of silica. Mr. Hutchings says of his analyses that the samples were dried at 220° F., and that- the titanic oxide was not determined but is contained in the silica and alumina. Concerning - the clays, he writes: From these analyses it will be seen that these clays would be capable, chemically considered, of transformation into very typical “‘clay-slates.” Mineralogically they are clay-slates, having already undergone all, or nearly all, the mineral changes requisite to constitute the normal (unaltered) slates. Nothing more is needed but physical changes, such as compacting, arrangement of mica in a plane, increase of size of mica, etc.” The great similarity of these clays with the Mansfield clay slate is very evident. The only material difference which exists between them is in the 'Notes on the composition of clays, slates, etc., and on some points in their contact metamor- phism, by W. Maynard Hutchings: Geol. Mag., Vol. I, 1894, p. 38. 2 Loe. cit., p. 38. PETROGRAPHICAL CHARACTERS OF MANSFIELD SLATE. 61 higher percentage of water contained in the clays. This difference is natural, clays usually containing about twice as much water as do the slates. COMPARISON OF ANALYSIS OF MANSFIELD CLAY SLATE WITH ANALYSES OF OTHER CLAY SLATES. In the following table there are given, for purposes of comparison with the Mansfield clay slate, analyses of typical clay slates, roofing slates from the Cambrian of Vermont and New York. Analyses of typical clay slates. Constituent. 1. 2 3 4. 5 SiO} 23 cabScRORA BOs Ceeme 60. 28 62. 37 59. 70 67. 61 67. 89 (RI @owe eer eee sGecache : 69 74 27) 56 49 AUS Ostprie carat ee claeecees 22. 61 15. 43 16, 98 13. 20 11. 03 OOS Gocasdocopecesodees 2.53 1.3t 2 5. 36 1.47 OO Saeeremeccckts ome 45 5. 34 4,88 1.20 3. 81 Wii Osaoce secEeaeESeaees Trace. 22 16 10 16 CaOWaee ema tascece sss: 13 3 Ht 1.27 11 1. 43 BaOeecaeecectecsoe aces O4 07 08 04 o4 Mip- Osean ease seecieees 1.35 3. 14 3.23 3. 20 4,57 Kis Oe ascee cess ects vette ase) | . 20 3.77 4,45 2. 82 Nab Olss eae) eas wasee late 54 1.14 1.35 67 77 TELO) ARF MOOS ssegnscsosese -60 | 34 30 a.45 a.36 H20 above 100°..-.....-- 3.62 | 3.71 3. 82 b 2.97 b3. 21 PS Oe sens A) aici toca a 703) . 06 .16 . 05 .10 (OhO Ses aed oso Ree eee None 87 1.40 None 1.89 HO Steers es tev chy -evttersyevaisicrcille metertieve 5/5 | 06 | 1.18 . 03 . 04 Chee ese hae tae Se .97 | Trace. wud tetera eval eerste Motal sess ees ees 99.57 | $9.80 | 100. 05 100. 00 100. 08 aH,.0 at 110°. b H2,0 above 110°. No.1. Black slate, Sp. 32497, N. 450, W. 1620, sec. 17, T.43 N., R.31 W., Michigan. Analyzed by George Steiger. No, 2. Sea-green slate, Griffith & Nathaniel Quarry, South Poultney, Vermont. W. F. Hillebrand. No. 3. Black slate, American Black Slate Company, Benson, Vermont. W.F. Hillebrand. No. 4. Red slate, three-fourths mile south of Hampton Village, New York. W.F. Hillebrand. No.5. Green slate, three-fourths mile northwest of Janesville, Washington County, New York. W. F. Hillebrand. Nos. 2, 3, 4, and 5 taken from Analyses of rocks and analytical methods, 1880-1896, Clark and Hille- brand: Bull. U.S. Geol. Survey, No. 148. Nos.2 and 3 are, respectively, C and F, p. 277, and Nos. 4 and 5 are A and D, p. 280. 62 THE CRYSTAL FALLS IRON-BEARING DISTRICT. The strong similarity between the composition of these clay slates is at once apparent, and needs no further comment. The only marked differ- ence between the Huronian clay slate and the Cambrian ones is the higher percentage of alumina present in the former. SIDERITE-SLATE, CHERT, FERRUGINOUS CHERT, AND IRON ORES. The two most interesting kinds of rock from the Mansfield slate belt are those known as the siderite- or sideritic slates and the cherts or ferrugin- ous cherts, according to the quantity of iron carbonate and iron oxide present. These alternate with each other, and are found also interstratified with the fragmental slates, and thus there can be no question as to their sedimertary character. The siderite-slates are of a light to dark gray color. They are well laminated, and in some places cleave rather readily along the laminze, though at other places they break with an almost conchoidal fracture. The weathered siderite slates are covered by a crust of reddish- brown hydrated iron sesquioxide. Microscopically the siderite slates are composed of siderite, or of sider- ite and exceedingly fine grained cherty silica. Roundish rhombohedra of siderite compose the purer sideritic portions. If one passes from the pure to the less pure slates, the siderite gradually diminishes in quantity, the silica grains increase correspondingly, and the rock grades imto the chert which, in bands, is commonly associated with iron carbonate in the Lake Superior region. As the carbonate alters to the oxide or hydrated oxide ferruginous cherts are produced. The cherts are white to red, depending on the amount of iron oxide present. The manner in which the siderite alters to limonite and hematite, and the various steps of the process have been so well described and beautifully illustrated in Monograph XXVIII, that the reader is referred to that volume for further information. None of the brilliant red jasper or jaspilite, such as that found in the Marquette district, is associated with the Mansfield slates. Iron ores of economic importance, however, are found associated with these slates, and are described in detail farther on. None of the sideritic slates, ferruginous cherts, or ores, although interbedded with the fragmental slates, show any evidence of fragmental origin so far as the individual grains of the minerals composing them are concerned. PETROGRAPHICAL CHARACTERS OF MANSFIELD SLATE. 63 RELATIONS OF SIDERITE-SLATE, FERRUGINOUS CHERT, AND ORE BODIES TO CLAY SLATES. Owing to the scarcity of the outcrops of the sedimentaries in the Mans- field Valley, it is practically impossible to decipher the relations of the individual beds. Neither the study of the surface exposures nor the expo- sures in the mine workings have given definite results. That the beds repre- sent interbedded strata is well understood, but the sequence of the strata is indeterminable. It is of especial interest to determine, so far as possible, the relations of the ferruginous rocks, in order that the possible iron-ore deposits associated with them may be found. A cross section through the Mansfield mine from east to west shows the following relations: The foot- wall of black hematitic slate is overlain by 25 to 30 feet of ferruginous chert and iron ore. ‘This stratum is succeeded by “red slate,” so called by the miners, which is probably weathered greenstone impregnated with iron. This is followed by a conglomerate, and this by amygdaloidal greenstone, of the overlying volcanic formation. The ore body extends north and south, agreeing thus with the strike of the slates. All drifts end on the north in mixed ore, and on the south in mixed ore, with ‘“‘quartz-rock” and “lime-rock” of the miners in some places. From these facts we may justly conclude that the ore-bearing ferruginous cherts exist in beds in the slates or as lenticular masses which agree in dip and strike with the surrounding slates. This conclusion is confirmed by test pits along the strike of the exposed beds, which have disclosed similar ferruginous cherts at various places for a distance of half a mile to the north. RELATIONS OF MANSFIELD SLATE TO ADJACENT FORMATIONS. RELATIONS TO INTRUSIVES. The Mansfield slates are surrounded on three sides—east, north, and south—by coarse-grained basic eruptive rocks. The fact that they are so surrounded by these rocks, which cut them off in the direction of their strike, points to the later origin of these eruptives. Moreover, the quartzitic character of some of the sedimentaries shows that they could not have been derived from the eruptives which stratigraphically underlie them, for in these no quartz is found. The quartzitic character would thus seem also to indicate that the slates are older than the intrusives. Wherever the 64 THE CRYSTAL FALLS IRON-BEARING DISTRICT. igneous rocks and slates are in contact or in close association, the latter have been metamorphosed, and adinoles, spilosites, and desmosites have been formed which are similar to those described as occurring in other areas along the contact zone of basic intrusives. Although no single instance of a dike penetrating the slates has been found, it ean hardly be doubted from the relations which have been outlined that the slates are older than the intrusive dolerites. RELATIONS TO VOLCANICS. The sedimentaries are ‘overlain by volcanics, both lava flows and tufa- ceous deposits. In these tuffs, at the northeast corner of sec. 7, T. 43 N., R. 31 W., angular black-slate fragments have been found similar in every respect to the slates of the Mansfield belt. From this it is clear that at least some of the volcanics are younger than part of the slate formation. In section 29 similar relations obtain, the only difference being that the masses of slate and graywacke are inclosed in rather larger fragments in a volcanic conglomerate, and still retain very closely their normal strike. In the conglomerates near the Mansfield mine are found chert fragments and in some places fragments of iron oxide. These latter were evidently not included as oxide, but as fragments of cherty carbonate. Like the great mass forming the ore body, the fragments have since their deposition been altered, forming iron-oxide bodies of small size. Further discussion of the relations between the voleanies and slates will be found under the heading “Hemlock formation.” STRUCTURE OF THE MANSFIELD AREA. It has already been seen that the Mansfield rocks strike north and south and have a high westerly dip. The two possible explanations of this structure which are compatible with the facts in other portions of the area are (1) that they form a westward dipping monocline, and (2) that they are the western limb of an anticline. THICKNESS. As the sedimentaries forming the Mansfield belt now dip west at a very high angle, and as there is no evidence of duplication of strata due to fold- ing, I feel comparatively safe in giving an estimate of their thickness. The belt is widest at the north end, and there has a breadth of about 1,950 feet. THICKNESS OF MANSFIELD SLATE. 65 The average dip of the beds is 80°, and this gives a maximum thickness of 1,900 feet. Toward the south the belt rapidly narrows, until it is cut out by the intruding dolerites. A thickness of 1,500 feet is probably not far from the average. To the east of the Mansfield slates is a belt, varying in width up to about 1,200 feet, im which are found large masses of metamorphosed slates, surrounded by intrusive dolerite. In this belt the slate masses still show a general north-south strike, with slight variations to the east or west, and a westward dip. One might, perhaps, consider this a slate area which has been completely saturated with intrusives. If it should be so considered, this thickness should be added to the estimated thickness of the slates as above given, but as intrusives predominate in it, the slate being, as it were, merely incidental, I have preferred not to include it in the belt with the slate. ORE DEPOSITS. Although a great deal of exploring for iron ore has been done in the Mansfield slates, only one large body of ore has thus far been discovered, in which is the Mansfield mine. This mine is situated on the west bank of the Michi- gamme River, in secs. 17 and 20, T. 43 N., R. 31 W. The mine was apparently prospering when, on the night of Septem- ber 28, 1893, a cave-in occurred, letting in the waters of the Michigamme River and drowning 28 miners. For two hours after the caving occurred, the bed of the river below the mine was bare, the water flowing into the mine workings. The accompanying figure, fig. 6, prepared by J. Parke Channing, October 8, 1893, shows the relative position of the shaft and the river, and the concentric cracks caused by the caving of d : : Fic. 6. Concentric cracks formed by the caving in the mine. (Plan copied from address of presi- of the Mansfield mine. dent: Proc. Lake Superior Inst. Min. Eng., Vol. III, 1895, plate opposite p. 42.) The timber shaft is near the center of these cracks. After the caving the mine remained idle until recently. At the present writing the DeSoto Mining Company has obtained control of the mine and, I understand, have freed it from water. MON XXXVI——95 66 THE CRYSTAL FALLS IRON-BEARING DISTRICT, In May they began the task of diverting the channel of the river to a point several hundred feet south of the old course. They have dredged out a cut 2,650 feet in length by 100 feet wide and 18 feet deep. At the upper end of the new channel a cofferdam containing 14,000 cubic yards of earth has been constructed, and where the waters join the old outlet several hundred feet below the mine another embankment has been constructed across the course of the old bed that has 8,000 cubic yards of earth. ‘This task was a very expensive one, and it has been well completed, the old channel being perfectly dry. The turning of the river’s course brings out with startling distinctness the criminal negligence or carelessness of those who were working the mine at the time of the accident. The upper tier of timbers in the mine are plainly seen, as also the ground that had been cut out to receive the set that was being gotten into place when the waters broke through. This shows the miners had worked up to within 12 feet of the water of the river. A great crack in the formation shows where the water first gained entrance. The ore made up the bed of the stream—was a portion of the bed in fact—and the walls of the mine were nearly vertical. The ore deposit had a width of about 20 feet. The water pressure must have been considerable, and the blasting of the ore (as it is hard, and explosives are needed to loosen it) shattered the thin protection over the miners, permitting the water to find ready and unimpeded entrance into the mine. An engineer could not have been employed and the wildest sort of guessing must have been done by those who had the work in charge. No sane man would have permitted the opening of the deposit so near the river’s bottom. Owing to the long abandonment of the mine, the direct sources of infor- mation have been closed. For a description of the ore body I am com- pelled to rely on such data as are available from existing notes and plats. I am especially indebted to a manuscript description of the mine by J. Parke Channing, and to Mr. C. T. Roberts, of Crystal Falls, for plats of the mine. The sketch of the mine here introduced, Pl. 1X, is compiled from an original drawing of J. Parke Channing, reproduced on the plate cited, and from data obtained from other sources. GENERAL DESCRIPTION OF MANSFIELD MINE DEPOSIT. The Mansfield mine has an ore body varying from 16 to 32 feet im width. It is in almost vertical position; it has well-defined foot and hang- ing walls composed of impervious rock; it has a somewhat indefinite longi- tudinal extent. The ore is Bessemer and occurs in an iron-bearing formation, which corresponds in every particular to those of the other iron-bearing 1 Report of Commissioner of Mineral Statistics of Michigan, George A. Newett, for 1896, p. 84. ‘uoljoas sold -Z BI4 ‘uoljoas jeuipnyisuoy “| “sI4 ‘€681 NI NI GHAVD Ll 3YOISE SVM LI SV ANIW GISISSNVW SHL 40 HOLES JIVHS H38WIL LAVHS NIVIN XI “Id IAXXX HdVHYSONOW ASAYNS 1V9INO103S “Ss “Nn ORE DEPOSITS IN MANSFIELD SLATE. 67 districts of the Lake Superior region. The ore was first found in a test pit which passed through 9 feet of drift. The main working shaft was then located about 100 feet west of this point. It was put down to a depth of 460 feet before ore was struck. From this shaft crosscuts were driven east at average intervals of 70 feet, and the ore body was met at a distance vary- ing from 74 feet at the first level to 10 feet at the sixth level. The cross- cuts, in every case after leaving the greenstone, pass through so-called red slate, at the maximum about 25 feet thick, before ore is reached, this rock constituting the hanging wall. From these data the dip of the ore body “may be calculated to be about 80° W., agreeing well with the observed dip of the slates, which outcrop over the area. The thickness of the ore, as shown by the cross sections, averages about 25 feet. The extreme variation in thickness ranges from two sets, or 16 feet, to four sets, or 32 feet. The strike of the slates is north and south, and the trend of the ore body agrees with this. This brings its southern end under the original course of the Michigamme River as the stream bends slightly to the west, south of the shaft. An examination of the longitudinal (north-south) section through the ore body does not determine whether or not it has a pitch. The southern boundary is nearly vertical from top to bottom, while the northern boundary lengthens about 140 feet between the first and the fifth levels. In the northern end of the mine—that is, in line with the strike of the sedimentaries—the ore body terminates, in a more or less irregular way, in so-called mixed ore. This mixed ore continues to the north for over half a mile, as shown by the numerous test pits which have been bottomed in it. To the south of the mine shaft the ore body proper extends for 200 feet. It then changes its character, becoming a lean non-Bessemer ore. A long drift (335 feet) at the second level was run through this ore, and after leaving it penetrated a mixed ore, the so-called lime rock (siderite?) and quartz rock (chert?) of the miners. Three crosscuts along this drift show the ore body to vary from 20 to 30 feet in thickness, with the same foot and hanging wall as for the remainder of the mine. The same condition exists also lower down, as shown by a drift from the fourth level, 260 feet south. The figures on Pl. 1X, giving longitudinal and cross sections of the mine, show clearly the dimensions of the ore body. 68 THE CRYSTAL FALLS IRON-BEARING DISTRICT. RELATIONS TO SURROUNDING BEDS. The foot wall of the ore is a black slate, described as being rich in hematite and bearing large crystals of iron pyrite. No crosscuts have been driven for any distance into the foot wall, so that it is impossible to say what thickness of the hematitic black slate there may be before the greenish pyritiferous slate begins. In places a gray “‘soapstone” takes the place of the black slate as the foot wall. The dump obtained by sinking the shaft in the material overlying the ore shows large masses of conglomerate, the pebbles of which are rounded and predominantly of volcanic rocks, with pebbles of chert and slate from the iron formation and slates below. These fragments are well rounded. The microscope also shows quartz grains with secondary enlargements, so that there can be no doubt that the rock is a true conglomerate. Similar conglomerates, except that the sedimentary fragments are wanting, have been noticed farther north along the west side of the river. Just west of the bridge at Mansfield, near the mine, there is also a small exposure of con- glomerate, which shows an alternation of coarse and fine sediments, with a strike nearly north and south, and a dip of 80° W. ‘To the west, above this conglomerate, and not more than 15 to 20 feet distant, are found the lavas of the Hemlock volcanics. According to the mine captain, the succession west from the ore body in the hanging wall is 20 to 25 feet of paint rock, or, as it is usually called, red slate, then conglomerate, then greenstone. It is difficult to diagnose the paint rock, as no specimens are to be had, but it is highly probable that it is a ferruginous and extremely altered lava sheet. Similar rocks are commonly found thus altered in association with the ores in the Penokee-Gogebie and Marquette districts. Lending weight to this conclusion is the fact that in some places an amygdaloidal green- stone has been exposed in test pits immediately above the iron-bearing formation. COMPOSITION OF ORE. The Mansfield mine up to the present time has raised only Bessemer ore, and is the only mine in the Crystal Falls district which has supplied any considerable quantity of ore of this character. An average of a num- ber of analyses gives the following composition for the Bessemer ore: ORE DEPOSITS IN MANSFIELD SLATE. 69 Metallic iron, 64.80; phosphorus, 0.037; silica, 3.70.1 According to Dr. N. P. Hulst,” those ore deposits in the Menominee range which have poorly defined walls carry a minimum of phosphorus. This body, however, shows that the same conditions do not exist at the Mansfield mine, since, while it has both sharply defined foot and hanging walls, it contains but a low per cent of phosphorus. From an examination of the analyses from which the above average was obtained I find that the percentage of phosphorus shows a marked increase in the lower levels of the mines over that of the higher, and there is also a slight corresponding decrease in the content of metallic iron. Increase of phosphorus with depth is also found in the adjoing Menominee range, as noted by Messrs. E. F. Brown,’ of the Pewabic mine, and Per Larsson,* of the Aragon. It is impossible to state whether or not this distribution is due to the action of percolating water, as suggested by Hulst,’ Larsson,* and other Michigan mining engineers. Only a large number of good analyses from carefully selected ores and asso- ciated rocks and a detailed study of conditions of occurrence could lead to any accurate determination of the reason for such distribution, and a dis- cussion of these reasons is by no means warranted by the few and imper- fect analyses of the Mansfield ores, which I have been able to obtain. The ore body changes in composition to the south of the shaft, as shown by the drifts in this direction. The ore in this part of the mine contains more phos- phorus, alumina, and calcium, and less iron. This low-grade lean ore then passes over into the banded chert and ore mixed with the lime and quartz rock mentioned above. MICROSCOPICAL CHARACTER OF THE ORES AND ASSOCIATED CHERT BANDS. The ore varies from a soft limonitic hematite to a moderately hard hematite. It is for the most part opaque under the microscope, but in places shows bright-red to brownish-red color in transmitted light. In incident light the ore for the most part shows a dull-brown or reddish color, though in places it has a bright metallic reflection. In places in the ore ' An average of 62 per cant metallic iron and .030 per cent phosphorus is reported in Report of Commissioner of Mineral Statistics of Michigan (G. A. Newett) for 1896, p. 85. 2The geology of that portion of the Menominee range east of the Menominee River, by N. P. Hulst: Proc. Lake Superior Inst. Min. Eng., Vol. I, 1893, p. 28. 3 Distribution of phosphorus and system of sampling at the Pewabic mine, Iron Mountain, by E. F. Brown: Proc. Lake Superior Inst. Min. Eng., Vol. ITI, 1895, p. 49. 4Op. cit., p. 52. . 5 Op. cit., p. 28. 5 Op. eit., p.53, 70 THE CRYSTAL FALLS [RON-BEARING DISTRICT. are spots, in which is a large quantity of chert mixed with iron oxide. As such ferruginous-chert areas increase in quantity the ore grades mto the ferruginous chert and chert which is found associated with it m bands and lenticular areas. ORIGIN OF THE ORE DEPOSITS. The mode of occurrence and general characters of the ore body hav- ing been described, we are now prepared to determine the cause of concen- tration of the iron at this particular point and the source. From the description it was seen that the appearance of the body of ore was that of a bedded deposit. The microscopical examination shows, however, that the ore presents no evidences of clastic origin. An examination of the cherts and rocks of the area which are interbedded with the ore, and also a study of the southern contact of the ore body, shows that the ore is a chemical deposit, or the result of a replacement process, by which the original rock was largely removed, and its place taken by the present ore. It has been shown (p. 62) that the siderite bands pass into hematitie and limonitic chert bands. It has been seen that m the southern end of the mine the lean ore merges into a mass of ore bedded with chert and mixed with a rock called by the miners lime and quartz rock. I interpret this rock to be banded siderite and chert, possibly with some quartzite bands, all of which are found outcropping at the surface. The siderite evidently has been changed mto iron oxide and the silica replaced by iron oxide, the banding of the original rock not having been destroyed thereby. Irving’ considered siderite to be the source of similar ore and associated chert and jasper. Van Hise” has fully explained the process of the concentration of the ores of the Penokee-Gogebic and Marquette districts, and has applied the explanation to the other districts 1 Origin of the ferruginous schists and iron ores of the Lake Superior region, by R. D. Irving: Am. Jour. Scei., 3d series, Vol. XXXII, 1886, pp. 255-272. 2 The iron ore of the Marquette district of Michigan, by C. R. Van Hise: Am. Jour. Sci., 3d series, Vol. XLIII, 1892, pp. 116-132. Iron ores of the Penokee-Gogebic series of Michigan and Wisconsin, by C. R. Van Hise: Am. Jour. Sei., 3d series, Vol. XXXVII, 1889, pp. 32-48. The Penokee iron-bearing series of Michigan and Wisconsin, by R. D. Irving and ©. R. Van Hise, Tenth Ann. Rept. U.S. Geol. Survey, Part I, 1890, pp. 341-507. The Penokee-Gogebic iron-bearing series of Michigan and Wisconsin, by R. D. Irving and C.R. Van Hise: Mon. U.S. Geol. Survey, Vol. XIX, 1892, pp. 245-290. The Marquette iron-bearing district of Michigan, by C.R. Van Hise and W.S. Bayley, witha chapter on the Republic trough, by H. L. Smyth: Mon. U.S. Geol. Survey, Vol. XX VIII, 1897, pp. 400-405. ORE DEPOSITS IN MANSFIELD SLATE. 71 in the Lake Superior region. I shall not do more, therefore, than to add that the investigations in this area have shown the probable correctness of this explanation. It is very interesting from an historical standpoint to note that as far back as 1868 Credner had made the suggestion, with reference especially to the Marquette district, that the ores were derived from an original iron carbonate. The following quotation will show his idea of the processes of development of the ore:* Sphaerosiderit wurde aus kohlensiurereichen Gewiissen abgesetzt, durch eine theilweise Oxydation desselben entstand Magneteisenstein, durch weitere Aufnahme von Sauerstoff das Gemenge von Magneteisenstein und Rotheisenstein und endlich reiner Rotheisenstein; aus diesem sporadisch durch Zutritt von Wasser Brauneisen- stein. Credner’s suggestion seems to have been lightly considered by other workers in that area. In 1886 Irving’ suggested the theory of replacement of an original ferruginous carbonate to explain the Penokee-Gogebic iron ores. This theory has since then been elaborated by Van Hise, and shown to have a wider application to the other Lake Superior ore districts. He has also traced the iron to its source in the rocks removed by denudation, and shows why it occurs in the positions in which the ore bodies are at present found to occur. Moreover, Van Hise has also explained the process of development in detail, and, what is perhaps far more important, the reason certain ores develop and not others. In its essentials, however, the process is the same as that suggested by Credner in the lines quoted above, though in them no suggestion of the replacement to which is due the enrichment of the ore bodies is made. Much of the iron of the Mansfield ore is presumed to have resulted directly from the alteration of the ferruginous carbonate in place, but a large amount was brought in from above by infiltrating waters. The ferruginous matter, which was taken into solution during the denudation of the area, has been carried down by percolating waters and deposited at places favorable for its accumulation. The beds are now on edge, offering the most favorable condition to percolation. The conclusion is obyious that these deposits 1 Die vorsilurischen Gebilde der ‘‘Oberen Halbinsel von Michigan” in Nord-Amerika, by H. Credner: Zeitschr. deutsch. geol. Gesell., Vol. XXI, 1869, p. 547. 2On the origin of the ferruginous schists and iron ores of the Lake Superior region, by R. D. Irving: Am. Jour. Sci., Vol. XXXII, 1886, p. 268. (2 THE CRYSTAL FALLS IRON-BEARING DISTRICT. were formed after the beds were tilted, and the iron derived from the upward extension of the rocks, which has been removed by erosion. CONDITIONS FAVORABLE FOR ORE CONCENTRATION. The conditions favorable for the accumulation of ore deposits have been ascertained by Van Hise from studies in the other iron-bearing dis- tricts of the Lake Superior region. He summarizes these results as follows:’ [1] The iron ore is confined to certain definite horizons, known as the iron-bearing formations. . . . [a| All ore bodies have been found to be distributed very irregu- larly in these iron-bearing formations. This is due to the fact that they are secondary concentrations produced by downward percolating waters, and the ore bodies therefore occur at the places where water is concentrated, in accordance with the laws of the underground circulation of waters. [b] These places are just above an impervious formation, at the contact of the Upper Huronian and Lower Huroniau and where the rocks are shattered. [c| The impervious basement formation may be a surface volcanic, a subsequent intrusive, an argillaceous stratum, or any other impermeable formation. [d| These impervious basements are most effective when they are in the form of pitching troughs, thus concentrating the waters from the sides along a well- defined channel. These pitching troughs may be formed by a singie one of the above rocks or by a combination of two or more of them. The horizon marked by the uncon- formity between the Upper and Lower Huronian is a great natural zone of percolating waters. Here oftentimes the basement formation of the Upper Huronian is itself a lean ore, having derived its material from the Lower Huronian, but in this case a secondary -concentration has occurred in order to produce the present ore bodies. [e] Finally, as a result of folding, the iron-bearing formations have been shattered, thus producing natural water-courses. More frequently than not, more than one of these classes of phenomena are found together where the great ore bodies occur, and in many cases all are combined. The original source of the iron ores has been ascer- tained to be in many cases a lean carbonate of iron, often with a good deal of carponate of calcium and magnesium, formed as an ocean deposit. Van Hise adds to the above statement that generally the ore bodies, as a result of their methods of concentration, somewhere reach the rock surface. The Mansfield ore body has well-defined foot and hanging walls of normally impervious rock. The iron-bearing formation is much fractured. We thus have certain of the conditions favorable to the concentration of an ore body. Whether a trough is completed by a slight cross fold in the formation, or possibly by an intersecting dolerite dike, has not been determined. 1Fourteeuth Ann. Rept. U.S. Geol. Survey, Part I, 1893, pp. 107-108. ORE DEPOSITS IN MANSFIELD SLATE. 73 EXPLORATION. Exploration has developed no other deposits along the Mansfield slate belt+ If other deposits exist, it is highly probable that they extend to the rock surface—that is, are covered by the drift mantle alone. The intervals between possible ore bodies along the strike of the slates are probably occupied by mixed chert and ore or ferruginous chert. Explora- tions should extend from the impervious slate below the iron-bearing forma- tion to the impervious rock above the iron-bearing formation. In order to explore the belt thoroughly, rows of pits cross-sectioning the formation ought to be made at intervals not greater than 100 feet, and even with such inter- vals an important deposit might be missed, for it frequently happens that at the surface of the rock an ore deposit is smaller than it is at a moderate depth. SECTION III.—THE HEMLOCK FORMATION. This formation, the most interesting petrographically in the Crystal Falls district, consists almost exclusively of typical volcanic rocks, both basic and acid, with crystalline schists derived from them. Sedimentary rocks play a very unimportant réle. With one exception they have been formed directly from the volcanics, and occur interbedded with them. Cutting through the volcanics are intrusive rocks, which likewise: include both basic and acid kinds. Chemically the intrusive and extrusive rocks show very close relationships. The name Hemlock has been given to this volcanic formation because the river of that name flows through it for a number of miles, and in places affords excellent exposures. DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY. Beginning in sec. 86, T. 46 N., R. 32 W., the place where the Hemlock formation enters the part of the district studied by me, the formation has a width of one-half of a mile. From this place the formation has a north- western course for about 5 miles, gradually widening. It then bends to the west, and after a short distance to the south, which course it follows for about 9 miles. In township 45 N., Rs. 32 and 33 W., the belt has a maxi- mum width of 5 miles. At the end of the southern course the formation ‘Since the above was written I have been informed that Mr. George J. Maas, of Negaunee, has, with a diamond drill, located a body of bessemer ore 30 feet thick on lot 6, sec. 20, T.43 N., R. 31 W., 1 mile south of the Mansfield mine. . 74 THE CRYSTAL FALLS IRON-BEARING DISTRICT. bends to the southeast, and continues with this general trend for about 16 miles into T. 42 N., R. 31 W., where my field study of it ended. At the north the belt runs into the eastern half of the district described by Smyth, and swings south, which course is followed for some 15 miles. The entire belt thus forms an oval surrounding the sedimentaries, except in the southeastern part of the district. Another area of Hemlock volcanics is found in T. 43, Rs. 32 and 33, just north of Crystal Falls. This area is about one-half a mile wide just north of the city of Crystal Falls, but rap- idly widens as it is followed to the west until at the western limits of the area it is about 34 miles wide. A third small isolated area is found in sees. 7, 18, 19; and 20, 142 NG. 32: We, and! sec. 247 Wi aD ONE i oo Ves about 4 miles south of Crystal Falls. The topography of the Hemlock formation is exceedingly rough where- ever erosion has succeeded in cutting through the drift mantle. This occurs only adjacent to some of the streams. The rough topography at these places is due to differential erosion working upon rocks approximately on edge, and of varying hardness. The valleys usually indicate the location of beds of tuff and the higher grounds are almost universally occupied by dense rocks forming the lava flows, or of the coarse-grained massive intrusive rocks. In a few places, however, the thoroughly consolidated and indurated tuffs form high hills. In traversing the Hemlock formation one makes an abrupt ascent, followed by a sharp descent into a narrow swamp, then another ascent, and so on. Exposures appear for the most part in small areas along the edges of the swamps and scattered over the faces of the hills. These are fairly numerous, but so small and disconnected as to prevent the tracing out of the individual flows, although this might be pos- sible if the traverses were made at very short intervals and the area mapped in great detail. THICKNESS. As has been seen, the belt of eruptives varies in width from one- half of a mile to nearly five miles. The dip of the rocks is about 75° W. The enormous thickness of 25,500 feet which these data would give is probably illusory. In the case of the assumption of the thickness of a series of lava flows and tufts, it is important that the initial dip, which these deposits must have, be considered. This dip varies greatly, depending on the slope of the THICKNESS OF HEMLOCK FORMATION. 75 cone, which in its turn, is dependent on the viscosity of the lava and the — presence of varying quantities of fragmental products. If we assume these pre-Cambrian volcanic products to have had an initial dip of 15°, I believe we are within limits for products consisting, as these do, of what was prob- ably moderately viscous basalt and vast masses of fragmental material. This estimate is based on the assumption that the voleanies here represented were deposited for the most part upon the westward slope of a volcano, or a series of volcanoes. ‘This initial dip of 15° is then to be deducted from the present dip, 75°, of the flows. ‘Taking this into consideration, we get a thickness of 23,000 feet for the volcanics. , It is highly probable that the rocks have been subjected to close folding, and for this reason also the apparent thickness would be much greater than the true thickness. The schistose character of some of the rocks shows clearly that they have been severely mashed, and this mashing was probably produced in connection with folding. It is probable that this possible maximum thickness should be very materially reduced, possibly to one-half or one-third of the amount. However, even the maximum above calculated is probably paralleled by the vast masses of volcanic material accumulated in certain voleanic areas, such as those of Hawaii or Iceland. Geikie writes:* The bottom of these Iceland Tertiary basalts is everywhere concealed under the sea. Yet their visible portion shows them to be probably more than 3,000 meters in thickness. An especial interest belongs to this Icelandic plateau because volcanic action is still vigorous upon it at the present day. RELATIONS TO ADJACENT FORMATIONS. In the northern part of the Crystal Falls district the voleanics overlie the quartzose dolomite formation known as the Randville dolomite. In the central part of the district, through which the Deer River runs, as shown in section G—H, Pl. VI, outcrops are so scarce that it has been found impossible to trace the boundaries of the formations with any degree of accuracy. Consequently this part of the district is mapped as Pleistocene. From the few outcrops of slate, probably equivalent to the Mansfield slate; which have been found in the Deer River area, it has been thought ‘The Tertiary basalt-plateaux of northwestern Europe, by Sir A. Geikie; Quart. Jour. Geol. Soc, London, Vol. LII, 1896, p. 395. 76 THE CRYSTAL FALLS [RON-BEARING DISTRICT. highly probable that this slate in an extremely plicated condition may underlie the voleanics of this area, and it is so represented in section G—H, Plate VI. As evidence of this, in T. 43 N., R. 31 W. the volcanics overlie the Mansfield slate unconformably. In places test pits have disclosed an amygdaloidal lava flow immedi- ately overlying the Mansfield slates. At one place, at the northeast corner of sec. 7, T. 43 N., R. 31 W., angular fragments of the underlying black slate have been found in the tufaceous deposits of the Hemlock volcanics. Farther south, along the contact just west of the Mansfield mine, a con- glomerate is exposed, which contains fragments of slate, lava, and rounded erains of quartz with secondary enlargements. The rock is evidently water deposited. There is also obtained from the workings of the mine a conglomerate, taken from just above the ore, which consists of lava frag- ments and pieces of chert and ore, as mentioned on pp. 64, 68. From these occurrences it is clear that some of the sedimentaries are unquestionably older than some of the volcanics, and yet the conglomerates bearing the fragments of ore and slate contain also fragments of lava, showing the existence of some of the volcanics before the deposition of this conglom- erate. The only explanation of all of the facts which has occurred to me is as follows: After the ore-bearing Mansfield slate was deposited, an erosion interval occurred. Then followed a volcanic outbreak. It is highly probable that this outburst began far north of the Mansfield mine, coincident with the upheaval which resulted in the erosion of the Mansfield slate. The volcanic ejectamenta were mixed with the sedimentary fragments and all together were rounded and bedded, forming in places conglomerates. In places along the shore lava flows descended, some reaching into the sea and covering the sedimentaries along the shore where no conglomerate had been formed. At other places deposits of scorize, etc., including fragments of slates from the sedimentaries through which the volcano burst, were made, and thus deposits of tuff are found overlying the sedimentaries. The various deposits, though really separated by a slight physical break, are practically conformable with the series below, all having a north-south strike and a high westward dip. The formations which underlie the volcanics in the northern and southern parts of the district are of different character. This difference RELATIONS OF HEMLOCK FORMATION. a may be explained by supposing the voleanoes broke out in the northern part, while the Mansfield slate was still being deposited in the south. Gradually, however, the voleanic activity spread toward the south, proba- bly following a fissure along the pre-Cambrian shore, and igneous materials buried the Mansfield slate. Hence, while on the whole these volcanics are younger than the Mansfield slates, some of the lower of them are con- temporaneous with some of the upper Mansfield beds. The volcanics invariably overlie the Randville dolomite, and are unquestionably of later age than that formation. The Hemlock voleanics are overlain throughout their extent by the Upper Huronian series of graywackes and slates. Near the contact line with the voleanics wherever the Huronian outcrops, or has been exposed by exploration, it has been found to be characterized by a line of magnetic attraction. By means of magnetic observations the line of contact has been traced, where owing to lack of exposures it would have been otherwise impossible to connect the isolated outcrops. RELATIONS TO INTRUSIVES. High ridges composed of dolerite are found extending in a general north- west and southeast direction through the volcanics. That these masses were forced up through the Hemlock formation is indicated by the folding which they cause in certain places. Such rocks are unquestionably younger than the voleanie series. There may be seen also on the map, in T. 44 N., R. 32 W., a number of isolated knobs. These are also doleritic, and are presumed to be, like the larger ridges, intrusive in the voleanics. The dolerites have in their turn been cut by acid dikes. These are coarse micropegmatitie granites. Similar acid dikes have been found cutting the surrounding volcanics. This set of acid dikes may be looked upon as the youngest intrusive igneous rocks occurring in the Hemlock voleanic formation. Cutting the volcanics are also basic dikes varying from fine to moder- ately coarse grain. It is well known that during a volcanic epoch the out- poured lavas and clastic voleanic deposits are penetrated by dikes coming from the same magma. Whether or not these dikes are of this origin, and are hence contemporaneous with the later volcanics, or are of later age, and 78 THE CRYSTAL FALLS IRON-BEARING DISTRICT. correspond to the coarse dolerites, it is impossible to determine with certainty. They are presumed, however, to form an integral part of the Hemlock voleanies, as no connection between the dikes and the unques- tionably intrusive dolerites could be traced in the field. VOLCANIC ORIGIN. In spite of numerous occurrences of ancient volcanics which have recently become known, the late Professor Dana makes the following statement :? It is not yet certain that a voleano ever existed on the continent of North America before the Cretaceous period; for the published facts relating to supposed or alleged volcanic eruptions in the course of the Paleozoic ages are as well explained on the supposition of outflows from fissures and tufa ejections under submarine conditions; and none of the accounts present evidence of the former existence of a voleanic cone, that is, of an elevation pericentric in structure made of igneous ejections. The presence in the Hemlock formation of a quantity of pyroclastics, great in proportion to the solid lavas, and the absence of any great sheets of lava, so important a product of great fissure eruptions, seem to point to the derivation of the Hemlock rocks from a volcano or volcanoes situated near the border of the contemporaneous Huronian sea, rather than from a simple fissure. While some of the eruptives may have been submarine, the occurrence of large quantities of clearly subaerial deposits shows that the eruptives were largely on the land. Thus it appears that neither a fissure flow nor a submarine volcano will wholly explain the Hemlock formation. However, it is highly probable that this volcanic outburst, which piled masses of volcanic material upon the land, was accompanied, as have been all or nearly all the great outbursts of recent times, by submarine lava flows and tuff ejections. No such clear evidence of the presence of a Pale- ozoic or pre-Paleozoic volcano on the North American continent has been adduced as that given by the English geologists for certain volcanoes in the British Isles. But while the presence of a central cone with peri- centric arrangement in the Hemlock district is not conclusively proven, the presumption in favor of such a cone or cones having existed is certainly strong. !Manual of Geology, by J. D. Dana: 4th ed., 1895, p. 938. VOLCANIC ORIGIN OF HEMLOCK FORMATION. 19 An attempt was made to locate the vent or vents from which the material was derived, but no evidence could be found, unless we consider the vents to have been where the accumulations were the greatest. The coarse-grained rocks which were first supposed to represent:the plugs of ancient volcanoes, on careful and detailed examination appear to be later intrusives, or else are indeterminate. CLASSIFICATION. The general character and distribution of the Hemlock formation hay- ing been given, we may now proceed to a petrographical consideration of the rocks comprising it. This will be given in more detail than for the other rocks of the Michigamme district because this great pre-Cambrian volcanic formation possesses peculiar interest. The rocks of the Hemlock formation are chiefly of direct igneous origin. Some interleaved sedimentary rocks occur, which, however, with a single exception are composed of fragments of the igneous rocks. For the sake of easy reference, the usual classification into igneous and sedimentary rocks will be used. ‘The massive igneous rocks are subdivided according to chemical composition into acid and basic rocks. The acid rocks include rhyolite-porphyries,’ aporhyolite-porphyries, and acid pyroclastics. ‘The basic rocks include altered nonporphyritic basalts, porphyritic basalts, and yariolite and basic pyroclastics. The sedimentary rocks are divided into the voleanic sedimentaries and the nonvolcanic sedimentaries or normal sedimentaries. The first include tuffs and ash beds—the zeolian deposits, and voleanic conglomerates—subaqueous deposits. The normal sedimen- taries are represented by slates and limestones. Various schists are locally produced from these numerous kinds of rocks through metasomatic changes and dynamo-metamorphic action. Many of these schists resemble one another very closely, though, as will be seen later, they are derived from both the massive rocks and from the clastics. These have been described in connection with the rocks from which they have been derived. 1 According to a late ruling of the Director of the United States Geological Survey, based on the recommendation of a committee on nomenclature for geologic folios, ‘‘ porphyry” is to be used only with a textural significance. Hence “‘quartz-porphyry,” according to this ruling should no longer be used as arock name. The rhyolite-porphyries here described are what have been known as normal quartz-porphyries. 80 THE CRYSTAL FALLS IRON-BEARING DISTRICT. The following table will show the arrangement outlined above, which will be followed in the descriptions: Classification of the rocks of the Hemlock formation. § Rhyolite-porphyry -.-.- : . (Acid , § Lavas..---.-- ) Aporhyolite-porphyry - Schistose acid lavas . | PyToclastics icles cee eosin see ee eters See eter eeeee 3 Nonporphyritic.-..--. Igneous ...-.. ) ( WANE Saceo cose Metabasalte.-c-ss-—---=- S Roxphyaitieeeeseeeee Basic - . Variolitic ..... eee ( Pyroclastics..Eruptive breccia --...-.- } rapuive receis “77" > Crystalline RAIL tA schists. ; 4 GMOS =5 Selene ec ces Volcanic sediments. .- - J Eolian deposits. ..---.. d Ash beds...........- i Subaqueous deposits...Conglomerates ..----.. Sedimentary - Sie TBR Sean RMR Ail Mme kA iN ane aU | Normal sediments -.... - piimibetone stot soak Omit ett eet ite ACID VOLCANICS. The acid volcanics are comparatively unimportant in quantity. They may be conveniently subdivided into the lavas and pyroclastics. ACID LAVAS. The acid lavas occur in such small quantity as to make it impossible without very great exaggeration to place them upen the accompanying small-scale general maps, though they have been introduced upon the “detail maps wherever the scale permitted. They usually form isolated ridges, and their relations to the surrounding basic voleanics are obscured by lack of exposures. The trend of the individual ridges agrees with the general strike of the banding in the basic tuffs. Moreover, in nearly all cases the isolated exposures which are closest together lie in such relations to one another that when connected the large sheets thus formed follow the strike of the tuff banding, as do the individual ridges, and they are there- fore confidently assumed to be the isolated portions of acid flows inter- bedded with the basie voleanic rocks. The rock types represented are the two closely related rocks—the rhyolite-porphyry and the aporhyolite-porphyry. Under the rhyolite- porphyries are included the porphyritie acid lavas, which have, so far as can be determined, an original holocrystallme groundmass. Under the aporhyolite-porphyry, following Miss Bascom’s use of apo,’ I include those 1 Structures, origin, and nomenclature of the acid volcanic rocks of South Mountain, Penn- sylvania, by Miss Florence Bascom: Jour. Geol., Vol. I, 1893, p. 816. ACID VOLCANICS OF HEMLOCK FORMATION. 81 acid lavas which are now likewise holocrystalline, but which owe this character to the devitrification of an original glassy base, supposing them in their original vitreous condition to have corresponded to the modern hyalorhy olite-porphyries. RHYOLITE-PORPHYRY. The rhyolite-porphyries on fresh fracture are dark grayish-blue to black. From this they grade with advancing alterations through chocolate brown to purplish. The weathered surface varies from white to reddish. The weathering has in one case brought out very well the fluxion banding of the rock. Their texture is very pronouncedly porphyritic. The quartz and feldspar crystals stand out plainly from the groundmass, which is usually dense with a somewhat resinous luster. The porphyritic quartzes average perhaps the size of a small pea, and hence are macroscopically very plainly visible. .They frequently stand out on the weathered surface and show their crystal forms, and in other cases we see the angular cavities out of which they have fallen, like the kernel from the nut. Under the microscope the rocks are seen to be typical rhyolite-por- phyries. The phenocrysts are chiefly corroded dihexahedral crystals of quartz. Crystals of plagioclase and orthoclase are less common. ‘These lie in a very fine-grained holocrystalline groundmass, composed largely of feldspar and quartz, with some zircon in small crystals, and here and there magnetite. These are presumed to be the original constituents of the groundmass. Associated with them are considerable quantities of secondary chlorite, epidote, biotite, muscovite, calcite, and reddish to brown alteration products of the magnetite. Included in the groundmass are here and there oval areas of finely crystalline secondary quartz, probably fillmg former amygdaloidal cavities. In thin section the crystal contours of the quartz phenocrysts are more or less rounded, with here and there embayments of the groundmass projecting into them. The crystal form is, however, always clearly marked. In some cases the individuals have been broken before the cool- ing of the magma, the fragments of an individual, though now separated, being seen to conform to one another. That they have been subjected to pressure is shown by the undulatory extinction and also by the separation MON XXXVI——6 82 THE CRYSTAL FALLS IRON-BEARING DISTRICT. of the black cross of uniaxial minerals into hyperbole. HKmbayments of groundmass, and liquid inclusions in which a dancing bubble may be seen, are in places rather thickly distributed through the quartz. The liquid inclusions have very commonly an hexagonal form, corresponding to the contours of the inclosing quartz. These liquid inclusions are certainly in some cases secondary. This character is well shown in some of the crystals, which are broken across, giving along the line of fracture a very wavy extinction. Along this lme of fracture the greatest quantity of inclusions are seen, both with and without bubbles. As the distance from a fracture increases, both the undu- latory extinction and the number of inclusions diminish. (See fig. 4, Pl. XIX.) These fractures in the quartzes are but continuations of those which extend in many cases all the way across the section. ‘The fractures have since been healed by secondary quartz. This secondary quartz has also in some cases healed the fractured quartz phencerysts, and then agrees with them in orientation. The possession of an imperfect rhombohedral parting is very noticeable in a number of quartzes, and especially those which, being on the edge of the section, are very thin. (See fig. B, Pl. XIX.) Similar parting in the quartz occurs im various rocks studied in this district. The phenocrysts of the porphyries are traversed by fractures, some of which are more or less circular, and simulate very imperfectly perlitic cracks. With the exception of those in porphyries in two localities, the quartz phenocrysts are surrounded by zones, largely of quartz, of varying widths, and considerably lighter than the remainder of the groundmass. Much of the quartz of these zones has the same optical orientation as the phenocrysts. In those sections in which the zones are observed they occur around every section of quartz. The feldspar phenocrysts are orthoclase and plagioclase, the latter apparently predominating. They occur usually in rounded, badly corroded crystals, with indentations filled with groundmass. They are always altered, and have associated with them as secondary products calcite, epidote, muscovite, biotite, and chlorite No large original ferro-magnesian phenocrysts appear to have been present in the porphyry. Their former presence is at least not indicated by any aggregates of secondary products. Whatever ferro-magnesian min- ACID VOLCANICS OF HEMLOCK FORMATION. 83 erals were present must have been scattered through the groundmass, and have been completely altered. The secondary minerals contained in the groundmass are chlorite, calcite, epidote, muscovite, and biotite. TEXTURE OF THE PORPHYRIES. The texture of the dense groundmass varies according to the mode of association of the two chief minerals—quartz and feldspar. The commonest variety is the rhyolite-porphyry with microgranitic groundmass (porphyre granulitique of Michel Lévy). A second variety is the rhyolite-porphyry with micropoikilitic groundmass.'. The microgranitic texture is too well known to warrant a description of it here. The micropoikilitic texture presents certain characters which render a further description desirable. This peculiar phase of the micropoikilitic texture was briefly described by the writer and illustrated by microphoto- graphs in 1895.” Shortly after the separates of this article were distributed, I received from H. Hedstrém, of the Swedish Geological Survey, an article published in 1894 containing a description of what appears to be very nearly the same texture.* If I have understood the description correctly, however, there seems to be one essential difference. In order to explain clearly this difference, I shall describe the texture in detail. In certain of the rhyolite-porphyries, as already mentioned, the quartz phenocrysts are surrounded by certain zones. These zones in the rocks having a micropoikilitic texture possess exactly the same texture as does the groundmass. The zones are composed of minerals which are of suffi- cient size to permit readily their determination. Quartz and feldspar are the essential components, with some chlorite, epidote, muscovite, and iron oxide. The first two are the important minerals, and will alone be referred to in the further description. The chief peculiarity of the zone is in the arrangement of the two minerals, and this character is best shown on the accompanying microphotographs. This texture can be seen even in ordinary light. It is brought out better when the field is partly shaded, so 'Eruptive rocks of Electric Peak and Sepulchre Mountain, by J. P. Iddings: Twelfth Ann. Rept. U.S. Geol. Survey, 1891, p.589. On the use of the terms poikilitic and micropoikilitic in petrography, by G. H. Williams: Jour. Geol., Vol. 1, 1892, pp. 176-179. * Volceanics of the Michigamme district of Michigan, by J. Morgan Clements: Jour. Geol., Vol. IIT, 1895, pp. 814-816, figs. 1 and 2. *Studier 6fver Bergarter fran Moriin vid Visby, by H. Hedstrém: Geol. Féren i Stockholm Forhandl, Bd. 16, H. 4, 1894, pp. 5-9. 84 THE CRYSTAL FALLS IRON-BEARING DISTRICT. as to exhibit the difference in relief of the minerals, and, best of all, between’ crossed nicols. (See fig. 4, Pl. XX.) The zones are seen to be made up of reticulating areas of clear quartz, in which lie embedded irregular pieces of feldspar. Where two or more of the quartz stringers or needles unite, one sees broad areas of limpid quartz. The network of quartz is best seen when it exhibits its highest polarization color, as then the feldspar is for the most part dark. The pieces of feldspar in such a quartz area for the most part have irregular orientation, as is shown by their varying extinction, although a number extinguish simultaneously. This quartz net is connected with the quartz phenocrysts, as shown by the continuation of the quartz of the phenocrysts and that of the zone, and the consequent agreement in orientation. The lack of a uniform optical orientation of the feldspar grains is made especially apparent when the quartz is cut perpendicular to the ¢ axis, and consequently remains dark under crossed nicols. Under the above circumstances we see certain feldspar grains polarizing in the zone around the quartz, and as the stage revolves other particles lighten as those which polarized in the previous position of the stage become dark. From this description it is evident that the texture is not micropegmatitic according to the generally accepted definition of the term, but corresponds to the micropoikilitic, as described by Iddings and Williams.* A gradation toward a spherulitic texture was noticed in one instance where a number of long quartz stringers were arranged perpendicular to the periphery of the quartz phenocryst. (Fig. B, Pl. XX.) The texture of this micropoikilitic mass, it will be observed, is finer than that before described. The groundmass of the porphyries is formed of irregular roundish areas having exactly the same micropoikilitic texture as the zones surround- ing the quartz phenocrysts. An explanation of the origin of the zones should therefore also explain the texture of the groundmass. Certainly im many cases, probably in most: cases, the groundmass areas result from tangential sections through one of the micropoikilitic zones surrounding the quartz phenocrysts. The description given by Hedstrém? of this same structure as observed by him is essentially the same as the above, if I have understood him cor- rectly. The following difference is, however, to be noted. In speaking of 1 Op. cit., pp. 589 and 179. 2Op. cit., p.8. ACID VOLCANICS OF HEMLOCK FORMATION. 8) the structure where the quartz is surrounded by this micropoikilitic zone, he ealls it the granophyric structure. As I have already emphasized above, the feldspars in the network of quartz have varying orientation, and the structure is, strictly speaking, micropoikilitic, and in no sense granophyric (micropegmatitic). Moreover, he describes in addition to the above type one in which are found phenocrysts of quartz lying in a micropoikilitic groundmass with the above reticulating texture, but the phenocrysts abut sharply against the groundmass, instead of being connected with it by means of these zones. The micropoikilitic texture has been held in some cases to be of sec- ondary origin and the result of devitrification. While recognizing that there may be certain unquestionable cases where a micropoikilitie structure results from the devitrification of a glassy groundmass, I can find no evi- dence in the rocks here described that points to this origin for the micro- poikilitie texture under discussion. On the other hand, there is an absence of evidence that indicates its unquestionably primary character. Rather than to regard the quartz as secondary and influenced in its orientation by the phenocrysts, as in the enlargements of quartz grains, it seems natural to suppose that when the lava was extruded after the crystallization of the phenocrysts, there began, consequent upon the diminished pressure and temperature and other factors, a rapid crystallization of the mineral elements from the remaining magma. This resulted in the production of the feldspar in very imperfect and small crystal individuals. At the same time the quartz of the phenocrysts continued to grow, and in so doing inclosed these small feldspars in its meshes. In certain rhyolite-porphyries the micropoikilitie texture is somewhat different from that above described. In these the quartz phenocrysts are surrounded by zones which are illustrated in figs. 4 and B, Pl. XXI. These appear to correspond very closely to the ones described by Michel Lévy* and Williams,* and since described by many other writers. The zones have a much higher index of refraction than the quartz of the phenocrysts, and hence contrast strongly with it. Examined closely, they are seen to be composed of chlorite, epidote, and black or reddish ‘Annales des Mines, Vol. VIII, 1875, pp. 378, 381. 2Die Kruptivgesteine der Gegend von Triberg im Schwarzwald, by G. H. Williams: N. Jahrb. fiir Min., Bd. II, 1883, p. 605. 86 THE CRYSTAL FALLS LORN-BEARING DISTRICT. ferruginous grains, which lie in a white matrix. This matrix shows the following characters: The greater part of it extimguishes and lightens simultaneously with the quartz phenocrysts which it surrounds, and is consequently believed to be quartz. When the matrix and quartz pheno- erysts are dark, one sees scattered through the matrix, making up a very small proportion of the total zone, certain irregular areas which show polarization effects. These are believed to be feldspar grains, though this could not be determined. With the highest magnification no radial arrangement of the quartz and feldspar could be observed which would warrant the inclusion of these aureoles under Michel Lévy’s term “ sphéro- lites & quartz globulaire.”* Where two quartz crystals with different orienta- tion are in juxtaposition, each possesses its own zone corresponding with it in orientation. The way in which the zones about the quartzes are confined to the quartz is clearly shown in one case in which a very much altered feldspar phenocryst was found, one portion possessing a typical coarse micropegmatitic texture. In this case where the quartz of the micropeg- matitic intergrowth touches the groundmass, it grades into a amo OL ine area, whereas the feldspar does not do so. The texture of the zones about the quartzes is apparently but a fine- grained variety of the micropoikilitic texture, the coarser phases of which are illustrated on Pl. XX. The groundmass of the rocks showing the texture is composed of roundish areas of exactly the same composition as the zones around the phenocrysts, with a feldspar of small dimensions here and there between these areas. (See fig. B, Pl. XXI.) The texture approaches very closely if it does not correspond exactly to the quartz épongeuse phase of the quartz-globulaire texture of the French.” In one part of a section of rhy- olite-porphyry the quartz phenocrysts have aureoles and the groundmass has the texture just described. In another portion of the section the quartz phenocrysts have no aureoles and the groundmass possesses an imperfect microgranitic texture (structure microgranulitique of Michel Lévy). This shows the passage of a micropoikilitic textured rock into one with a micro- granitic texture. JI explain the aureoles and the roundish areas in the 1 Structures et classification des roches éruptives, by A. Michel Lévy, Paris, 1889, p. 21. 21¢ is found to show exactly the same texture as seen in a section obtained from Paris and labeled ‘‘ Porphyre 4 quartz globulaire dela Sarthe.” CO OO a a a o em ¢-400 ACID VOLCANICS OF HEMLOCK FORMATION. 87 groundmass as original, in exactly the same way as has been suggested by Williams? for those which he described. This is essentially the same expla- nation which I have given on a previous page for the less common, coarse micropoikilitic phase. The cause of the formation of the microgranitic phase appears, however, rather difficult to discern. Its development seems to depend upon peculiar local conditions. APORHYOLITE-PORPHYRY. Intimately associated with the rhyolite-porphyries are rocks very similar to them in mineral constituents, both macroscopically and microscopically, so that the description of the rhyolite-porphyries will largely answer for the aporhyolite-porphyries. Flow texture, however, is well shown ‘by the aporhyolite-porphyries. A beautifully developed perlitic parting, fig. A, Pl. XXII, is taken to indicate the presence of an original glass; hence the rocks are classed with the aporhyolites. The perlitic cracks are well brought out in ordinary light by the chloritic flakes along them. Between crossed nicols these disappear, and the groundmass resolves itself into a fine-grained mosaic of quartz and feldspar. (Fig. B, Pl. XXII.) This groundmass has all the characters of that of a microgranite. No evidence which would point to the devitrification of a glass could be seen other than the presence of a perlitic parting, as described. For recent excellent descriptions of similar devitrified lavas in which varicus structures characteristic of vitreous lavas have been identified, the reader is referred to the articles already mentioned, and the one by Dr. Bascom,” in which a moderately full bibliography is found. SCHISTOSE ACID LAVAS. The results of the ordinary alterations of the acid lavas, chiefly meta- somatic in character, by which the phenocrysts and the matrix have been changed, have been briefly described. The results produced by dynamic action are more interesting and perhaps more striking. The mashing, result- ing in chemical changes and schistose structure, has in many cases almost obliterated the porphyritic texture, and in extreme cases destroyed all inter- nal evidence of igneous origin. Even the fluxion banding, as is well known, at times simulates very closely sedimentary bedding, and thus increases ‘Op. eit., p. 607. 2Acid voleanic rocks of South Mountain, by Dr. Florence Bascom: Bull. U.S. Geol. Survey No. 136, 1896, p. 87. 88 THE CRYSTAL FALLS IRON-BEARING DISTRICT. the difficulty of determining the igneous character of the rock. In the rocks- to be described the phenocrysts may still be observed, though more or less. deformed, and the fluxion banding has been in one case exceptionally well preserved, so that no doubt is felt as to their igneous character. Dynam- ically metamorphosed rhyolite-porphyry flows have been found in two areas in the Hemlock formation. In the following each area will be described separately, the one in which the original character of the porphyry is least in doubt being considered first. The Deer River schistose porphyries are found in the SE. { sec. 36, T. 44 N., R. 32 W., beginning at 400 N., 250 W., and continuing to 600 N., 350 W., of the southeast corner near the bridge on the Floodwood road. They occur in several outcrops which are practically continuous, being separated by very short distances, and are so much alike both macroscopic- ally and microscopically that there is sufficient reason for the conclusion that they belong together. Their field relations to other rocks have not been observed. No data have been found which offer any clue as to the time of eruption of these rocks other than the fact that they are surrounded by the basic voleanics of the Hemlock formation and have undergone the same dynamic action. The porphyries are dense, bluish-black rocks, with porphyritic crystals. of red feldspar. A fluidal structure is not present in them. The schistose structure is apparent to the eye, especially upon the weathered surface, and the cleavage of the rock also indicates it. The cleavage face of the rock has a silky luster, due to the sericite and biotite flakes parallel to it. The rock breaks readily in various directions at angles to the cleavage, so that it is impossible to obtain well-shaped hand specimens. The schistosity in these porphyries is clearly brought out by weathering, the weathered rocks showing perfect schistosity, while fresh specimens from the same exposure, although splitting easiest in one direction, appear perfectly massive in hand specimens when broken across the schistosity. That the dynamic action was greatest along certain zones of the rock, other portions being more or less exempt, is shown by the fact that of several specimens collected with the view of obtaining different stages of alteration from different portions of the same exposure some are markedly schistose, while the least altered approach a fairly massive character. I shall give a brief description of this least altered phase, and then ACID VOLCANICS OF HEMLOCK FORMATION. "G9: consider the changes which have taken place and the character of the rock which has resulted in the more altered phases. The slightly schistose rock, like all the porphyries, is very fine grained and black, with a more or less silky luster on fresh fractures parallel to the schistosity. The porphyritic character is not very strongly marked. Maeroscopically, comparatively few small feldspar phenocrysts are visible. Under the microscope the rock is seen to be a micropegmatitic rhyolite- porphyry in which the silica has not crystallized as quartz phenocrysts, but has remained in the groundmass. The feldspar phenocrysts are both orthoclase and plagioclase. The latter shows its usual characters, but is not present in well-formed crystals. The orthoclase, on the contrary, is well crystallized, occurring im Carlsbad twins. While some of the feldspar crystals are broken, they as a rule do not show many signs of pressure The fine-grained micropegmatitic groundmass is made up of the quartz and feldspar intergrowth and of secondary mica, both muscovite and _ biotite, and remnants of iron oxide. Micropegmatitic intergrowths of quartz and feldspar occur im irregularly shaped areas which frequently have a fairly large quartz at the center. Very similar irregular areas which seem to be composed altogether of unstriated feldspar also occur. These two kinds of areas compose the greater part of the rock. The mineral particles fre- quently show undulatory extinction. Between the micropeematitic inter- growths oue finds here and there granular aggregates of quartz and striated and unstriated feldspar. These feldspar grains, and likewise the feldspar intergrown with the quartz, are considerably altered. Sericite and biotite are present in considerable quantity. The former possesses the better erys- tallographic outlines, the biotite being usually found in ragged fragments. The two micas occur in the feldspars and lie between the quartz grains, but not in them. They appear to be secondary products from the feldspar. The micas lie with their long directions approximately parallel, and impart to the rock its schistose character. A few automorphic crystals of apatite were found. There occur also a few irregular grains of a dark reddish brown mineral with high single refraction, but which is isotropic. This. mineral is presumed to be allanite, though conclusive tests could not be made. Some crystals of zircon were also observed. The iron oxide is evi- dently titaniferous, probably titaniferous magnetite. Secondary calcite is scattered through the rock in considerable quantity. 90 THE CRYSTAL FALLS IRON-BEARING DISTRIC1. In a more altered phase of the porphyry exhibited in a number of specimens, the schistose structure is much better marked both macroscopic- ally and microscopically. The macroscopical appearance is otherwise quite similar to the one just described. Under the microscope the pheno- crysts show up well. These are rounded and shattered orthoclase and plagioclase feldspars. They lie usually with their long direction in the lines of marked schistosity of the rock. The larger crystals have been much more generally fractured than have the smaller ones, and seem to have obtained relief from strain in that way, the individual fragments not showing very strong dynamic effects. The small crystals are more or less rounded. The crushing to which the rock has been subjected has severed the fragments in a number of cases. Triangular areas on two sides of the broken or unbroken feldspars in the direction of schistosity are filled with what appear to be secondary quartz and flakes of biotite. The feldspars as a whole have undergone considerable chemical changes, the freshest being red and very cloudy. Those more altered show secondary muscovite and biotite scattered through them. The character of the triclinic feldspar could not be determined. It appears, however, to be very rich in calcium, as in some of the badly weathered sections the feldspar fragments may almost be said to lie in a calcite matrix, resulting apparently from the alteration of the feldspar and not from infiltration. No quartz phenocrysts retaining their normal character are found. There occur here and there, however, small rounded mosaics of quartz, the individual grains of which show undulatory extinction. These are evidently the result of the granula- tion of quartz grains, such as occur in the freshest specimens. It is well known that the quartz is more easily affected by pressure than feldspar, and Futterer’ has shown that they may be found in a completely crushed condition, in the same section with feldspars which still retain their regular crystal contours. The groundmass of the porphyry is made up of quartz and feldspar, in and between which lie leaflets of biotite and sericite. The holocrystalline granular mixture of quartz and feldspar is very fine grained, and the pres- ence of the feldspar was only determined by difference in the refraction of the two minerals. No striated feldspar grains were observed. The second- 1 Die “Ganggranite” von Grosssachsen, und die Quartzporphyre yon Thal im Thiiringerwald, by Karl Futterer, Heidelberg, 1890, pp. 31, 126. ACID VOLCANICS OF HEMLOCK FORMATION. 91 ary micas appear usually in ragged flakes, though the slightly greenish- yellow sericite flakes approach crystal outlines rather frequently. The biotite is brownish-green and strongly pleochroic. A few spots of brown iron hydroxide and small heaps of grains of sphene probably indicate the former presence of titaniferous iron ore. The few erystals of apatite present are broken and separated, but otherwise retain the usual characters of this mineral. The groundmass has a very marked schistose structure, brought out especially well by the parallel arrangement of the mica flakes. The way in which these lines of schistosity flow around the mashed phenocrysts, one line never coalescing with another, but remaining continuous, may be seen with great distinctness where the lines abut sharply against the crystal at a very obtuse angle. As the angle becomes less and less obtuse, the ends of these lines bend up slightly in the direction which would enable them to pass the erystal, and then end, so that along the face of the crystal one can follow them, as it were, in a series of steps until those lines which strike the erystal near enough the edge to flow around it bend slightly, and passing around continue on the opposite side. The fact noted by Fiitterer’ that an increased amount of sericite occurs on the two sides of the feldspar crystals parallel to the schistosity is very patent in these porphyries. The diminu- tion in grain of quartz and feldspar seems to accompany the increase in the amount of the sericite. The slides are crossed by narrow fractures cutting the planes of schistosity, which are filled with secondary quartz, showing marked strain effects. Associated with the quartz were observed some erystals of brown rutile. In one of the more altered slides these fissures have been filled with calcite, whether or not as a replacement of the quartz could not be told. Schistose porphyries showing the extreme alteration phases are found from N. 300, W. 300, to N. 400, W. 250, in the SE. 4 sec. 4, T. 44 N., R. 32 W. They form a rough escarpment upon the southeast side of and near the base of a large hill, and overlook McCutcheon’s Lake. The exposure is not continuous throughout, though practically so, but the unexposed parts are sufficient to prevent a perfect sequence being traced. The appearance of the rock is strongly like that of sedimentary rocks. Different bands nearly on edge may be seen, dipping 60°-90° SW. and striking N. 30° W. ' Op. cit., p. 40. 92 THE CRYSTAL FALLS IRON-BEARING DISTRICT. At a point about 100 feet higher and three-fourths of a mile distant, on the very northwest flank of the same hill, at N. 1725, W. 775, from the south- east corner of sec. 4, T. 44 N., R. 32 W., there is a small ledge of schistose porphyry, macroscopically and microscopically similar to those to the south- east of it, and with its schistosity striking N. 20° W. and dipping 80° SW. The striking agreement in strike, dip, and general character of these two separated outcrops points to their being merely isolated portions of the same mass. There seems to be no discrepancy between the dip and strike of the schistosity and that given above for the bands. The most striking macroscopical characteristic of these mashed por- phyry flows is the occurrence of phenocrysts in a schistose and beautifully banded rock. These phenocrysts stand out clearly from the groundmass in all cases. The general appearance of the rocks is that of the well-known very dense banded hiilleflintas of Elfdalen, Sweden. The bands vary in color, ranging on the weathered surface from light creamy white, through light greenish, to red and almost black. The rocks which have very light colored weathered surfaces are always bluish black on a fresh fracture, and very dense, and those weathering red are usually cream colored on freshly fractured faces. Many of the areas which appear macroscopically to be single phenocrysts are resolved under the microscope into tangled groups of individuals, though in rare cases the individuals show the imperfect radial arrangement rather frequent in medium-grained micropegmatitic rhyolite-porphyries. The feldspar has undergone considerable alteration. In the least- changed grains there is a cloudiness caused by numerous indeterminable specks, probably of iron oxide, which give a reddish tinge to the mineral. Further changes result in the production of muscovite and epidote, with biotite in rare cases, accompanied by the obliteration of the twinning lamelle. The greater part of the phenocrysts seem to be orthoclase, though associated with them are found pieces which show indistinct traces of the polysynthetic twinning of plagioclase feldspar. The feldspars exhibit marked strain effects, especially in their flattening into long oval and spindle-shaped areas. Some crystals have been broken and separated perpendicular to the direction of the schistosity. The spaces between the fragments are filled with secondary muscovite, quartz, and feldspar. Sur- rounding the phenoerysts—that is, between the phenocrysts and the ground- ACID VOLCANICS OF HEMLOCK FORMATION. 93 mass proper—we find a mass of small angular, finely striated, limpid grains of feldspar, associated with similar grains of quartz, the two having in places between them sericitic flakes. In one especially clear case, this secondary aggregate fills half the space formerly occupied by an individual feldspar, the other half being still occupied by the remnant of the appar- ently simply twinned feldspar from which it was derived. (Fig. A, Pl. XXIII.) While no large quartz phenocrysts were observed, a mosaic of quartz is found in small round or oval areas in various sections. The individual fragments exhibit the usual strain effects of crushed minerals. (Figs. 4, B, Pl. XXIV.) The groundmass consists of the same preponderant minerals as the schistose porphyries, which have been previously described. The accessory minerals are apatite, which is present in very small quantity, and rutile, which in one of the slides is present in very considerable quantity in the form known as ‘‘thonshiefer-niidelchen.” Calcite is found in all of the slides, the amount varying very much. Those which contain a great deal have a scoriaceous-looking surface, due to weathering out of the calcite. The flow structure mentioned as having been observed in the schistose porphyries of the Hemlock formation is perhaps of sufficient general interest to warrant a few comments. This is well marked only on one hand speci- men. In this there is an alternation of pink and dark grayish-blue bands which are rarely more than a fraction of an inch thick. Some, especially the thicker bands, are remarkably persistent. Even macroscopically on the weathered surface the pinkish bands can be distinctly seen to wrap around the pink feldspar phenocrysts and oval areas of the grayish-blue part of the rock. Under the microscope the bands which macroscopically are the darkest are clear and transparent, while the pink bands are much less trans- parent. The microscope shows the difference in the color of the bands to be due chiefly to the fineness of grain, and brings out the flow structure even more clearly than the weathered surface. (Figs. A and B, Pl. XXIV.) Accompanying this variation of grain there is also a difference in mineral- ogical composition. The dark bands are composed essentially of quartz grains, with feldspar, sericite, some magnetite, considerable calcite, and rare crystals of apatite and rutile, the quartz including many black and indeterminable specks. The pink bands are very fine grained, so much 94 THE CRYSTAL FALLS IRON-BEARING DISTRIOT. so that the clear white mineral grains composing it can not be determined, though probably both quartz and feldspar are present. These bands are darkened by innumerable black indeterminable specks and long rutile needles, with a small amount of biotite. It is possible that some of the minute biotite flakes have been mistaken for rutile needles when viewed on edge, but it is certain that, these bands contain a great deal more rutile than do the others. Whether or not there is a still more essential chemical difference between the bands than that indicated by the increased quantity of rutile, was not determined. It has become more or less common of late to attribute the banding found in metamorphosed eruptives altogether to the pressure to which they have been subjected. In the present instance I can not but consider the banding as being an original fluxion structure, with the slight original differences between the bands emphasized, as it were, by subsequent pressure. It appears highly probable that the rock was originally more or less glassy and showed a flowage structure, and that the present miner- alogical character of the groundmass is due to the process of devitrification which did not destroy the banding of the original glass. ACID PYROCLASTICS. The only acid pyroclastic rock found was formed from the aporhyolite- porphyry. This is a true eruptive breccia. The fragments are angular to rounded in shape, weather to a pure white color, and have an exceedingly rough surface. This roughness is due to a great extent to perlitic partings, which are macroscopically visible, and give the rock an almost scoriaceous appearance. Other inequalities on the surface adding to its roughness are caused by the leaching out of feldspars, and by the fact that many of the quartz phenocrysts have fallen out of the inclosing matrix. The fragments are all aporhyolite-porphyry, containing a very large proportion of quartz and feldspar phenocrysts. The cement of the breccia is aporhyolite. This contains far less numerous phenocrysts, and is, therefore, on the whole much finer grained than the fragments. The weathered surface of the cementing aporhyolite appears a bluish gray, and is very smooth compared to the scoriaceous appearing surface of the fragments already described. This dif- ference in weathering shows the brecciated character admirably, as the finer- grained matrix stands out sharply and delimits the contours of the fragments. BASIC VOLCANICS OF HEMLOCK FORMATION. 95 Movements of the magma are shown by a flowage structure in the matrix and by the fracturing of the quartz and feldspar phenocrysts and separation of the pieces in both the cement and fragments. (Fig. B, Pl. XXIIL.) This eruptive breccia can be seen in its best development in the NW.- SE. trending ridge, just west of the small lake, crossed by the Chicago, Milwaukee and St. Paul track in sec. 32, T. 44 N., R. 32 W. BASIC VOLCANICS. The basic volcanics are considered under the main headings of lavas, pyroclastics, and Bone Lake crystalline schists. BASIC LAVAS. GENERAL CHARACTERS. The basic lavas are so very characteristically developed that no one could for a moment doubt their true nature, even upon the most superficial examination. One of the nearly general characters is the presence of a well-marked amygdaloidal texture. (Figs. A, B, Pls. XXV and XXVI, and fig. d, Pl. XXVIII.) Some of the lavas are so full of amygdules that they may be correctly said to have been scoriaceous. The amygdaloidal portions of the rock masses—which may be considered the surface parts— gerade over into other portions, the interiors of the lava flows, which are, macroscopically at least, nonamygdaloidal. Owing to the homogeneous character of the basic magmas, a fluxion structure is rarely shown macro- scopically, though microscopically it may be more or less well developed. Columnar jomting was nowhere observed. An ellipsoidal parting, on the other hand, is common. NOMENCLATURE. In a preliminary article on the Hemlock volcanics, I made a brief mention of the occurrence on the Upper Peninsula of Michigan of the basic pre-Tertiary equivalents of the post-Tertiary and Recent family of basalts. Followmg the Danas, Wadsworth, Williams, Iddings, Kemp, Darton, and Diller, some of the most influential of the men who, in the ‘The voleanics of the Michigamme district of Michigan, by J. Morgan Clements: Jour. Geol., Vol. III, 1895, pp. 801-822. 96 THE CRYSTAL FALLS IRON-BEARING DISTRICT. United States, have advocated the simplification of petrographical nomen- clature, I used the term basalt, now ordinarily used for the Tertiary or post- Tertiary basic rocks. This term was, however, modified by the prefix “Cano,” as indicating their altered condition and the presumed presence of a glassy base.’ This was a logical continuation of the use of the prefix as proposed by Dr. Bascom” for devitrified acid lavas. More detailed studies upon the Hemlock volcanics have shown the presence of rocks which were apparently originally holocrystalline, and therefore do not belong with the altered vitreous basalts, the apobasalts, and others in which some of the glass is apparently unaltered. Consequently, since the apobasalts comprise only a portion of the Hemlock voleanies, the replacement of that term as a general heading by the older, more general, one of basalt was considered. The use of this term is, however, not altogether satisfactory, for the rocks, while clearly recognizable as basalt derivatives, do not possess the mineralogical composition of the basalts. The term “apo” having been restricted, as above indicated, can not be applied to them, for their altera- tion is in many cases metasomatic and dynamic, and in most cases not devitrification. If we adopt the prefix ‘‘meta” to indicate alteration of all kinds, then these rocks could be called ‘‘metabasalts.” The terms “‘metadolerite,” “‘metadiabase,” etc., were proposed by Dana’ for metamorphic dolerites, diabases, etc., and first used by Hawes* in the description of the altered rocks around New Haven. Recently these terms have been revived, but with a very different significance from that with which they were first used. It is proposed to designate by such terms “rocks now similar in mineralogical composition and structure to certain igneous rocks, but derived by metamorphism from something else.” Fol- lowing this suggestion, an uralitized dolerite (diabase) would be called a metadiorite. Such a use of the term does not seem justified, and the ' Loe. cit., p. 805. *The structures, origin, and nomenclature of the acid volearic rocks of South Mountain, by Florence Bascom. Jour. Geol., Vol. I, 1893, p. 828. 3 Chloritic formation of New Haven, Connecticut, by J.D. Dana: Am, Jour. Sci., 3d ser., Vol. XI, 1876, pp. 119-122. 4The rocks of the ‘‘ chloritic formation” on the western border of the New Haven region, by G. W. Hawes: Am. Jour. Sci., 3d ser., Vol. XI, 1876, pp. 122-126. 5On a series of peculiar schists near Salida, Colorado, by Whitman Cross: Proc. Colo. Sci. Soc., p.6, footnote. Paper read Jan. 2, 1893. BASIC VOLCANICS OF HEMLOCK FORMATION. 97 objection to it can not be given better than by quoting the words which Zirkel uses in the discussion of the metamorphism of rocks:1 Bei solchen metamorphisch veriinderten Gesteinen ist es nicht zweckmiissig, sie mit dem Namen desjenigen Typus zu belegen, dem sie durch die Verinderung abnlich, oft blos scheinbar ahnlich geworden sind. Eine solehe Bezeichnung werde nur zu missverstéindlichen Auftassung der von dem Gestein gespielten geologischen Rolle fiihren, welche niemals ausser Acht gelassen werden darf. Und so ist es denn ent- schieden vorzuziehen, der Benennung solcher Gesteine eine Form zu geben, in whelcher zuyvorderst auch zum Ausdruck kommt, was sie friiher gewesen sind, und nicht einen Namen zu wiihlen, der sie in erster Linie zu etwas stempelt, mit welchem sie genetisch keine Gemeinschaft haben. Using these terms in the way suggested by Cross, attention is most pointedly directed to that variety of rock which the secondary product now resembles mineralogically, rather than to the type from which it was derived, and which in all likelihood it still resembles most closely in its chemical constitution. Whether or not a petrographer will use the term “metadio- rite” or the term ‘‘metadolerite” (diabase) for a metamorphosed dolerite will depend on whether or not he prefers to emphasize the present miner- alogical composition of the rock, or its original characters, and thereby its chemical constitution and genetical relations. In the present report the terms ‘‘metabasalt” and ‘metadolerite” are used as including all those altered rocks which demonstrably were originally basalts and dolerites. These same strictures hold good in the case of Giimbel’s term ‘‘epidior- ite,” when used, as it is very commonly, in the literature of the Lake Superior region and elsewhere, for rocks avowedly derived from dolerites (diabases), and characterized by the presence of fibrous secondary amphi- bole. It is preferred, in accordance with the above statement, to use the term “epidolerite” (epidiabase) instead of ‘‘epidiorite” for such altered dolerites. None of these rocks, unless extremely changed, would resemble chemically a diorite, and we have come of late years to rely more and more upon the chemical composition, combined of course with the mineralogical composition and textures of the rocks, to separate the various kinds from one another. As stated above, the term ‘‘epi,” associated with the rock name, has come more and more to be restricted in its use solely to a rock, the epidiorite, characterized by a specific alteration product, the amphibole. In respect to this restriction to specific alteration, the term corresponds to ‘Lehrbuch der Petrographie, F. Zirkel: 2d ed., Vol. I, p. 573. MON XXXVI U 98 THE CRYSTAL FALLS IRON-BEARING DISTRICT. “apo,” and it is unfortunate that these two terms should have been so nar- rowly confined. As it is, the epi- and apo-basalts would be subordinate to and therefore included under the metabasalts, as this term is used in this report. In the first the production of secondary hornblende is character- istic; in the second the process of devitrification, and hence the original presence of a vitreous base, is the chief characteristic.’ METABASALTS. All of the basalts belong to the plagioclase type. They may be most conveniently divided into nonporphyritic and porphyritic kinds, according to their most obvious macroscopical characters. There has also been found a single occurrence of a spherulitic basalt, which will be described under the head ‘ variolite.” NONPORPHYRITIC METABASALT. The nonporphyritic rocks possess a fine-grained or aphanitic structure and are amygdaloidal or nonamygdaloidal. There are included under this general name the microophitic-textured fine-grained pre-Cambrian basalts (diabases in part), the very amygdaloidal forms of the basalts (spilites),? and the melaphyres in part. In these rocks the former presence of a considerable amount of original glass is probable, and they show the various textures known as navitic, intersertal, pilotaxitic, and hyalopilitic. With the nonporphyritic basalts there have been included some rocks which are to a considerable extent devitrified glasses, and others m which only a few microlites have developed. These last two vitreous types occur more especially in fragments in the tuffs, and are quantitatively unimportant. Petrographical characters.—In color the nonporphyritic basalts on fresh fracture show various uniform shades of green, dark olive green usually prevailing. Much less common are purplish-black rocks, and these are much more vari- able in color. In one of them is seen lighter-colored schlieren, which pass over into the ordinary dark colors. The lighter-colored portions are seen on microscopical examination to be due to a smaller quantity of the iron in them and to a greater quantity of chlorite than occurs in the rest of the rock 1 The above discussion was written and the determination to use the terms porphyry—without textural significance, as in rhyolite-porphyry—metabasalt, etc., was reached, in 1896, before the com- mittee on petrographic nomenclature of geologic folios was appointed by the Director of the United States Geological Survey. 2Microscopic characters of rocks and minerals of Michigan, by A.C. Lane: Rept. State Board of Geol. Survey for 1891-92, 1893, p. 182. BASIC VOLCANICS OF HEMLOCK FORMATION. 99 mass. Where weathered, the rocks are usually covered by a thin crust, in which gray, brown, and pinkish tints prevail. The rocks vary in texture very much, from the dense aphanitic kinds to medium fine-grained varieties. The latter are usually less amyedaloidal than are the aphanitic forms, and approach in appearance both macroscopic- ally and microscopically the coarser-grained basalts or dolerites represented in the Michigamme district by the coarse-grained intrusives. Owing to the basic nature of the rocks, they have generally suffered much alteration, and as a result the original texture is in many cases poorly preserved. On the whole, however, it is remarkable, considering their age and basic character, how well preserved it is. Where it is preserved it varies from the micro- ophitic to the various microlitic textures, such as intersertal, navitic, pilo- taxitic, and hyalopilitic, and lastly glassy. In places a flowage structure is beautifully brought out by the position of the feldspar microlites, especially | around the amygdules. The constituents present are plagioclase, light-green fibrous hornblende, epidote-zoisite, chlorite, calcite, muscovite, apatite, sphene, quartz, magnet- ite, and pyrite. Of these the feldspar, apatite, and iron oxide alone are original. In some places the hornblende is wanting, the chlorite then appearing in correspondingly greater quantity. The feldspar ordinarily occurs in lath-shaped crystals showing twins of the albite type, but where the texture is fine the feldspars are microlitic, and, while showing their prominent long extension, the edges of the various erys- tals interfere, and the outlines consequently are less sharp. In some of the rocks which appear to have been vitreous the feldspar forms feather and sheaf like aggregates (figs. A, B, Pl. XXVI), apparently quite similar to those described by Ransome in rocks from Point Bonita, California.’ No reliable measurements could be made upon the microlites, and consequently their character could not be determined. The feldspar is more or less completely altered to aggregates of epidote-zoisite which have chlorite associated with them or are altered to sericite. In a number of places minute limpid spots of secondary quartz and albite are present: The very small quantity of apatite present shows its usual characters. Titanif- erous magnetite ore is apparently the only oxide present. It oecurs in erys- tals and in irregular grains, which in a few cases are not entirely altered, ! Eruptive rocks of Point Bonita, by F. Leslie Ransome: Bull. Univ. of Cal., Vol. I, 1893, p. 84, fig. 6. 100 THE CRYSTAL FALLS IRON-BEARING DISTRICT. though in most cases they are replaced by sphene. In some cases the alteration product is not well enough individualized for one to diagnose it as sphene, and it should perhaps be called leucoxene. In some of the fine- grained rocks the material in the angles between the feldspars consists pre- -dominately of grains of magnetite. This abundant magnetite renders the rock very dark, giving the rare purplish-black lavas. The most of the hornblende has a light-green color. A lesser portion shows a decided bluish tinge, and gives fairly strong pleochroism. This resembles the hornblende, which in the coarse dolerites is undoubtedly sec- ondary after the augite and it is considered to be secondary after the origi- nal augite in these rocks. The original augite was presumably in most cases present in wedge- shaped pieces filling the spaces between the feldspars, and consequently the _ hornblende pseudomorphs never show augite outlines. No unaltered augite was observed amongst the hornblende fibers. The fine fibers frequently form a fringe beyond the original boundaries of the pieces and penetrate the ‘adjacent feldspar. Quite frequently the secondary hornblende shows partial alteration to chlorite and epidote. Though careful search was made for olivine or indications of its pres- ence, no traces of it were found, and I have concluded that these basic vol- canics were essentially nonolivine bearing, though it would be rash to state that such rocks did not contain some olivine. The calcite is usually found in irregular secondary granular aggregates scattered through the rock, and evidently replaces the other mmerals. Less commonly it is seen as an infiltration product along fissures. A second form of the occurrence of calcite in the nonporphyritic meta- basalts, and one not so common as the granular aggregate, is that of large porphyritic automorphic rhombohedra and scalenohedra which lie embedded in the eruptive groundmass. Such a rock, as, for instance, Sp. 82472, shows macroscopically large rhombohedral phenocrysts in a green groundmass. On the weathered surface are ferruginous rhombohedral cavities, once occu pied by the carbonates. The groundmass consists of rather fresh plagioclase microlites, between which are observed some quartz, fresh magnetite crys- tals, and lastly chlorite flakes as alteration products of originally present bisilicates or glass, or both. The texture is undoubtedly that of an eruptive. The carbonate is more or less ferruginous, brown iron hydroxide resulting BASIC VOLCANICS OF HEMLOCK FORMATION. 101 from its alteration, and as it effervesces quite readily with cold HCl, it is supposed to be ferruginous calcite. Sericite is found in minute flakes replacing the feldspars, and it is also found in large porphyritie plates occurring in the eruptive groundmass asso- ciated with the porphyritic carbonate above described. In some cases we find epidote in these altered basalts, in others zoisite. In a great number of instances the same individual exhibits the high interference colors of epidote and the low blue interference color of zoisite in different parts. These different portions, formed respectively of the epidote and zoisite mole- cules, are most closely intergrown, and I have therefore used the compound term “‘epidote-zoisite,” indicating this fact. Associated with this, one finds in many of the specimens small mineral aggregates which merit somewhat further notice. These aggregates have a brownish-yellow color and possess a very high single and also a high double refraction. In these masses the single and double refractions of the granules composing the aggregates appear to be higher than that of epidote. In shape the aggregates vary from perfectly round, zonally arranged spheres and irregular, elongated, rounded aggregates to forms giving oblique quadratic sections. All of these aggregates are found at times included in the epidote-zoisite crystals. In a few cases the oblique quadratic sections were seen to occupy the centers of the epidote-zoisite crystals, having exactly identical outlines. It is believed that they are composed of an epidote much richer in iron than the common variety with which they are associated. This increase in iron explains the darker color and the increase in single and double refraction, as shown by Forbes.! Why it should appear, especially in the aggregates, can not be explained. The chlorite does not appear to be entirely an alteration product of the secondary hornblende with which it is associated. There is usually rather more chlorite than it would seem could possibly have been formed from the alteration of the hornblende alone. In some of the rocks the larger angles, as well as the extremely fine areas between adjacent feldspars, are occupied by a very fine felt-like chloritic mass. The chlorite which is not secondary after hornblende is considered as the product of an altered glassy base. No glass was observed in the nonporphyritic basalts occurring in large masses, but in one of the fragments of basalt in a tuff a dark chocolate- brown glass forms the matrix in which are lying well-developed plagioclase 'Epidote and its optical properties, by E. H. Forbes: Am. Jour. Sci., 4th ser., Vol. I, 1896, p. 30. 102 THE CRYSTAL FALLS IRON-BEARING DISTRICT. microlites. The glass where thick appears isotropic, but where thin appears to be full of globulitie devitrification products, which show slight polariza- tion effects between crossed nicols. The original presence of glass in other basalts is considered to be indi- cated by the occurrence of amygdaloidal cavities, with very sharply defined walls marked by accumulations of magnetite. The character of one basalt points strongly toward its glassy condition. It is amyegdaloidal, the amygdaloidal cavities being sharply defined. The eroundmass contains at present no indication of the existence of any orig- inally crystalline elements whatever. It is now a dense mass of felty chlorite and minute epidote grains. Threugh this mass and around the amygdaloidal cavities wind lmes which are somewhat differently colored from the rest of the matrix, and seem to indicate the direction of flowage. The amygdules are not all elongated, though some are, and these agree in direction of elongation. It is really impossible to describe the groundmass so as to do justice to its appearance and convince one who has not seen it of its devitrified character. The general impression it makes is that of a devitrified glass, and the photomicrograph (fig. b, Pl. XXV) gives a fairly good idea of its appearance under the microscope, and will probably prove more convincing than any description that might be given. Fig. 4, Pl. XX VII, represents a polished face of the specimen in its natural size. Another kind of glassy basalt is represented in this district. This rock resembles the one just described, but differs from it in that it was not alto- gether glassy. In it one sees long, slender, much-altered feldspar microlites scattered through the matrix. These feldspars occur in needles, which fringe out at the ends. They do not give the groundmass-textures usually found in the basalts, but occur in sheaves and imperfect spherulitic forms; the rock thus approaches in texture the variolites. ‘The base in which the feldspars lie is brownisk gray, and consists of recognizable chlorite, epidote, some clear mineral in minute particles, probably quartz or feldspar, or both, and aggregates of yellowish granules, which are apparently of a single kind and are so minute as not to permit of determination. The granules show very slight polarization effects under crossed nicols, and the groundmass in many places where they occur in great quantity appears almost isotropic. It seems highly probable that a large portion, if not all, of the ground- mass was originally a glass. Further evidence of the originally glassy BASIC VOLCANICS OF HEMLOCK FORMATION. 103 nature of the groundmass is afforded by the groundmass, which, under the microscope, shows variable lighter and darker shades of brown, and these portions interpenetrate, forming an imperfect eutaxitic structure. Such structures are especially common in the glasses. The photomicrographs (figs. 4, B, Pl. XX VI) show very well the general microscopical characters of this rock. Chemical composition.— [he following complete analysis, for which I am indebted to Dr. Henry Stokes, of the United States Geological Survey, shows the chemical composition of one of these pre-Cambrian nonporphy- ritic metabasalts. The rock is very fine grained and microophitic, with a marked amygdaloidal character. The amygdaloidal cavities are filled with chlorite, into which project crystals of epidote-zoisite and calcite. The altered condition of the basalt is very clearly shown by the high percent- ages of water and carbon dioxide. The other oxides show nothing remarkable except that the percentage of titanium oxide is quite high. On the whole, however, the analysis is very similar to those of recent fresh rocks of the same character. Analysis of pre-Cambrian nonporphyritic metabasalt. Constituent. Per cent. | Constituent. | Per cent. SOs Vas teat ea AGATA ll K sO» Gees gue eran 0.21 WU Oh GScacoBeacatnesesceaee 128) +|) RsOs\to seb eee sae ° 183 ANGO@s sce teperase emia siecle ce | 16.28 CODST Reta ase ee oeeseee 1. 26 TBE MO) sar, Sal apo eee Sebi) lll \CLLAG,. Aa Os eee ae era IMG ON saersm elas seisivecs ave wise Ee inl tas Owe aetsmicacisec anda soccad| searealecos INGO) Sees dean aes aeon age aer . 09 18GO) ENGIN. cco scoussses . 28 WIGKO) sseAccupicoeebnasseaas 6. 56 | HO above 110°..-..---.-. 3. 89 CaOl eres hee. oh 7.90 | aise ere Unt Nias O foe eres cyesteisicise anise 3. 64 PORPHYRITIC METABASALT. The porphyritie rocks are fine grained, and may be or may not be amyegdaloidal. They include diabase porphyrites and porphyritic forms of the melaphyres. These last in the textures of their groundmass correspond to the labradorite-porphyrites, the equivalents of the andesite-porphyries, though more basic than they. The phenocrysts lie in a fine groundmass which shows the same kinds of texture already mentioned as having been 104 THE CRYSTAL FALLS IRON-BEARING DISTRICT. observed in the corresponding nonporphyritic basalts, the microophitic, inter- sertal, navitic, pilotaxitic, and hyalopilitic. The various basalts are connected by transition phases. The close connection between the different varieties is well shown where one passes from the fine-grained amygdaloidal rock through the fine-grained nonamygdaloidal over to the porphyritic macro- scopically nonamygdaloidal type. Petrographical characters—As stated in the general description, these rocks do not differ essentially from the nonporphyritic basalts just described. The most important difference is in the presence of the feldspar phenocrysts, giving them a porphyritic texture. Measurements upon the phenocrysts, made against the albite twinning plane on zone _L to 010, according to the Michel Lévy method, give an average extinction angle of about 18°, which points toward its character as labradorite. However, angles obtained lower than this indicate the possibility of the association of andesine with the predominant labradorite. The feldspars show the usual alteration products. One very infrequently finds augite phenocrysts which have been completely uralitized associated with the feldspars. Other phenocrysts are now represented by masses of chlorite, with or without epidote, evidently pointmg toward the basic and magnesian nature of the original mineral. As uralite is the common sec- ondary product of pyroxene in these volcanics, the altered phenocrysts represented by chlorite masses are not believed to have been pyroxene. The original mineral was perhaps olivine. The very noticeable scarcity of augite phenocrysts in the basalts stamps them as different from the great majority of basaltic rocks and as being very similar to the basalts described by Judd,’ from the Brito-Icelandi¢ petrographical province, in which por- phyritic crystals of augite are seldom if ever seen and in which the pheno- erysts are feldspar and sometimes olivine. The groundmass in which the phenocrysts lie have generally the same mineralogical composition and texture as the nonporphyritic rocks already described, and the two kinds are supposed to have been originally similar.’ ‘On the gabbros, dolerites, and basalts of Tertiary age in Scotland and Ireland, by J. W. Judd: Quart. Jour. Geol. Soc., Vol. XLII, 1886, p. 79. ? The groundmass of one of these porphyritic forms differs somewhat in one important respect. In it were observed numerous round areas of small size occupied by a clear white aggregate, polariz- ing in low gray colors. The centers of some of the areas were occupied by clumps of yellow grains, with here and there a minute flake of chlorite. Others contain only the white material, which is apparently secondary. The round areas are not sharply delimited, and hence are most probably not microamygdules. Their general appearance is strikingly like that of leucite in those plagioclase BASIC VOLCANICS OF HEMLOCK FORMATION. 105 Measurements made on the feldspar microlites of the groundmass gave 17° as the maximum extinction in zone perpendicular to 010. This angle points toward the microlites being acid labradorite. The microlites thus seem to agree essentially with the phenocrysts in composition. Flowage structure around the phenocrysts is most distinctly shown by the arrangement of the feldspar microlites. In one case in which the porphyritic texture and the flowage structure are very good, secondary actinolite crystals have devel- oped parallel to one another and parallel to the flowage direction, giving the rock under the microscope a distinctly schistose appearance. Chemical composition—In the preliminary article upon these Hemlock vol- canics published in 1895,* the occurrence of andesites as well as basalts was mentioned. This determination was based solely on the microscopical study of the rocks, and the rocks which were presumed to be andesites were those porphyritic forms which have just been described. Since the publication of that article the following analyses (Nos. 1 and 2) have been obtained of the porphyritic rocks. The rocks selected for analysis were those which appeared to be especially rich in feldspar, and, having a rather lighter color than the others, seem to be somewhat more acid than the average. These, it was thought, might have the composition of andesite. The comparison of series of rocks derived presumably from the same magma is more profitable than the study of smgle analyses. This line of investigation, as followed by Rosenbusch,’ Iddings,*? Lang,‘ Broegger,® Becke,® and Michel Lévy,’ has been very fruitful. basalts in which it is present in very small quantity, filling irregular but in general rounded areas between the other constituents. It would, of course, be impossible to base the determination of the former presence of leucite in these pre-Cambrian rocks upon such scant evidence as has been obtained. Still it is worth while to notice even such a doubtful indication of its former presence as has been mentioned above. 1 Jour. Geol., cit. pp. 805-806. 2 Ueber die chemischen Beziehungen der Eruptivgesteine, by H. Rosenbusch: Tsch. Min. u. Pet. Mitt., Vol. II, 1889, pp. 144-178. 3 Origin of igneous rocks, by J. P. Iddings: Bull. Phil. Soc. Wash., Vol. XII, 1892, pp. 88-214. The eruptive rocks of Electric Peak and Sepulchre Mountain, by J. P. Iddings: Twelfth Ann. Rept. U.S. Geol. Survey, 1892, pp. 571-664. 4 Ordnung der Eruptivgesteine nach ihrem chemischen Bestand, by H. Otto Lang: Tsch. Min. u. Pet. Mitt., Vol. XII, pp. 199-252. Beitrage zur Systematic der Eruptivgesteine, by H. Otto Lang: Tsch. Min. u. Pet. Mitt., Vol. XIII, 1892, pp. 115-169. ° Die Eruptivgesteine des Kristianiagebietes, by W.C. Broegger. I. Die Gesteine der Grorudit- Tinguait serie. II. Die Eruptionsfolge der triadischen Eruptivgesteine bei Predazzo in Siidtyrol. Videnskabsselskabets Skrifter, I Mathematisk-natury. klasse. No.4, 1894; No.7, 1895. £ Gesteine der Columbreies, by F'. Becke: Tsch. Min. u. Pet. Mitt., Vol. XVI, 1896, pp. 308-336. 7Note sur la Classification des Magmas des Roches Eruptives, by A. Michel Lévy; Bull. de la Soc. Géol. de France, 3d ser., Vol. XXV, No. 4. July, 1897, pp. 326-377. 106 THE CRYSTAL FALLS [RON-BEARING DISTRICT. The value of such an investigation largely depends on the freshness of the rocks examined and the amount of variation. The Hemlock volcanics are all more or less altered, and the variation in character is slight. I wish, however, to call attention to the close relationship exhibited by the types of which analyses were made, and to that end the analysis of the nonpor- phyritic very basic appearing basalt (No. 3 of Table I) is repeated and is placed by the side of the analyses of the porphyritic ones. ‘The complete analyses made by Dr. Henry M. Stokes, of the United States Geological Survey, are found in the first table. In the second table there is given the molecular proportion of the chief oxides, those which are not here repre- sented having been first proportioned among them. From this table, following Rosenbusch,' there were obtained the figures in the third table, showing the atomic proportions of the metals present.” The analyses are arranged according to the increasing percentage of ~ calcium. Analyses of porphyritic metabasalt. TABLE I.—COMPLETE ANALYSES. Constituent. is | 2. a3. SiO Skee eer eae 47. 20 | 52.59 46, 47 Dy ORIG aE ee ee ellin 1 BL) 1.36 1, 28 TUG Wear es eee nal GL els 15. 93 16. 28 TST ON MASE ea ese se sea ae 3. 06 6. 12 3.15 BeO ee ne AN 8. 87 3.96 8. 96 MnO x tsscce See ea eee .20 25 .09 CaO. 2 ai ee ees 5. 05 5 7.90 MpOlc sean antes eaeeee 4,20 5. O4 6.56 INAS OS3 1a. sce eee 4,72 5.79 3. 64 KO ghee ak ae ieee 1.40 67 21 PiOg econ eee ee 36 15 13 GT Uae eee eres SA ke Bee Deeame rritea | Beso BO ge. ogee see ea ee eat ese lee aie ae | Cee OOgs 23 34s Sarees 3. oF None. 1. 26 TELCO) AN) AUS ns ssceecoce .16 .16 .28 HyO above 1100s s5---- + | iaSHOL | eeaG 3.89 Totally sse = hee 100.26 | 99.73 | 100.11 aNo. 3 is the analysis by Dr. Stokes of the nonporphyritic basalt, and is given for comparison, 1 Uber die chemischen Beziehungen der Eruptivgesteine, by H. Rosenbusch: Tsch. Min. u. Pet. Mitt., Vol. XI, 1890, p. 144. 2 Tables Nos. II and III were calculated for me by Mr. Victor H. Bassett, assistant in chemistry in the University of Wisconsin. BASIC VOLCANICS OF HEMLOCK FORMATION. 107 TABLE II._MOLECULAR PROPORTION OF THE CHIEF OXIDES. SHO he ee ak os arene 50. 55 54.07 | 49.15 Obes een tata a. cie-cel\s aoe5d 1.40 1.35 YATE (G ie ha Soar as a tea eee a Vp 16.38 | 17.22 | He, Omer eee teres 3.28 6.29 | 3.33 Te O eee ry eee) ohn | 97 4,33 9.57 CaO Pe ere nee el B5ediT 5.71 8. 36 Mo Ommene see Aire ee FoR | Berk Gy NCH O Sas eee ead | ent 5.95 3.85 Ke O Mee mtene acon tC E | 1.50 8 | 29 Totalasseccesn oseee | 100.00 | 100.00 | 100.00 TABLE I11.—ATOMIC PROPORTION OF METALS. Sliced ee bee 46. 97 49.49 | 45.41 rien meme LN Mohae 97 94 Alpine teen eu tell RAO 17.73 18. 80 TG ois sense ae 9, 87 7 6 9. 74 (Ca E Te cok 5, 42 5.63 8.33 Wifer ee spent saa eco ammie 6.25 7.09 9.58 Naeem ee SoS) G66 10. 61 6.93 1 aye ee Wie jh ae 27 ARO REN ome eee 100.00, | 100.00 | 100.00 As tne calcium increases there is a corresponding increase of magne- sium and a diminution in potassium. A decrease in sodium is also shown if Nos. 1 and 3 alone are compared. The percentage of sodium present is rather high and with the potassium indicates a magma family rich in alkalies. The magnesium is notably high; such high percentages as we have here usually accompanying much lower percentages of alkali. It may also be noted here that the presence of the magnesium in such amounts indicates the former presence of olivine or the presence still of its alteration products, a point to which attention was directed in the microscopic description of the rocks. No.1 is remarkable for its percentage of titanium, which is very high, even when compared with that contained in the others, which are themselves considerably above the average. All of the rocks contain a large amount of water of hydration. The percentage of CO, contained in Nos. 1 and 2 indicates also that they are much altered. These analyses show that the rocks can not be classed with the typical andesites. Should they be called andesites at all, they must be classed with 108 THE CRYSTAL FALLS IRON BEARING DISTRICT. the augite-andesites, and placed on the border line between them and the plagioclase basalts. It is preferred to include them under the basalts, though it can not be doubted but that if analyses of perfectly fresh rocks could be obtained, there would be found some which would incline more decidedly toward andesites than do the above specimens. VARIOLITIC METABASALTS. Variolites are spherulitic basalts, usually very vitreous. Since the tend- ency to crystallization is so much stronger in the basic than im the acid rocks, it is not surprising that they should be far less common than the cor- responding acid kind. Moreover, the basic glasses are very susceptible to alteration, which naturally obscures the original characters of the rocks. This probably partly accounts for the fact that they are very imfrequently observed. This spherulitic phase of the basalts is well known in Europe, but there has thus far been .found only one reference to its occurrence in the United States. Ransome’ has described a variolite from Point Bonita, California. To this there may now be added a single occurrence in the Crystal Falls district of Michigan. This variolite exposure occurs at N.375, W.900, see. 4, T. 44 N., R. 33 W., in close proximity to the remnant of a basalt stream which shows well-marked flowage structure. The relations of the two rocks are not determinable from the exposures. The rock presents a very rough mammillated surface, due to differential weathering. The varioles, being more resistant than the groundmass sur- rounding them, form the protuberances. These protuberances vary in shape from round to oval, and very rarely are irregular. The varioles vary also in size from minute ones to those about one-half inch in diameter, and con- stitute by far the greater part of the rock. These general characters may be seen on the photograph, fig. 4, Pl. X, taken from the hand specimen. The color of the weathered surface of the rock is gray or light brown, while the fresh surface is in general a dark green. Upon the polished surface of a fresh rock the varioles have an olive-green color, with, in the majority of cases, a distinctly darker center of purplish color. Less frequently this center is lighter green than the remainder of the variole. The varioles are usually separated from each other by narrow areas of groundmass, darker than the varioles themselves, with a purplish or very dark olive-green 1 The eruptive rocks of Point Bonita, California, by F. Leslie Ransome: Bull. Dept. Geol. Univ. of Cal., Vol. I, 1893, p. 99. in , i i . j t 4 ' | 1 Wh \/ | 4 i i ; » rd ideas 4 r Ms . 1 t v " eee ve ‘ { . ; A W Yee Dore t them il Maran i ed { 4 ‘ , i 4 ' F + r i " Je Wy I; OC Fie. A. (Sp. No. 32273. Natural size.) : Photographic reproduction of the weathered surface of a variolite. This brings out very clearly the inammillated surface of the rock, which is due to the differential weathering of the varioles and of the groundmass between them. The rounded character of the varioles, and their gradation 1rom those of very small to those of much larger size can readily be seen. (Desc., p. 108.) Fie. B. (Sp. No. 32273. Natural size.) Reproduction of the polished surface of a variolite. This is designed to show the circular character of the varioles, and the fact that each is separate and distinct from the one adjoining it. It can be seen that some of the varioles have very dark and others much lighter centers. (Desc., p. 108.) 110 U. S. GEOLOGICAL SURVEY MONOGRAPH XXXVI PL. X (A) Weathered surface of variolite. (B) Polished surface of variolite. BASIO VOLCANICS OF HEMLOCK FORMATION, 111 color. In places the varioles are in juxtaposition. However, they do not coalesce, but each is separate and distinct (fig. B, Pl. X). The rock when examined under the microscope is seen to be extremely altered. The only original minerals present are feldspar, apatite, and pos- sibly some magnetite. The groundmass consists of a fmely crystalline secondary aggregate of flakes of chlorite, associated with minute limpid grains, some of which are probably quartz and others feldspar. Scattered through this aggregate are grains of epidote, calcite, a few crystals of original apatite, and mag- netite, and numerous dark reddish brown and black ferruginous specks. The varioles are readily distinguishable from the matrix. From this, as well as from each other, they are invariably separated by a crack, along which reddish-brown ferruginous matter has been infiltrated. The varioles are in general much finer grained than the groundmass, and at times exhibit phenocrysts of feldspar. The composition of the varioles is the same as that of the groundmass, except that apatite is more common in them, and that in addition to the minerals mentioned as occurring in the groundmass a small quantity of original feldspar may be recognized, both as phenocrysts and as part of the groundmass of the varioles. Where these feldspars occur, they are to a great extent replaced by a mass of epi- dote, chlorite, sericite, quartz, and feldspar. The phenocrysts are found near the center of the varioles, and the occasional light-colored centers which were observed macroscopically are due to the presence of these altered feldspar phenocrysts. The more frequent dark centers are due to an accumulation of the dark ferruginous specks in varioles in which the phenocrysts are wanting. No textures could be determined from the remnants of the original minerals. In one variole aggregates of secondary epidote grains and fer- ruginous specks lie in such a position as to produce a distinct radial arrangement. With advancing alteration, spherulites in acid rocks are frequently found to have between their radial fibers secondary deposits of epidote and ferruginous matter, which mark very clearly their radial arrangement. The similar radial arrangement in these varioles of epidote and ferruginous matter seems to point to the varioles having possessed the spherulitic character, though it is now impossible to determine the nature of the fibers forming the spherulites. 112 THE CRYSTAL FALLS IRON BEARING DISTRICT. THE ELLIPSOIDAL STRUCTURE IN THE METABASALTS. Upon examining the flat surfaces of many of the lavas one is immedi- ately struck by their resemblance to a conglomerate formed of round bowlders, all of the same kind of rock, lymg in a matrix of very small quantity and of very different color. Fig. 7isa sketch showing a portion of sucha lava flow. I find that these ellipsoidally parted rocks have been called ‘massive conglomerates,” and the blocks have been spoken of as “bombs” in the manuscript notes of some of the men who have worked among them. The latter term was undoubtedly due to the resemblance of the ellipsoids to the spindle-shaped pieces of lava git Ra eS ER Fila. 7.Sketch of the surface of the outcrop of an ellipsoidal basalt, showing the general character of the ellipsoids and matrix. which one finds around the modern volcanoes. Ellipsoidal basalt is very common throughout the Hemlock volcanic area. It is found most frequently in isolated ledges. However, it is also associated with and grades into non- ellipsoidal varieties. In one good exposure it is overlain by a fragmental scoriaceous mass which separates it from another mass of similar ellipsoidal basalt. While the scoriaceous portion may represent the brecciated surface of a lava flow, it is not so considered, but is presumed to be a tuff deposited upon the flow represented by the ellipsoidal basalt. According to this view the ellipsoidal basalt is on the surface. In another exposure an ellipsoidal basalt overlies a bed of water-deposited clastic rock. There is no passage between the two kinds of rock. The contact between the two is an undu- lating one, and is marked by a mass of schistose material about 2 inches thick and similar to that which is between the ellipsoids. This particular BASIC VOLCANICS OF HEMLOCK FORMATION. 113 basalt is very dense, with only occasionally small chlorite-filled vesicles in it, and there is no true flow structure observable. The facts cited seem to show that this ellipsoidal portion was the surface of a lava flow, whether the top or the bottom is immaterial. In certain cases the ellipsoidal facies may constitute an entire flow. Where the direction, of flow could with any degree of certainty be determined, it was seen that the two longer axes of the ellipsoids are in the plane of the flow. The ellipsoids vary in size from a few inches to 6 or 8 feet in diameter, and are usually spoken of as spheroids. Attention has already been called to the mcorrect usage of this term by F. Leslie Ransome, in his interesting paper on ‘The eruptive rocks of Point Bonita, California.”? The outlines of the bodies are circular only in exceptional cases. On the other hand, sections in all directions through them give almost in- variably ellipses, and there- fore they are more properly ellipsoids than spheroids. On the surfaces exposed the long axes of the ellipses lie in the same general direction. The ellipsoids are formed of a very fine-grained porphy- DO ] ole : k Fic. 8.—Sketch showing the concentration of the amygdaloidal cavities ritic or honporp lyY tle YOCK. on one side of an ellipsoid, this side probably represeuting the side nearest the surface of the How. This is amygdaloidal or non- amyedaloidal. Where amygdaloidal, the amygdules are as a rule dis- tributed throughout the ellipsoids, though on the whole the masses are more scoriaceous on the periphery than near the center. In exceptional cases, the amygdules are much more numerous on the west side of the ellipsoids than on the east side (fig. 8). In such cases the west sides are toward the tops of the lava flows. The ellipsoids are very commonly split up by cracks. Some of them have aroughly radiate arrangement. These may be due to the effects of contraction in the early stages of the existence of the ‘Op. eit., p. 75. MON XXXVI——S 114 THE CRYSTAL FALLS IRON-BEARING DISTRICT, ellipsoids. Others, and by far the greater number, have one set of lines par- allel and another parallel set which in different cases cut the first set at dif- ferent angles, very rarely at a right angle (figs. 8, 9). One of these sets is usually tranverse to the long axes of the ellipsoids (figs. 7, 8). The blocks are separated from one another by a thin layer of a schistose matrix, rarely more than 3 inches in thickness, though exceptionally nearly 8 inches thick. (Cf. figs. 7 and 8 of this paper, and fig. 1 by Ransome.’) Since the above description of the Crystal Falls ellipsoidal lavas was written in 1896, there has appeared Sir Archibald Geikie’s valuable work on the Ancient Voleanoes of Great Britain,’ in which several similar oecur- rences are mentioned. His illustration of this structure on page 184, as can readily be seen on comparison, would answer, but for the absence of a well- defined schistose matrix between the ellipsoids, very well for a sketch of a Michigan pre-Cambrian ellipsoidal lava. The schistose matrix between the ellipsoids upon the weathered surface is seen to be made up of layers concentric with the ellipsoids. It is possible that these layers are not Fis. §—Ellipsoids absolutely concentric in the third dimension. However, no with sets of parallel : Per a : ; lines cutting each @Xposure permitted of the determination of this point. Fre- other at an angle. : ; quently certain layers seem to grade off into others of a some- what different character. The matrix between any two ellipsoids usually separates near the center; where apparently the greatest movement having occurred the schistosity is most developed. One can often as easily knock an ellipsoid out of its encircling matrix as one can the kernel out of a nut. In some cases there is no absolutely sharp line of demarcation between matrix and ellipsoid, but a gradation from one into the other. At the places where three blocks are in juxtaposition one frequently finds, mstead of a triangular space entirely filled by the matrix, in the center of the matrix a triangular area of infiltrated vein quartz (figs. 7, 8). In certain cases the minerals which compose the schistose matrix are not thoroughly cemented and give it a somewhat friable character, causing it on weathered surface to appear granular. In very rare cases a matrix with a distinctly brecciated character was observed, but in this as well as in the cases above described a certain degree 1 Op. cit., p. 76. 2 Ancient Volcanoes of Great Britain, by Sir Archibald Geikie, Vol. I, 1897, pp. 26, 184, 193. EME Ie IE Vt OG Ibe e . (Sp. No, 23675. Natural size.) This colored plate represents the polished ‘surface of an ellipsoid with a portion of the matrix which surrounds it and separates it from the adjacent ellipsoids, The dense character of the center isnicely shown. Around this oval area we get narrow concentric zones of alternating light-grecu and dark-green material. The light-green material corresponds to that in the center, and represents the least-altered basalt of the ellipsoids. The dark-green areas are the chloritized basalt. Beyonil this, forming the outermost greenish-gray zone, one finds the matrix, which possesses distinctly frag- mental characters, though in spite of this with a marked schistose character. The schistosity of the matrix conforms to the contours of the ellipsoid. 116 U.S. GEOLOGICAL SURVEY. MONOGRAPH XXXVI PL. XI. JULIUS BIEN &CO:LITH NY BASALT ELLIPSOID WITH MATRIX. 22 BASIC VOLCANICS OF HEMLOCK FORMATION. 117 of schistosity is noticeable. The matrix between the ellipsoids varies very much in degree of schistosity, color, and composition. The most schistose, and by far the most common variety, is the dark green matrix, which consists essentially of chlorite, epidote, and zoisite. This material is clearly the result of the chloritization and the epidotization of the original basalt constituting the ellipsoids, for we see it alternating with bands of and grading into the less altered basalt. (PI. XI.) A second facies of the matrix is that which possesses only a moderate degree of schistosity, and appears at times almost massive. This matrix may be light colored, almost white or greenish, or a dark bluish-black. It is ~-edium grained or aphanitic. The light-colored matrix consists essentially ot quartz and calcite. When a little chlorite or epidote is present, it has a greenish tinge. The very dark variety consists of quartz and siderite, col- ored with minute particles of iron oxide. The quartz-calcite or quartz-siderite rake) goregates owe their origin to essentially the same processes, calcification, or x sideritization, respectively, followed by silicification of the original basaltic material. They are therefore briefly described together here. Their charac- ters and origin will be found discussed in detail on page 130 et seq. Some of the peculiar characters of this matrix are illustrated in fig. B, Pl. XX VII. The least common variety of matrix found between the ellipsoids is of a light greenish-gray or brownish color, and possesses a noticeably brecciated character (fig. 6, Pl. XXXIV, and Pl. XI), but with at the same time a certain degree of schistosity. Its characters are best seen under the microscope by moderate maguification. Its breeciated character is then well shown. The fragments of such a matrix are of all sizes and are angular. They show quite commonly a separation into zones. The fragments now consist of chlorite and epidote, and in the fragments with zonal arrangement chlorite in exceedingly fine flaky aggregates occupies the center and epidote the out- side. Now and then there may be several alternating zones of chlorite and epidote. In all cases both epidote and chlorite are present in the zones, but the one mentioned is in great quantity, while the other is very subordinate. The epidote is very commonly the dark ferruginous kind mentioned on page 101, and marks off the outer limits of the fragments. Now and then the limits are outlined by a zone of brownish ferruginous material, whose exact character could not be determined. The fragments of the breccia show now neither original minerals nor textures. To judge from 118 THE CRYSTAL FALLS IRON-BEARING DISTRICT. the uniform character of the zones in the fragments, the original material was very homogeneous, most probably a basalt glass. The spaces between the fragments are occupied by a finely crystalline ageregate of quartz and chlorite, with a small amount of epidote. ‘This aggre- OOo sgt gate outlining the original fragments owes its origin most probably to the process of infiltration. The long axes of the quartz grains and chlorite flakes in this aggregate usually show a general parallel arrangement. Moreover, the long directions of the fragments are in general parallel with each other, and : with the quartz-chlorite aggregate between them. This parallelism results in giving an imperfect schistosity to the matrix. The schistosity of the matrix is in general parallel to the contours of the ellipsoids which it surrounds. Origin of the ellipsoidal structure—H]lipsoidal structures similar to those just considered have been described by various authors. } 1 On columnar, fissile, and spheroidal structure, by T. G. Bonney: Quart. Jour. Geol. Soc., Vol. XXXII, 1876, pp. 140-154. Ueher mechanische Gesteinsumwandlungen bei Hainichen in Sachsen, by A. Rothpletz: Zeitschr. deut. Geol. Gesell., Vol. XXXI, 1879, pp. 374-397; Vol. XXXII, 1880, p. 447. Report on the geology of northern New Brunswick, by R. W. Ells: Ann. Rept. Geol. and Nat. Hist. Survey of Canada, 1879-80, D, p. 24. EK. Dathe: Jarb. K. preuss. geol. Landesanstalt, 1883, p. 432. K. Dalmer: Cf. Zirkel Pet., Vol. II, p. 650. Report on the geology of ae Lake of the Woods region, by A. C. Lawson: Geol. and Nat. Hist. | Survey of Canada, 1885, CC, pp. 51-53. The greenstone-schist areas of the Menominee and wanders regions, Dy G. H. Williams: Bull. U.S. Geol. Survey, No. 62, 1890, pp. 137, 166-168, 175, and 203. On the variolitic mone of Mont Genevre, by G. A. J. Coleand J. W. Gregory: Quart. Jour, Geol. Soc., Vol. XLVI, 1890, pp. 295-332. On a yariolitic diabase of the Fichtelgebirge, by J. W. Gregory: Quart. Jour. Geol. Soc., Vol. XLVII, 1891, pp. 45-62. The Kawishiwin agglomerate at Ely, Minuesota, by N. H. Winchell: Am. Geol., Vol. IX, 1892, pp. 399-368. The eruptive rocks of Point Bonita, California, by F. L. Ransome: Bull. Dept. of Geol. Univ. of Cal., Vol. I, 1893, pp. 71-114. Editorial note on the above paper, by N. H. Winchell: Am. Geol., Vol. XIV, 1894, p. 321. The geology of Angel Island, by F. L. Ransome: Bull. Dept. Geol. Univ. of Cal., No. 7, 1894, p. 202. Variolite of the Lleyn and associated voleanic rocks, by C. Raisin: Quart. Jour. Geol. Soc., Vol. XLIX, 1893, pp. 145-165. On a radiolarian chert from Mullion Island, by H. Fox and J. J. H. Teall: Quart. Jour. Geol. Soc., Vol. XLIX, 1898, p. 211. On greenstone associated with radiolarian chert, by J. J. H. Teall: Trans. Roy. Geol. Soc, of Cornwall, 1894: Cf. Rosenbusch, Mikroskopische Physiographie, 3d ed., p. 1064. The volcanic rocks of the Michigamme district, by J. M. Clements: Jour, Geol., Vol. IIT, 1895, p. 808. The geology of Point Sal, by H. W. Fairbanks: Bull. Dept. Geol. Univ. of Cal., Vol. II, 1896, p. 40. Geology of the Fox Islands, Maine, by G. O. Smith, 1896, pp. 16-18. The Ancient Voleanoes of Great Britain, by Sir Archibald Geikie, London and New York, 1897, }p. 26, 184, and 193. BASIC VOLCANICS OF HEMLOCK FORMATION. 119 Various attempts have been made to explain this peculiar structure. Bonney, Dathe, and Raisin regard contraction as the force which produced the rounded masses. Dathe and Dalmer show by the presence of the concentrically arranged amygdules that the ellipsoids were units, and were formed before solidi- fication of the rock. his arrangement of the amygdules, as well as the arrangement illustrated in fig. 8 on page 113, precludes at once the idea that the structure owes its origin to the well-known weathering process which by exfoliation produces spheroidal blocks. Rothpletz and Williams look upon the ellipsoids as due to mechanical forces which ground down the angles and edges of a fractured lava dow, the idea of both authors apparently being that the fractures were long sub- sequent to the movement of the flow. Ells and Lawson mention the structure as concretionary. Winchell considers the cases described by him as agglomeratic accumulations. Cole and Gregory see in the masses evidence of lavas rolling over among themselves. In the later paper, published alone, Gregory definitely states that the lava first contracted into spheroids, which then rolled over one another. Ransome explained the Point Bonita occurrence as a basalt which flowed ‘‘as a viscous pahoehoe, one sluggish outwelling of lava being piled upon another to form the whole mass of the flow.”* In the description of the basalts, he writes: “A certain amount of crushed and sheared material fills the interstices between the spheroids and seems to be made up of com- minuted fragments of the same rock. It is, however, too crumbling and too full of secondary products for a satisfactory determination.”” In the second occurrence, in a fourchite (augitite?), the relations of the rocks are such as to prove “conclusively that such structure can not be rigidly restricted to surface flows, although it is still believed that lavas exhibiting it must have been erupted under very nearly surface conditions.”° Teall agrees with Ransome in comparing the ellipsoidally parted masses of basalt to pahoehoe lava. Teall concludes that such ellipsoidally parted basalts are submarine flows. In a recent paper Smith has described from certain volcanics bodies 1 Op. cit., p. 112. 2Op. cit., p 78. * Angel Island, op. cit., p. 202. 120 THE CRYSTAL FALLS IRON-BEARING DISTRICT. which in cross section give elliptical figures, but whose indeterminate down- ward extension shows them to be columns. The rounding of the columns, which were presumably originally prismatic, he ascribes to dynamic action- He also suggests that ellipsoidal masses could result from a similar dynamic modification of a mass of lava parted into shorter prisms, or even ellipsoids. In the description of the eruption at Santorin, Fouqud* mentions a viscous lava exuded in the form of a mass of blocks. These blocks, tum- bling over one another as the mass is pushed from behind, have accumulated in a rough pile, Pl. XI. Fouqué climbed these piles of block lava shortly after their production, and noticed the breaking off of pieces from the sides, due to the cooling and contraction of the individual blocks.” In general this character agrees well with that of the aa lava of Hawaii, as described by the late Prof. J. D. RU aaig Se Shee 1 Dana.’ He describes the formation of Fig. 10.—Reproduction of illustration of aa lava, after the blocks as due to the slow for- Dana (Characteristics of Volcanoes). ward movement and contemporaneous breaking up of the viscous lava. The surface contrasts with the ropy surface of the more liquid pahoehoe. The aa is as a rule compact as compared with the pahoehoe, though the exterior “is roughly cavernous, horribly jagged, with projections often a foot or more long that are bristled all over with points and angles.” From the illustrations of this lava (see fig. 10, taken from Dana) the blocks may be seen to be, while irregular, still in general distinctly rounded. This is the shape which viscous material would naturally tend to take when subjected to the rolling action attendant upon the onward motion of the stream of which they form an outer portion, or in certain cases the entire thickness. This is clearly shown from the following quotation from Dana’s description of the constitution and condi- 'Santorin ét des Lruptions, by F. Fouqué: Paris, 1879, Chap.II. Compare especially Pls. VIII and XIII. 2Op. cit., p. 54. 8Characteristics of Volcanoes, by J.D. Dana: New York, 1890, pp. 9, 241,and Am. Jour. Sci., 3d ser., Vol. XXXIV, 1887, p. 362. “An aa or arate lava stream consists of detached masses of lava as far as is visible from the outside. The masses are of very irregular shapes and confusedly piled up to nearly a common level, although often covering areas many miles long and half a mile toa mile or more wide. The size of the masses in the coarser kind varies from a few inches across to several yards.” ee —T ‘enbnoy *4 Aq ,,‘suoncnig sas je UOJUeS ,, WO |] ‘Jg JO UONONpoday 'SVAV1 XOO1E8 DILSIMSLOVYVHOS SHL ONILVYLSNIT! 998 ‘TIYdVv NI ‘MNV14 LSSM SLI WOYS GSMFIA 'SOINYOID LNNOW WX “Id IAXXX HdVYSONOW AGAYNS 1V91901039 ‘Ss ‘sn i] te i > : eae : \ ~ i ) i BASIC VOLCANICS OF HEMLOCK FORMATION. 121 tion of the aa stream wher in motion:' “(1) A mass of rough blocks outside, precisely like the cooled aa stream; (2) the motion extremely slow, indi- cating a semifluid condition beneath; . . . (5) the blocks of the upper part of the front, as the stream creeps on, tumbling down the high slope, owing to retardation at bottom from friction, and thus a rolling action in the front part.” Dana describes the gradation of pahoehoe into aa lava. He writes, ‘a lava stream may change from the smooth-flowing or pahoehoe condition to the aa and back again to the smooth-flowing.”* Platania® describes from Aci-Trezza and Aci-Castello basalts with globular structure. The interspaces between the globes are filled with silt, or silt and tuff, and the exterior of some of these globes presents a thin vit- reous cracked crust (cf. p. 117). These globular basalts are apparently but a modification of the block or aa lavas described by Fouqué and Dana, in which the separate portions of the lava have assumed a sufticiently rounded character to be called globes. However, Platania’s further descriptions show this term to be clearly inapplicable unless the word ‘“elobe” is used with considerable latitude. The Santorin block lava, the Hawaiian aa lava, and the Aci-Castello globular lava are all products of a slowly-flowing comparatively viscous mass. They will in the further description be included under the general term ‘‘aa lavas,” as this is the most common form of occurrence of such viscous lavas. The ellipsoidal basalts of the Crystal Falls district appear to be com- parable to the Hawaiian aa lava and block lavas of the kind described by Fouqué. The lavas have subsequently been exposed to great pressure and are considerably altered. The most obvious character of these masses, their rounded outline, is believed to be due to considerable extent to the onward motion of the stream as described by Dana. Contraction caused by cooling, accompanied by falling off of fragments from the outside, as observed by Fouqué* in the Santorin block lava, would also tend to round blocks which were originally angular. (PI. XI.) In 1\Characteristics of Volcanoes, by J. D. Dana, New York, 1890, p. 242; and Am. Jour. Sci., 3d ser., Vol. XXVI, p. 100. 2Am. Jour. Sci., 3d ser., Vol. XXXIV, p. 363. 3Geological notes of Acireale, by Gaetano Platania: The Southern Italian Volcanoes, H. J. Jolnston-Layis, editor, Naples, 1891, Chap. IL., p. 41. 4Op. cit., p. 54. 22 THE CRYSTAL FALLS IRON-BEARING DISTRICT. some cases the separate portions of the lava may have been originally nearly globular, similar to the ones described by Platania. The ellip- soidal basalts, however, are so common in the Crystal Falls district and such globular basalts are so rare that this peculiar form is not considered worthy of much consideration in the further discussion, the first two kinds being chiefly the forms from which these were derived. The lava blocks rolling over one another as the lava stream advanced, would lie with their axes in all positions, but pressure and the onward movement of the flow would, in the lower portion of the stream at least, be sure to produce from the blocks ellipsoidal bodies with their two longest axes corresponding—the one to the direction of flow and the other to the lateral extension of the stream. After the stream ceased to flow and the lava solidified, there would be a gradation from the ellipsoidal into the non- ellipsoidal portion of the flow. An aa stream, such as described and shown in fig. 10, when subjected to great pressure subsequent to burial beneath thick deposits, would be compacted by the breaking up of the jagged outer portions, which, falling down, would fill the spaces between the blocks. This broken material fillmg the spaces would be most exposed to movement and to the action of percolating waters. It would consequently be very much altered, as in the material described above (p. 119) by Ransome. Such alterations would result in producing a matrix of exactly the same general composition as the altered ellipsoids. It is the common case of metamorphic action producing from rock masses of essentially the same chemical composition, but of different character, similar end products. This brecciated character of parts of this matrix is well shown in parts of Pl. XI, and fig. B, Pl. XXXIV. In this case silica has been introduced, filling the spaces and marking out the outlines of the fragments. Where mashing has been excessive, the outlines of the fragments are obliterated and the matrix rendered schistose. There may even be a gradation from the schistose matrix imto the altered basalt of the ellipsoid, which at the center is massive. Let me recall the statement made on previous pages concerning the distribution of the amygdaloidal cavities in the ellipsoids. This is one of the characteristic features of the lavas. We have (1) amygdaloidal cavities distributed about evenly throughout the ellipsoids, the cavities being some- what smaller in the center than upon the periphery; (2) the cavities are BASIC VOLCANICS OF HEMLOCK FORMATION. 123 concentrated upon the periphery with few or only microscopical cavities in the center; (3) they are concentrated on one side of the ellipsoid, this side representing apparently that side of the ellipsoid turned toward the upper surface of the lava stream. The following explanation is offered for this difference in occurrence. he distribution of cavities is determined by three factors: The viscosity of the lava; the difference in specific gravity between the bubbles filling the cavities and the lava; and the expansive action of the gas. In the case of (1) the ellipsoids are considered to have consisted of lava in a viscous condition through which the gas pores formed, but in which, owing to the high degree of viscosity, they remained nearly or quite in the positions in which they were formed. Here viscosity was the determining factor. In case (2) the gas pores, influenced chiefly by the expansion of the gas, collected upon the periphery—just as, for instance, in the steel ingot while the center is compact the outer surface is porous. The lava in this case was probably less viscous than in the former. In the last described condition of distribution (3), where the gas cavities are on one side, which is the upper surface, the lava was still less viscous than in the preceding cases. Here specific gravity was the controlling factor, and, as a result of the specific gravity and the less viscous nature of the lava, the gas bubbles rose and collected upon the upper surface. The explanation of the ellipsoidal basalts which has been offered—viz, that they are comparable with aa or block lava—seems to offer a ready explanation for all of the observed characters. On the whole, the ellip- soids owe their origin and certain peculiarities to the viscous nature of the lava. They possess also characters which are due to contraction, others which are due to original flowage, and still others which are the result of subsequent orogenic movements. In certain places we may find the ellipsoids only half formed—that is, attached by one side to the main unbroken part of the lava flow, the other side showing a rounded outline. This probably represents a place where the aa grades into a pahoehoe or smooth-flowing form. Such an instance is possibly that illustrated by Ransome.’ Both Ransome and Teall compare the ellipsoidal basalts studied by them with pahoehoe lava. The latter also suggests a submarine origin for the basalts studied by him. It should be noted that pahoehoe lava in its ‘Point Bonita, op. cit., fig. 2, p. 77. 124 THE CRYSTAL FALLS IRON-BEARING DISTRICT. typical occurrence in Hawaii is found only in dry places, whereas the aa is confined to those parts of the lava stream—which in other portions of its course is perhaps developed as pahoehoe—where it crosses moist valleys or other depressions presumed to have contamed a considerable amount of moisture.’ In the case of some of the block lava of Santorin described by Fouqué,* with which this may be compared, the conditions were such that the lava practically welled up through the water. ‘From Dana’s description it appears that lava in the pahoehoe form can not exist in the presence of moisture, being changed to the aa form. It would thus seem that Téall’s statement of a submarine origin for the pahoehoe lava is untenable. Wherever the ellipsoids have been studied in the Crystal Falls district, they have been found to exist as separate units, thus indicating the extremely viscous character of the lava. It would seem that the analogy between these basalts and the aa or block lava is much greater than that which exists between them and the pahoehoe or smooth-flowing lava. AMYGDALOIDAL STRUCTURE, The amygdules in the basalts are composed of nearly the same min- erals as those which occur secondarily in the rock mass itself. Arranged in order of frequence of occurrence, they are as follows: Chlorite, epidote- zoisite, quartz, calcite, feldspar, iron oxide, and biotite. An amygdule may consist entirely of one of the above minerals, or, as is most commonly the case, of two or more of them. In the latter case the minerals are usually arranged in concentric layers. The nonoccurrence of zeolites is very noticeable. Their absence from these Huronian volcanics is especially striking since they are so common in their altered modern equivalents, and also occur in basalts as old as those of the Keweenawan of Lake Superior® and of the South Mountain of Pennsylvania.* ! Cf. Characteristics of Volcanoes, by J.D. Dana: New York, 1890, p. 243. 2Op. cit., Chap. II. ‘’Paragenesis and derivation of copper and its associates on Lake Superior, by Raphael Pum- pelly: Am. Jour. Sci., 3d ser., Vol. II, 1871, p. 188; also Geol. Survey, Michigan, Vol. I, part 2, 1873, pp. 19-46; Geol. of Wisconsin, Vol. IIT, 1880, p. 31. The copper-bearing rocks of Lake Superior, by R. D. Irving: Mon. U.S. Geol. Survey, Vol. VY. 1883, p. 89. 1The voleanic rocks of South Mountain im Pennsylvania and Maryland, by G. H. Williams: Am Jour, Sci., 3d ser., Vol. XLIV, 1892, p. 491. ea BASIC VOLCANICS OF HEMLOCK FORMATION. 125 It is also of interest to notice that there is a total absence of indica- tions of copper in these Huronian volcanies, as well as in those of the Penokee- Gogebic, although it is associated with similar rocks in the areas above referred to as well as in many others. The amygdules, with the exception of those of chlorite and of biotite, are of much lighter color than the body of the rock, and from a short dis- tance give the rock the appearance of a porphyry. Weathering gives the rock a different appearance according to the materials filling the vesicles. Where these weather readily they are removed and the rocks become scoriaceous. Where, on the other hand, as frequently happens, the vesicles are filled with quartz, the matrix weathers more rapidly and the rounded quartz cores stand out on the face of the rock like the quartz pebbles from the softer matrix of a conglomerate. In a few cases hematite is disseminated through the quartz of the amyedules, giving it the bright-red color of jasper, and by some these anygdaloidal fillings have been taken for included jasper pebbles. Careful study was made of the filling of the vesicles with the object of © determining the order of deposition of the minerals. However, it was found that the amygdules in a single slide contain very different fillings, one chlorite, another calcite, a third epidote, and so on; and that even in the same slide the relations are not always the same, a mineral which here occupied the center of an amygdule being found there on the periphery. Moreover, the same mineral species was found at times occupying the out- side and the center of the same amyedule. It is clear that the fillings are not the result of a solution common to all the lavas, but that the same kinds of solutions were active in the various lavas at different times and even in the same lava at different times. How- ever, the conciusions reached were that the chlorite was generally the first product deposited and the quartz usually the last. From the study of the related amygdaloids upon Keweenawan Pomt, Pumpelly' long ago reached the conclusion that chlorite was the earliest product of alteration—hence we may conclude the first to be deposited in the amygdaloidal cavities; and that the latest mimeral deposited in the cavities, omitting copper from 'The paragenesis and derivation of copper and its associates on Lake Superior, by Raphael Pumpelly: Am. Jour. Sci., 3d ser., Vol. II, 1871, p. 29. Metasomatic development of the copper-bearing rocks of Lake Superior, by Raphael Pumpelly: Proc. Am. Acad. Arts and Sci., Vol. XIII, 1878, p. 307. 126 THE CRYSTAL FALLS IRON-BEAKING DISTRICY. consideration, was quartz, the tendency naturally being to replace more alterable with less alterable minerals. Flattening of amygdaloidal cavitie.—In some of the amygdaloids (fig. B, Pl. XXY) the cavities retain their circular shape, as though the rock had not flowed to any great extent. More commonly the cavities are drawn out into irregular (fig. 4, Pl. XXV) or lenticular shapes, the long axes agreeing with the direction of flowage in case their deformation resulted from this, or with the direction of schistosity in those cases where the rocks have been extensively mashed. In some cases the cavities have been so extremely flattened that the amygdules appear almost the shape of a melon seed, showing a mere streak of chlorite in the sections cut perpendicular to the schistosity, and in the planes of schistosity large lustrous oval areas. In some few of the basalts the groundmass immediately surrounding the amygdules is characterized by an accumulation of ferruginous matter. In most cases, however, this part of the groundmass does not differ in any respect from the rest of the groundmass of the basalts and points to a very eradual cooling. ALTERATION OF THE BASALTS. The descriptions given are of the freshest and most characteristic basalts. As already explained, the mineral constituents in even these freshest ones have undergone a very far-reaching alteration. The rocks which show a more advanced stage of alteration exhibit merely a difference in degree rather than in kind, and the minerals which result are in all cases the same. They are uralite, actinolite, epidote-zoisite, chlorite, white and brown mica, calcite, sphene, quartz, and feldspar. The amount of these secondary minerals varies greatly, showing that the alteration products resultmg from the same kind of original rock may differ very materially according to the process of metamorphism. In a general way the alteration of the basalts, as observed under the microscope, has taken the following course: Even in the rocks nearest their original condition the augite has largely changed to uralite. The vitreous base, if any was present, has become devitrified. Rocks in this stage of change still show the more important external characters of igneous rocks, including in many cases those which are characteristic of glass. Some of the rocks at this stage are light gray to green and exceedingly tough. Many of these break with a ringing sound almost like phonolites. At a BASIC VOLCANICS OF HEMLOCK FORMATION, 127 further stage of change the feldspars are partly altered to a granular ageregate of various minerals. In ordinary light the textures of igneous rocks are still preserved, but in polarized light none are seen, with the exception of amygdules which may be present. In some cases even these are obliterated, and the original nature of the rock can only be determined from its mode of occurrence and its association. Further changes may produce rocks which consist practically of calcite, and may be nearly white. Again, from these basic rocks there may be produced in extreme cases, by a process of silicification, a rock which consists practically of pure silica. Description of some phases of alteration—As illustrating some cases in which the same alteration products, but in different proportions and arrangement, give rocks differing very essentially, there are given the following brief deserip- tions of some of the rocks studied. The flow structure was noted as being exceedingly well developed in the microlitic rocks, and in some of them the production of amphibole needles and chlorite flakes has taken place parallel with the long direction of the feldspar microlites (the flowage direction), thus developing, in com- bination with the unaltered microlites, a well-marked schistosity. The feldspars are still fairly well preserved. Tn another case the feldspar microlites have become completely sericit- ized, the interspaces between them being occupied by epidote, chlorite, and iron oxide. The preservation of the feldspar shapes, showimg in ordi- nary light the igneous texture of the rock, gives the only clue to its origmal nature. (Figs. 4 and B, Pl. XXVIII.) In some of the basalts the feldspar is replaced chiefly by epidote-zoisite, and, as in the above case, such rocks show their igneous character only when examined in ordinary light or by uncrossed nicols. (Figs. A and B, Pl. XXTX.) In still other rocks calcite is very abundant. Its occurrence in por- phyritic rhombohedra and scalenohedra was mentioned in the description of some of the rocks. These porphyritic calcites have thus far been found only in the fine-grained microlitic types of groundmass, the coarser ophitic rocks haying it only in the usual granular aggregates. Muscovite, occurring in large porphyritic plates, conforms in occurrence to the calcite. When muscovite is present, calcite is found associated with it im every case, though the calcite may oceur alone, and this latter is also by far the more 128 THE CRYSTAL FALLS IRON-BEARING DISTRICT. common. These crystals give a secondary porphyritic character to the lavas, and the microscopical appearance of the rocks varies somewhat according to the occurrence of the caleite. Such rocks, for instance where the rhombohedra oceur, look on fresh surface by rapid examination like porphyrites in which the feldspar sections are all quadratic. In the others the scalenohedral sections resemble in general lath-shaped feldspar pheno- erysts lying scattered in all directions on the surface of the rock, Another case of extreme alteration is shown in a light greenish-gray, much-altered schistose rock from sec. 21, T. 46 N., R. 32 W. Upon the weathered surface long grooves are noticed—one measuring 60 mm. long by 5 mm. wide—which on the fresh surface are filled with calcite. On faces perpendicular to the long extension of such grooves they appear as narrow slits, with the long direction of the slit, that is, the width of the erooye, agreeing with the schistosity. These are clearly flattened amygda- loidal pores, and but for them the igneous nature of the original rock could not have been determined. The extreme flattening of these amygdaloidal cavities and the schistose nature of this rock produced from an original volcanic, points toward mashing as one of the causes, if not the main cause, of its present characters. It is now composed of fairly large automorphic actinolite individuals, a very small amount of biotite and chlorite flakes, and masses of grains of quartz, calcite, epidote-zoisite, magnetite, with ilmenite aud hematite in thick plates fillmg in the spaces between the actinolites. If any feldspar was originally present, it is now entirely concealed by the calcite and epidote-zoisite. The calcite phenocrysts are found in the fairly fresh lavas. They are beautifully automorphic and are certainly not replacement pseudomorphs of some original phenocrysts, but replace the various minerals of the fine- grained mass. Moreover, it is clear that they were formed subsequent to all dynamic action, as their erystal outlines are perfect and they never show any evidence of pressure. This is so even in those cases where the amygdules which have been markedly elongated are filled with calcite The process of replacement could not be followed, but it is evidently con- nected with the development of chlorite, those rocks in which a great deal of the calcite occurs having chlorite developed instead of actinolite. In other sections in which the amount of porphyritic calcite or calcite and muscovite is much greater than in the rocks just described, the amount ee ee ee BASIC VOLCANICS OF HEMLOCK FORMATION, 129 of chlorite, iron oxide, rutile, and quartz is also greater. The quartz is in very fine grains. The presence of the feldspar can only be determined with difficulty, and usually only on the edges of the sections, as the large amount of chlorite in the center conceals it. The textures caused by the feldspar and the amygdules still indicate the original character of such extremely altered stages. Figs. 4 and B, Pl. XXX, illustrate such a rock, showing the secondary porphyritic muscovite and calcite, and also the original amygdaloidal character. A still further stage of alteration gives a rock whose groundmass is composed of the finest-grained quartz and of grains and needles of brown rutile (anatase?). In this lie rhombohedra of ferruginous calcite, plates of muscovite, and irregular flakes of chlorite. The rock is macroscopically eray, hard, and quartzitic, has a ferruginous, brown, weathered crust, effervesces with cold HCl, and yet shows its volcanic character by the numerous beautiful amygdules. These stand out on the surface like pebbles in a conglomerate. In some cases the weathering brings out the concentric character of the fillmg very nicely. For example, some may be seen in which the core is quartzitic, and is standing surrounded by a ring-like depression, showing by difference in the weathering the different character of the mineral filling. Under the microscope the only amyg- dules which happened to be cut by the section were found to be filled with fine-grained quartz, with chlorite in automorphic flakes at the center of the amygdules, and lying in the quartzitic mass. The macroscopical appear- ance of some of the amygdules shows that just the reverse condition also exists, that is, that quartz forms the centers and chlorite surrounds it. The extreme stage of such an alteration is a rock which shows no amygdules macroscopically or microscopically, but is otherwise like the groundmass of the above last-described rock. It would be impossible to determine the original character of such a rock except by its association. The extremes of texture obtained in the alteration processes are, on the one hand, a porphyry with eruptive groundmass and secondary pheno- erysts; on the other, a porphyritic schist, in which all elements are secondary, These extremes are connected by gradation varieties, in some of which the calcite and muscovite approach more closely to the size of the elements composing the groundmass, and which consequently approach the ordinary schists in structure. MON XXxvVI——9 e 130 THE CRYSTAL FALLS IRON-BEARING DISTRICT. In these rocks the porphyritic characters are unquestionably due to the production of secondary phenocrysts of mica (muscovite) and calcite, not by contact metamorphism but by dynamic action.’ It has not been found possible to determine definitely from a study of the specimens, in many cases from widely separated exposures, on which the above observations were made, whether the process which has taken place in the production of such rocks has been a combination of calcification and silicification, or a process by which carbonate is being replaced by silica or the reverse. The replacement of carbonate by silica, as shown by Irving and Van Hise,” has taken place extensively in the case of the ferru- ginous carbonates of the Penokee-Gogebic and Marquette iron ranges of Wisconsin and Michigan. The automorphic character of the carbonate would seem to point toward calcification as the controlling process in the Crystal Falls rocks. Though the presence of quartz as the last filling of the amygdaloidal cavities points toward silicification as being the process which would eventually predominate, it is most probable that both processes of calecifi- cation and silicification are active; but whether the one or the other is the controlling one depends upon the depth of burial of the rocks which are altering. This statement appears to be supported by the facts to be described in the following pages. The following observations, which were made upon sections taken from an ellipsoidally-parted basalt occurring on top of the hills to the west of and overlooking Mansfield, illustrate the changes which take place in the passage from the massive rock of the ellipsoids into the schistose material of the matrix. The change is one of increasing altera- tion. This alteration is largely one of carbonation followed by silicifica- ! Metamorphism of clastic feldspar in conglomerate schist, by J. E. Wolff: Bull. Mus. Comp. Zool., Vol. XVI, 1891, pp. 173-183, Pls. I-XI. Cf. also Wolff on Green Mountains, Mon. U.S, Geol. Survey, Vol. XXIII. ; Prineiples of North American pre-Cambrian geology, by C. R. Van Hise: Sixteenth Ann. Rept. U.S. Geol. Survey, Pt. I, 1896, p. 692. Phases in the metamorphism of the schists of Southern Berkshire, by W. H. Hobbs: Bull. Geol. Soe. Am., Vol. IV, 1894, pp. 169-177. 2 Origin of the ferruginous schists and iron ores of the Lake Superior region, by R. D. Irving: Am. Jour, Sci., 3d ser., Vol. XXXII, 1886, pp. 255-272. The iron ores of the Penokee-Gogebie series of Michigan and Wisconsin, by C. R. Van Hise: Am. Jour. Sci., 3d ser., Vol. XXX VII, 1889, pp. 32-48. ‘The Penokee iron-bearing series of Michigan and Wisconsin, by R. D. Irving and C. R. Van Hise: Tenth Ann. Rept. U.S. Geol. Survey, 1889, pp. 341-507; Mon., Vol. XTX, 1892, pp. 254-257. ‘ BASIC VOLCANICS OF HEMLOCK FORMATION. 131 tion. It may be characteristic also of basalts with no ellipsoidal parting, but it has been possible to follow the successive changes only in the ellip- soidal basalts. This is due to the fact that each ellipsoid shows all stages from the comparatively fresh material of the center to the much altered material on the periphery, and to the most altered basaltic material forming the so-called matrix surrounding the ellipsoidal bodies (p. 114). The freshest part of the interior of an ellipsoid from this occurrence is a very fine-grained micro-amygdaloidal basalt, in which in ordinary light lath-shaped feldspar microlites can be readily distinguished. Upon close examination the feldspars are found to be much altered, and in many cases their crystal outlines are almost completely filled out by grains of calcite and flakes of sericite and chlorite in a quartz-albite (?) aggregate. The spaces between the feldspar laths are now occupied by large erystals of epidote-zoisite, grains of iron oxide, a few flakes of chlorite, and innumera- able small round yellowish-brown and greenish indeterminable bodies. The epidote-zoisite crystals also include large quantities of the brown and green globular bodies, showing that they were produced previous to the epidote- zoisite. ‘The substance in which this aggregate is embedded could not be determined, as the aggregate is either so dense that nothing could be dis- cerned or else underlain by feldspar. In the last case the substance is :2en to be clear white. The minerals mentioned, with the exception possibly of the iron oxide, have evidently been produced secondarily from the sub- stance or substances originally fillmg the spaces between the feldspars. Nothing points toward the original substance or substances having been crystallized, and’ am inclined to believe that it was glass. Toward the exterior of the ellipsoid the rock is more altered. The zoisite and calcite are more abundant. The calcite occurs in the spaces between the feldspars, as well as occupying parts of their outlines. (Figs. A and B, Pl. XXXI.) All of the other products drop into the back- ground, owing to the fact of nonproduction, or concealment by the zoisite- calcite aggregates. Still nearer the exterior of the ellipsoid the calcite frequently fills the spaces once occupied by the feldspars with long scalenohedral crystals, which in a way maintain the original igneous structure. The calcite is, howeyer, not confined to these feldspar areas alone, but, as stated above, also occurs between them. V2, THE CRYSTAL FALLS IRON-BEARING DISTRICT. The matrix, representing the most altered phase, is a granular aggre- gate of calcite, in which one may here and there discern small clear limpid grains of secondary quartz and feldspar (?) and flakes of chlorite. The calcite includes in considerable quantity the globular bodies mentioned. These are found also in the spaces between the calcite grains, as though pushed away from the grains as they crystallized. Some of the calcite in the first stages of the alteration of the rock may have been derived from a basic feldspar It is clear, however, that the great mass can not owe its origin to this process, but must be the result of infiltration. The calcite grains derived from, and lying in, the feldspar acted as nuclei, around which the infiltrated calcite was gradually col- lected, producing pseudomorphs after the feldspar laths. Quite recently Dr. W. 8. Bayley’ has noted in the Clarksburg submarine voleanic forma- tion of the Marquette district, Michigan, the occurrence of tuffs, in which calcite has been introduced in such quantity that they may almost be called limestones. In another case in which the alteration of the ellipsoid (Pl. XI) appar- ently proceeded along the lines of shearing, and produced the kind of aggregates of chlorite, including crystals and aggregates of epidote- zoisite, which were described (p. 117) as the usual matrix of such ellipsoids, one can see in thin section the calcite entering the chlorite aggregate along minute fissure lines. The calcite literally eats its way into the chlorite, and produces by an interchange of elements a mass of calcite (magnesian ?) and epidote, besides including epidote which originally occurred scattered through the chlorite aggregate. The carbonation of the original basalt or of the secondary chlorite mass results in producing a mass of carbonate which has associated with it some secondary quartz, chlorite, and epidote. This carbonate mass may be almost massive or it may be decidedly schistose. When schistose, the grains of calcite and quartz have a uniform elongation, and the schistosity is materially enhanced by flakes of chlorite, which are not uncommonly found in thin streamers or thick masses in the carbonate aggregate, at times in sufficient quantity to give it macroscopically a decided green tinge. T have used the term ‘‘carbonate,” although having described in detail above the calcification of the basalt, for the reason that at times, and for no ‘Mon. U.S. Geol Survey, Vol. XXVIII, p. 473. BASIC VOLCANICS OF HEMLOCK FORMATION. 133 discernible reason, the iron carbonate (siderite) may replace the calcite, in which case we get a dark bluish-black variety of matrix (p. 117). The siderite masses do not differ essentially from the calcite, though in some of them a very small quantity of actinolite is found associated with the chlorite. As illustrating the purely local development of these two carbonates, I would mention having observed in one section a band of siderite separated from a band of carbonate, which from its color appeared to be quite pure calcite. One may also see commonly in exposures areas of pure white calcite, almost in juxtaposition with areas of siderite. It is a fact generally recognized that carbonation is a process confined to the outer crust of the earth, so that we may perhaps best explain the local occurrence of these carbonates replacing the basalt as products of carbonate-bearing waters. That such carbonation of the igneous rocks through which these waters percolate is now taking place seems certain. The carbonate grains in the rocks described are shattered and elongated, or at least show undulatory extinction. They thus give evidence of having been more or less mashed since their production, and this mashing probably took place after they had been more or less deeply buried, and was, as a matter of fact, to some extent due to the pressure of the superincumbent rocks. The probability that these rocks have been thus deeply buried subse- quent to their formation is to be borne in mind with special reference to the next process to which they have been subjected, that of silicification. This process is most clearly shown in the siderites, and the phases of alteration noted in their study will be briefly described. _ The microscope shows the siderite matrix to be a coarsely granular agoregate composed essentially of crushed siderite grains. Between these grains in a few places are small grains of quartz, flakes of chlorite, and very rarely needles of actinolite. Large quantities of black ferruginous specks are included in and also lie between the quartz grains, and such specks are also to be seen included in siderite areas, but close inspection shows that they are also associated with blebs of quartz. The chlorite flakes and quartz grains are generally elongated in the same direction, and the quartz shows wavy extinction. A more advanced stage in the process of silicification was studied in the case of a rock which is bluish-black in color, exceedingly fine grained, and minutely schistose, the schistosity agreeing with the contours of the 134 THE CRYSTAL FALLS IRON-BEARING DISTRICT. ellipsoid from around which it was broken. This is essentially an exceed- ingly fine-grained quartz rock, with chlorite flakes and black ferruginous specks scattered through it, and here and there an irregular oval siderite grain remaining. Very few and unimportant grains of epidote were also noticed. This rock represents nearly the last stage in the process of silici- fication by which the siderite has been replaced, and a part, probably the greater part, of its iron content oxidized. Some chlorite and epidote has been produced, clearly from the lime and magnesian impurities in the siderite. Essentially the same process of silicification has been described by Van Hise in his various articles on the Penokee-Gogebie and Marquette iron ranges, to which references have been so frequently made. I have desired especially to call attention to it here, however, on account of the fact that it shows the possibility of the production ofan iron ore from an original eruptive rock by the combined processes of carbonation and silici- fication. It is true that the end product in the case described does not contain enough iron to be an ore deposit, but that is a mere detail. May not this serve also to some extent to explain the numerous clearly marked belts of magnetic attraction which occur throughout this area of altered basalts, in which little of the original magnetite remains unaltered to exert an influence upon the magnetic needle? To explain this we must suppose the influence to be exerted by secondary magnetite accumulated along cer- tain lines. The magnetic lines traced out agree in a very marked way with what has been determined to be the trend of the lava flows and tuff beds. The condition which would determine the presence of such a line of carbonation, if we may so put it, may be the presence of a scoriaceous lava flow or a bed of tuff, which offers exceptional facilities for the passage of carbonate-bearing waters. It is thus intimated that there is possibility of finding purely local ore bodies of small size even in the midst of this volcanic area. The process of silicification is generally considered as a deep-seated one, occurring far below the outer weathering zone. When the rocks exhibiting these various phases of silicification are exposed in the zone of weathering, certain interesting results are obtained which are worth noticing. Rocks are produced from these which upon the surface strongly resemble amygdaloids, but in which the pseudo-amyegda- loidal cavities are of purely secondary origin. For instance, when the siderite mass has become only partially replaced by silica, weathering BASIC VOLCANICS OF HEMLOCK FORMATION. 135 agencies leach out the remaining siderite areas and leave the thin films of silica which lie between them standing up, thus giving the rock the appear- ance of a very dark pumice. As the silicification progresses the siderite is very much reduced in quantity, the intervening siliceous areas increasing correspondingly. The pressure exerted upon the rock has caused the isolated siderite areas to take on an oval character, the longer axes in general agreeing and being perpendicular to the pressure. When such siderite areas are leached out, the silica bands remain, and pseudo-amyg- daloidal cavities are produced, giving a very perfect pseudo-amygdaloidal structure to the hand specimen. This is the origin of that character of matrix which some of the geologists have described in their field notes as like rotten, worm-eaten wood (fig. B, Pl. XXVII). Although at present the material between the ellipsoids differs so markedly from the rock forming the ellipsoids themselves, nevertheless there is no reason for supposing the original composition of that part of the rock mass to have been essentially different. The change in the character of the basalt in passing from the ellipsoids toward the schistose matrix is in mineralogical character much as has been described for other basalts from this same district. The reason for the more complete degree of the replacement process in passing away from the ellipsoids may be readily understood from the discussion of the origin of the ellipsoidal parting of the basalts, where the conclusion was reached that the matrix between the ellipsoids resulted from the comminution of basaltic material of the same general character as that of the ellipsoids. This matrix was ot course more porous and probably more vitreous than the basalt, and hence more liable to be altered. PYROCLASTICS. The majority of the clastic rocks have been derived from the basic volcanic rocks already described. These clastics are very characteristic of the Hemlock formation and constitute the greater part of it. They comprise several classes, the more important of which are the eruptive breccias, voleanic sedimentary rocks, and schistose pyroclastics. ERUPTIVE BRECCIA. The term ‘‘eruptive breccia” is here used to include those clastic rocks in which angular fragments of an igneous rock are surrounded by a matrix 136 THE CRYSTAL FALLS IRON-BEARING DISTRICT. also of igneous origin. In an eruptive breccia the fragments may be like or unlike. Likewise the matrix may be like or unlike the fragments. Where the fragments have been rounded during the movement of the erup- tive magma surrounding them, the resulting rock may be called an eruptive pseudo-conglomerate. Eruptive breccias are not very common in the Crystal Falls district. Where they do occur, the fragments, while predominantly angular, are to some extent more or less rounded, and are similar in nature to the matrix in which they lie. Since the rocks which form them preserve the main characters of the massive lava flows which have just been described, they will not be discussed in detail. The exact method of the formation of these breccias could not be told. In one ease, in which both fragments and matrix are amyegdaloidal, it appears probable that the occurrence represents a true flow breccia in which the broken surface of a lava flow had been recemented by a later lava flow of the same kind of rock, or that it represents a very possible case in which the lava welled up through and flowed over portions of its own crust, cementing the fragments. In such breccias a flow structure around the fragments is quite plainly shown and the matrix possesses a peculiar ropy appearance. In one instance, in which both the fragments and matrix were macroscopically nonamygdaloidal, it is probable that they were formed under considerable pressure, and that this was a case in which lava was forced up through a previously consolidated mass of rock of like character, and in its passage carried with it various fragments, forming an eruptive “‘reibungs-breccia” or friction breccia. VOLCANIC SEDIMENTARY ROCKS. Under the term ‘‘tuffs” have been very generally included all kinds of voleanic clastic rocks.’ This is probably due to the fact that there is fre- quently considerable difficulty in discriminating between eolian deposits and those which have been deposited in water. It seems desirable, wherever it is possible, to make this discrimination. To that end I shall in the fol- lowing pages restrict the term ‘“‘tuff” to eolian deposits. The term “volcanic conglomerate,” or, for the sake of brevity, simply “conglomerate,” will be used for those coarse deposits which have been sorted by and deposited 'Text-book of Geology, by Sir Archibald Geikie: 3d ed., p. 135. PYROCLASTICS OF HEMLOCK FORMATION, 137 in water, and whose fragments show a rounded character. Should the fragments be angular, the rocks may be called ‘volcanic breccias.” It has been found practicable to maintain this distinction in earlier studies on Tertiary volcanics,’ and it is also maintained in the present study of pre-Cambrian volcanics. I am confident the same distinction could be made more generally than it is, and would in that case tend to a greater precision in the separation of rocks of different characters. However, it is rather difficult to separate true eolian deposits of voleanic fragmentary materials from those in which the fragments have been deposited rapidly through water without having embedded organic remains and without having undergone sufficient attrition to be much rounded. More or less rounding, it is well understood, results from the attrition of the voleanic ejectamenta during their ascent and descent through the air, so that they may in this respect resemble many of the sedimentaries. The exact mode of origin of many of the volcanic fragmental deposits of the Michigamme district is not clear. The greater portion appear to be of true eolian origin, and where the origin of any is in doubt it has been put with those of eolian origin. COARSE TUFFS. The coarse tuffs include rocks composed of fragments of all sizes, from the large volcanic blocks to the fine-grained particles of sand and dust which fill in the interstices. The ejectamenta may be more or less rounded by attrition during their progress through the air, so that if a refinement of the nomenclature should be needed one might very properly be justified in speaking of tuff breccias and tuff conglomerates. Tufts are very common and characteristic for the district. The char- acters of the beds is best shown on the weathered surfaces. Here the scoriaceous and dense light-green fragments stand out well from the brownish-red matrix of more altered, finer fragments and cement. On a fresh surface the interstitial material usually has a darker green color than the fragments. The fragments have a prevailing green color, but many, especially in sections, are brown, much darker than any of the rocks forming the lava flows. The larger fragments are usually sharply angular, but in many cases are more or less rounded because of attrition during 1Die Gesteine des Duppauer Gebirges in Nord-Bohmen, by J. Morgan Clements: Jahrbuch K.-k. geol. Reichsanstalt, Vol. XL, 1890, p. 324. 138 THE CRYSTAL FALLS IRON-BEARING DISTRICT, their progress through the air. (Pl. XIIL) They are for the most part not scoriaceous, though rather commonly amygdaloidal. The macroscopi- cally dense fragments seem to predominate, though the amygdaloidal ones do occur in some specimens in nearly equal quantity. The fragments of the tuffs are derived from the various kinds of basalt already described as forming the lava flows. Among the fragments some of the most typical of these rocks have been found, and remarkable as it may seem, some of the thin sections from them show the least-altered basalts. In addition to the kinds mentioned under the basalts there are a number which differ slightly from them, and apparently represent more glassy modi- fications of the basalt magma. In one of these the amygdules are more sharply outlined by the accumulation of iron oxide around the edges of the amygdule than is the case in the crystalline flow rocks. An especially well-preserved fragment shows perfectly fresh plagioclase microlites exhib- iting well-developed fluidal structure lying in a dark-brown apparently isotropic glassy base. Where the section is thin, globulitic devitrification products can be seen, and there also the base no longer appears isotropic, but very feebly double refracting. There is very frequently found among these tuffs amygdaloidal fragments which appear to have been derived from what was originally a completely glassy rock, no indication of the presence of any original crystals having been preserved. The background of these fragments consists of a fine felt of a green chloritic mineral, dotted with innumerable grains of epidote, in which one may distinctly discern concen- tric circles and ares of circles outlined by aggregates of epidote grains. These circles probably represent perlitic partings. (Fig. 4, Pl. XXXII.) The dark-brown fragments mentioned as occurring with the prevailing green ones are very dense, appear to be very rich in iron, and may possibly represent a very basic devitrified glass. Should accumulations composed essentially of such glassy fragments be found, they could properly be called “palagonite tuffs.” In addition to the rock fragments, a few’rare ones of large plagioclase erystals were found, and also in one case a fragment of a violet-brown augite, the only specimen of fresh pyroxene thus far found in any of the volcanics. The tuffs show in places fairly well-developed banding, caused by the 139 Ee AMM So aepeNeal alin, (Sp. No. 23644. Natural size.) This illustration is a faithful representation of the appearance of the polished surface of a pyroclastic from the Hemlock formation. It is somewhat doubtful whether or not the fragments composing the rock have been deposited through the mediation of water or air alone. The larger fragments are rather dense. Vesicular fragments are more common among the smaller particles. Pyroclastics similar in appearance to this are of very common occurrence in the Crystal Falls district, and huge cliffs of it are readily accessible from the railroad. 140 U.S. GEOLOGICAL SURVEY. MONOGRAPH XXXVI PL. X{II. JULIUS BIEN ACO LITHINY BASALT TUFF. PYROCLASTICS OF HEMLOCK FORMATION. 14] interbedding of layers in which coarse and finer fragments prevail, illus- trating well the varying intensity of the volcanic discharges. Owing to the fragmental nature of the exposures, it is impossible to get a correct idea of the maximum thickness of any of the tuff deposits. Exposures were seen in the north half of sec. 5, T. 43 N., R. 32 W., which gave a thickness of over 500 feet for some of these deposits, but as their farther continuation had been cut off by valleys, most probably eroded in the tuffs, no means was afforded of determining their total thickness. It is almost needless to state that the most of the tuffs have undergone a great amount of alteration. The alterations were apparently due to an interchange of the various elements without any essential variation in the chemical nature of the rock as a whole. Since water is the chief agent through which alterations occur, these always begin along the interstices. In the case of the fragments the alteration accordingly proceeds from the outside inward, and ordinarily at an equal rate all around the fragments, following its contours. In this way zones of somewhat different mineralogical composition are formed, surrounding the less altered part of the fragment. This secondary zonal structure may be observed more or less imperfectly in almost all of the sections made from the breccias, but is much better shown in the field, where the concentric zones are well brought out on the large weathered surfaces of the bowlders. In each case the outside, lighter-colored zone is chiefly made up of chlorite, from which project light-green hornblende needles into the matrix beyond. Less commonly we find it composed of epidote grains and chlo- rite. Inside of this zone the mineral elements composing, the fragments sometimes can not be determined with any great degree of certainty. Where determinable, the alteration products are found to be the same as are produced from the corresponding rocks in the lava flows. As also in the lava flows, some of the fragments of the denser rocks have become almost opaque from the quantity of minute secondary epidote and sphene grains. These have a lighter green color than the less altered fragments. In examining many of the tuffs, one is repeatedly struck by the large amount of space occupied by the cementing material. In some cases cavi- ties of very considerable size were left between the fragments. It appears that the fragments must have been lying very loosely. This fact tends to confirm the eolian origin of the rocks, since water deposition tends to bring 142 THE CRYSTAL FALLS IRON-BEARING DISTRICT. the fragments in close contact, and also to fill the intervening spaces with fine detritus. Those cases must of course be excepted where the material fell upon water so deep that after sinking to the bottom the action of the waves was not felt. Under such circumstances one can imagine the blocks, being partly supported by the water, coming to rest in a more unstable position than they would in the air. The cement differs in different specimens. The minerals constituting it are quartz, feldspar, calcite, chlorite, epidote, and hornblende. The minerals are found including one another in such a way as to make it prob- able that they usually formed simultaneously. The calcite is an exception, asit is usually present in greater quantity near the surface of the exposures, and is therefore a weathering product. It was noted above that the horn- blende and chlorite frequently extend from the rock fragments into the clearer elements composing the cement. Hornblende needles in many cases constitute a large part of the cement. Where two fragments are very close together, a perfect network of needles may extend from the one across the intervening space and penetrate the other, and the fragments thus prac- tically grow together. The cavities—especially the large ones mentioned above—have quite frequently been filled with concentric growths of vari- ous minerals. In general, chlorite seems to be the first mineral deposited, and quartz the last, but in weathered specimens calcite is the last. FINE TUFFS OR ASH (DUST) BEDS. The fine tuffs or “‘ash” beds occur plentifully in the Crystal Falls district. In many cases they possess a very well developed cleavage, and were very puzzling in the field on account of their striking resemblance to true sedi- mentary slates. They are of interest as emphasizing the resemblance between pre-Cambrian ejectamenta and Tertiary and Recent ones. In one respect they differ from the modern forms. The dust from Krakatao in 1885 and from other volcanic explosions consists mainly of fragments of minerals and glass. The constituents of the Crystal Falls beds are usually fine lava and glass fragments and less commonly minerals. The rock fragments are angular, vesicular, and completely altered. The glass fragments are likewise angular, and have the characteristic curved shapes from which they are usually described as sickle-shaped bodies. (Fig. B, Pl. XXXII.) Such are formed when a pumice is broken up, and each PYROCLASTICS OF HEMLOCK FORMATION. 143 represents a portion of the glass bounding the vesicles. Here and there is a fragment with a more or less perfect vesicle remaining. The few mineral fragments found—feldspar—were angular, but quite fresh. The rocks show no intermixture of rounded fragments, and they are consequently regarded as volcanic dust deposited through the air. These ash beds show a delicate banding of finer and coarser-grained fragments. In a single slide a grada- tion can be traced from a moderately fine grained sand composed of distinct volcanic fragments into a very fine grained mass composed of hornblende needles, biotite, chlorite, epidote, and sphene, cemented by what is prob- ably quartz, perhaps having associated with it some feldspar whose charac- ters could not be determined. Relations of tuffs and ash (dust) beds — The pyroclastics seem to predominate in the northwestern part of the district in the neighborhood of the small town of Amasa. Special opportunities for observing the relations between the tuff and the ash beds are offered by the third cut of the Chicago, Milwaukee and St. Paul Railway west of Balsam, Michigan. Gradation can be traced from coarse tufts to delicately banded fine tuffs. The average thickness of a single ash bed probably does not exceed 5 feet. In the same exposures the tuff beds are from 50) to 100 feet thick, and even more. VOLCANIC CONGLOMERATES (TUFFOGENE SEDIMENTS, REYER). That certain of the pyroclastics have been brought together and rearranged by the agency of water is made clear by their characteristic structure. Such rocks are the volcanic conglomerates. In very many respects they are strikingly like the various eolian deposits, tufts, etc., deseribed above. They agree with them in color. The same varieties of volcanic rocks are represented that are found in the tuffs. They are true basalt conglomerates. The pebbles are very commonly sharply outlined by accumulations of epidote grains on the periphery. Some of the fragments have a reddish- brown to purplish-black color, and stand out strongly from the green matrix. Such pebbles are found to contain large quantities of magnetite, the oxide being in beautiful sharp crystals and in absolutely fresh condi- tion, forming a sharp contrast to the altered condition of the fragments in which it occurs. In one case in which the main mass of the fragments now consists of chlorite and epidote, magnetite occurs in large quantity, 144 THE CRYSTAL FALLS IRON-BEARING DISTRICT. and in chains of crystals forming dendritic growths. The oxide is clearly secondary in these altered rocks. Since it also occurs secondarily in the cement, it appears highly probable that it is an infiltration product formed during or after the metasomatic process. In these conglomerates feldspar fragments are far more common than they are in the tuffs, and they show a well-defined, round, waterworn charac- ter. (Fig. A, Pl. XX XIII.) Likewise masses of uralite are commonly associated with the rounded feldspar. The uralite is taken to be altered pyroxene fragments, though no proof of this beyond its association could be offered, as the fragments show no characteristic pyroxene outlines. The well-rounded nature of the volcanic pebbles makes it certain that they have been deposited through the mediation of water and enables one easily to distinguish the typical examples in the field. In size the fragments differ from one another just as they do in the case of the eolian deposits (fig. B, Pl. XXXIIT). Many of the largest are several feet in diameter, but more commonly they vary from a couple of feet in diameter to small pebbles. Partly filling the interspaces and aiding in cementing the larger fragments, with which they are associated, are very fine grained fragments derived from the trituration of the water- worn lapilli and blocks. The coarse bowlder conglomerates grade through finer conglomerates into very fine material. This fine material shows beau- tifully marked false bedding. (Fig. C, Pl. XX VIL) In one of the finer-grained rocks, in addition to the usual sedimentary banding, there are bands which appear to have been caused by a further sorting of the materials, some of these bands being composed almost exclu- sively of uralite. They consequently represent bands which were originally composed mainly of pyroxene fragments. In this case, when the fine ejecta- menta settled through the water they were separated according to size of grain and specific gravity, as in ore-dressing processes. Under the microscope other points of difference in addition to those above mentioned are noted between the conglomerates and the tufts or eolian deposits. The fine-grained rocks corresponding nearly to the vol- canic sand, do not consist of distinguishable rock fragments, but of clearly rounded feldspar grains which have been enlarged by peripheral additions of feldspar substance, bunches of uralite, some chlorite, and of sphene secondary after titanic iron. The photomicrograph, Fig. 4, Pl. XX XIII, PYROCLASTICS OF HEMLOCK FORMATION. 145 illustrates the appearance of the thin section of such consolidated sand, which possesses in no place the structure of an igneous rock, and which, moreover, grades into a finely banded rock composed of minute needles of hornblende, chlorite, and grains of epidote, lying in a clear minutely crys- talline groundmass of quartz or of feldspar, or possibly of both. The finer material cementing the recognizable fragments is the same as it is in the tuffs. The large bowlders and pebbles lie in a matrix of smaller pebbles, and these in turn lie close together in a paste which has been com- pletely altered, and does not in all cases show clastic characters. The cement is composed of hornblende, chlorite, sericite, epidote, feldspar, and quartz, and in one case large porphyritic rhombs of a ferruginous carbonate are scattered through the finer-grained material of the cement. In the cement of the tuffs hornblende is present in large quantity and feldspar is not so abundant. In the cement of the conglomerates feldspar seems to be rather plentiful, hornblende is present in a comparatively small quantity, and chlorite is more abundant, thus reversing the order of these minerals in the conglomerates. SCHISTOSE PYROCLASTICS. At various places in the Hemlock formation there occur clastic rocks which have become schistose. Two isolated exposures of pyroclastics are known whose characteristics have been so changed that, while recognizable as Clastics, it is impossible to say whether they belong to the eolian or the water-deposited class. Upon weathered surface the rock is covered with brownish ochre, and on fresh fracture it is dark green and very schis- tose. Neither im exposures nor in hand specimens does it give any indication of its origin. In thin section, however, one may see macroscopically the fragmental characters. The fragments are elongated and rounded. The amygdaloidal texture is also seen, showing the volcanic nature of the fragments, though the majority of the fragments are dense. Under the microscope the frag- mental nature of the rocks as a whole and the volcanic character of the fragments forming it are still more clearly seen. In the centers the frag- ments are seen to be composed of chlorite flakes in such great quantity as partly to conceal the character of the clear white cement, which is supposed to consist for the most part of quartz, though lath-shaped areas with poly- MON Xxxvi——10 146 THE CRYSTAL FALLS IRON-BEARING DISTRICT. synthetic twinning, showing the presence of plagioclastic feldspar, were seen in places on the edge of the section. In the chlorite and quartz occur large grains of fresh titaniferous iron ore, altering on edge to sphene, and, most striking of all, large porphyritic, beautifully automorphic calcite rhombs and muscovite plates in isolated individuals as well as in heaps of erystals. The carbonate, which predominates, effervesces readily with cold HCl, but is evidently ferruginous as it is yellowish when altered, and from it results some of the ochre which colors the weathered surface of the rock. In other sections the calcite phenocrysts are scalenohedra, with very few rhombo- hedra. The terminal faces are not sharply defined. The sections resulting from the scalenohedra are long, lath-shaped, and have pointed or irregular ends, parallel extinction, and oblique cleavage. In passing from the centers toward the edges of the fragments, we note a marked diminution in the amount of carbonate, musccvite, chlorite, and iron oxide, causing a consequent lightening in color of the periphery. This gives the zonal structure noticeable upon the macroscopical examination of the thin section. The schistose character of the fragments is caused by the parallelism of the chlorite flakes. The cement between the fragments is composed of quartz in rather coarse grains, chlorite in larger flakes than in the fragments, and carbonate in large porphyritic crystals, and also in minute rhombs included in the quartz grains. Another phase contains considerable secondary plagioclastic feldspar associated with the quartz in the coarser-graimed portion of the cement. As in some of the conglomerates and tuffs, the fragments are observed lying in juxtaposition, the only cement between them being the secondary interpenetrating minerals. In some cases the edges of the frag- ments have been so welded that one may pass from one pebble to the adjoining one across an intervening lighter zone without detecting the transition, unless changes in the amount of chlorite, iron oxide, and carbonate are noticed. Nothing thus far mentioned would indicate the igneous origin of the fragments, but that is indisputably proven by the amygdaloidal texture of the specimens, than which I have seen none better, even in the freshest voleanics. The outline of the cavities 1s marked by an accumulation of grains of iron oxide, and the cavities themselves are filled by fine-grained quartz having a small amount of chlorite associated with it. (Figs. 4 and B, PYROCLASTICS OF HEMLOCK FORMATION. 147 Pl. XXX.) These specimens have the characters of the porphyritic schis- tose lavas described above, but show clearly that they have been derived from igneous clastic rocks. Other schistose clastics occur in large quantities in sec. 24, T. 46 N., R. 33 W. They are penetrated by a boss of coarse poikilitic dolerite which may have aided in rendering them schistose, though their schistosity agrees with the general strike of that of the rest of the district, and is probably chiefly due to the general folding. Macroscopically their clastic structure may be clearly seen. The peb- bles are dense greenish gray in color and oval in outline. The matrix is a much darker green. The schistosity of the rock is marked. The pebbles are uniformly elongated, and they have the appearance of having been mashed. The schistosity agrees in direction with the elongation of the pebbles. The microscope shows the pebbles to be basaltic, with a type of struc- ture intermediate between the navitic and intersertal structures, and another with approach to the trachytic structure. Considerable brown mica is present in both kinds, and occurs in flakes which are probably secondary, though the pebbles show few traces of alteration. The matrix consists essentially of actinolite im rather coarse needles, large grains of fresh magnetite, but very little mica, and that such as is seen in the pebbles, all lying in a cement of quartz and calcite. The passage between the cement and the pebbles is a more or less gradual one, there being a change as we pass from the center of the pebble, where isolated actinolite needles and epidote grains occur, toward the edge, where these minerals increase in amount until between them here and there are twinned feldspars. In the matrix proper the quartz is the predominant white silicate, though here and there limpid feldspar is also seen. The pebbles are gradually being eaten up, so to speak, by the actinolite, and we can imagine the final result to be an actinolite-schist showing no clastic structure, and giving absolutely no indication as to the rock from which it originated. There is no microscopical evidence of mashing in the minerals, and since this is absent from the quartz in the cement, I conclude that no original clastic cement is now present, and that the quartz is a secondary crystalli- zation product derived from infiltrated material and from material obtaimed from the adjacent pebbles. Whether the rock is an eolian deposit or a 148 THE CRYSTAL FALLS IRON-BEARING DISTRICT. waterworn sediment can not now be definitely determined, though that it belongs to the first appears more probable. Still another rock very similar in general character but differing in detail, and showing a slightly different result, has been examined. The pebbles are basalt, and in them the secondary nature of the biotite, which has chlorite associated with it, is clearly shown. Near the centers of the pebbles very little is present, but it rapidly mereases in amount toward the periphery, until at the edge only here and there the feldspars may be seen between the mica and chlorite flakes. The cement between the pebbles consists of angular fragments of altered orthoclase feldspar, quartz, and a ereat quantity of chlorite, and some biotite. This cement is present in large quantity. In both of these last the secondary minerals are parallel, and produce rocks of most decided schistose character. These schistose pyroclastics may be compared with the rocks described by Williams* and Bayley” from the related rocks in the adjoining Marquette district. THE BONE LAKE CRYSTALLINE SCHISTS. Under this name are included certain crystalline schists which are best developed in the northern part of the Crystal Falls district, in the vicinity of Bone Lake. If one examined isolated specimens of certain of these rocks, it would be impossible to determine their origin. Studied in connec- tion with the alteration of the altered and schistose lavas and pyroclastics already described, the problem becomes greatly simplified. ‘These schists, as will be shown on. the following pages, are but extremely meta- morphosed members of the Hemlock volcanic formation. Since in the limited area in which the rocks oceur the secondary characters are domi- nant, while the primary voleanic characters have nearly all disappeared, a brief separate description of these rocks seems warranted, but they are not represented by a separate symbol on the map. DISTRIBUTION. The crystalline schists predominate in T. 46 N., R. 32 W. Near the western limit of this township the belt occupied by these rocks is about 2 miles wide. As it is followed to the east past Bone Lake, and then to the 1Bull. U. 8. Geol. Survey, No. 62, cit., pp. 185-191. ?Mon. U.S. Geol. Survey, Vol. XX VIII, cit., pp. 160-169. BONE LAKE CRYSTALLINE SCHISTS. 149 southeast, it gradually narrows, until in sec. 36, T. 46 N., R. 32 W., the eastern limit of the area studied by me, it is only about half a mile wide. Except in the vicinity of Bone Lake, where erosion has uncovered some of the knobs, outerops are very scarce, since the drift is very heavy, and the drainage is poorly developed. FIELD EVIDENCE OF CONNECTION WITH THE VOLCANICS. If one examines attentively the Hemlock formation in its typical development, beginning, say, in sec. 27, T. 45 N., R. 33 W., and following its northward extension through secs. 22, 16, and 15 of the same township, he will observe instances of banding in the tuffs and of schistosity in the amyedaloidal lavas and pyroclastics. The strikes and dips of the primary and secondary structures approximately coincide, both having a general north-south strike and dipping high to the west. Throughout this area, however, the unmistakable massive volcanics are the predominant rocks. Continuing the examination farther north into sec. 34, T. 46 N., h. 33 W., rocks are found which possess almost invariably a strongly marked schis- tosity, but with their volcanic origin clearly shown by the flattened amyg- dules. his is also true for the exposures east of this place on the under side of the Hemlock belt, in sec. 31, T. 46 N., R. 32 W. The strike of the schistosity of the amygdaloids varies from N. 30° to 70° E., and the dip is high to the northwest. Farther along this belt to the northeast, in sec. 24, T. 46 N., R. 33 W., schistose pyroclastics were observed striking N. 80° E. The original characters of these pyroclastics have been almost entirely obliterated. The exposures next to the east in sec. 16, T. 46 N., R. 32 W., possess all the characters of crystalline schists. Somewhat farther east, however, associated with these schists are isolated outcrops in which traces of flow structure and remnants of amygdules were observed, and, in some, traces of igneous textures were seen under the microscope. The schistosity of these rocks strikes for the most part south of east, varymg from N. 65° to 80° W. and dipping to the northeast. Following the belt as it now turns to the southeast, the crystalline schist characters prevail, the volcanic char- acters being obliterated. The schistosity at the same time bends farther around to the southeast, pointing toward the continuation of this area of voleanics to the southeast, outside of the area studied. From field observations the conclusion seems necessary that these 150 THE CRYSTAL FALLS TRON-BEARING DISTRICT. schists are metamorphosed volcanic rocks, and this conclusion is strength- ened by detailed petrographical examination. PETROGRAPHICAL CHARACTERS. The crystalline schists are fine to medium grained schistose rocks which vary in color from a moderately light green for the more chloritic phases to a very dark green and purplish-black for those in which the hornblende, mica, and iron ores are prominent. The minerals of which the rocks are composed, arranged in order of importance, are hornblende, biotite, feldspar, chlorite, epidote muscovite, quartz, magnetite, hematite, ilmenite, and rutile. Under the microscope the schistose structure is seen to be produced by the general parallelism of the bisilicate constituents. The porphyritic texture is seen in a few specimens, and hornblende forms the phenocrysts. Hornblende occurs in fine needles and also in coarse crystals which are automorphic in the prismatic zone, but on which no terminations have been observed. It also occurs rather commonly in sheaf-like bundles of ragged crystals. The marked orthopinacoidal development so common for actinolite is quite noticeable. The crystals show the usual strong pleoch- roism: ¢=bluish-green, b=olive green, a=yellow, whereby c>b>a. The hornblende crystals frequently contain large quantities of the minerals of the groundmass, many of them in such quantity that there are really only skeleton hornblende crystals present. The general character of the hornblende in all these rocks is that of a secondary porphyritie constituent, and seems to be analogous to such minerals as garnet, staurolite, ete., which are produced in clearly metamorphic rocks. Brown biotite is rather common in some of the rocks. Though usually subordinate to the hornblende, it is at times the predominate bisilicate. It is light brown and shows the usual characters of biotite. It is present in small irregular flakes, and also in larger individuals which show poor pinacoidal development. In one case such a mica individual im perfectly fresh condition may be seen with its ragged edges interlocking with the fringed periphery of an altering feldspar crystal. The biotite appears to have derived some of its necessary elements from the feldspar and to be eating into it, and consequently to be a secondary product. Feldspar is not found as an original mineral in any of the crystalline BONE LAKE CRYSTALLINE SCHISTS. 151 schists. It occurs as a secondary constituent. It is found, however, as a primary constituent im a few rocks which, as they still possess remnants of original igneous textures, strictly speaking, should not perhaps be included with the crystalline schists. They represent more properly the transition stages to the crystalline schists, but the process of the alteration of the feldspar is so well shown in these that it is considered expedient to mention it at this place. The original feldspar occurs in this transition phase in the large tangled intergrowths commonly seen in andesitic and. basaltic rocks, as individual phenocrysts, and as microlitie lath-shaped individuals in the groundmass. The greatest interest centers in the phenocrysts, as in them the changes which take place are more clearly seen. The feldspar phenocrysts are always cloudy, due to numberless black ferruginous inclu- sions. They also inclose the various secondary dark silicates composing the groundmass, grains of epidote, flakes of biotite, and erystals of horn- blende. These are usually surrounded by very narrow clear zones, appar- ently feldspar. Near the edges of such altered crystals, and especially in the more altered individuals, these inclusions are more numerous, and are accompanied by grains of quartz and new feldspar (albite?). These last two have certainly been derived from the alteration of the feldspar, but that mineral may possibly also have contributed something to the produc- tion of the dark silicates. The secondary feldspar, that of the schists proper, is found in grains usually unstriated, though in a few cases striations were observed. This feldspar was not determined, butis probably albite The chlorite is in flakes seattered through the schists, showing the usual characters. Epidote, muscovite, quartz, and rutile appear as usual. I]menite is present in one case as micaceous titanic iron oxide, and is then in extremely thin plates which show a beautiful hexagonal develop- ment, though more frequently the plates are rounded. They are transparent with the characteristic clove-brown color. The thicker plates are thin enough to be transparent only along the edges. The iron oxides, magnetite, and hematite occur in some of these rocks in large quantity. In certain parts of the area underlain by these schists considerable excavations have been made in search of iron, the presence of which was indicated by the magnetic needle, and moderately large bodies of ore have been found, though in no case in sufficient quantity to 152 THE CRYSTAL FALLS IRON-BEARING DISTRICT. admit of successful mining. Such ore bodies probably owe their presence in great part to processes active subsequent to the formation of the schists. (See p. 134.) According to the quantity and association of the minerals described above as occurring in the schists, the followimg rocks may result from the complete metamorphism of the basic volcanics: Amphibolites, chlorite- schists, epidote-schists, mica-schists, mica-gneisses, and possibly siliceous hematite and magnetite ore. The complete metamorphism of dense basic lava flows into crystalline schists has been described by Williams’ for the Menominee and Marquette districts, and also by Van Hise and Bayley* for the Marquette district. Williams’ has also described the production of schists from the igneous clastics in the Menominee district and similar products have been described from the Marquette district both by Williams* and by Bayley.’ The above-described schists cover a considerable area, with only iso- lated exposures of rocks associated with them in which volcanic characters are recognizable. They are confidently believed to represent extremely metamorphosed volcanics of the same general original character as those constituting the Hemlock formation and belonging to the same relative period of extrusion. The same conclusions have been reached by Smyth for similar schists along the Fence River to the southeast of those described. It is noticeable that the most intense metamorphism of the voleanics has taken place in the northern and northeastern part of the Crystal Falls district, that part in which the crystalline schists have been produced, though the explanation for this can not be offered. NORMAL SEDIMENTARIES OF THE HEMLOCK FORMATION. The normal sedimentaries are in small quantity. It has been seen (pp. 64, 78) that the Mansfield slate is overlain by a conglomerate in which volcanic material predominates, but which contains partly rounded frag- ments of chert and slate and round quartz grains derived from the under- lying sedimentaries. But for the intermingling of this normal clastic débris ' Bull. U.S. Geol. Survey No. 62, cit. ‘Op. cit., p. 158. ?Mon. U.S. Geol. Survey, Vol. XXVIII, cit., pp. 152-159. 5 Op. cit., pp. 160-169. 8Op. ecit., p. 133. NORMAL SEDIMENTARIES OF HEMLOCK FORMATION. 153 with the pyroclastics, the conglomerate shows nothing different from the voleanic conglomerate already described. It is a transition rock between the tuffs and the normal sedimentaries. Similarly, in sec. 34, T. 45 N., R. 33 W., a gradation occurs in the upper horizon of the Hemlock formation from the yoleanic conglomerates to the true normal sediments. The sediments are slates about 175 feet thick, containing lenticular masses of limestone. These beds dip 80° to the west, generally strike north, but vary in places a few degrees to the west. They are underlain by conglomerates containing well-rounded volcanic pebbles. This volcanic conglomerate grades from the coarse conglomerate up into what might be termed a water-deposited volcanic sand. The peb- bles are all of voleanic material. Between the conglomerates and slates is a small area without outcrop. Overlying the slates is a succession of tufts and lava flows. The slates in color range from light gray and green to purplish red, and the lenses of limestone vary from cream color to purplish red. In thin section the slates are seen to be composed of a felt of sericite, chlorite, and quartz, with associated innumerable minute rutile crystals, and here and there a large spot of limpid quartz. A ferruginous carbonate is present in all of them in porphyritic rhombs. Where chlorite is abundant, the slates are a light green. Where iron oxide is abundant and the chlorite less plenti- ful, the slates are purplish. The lenses of limestone are rather pure, consisting mainly of calcite, with some few scattered areas of cherty silica. On the edges of the lenses some of the slate material is found forming bands inthe carbonate. These intermediate phases grade on the one hand into the pure carbonate, and on the other hand into the slate beds. From the crust of limonite, which may be seen on the weathered surface of the rock, the calcite is evidently rather ferruginous. The process of alteration is clearly seen under the micro- scope, where many of the grains are surrounded by rims of hydrated oxide of iron and hematite. ECONOMIC PRODUCTS. BUILDING AND ORNAMENTAL STONES. The rocks of the Hemlock formation are not likely to be much used for building purposes. The compact basalts possess in a high degree the 154 THE CRYSTAL FALLS [RON-BEARING DISTRICT. two essential features of strength and durability. For trimming in contrast with lighter stones they might be found desirable, and it may be suggested that they are especially suitable for mosaics in which rich greens are desired. They are of too somber a color to be used in large quantity for anything else than foundations. Moreover, the difficulty and consequent expense of quarrying them, and their remoteness from cities of large size, will operate strongly against their use. The pyroclastics are natural mosaics, and some of them have a very pleasing appearance (Pl. XIII) and are suitable for table tops, wains- coting, ete. ROAD MATERIALS. The importance of good roads in aiding in the material development of a region can hardly be overestimated, and in the building of good roads, especially in thinly mhabited regions, the proximity of good road material is of prime importance. Thus far the 15 miles of good road between Crystal Falls and the adjacent mining villages have been covered with the ferruginous chert and slates from the dumps of the mines, and unroll themselves to the traveler like red ribbons laid through the green woods. No rock is better suited for use in building macadamized roads than the basalt, and of this the Hemlock formation offers an inexhaustible supply. The fine-grained compact basalts are by far the best rocks obtaiable, and, other things being equal, should of course be chosen rather than the scoriaceous and consequently weaker facies, but these weaker kinds and also the pyroclastics are preferable to the cherts and slates which have been used. The cherts are very hard and durable, but the dust and sand from them possess but slight capacity for cementation. Consequently the roadways upon which quartzite and chert have been used are more likely to wash out than are the roads macadamized with basalt, since the dust in this latter case serves as a cement which binds the larger fragments more firmly together. The road commissioners have thus far used very little basalt, chiefly for the reasons that no crusher was at their disposal, and the chert and slates were at hand ready for use. ISLA PIO IR, WS THE UPPER HURONIAN SERIES. The upper series of this district is connected in the northeastern part of the area with the Upper Marquette series of the Marquette district already described in the Fifteenth Annual Report and Monograph XX VIII. In these reports the Upper Marquette series is regarded as part of the Upper Huronian. As has been stated, the Crystal Falls district is the southwestern extension of the Marquette district, and consequently we should expect the chief formations of the two districts to be continuous, as they are. Because of the drift and because of a change in the character of the rocks, in mapping the western part of the Crystal Falls district it has not been practicable to divide the Upper Huronian into several formations, corresponding to those m the Marquette district. No independent name will be given to it, but it will simply be called Upper Huronian, with the understanding that it corresponds stratigraphically to the Upper Marquette series. DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY. Beginning in the northeastern part of the area discussed by me (see Pl. II), this series covers the southern parts of T. 46 N., Rs. 31 and 32, where it is only 4 miles in width. It is here a northwest-southeast syncline. From this place it stretches beyond the northern limit of the map. With slight interruptions where intrusives occur, it extends in a broad area to the west and south about the Hemlock volcanics to a point lying beyond the limit of the map. On the eastern side of the district it abuts against and is folded in synclines in the Archean granite. Exposures are scanty for the greater part of the area in the Crystal Falls district underlain by the Upper Huronian series. This is due to two conditions, first, to the soft character of the rocks constituting the series, and, second, to the presence in places of the Cambrian sandstone, and more especially to the deep covering of glacial drift which is found spread over the entire district. The Upper Huronian is composed in great measure of 155 156 THE CRYSTAL FALLS IRON-BEARING DISTRICT, slates, which are interbedded with much smaller quantities of graywackes and chert. The slates are eroded much more readily than the associated harder beds, and therefore, except along valleys, we rarely find the soft slates exposed. The graywackes and cherty rocks are the ones which form the striking topographical features of the landscape, the slates forming softly-rounded hills. The drift is also an important factor in the scarcity of outcrops. In the northern and western parts of the districts especially the drift is very heavy. In this portion the youthfulness of the topography is emphasized by numerous swamps, lakes, and generally imperfect drain- age. In the southern and southwestern parts of the district, owing to the presence of larger streams, and consequently more advanced erosion, the drift has been removed to a greater or less extent, so that the topographical forms approach much nearer to those of an unglaciated region. For instance, the general strike of the graywacke and cherty ferruginous slate beds in the southern portion of the area, T. 42 N., Rs. 32 and 33 W., can be closely followed by the north-south to northwest-southeast ridges which they form, the intervening valleys being in all probability underlaim by the softer carbonaceous clay slates. Also in this vicinity, from the Chicago and Northwestern Railway eastward to the Michigamme River, exposures of intrusives with some sedimentaries stand out from the sand plains as rounded knobs. MAGNETIC LINES. A considerable amount of detail magnetic work has been done in the vicinity of the ore-bearing areas, in the hope that with the assistance afforded by the magnetic needle the iron belts might be better traced than they could be by means of the very scanty outcrops. I shall here describe those lines of magnetic disturbance which have been traced for considerable distances. They are indicated on the map, Pl. III, by solid blue lines. Magnetic lineD—This line of maximum magnetic disturbance was traced northwest from near the southeast corner of T. 46 N., R. 32 W., around Bone Lake, then southwest and south through T. 46 N., R. 33 W., until finally lost near the south side of sec. 34 of the same township. The tracing of this line was begun where outcrops were wanting, and it was not possible to connect it directly with any magnetic formation until sec. 34, T. 46 N., R. 33 W., was reached. Here it was connected with outcrops of magnetitic slate and graywacke which overlie the Hemlock formation, MAGNETIC LINES. 157 but with no contact exposed. Throughout its extent the line of disturbance is separated from the line of outcrops of the Hemlock volcanics by a short ‘interval. It is, however, always distinctly separated from them. Magnetic line E— This magnetic line passes directly through the open pits at the Hemlock. As the line is traced north from this point it passes just west of an amygdaloidal lava one-half mile north of the mine. From this point until it is lost in sec. 16, T. 45 N., R. 33 W., there is no evidence in regard to the nature of the rock causing the attraction. Tracing the line south from the Hemlock mine it is found to swing about 200 paces east of the Michigan mine, near the north line of sec. 9, T. 45 N., R. 33 W. A quarter of a mile farther south it swings back again, apparently in the line of the continuation of the iron-bearing formation, which it follows for one-half mile farther, where it is lost. The only place along this line where it has been possible to determine the rock causing the attraction is at the Hemlock mine. Here it was found that it is not the ore formatian proper which is magnetic, but that it is the foot wall. This is a magnetitic slate, about 42 feet in thickness, as shown by the diamond-drill borings. The above are the only lines of maximum magnetic disturbance of this part of the Crystal Falls district which it has been found possible to connect in any way closely with the iron-bearing rocks. A large number of lines of disturbance, however, were traced within the limits of the Hemlock formation, but on account of their slight economic importance they are not inserted on the map. In these cases the influence on the needle is evidently exerted by the magnetite of the lavas and pyroclastics, and in proof of this the lines can very commonly be connected with exposures of the various voleanic rocks. It is of interest to note that the trend of the lines in the voleanics invariably agrees with that of the tuff beds, and with the general strike of the formations of the district, and the reader is reminded of the suggestion already offered (p. 134) that they may be caused by magnetite accumulated by secondary processes, especially active in the tuff beds and scoriaceous portions of the lava flows. THICKNESS. Since the Upper Huronian sediments cover a broad area, their thick- ness must be very considerable. Owing, however, to the scarcity of exposures, it is impossible to give even an approximate estimate. 158 THE CRYSTAL FALLS IRON-BEARING DISTRICT, FOLDING. The extreme northwestern part of the area has not been studied in- such detail as to enable the minor folds to be determined. In general, the series may be said to fold around the Lower Huronian, following the general outline indicated by its color, as shown on PI..ITI, and having a steep dip away from it. In see. 20, T. 45 N., R. 33 W., large outcrops of chert are folded in a most complicated fashion and are locally brecciated. South from this point the evidence of subordinate cross folds is marked. As a result, the line between the Lower Huronian and Upper Huronian is undulatory. ‘The indentations in the Lower Huronian represent minor cross synclines, and the protuberances represent minor cross auticlines. CRYSTAL FALLS SYNCLINE. Near Crystal Falls is the most important of these synclines. This town and a number of small outlying mining villages are situated on a syncline. The character of this syneline is shown better by the distri- bution of the Hemlock voleanics than by the sedimentaries, owing to the scarcity of the outcrops of the latter (Pls. XVIE and XVIII). The broad belt of northwest-southeast trending voleanies, situated 3 miles northeast of Crystal Falls, bends in secs. 11, 12, and 13, T. 43 N., R. 32 W., to the south, and gradually changes to a slight southwest trend. In the reentrant angle of this voleanie formation is the Crystal Falls syncline, its course being that of a southwestward-opening U. The axial line of this U probably has a westward pitch, corresponding with the general folding of this part of the district. Near the center of the U and just a little northwest of Crystal Falls, in sees. 17 and 20, T. 43 N., R. 32 W., is an area underlain by volcanics, which trends east and west, and can be followed westward into sec. 1, T. 43, R. 35, beyond the limits of the area represented on the map. It varies in width from one-fourth mile to 4 miles, averaging about 2 miles. The contacts of these volcanics with the overlying Upper Huronian sediments are not exposed. Hence definite proofs of their interrelations can not be given. The volcanics have been folded with the sediments, and subsequent erosion has exposed them along the axis of an anticline. The southern arm of the curved syneline bends around the extreme southern projection of the Hemlock volcanics in sees. 1 and 2, T. 42 N., R. SLATE. XIV) Te bay OO IDEALIZED STRUCTURAL MAP AND DETAIL GEOLOGICAL MAP, WITH SECTIONS, TO SHOW THE DISTRIBU- TION AND STRUCTURE OF THE HURONIAN ROCKS IN THE VICINITY OF CRYSTAL FALLS, MICHIGAN. Idealized structural map of the vicinity of Crystal Falls. An attempt has been made to illustrate upon this map the distribution of the Huronian rocks, and at the same time our conception of the general features of the structure of this area, The drainage is merely introduced for the purpose of orientation. The topography as here represented does not agree with the true topography of the area. The bottom of the geological basin now occupies, as the result of erosion, the highest places topographically. b Detail geological map, with sections, to show the distribution and structure of the Huronian rocks in the immediate vicinity of Crystal Falls. This serves as a key to the accompanying idealized structural map. 160 A AVAVAVAVAVAN,| VAVAVAVAVAVAVA\ RAAAAT AS Wend ALMATOG NOWWWNOd HOOTWAH EM HAIS OULNI NVINOYNA Wadd) NVINOWQH YAMOT NVIMINOD TW SHINE = 4 ATVOS THAET VAS NVA AAOEVY LHW OOOT = SANIT ASV AO SNOMVAATS “LUA OOZI - HONI 1-SNOMLOUS 40 ATVOS TVOILHaA NVODIHOIN' STTVA TVISAUO HO KLINIOIA AHL NI SMOOU NVINOYNH AHL AO HYOLONULS CNV NOLO ETaLSIG AHL MOHS OL SNOLLOAS HLIM dV TVOIDO0TOd9 CaTIVLAd ANV dVW TVeOLOOMLS TVA ee ee WSS = " \ \ SSS 6G Ug Ww Ve Uy ZY XK VING SA a TERY. Aes Ms A\WS if ASS SH: 7 oe Ne ogni fi ae et XS es SS) A, AY Ne Weg Y OO A/\ ut PXAAAAL BALMS Hh Bees yy Z Os ly ROX XY ae K ROS AROCOR EEO OOK) AK 2 NX 1AXXX HAVHOONOW AJAYNS 1V91901039°S'N ee ee ee, ee ee a ee FOLDING OF UPPER HURONIAN SERIES. 161 31 W., and swings east north of Lake Mary into see. 32, T.43 N., R. 31 W. Here’ ferruginous slates are exposed, bordering the Michigamme River at the so-called Glidden exploration. The extension of these lowermost Upper Huronian beds east from this point soon passes under the sand plains and drift hills and is lost. The higher beds of the series are, however, exposed in the lower course of the Michigamme, Paint, and Brule rivers, which give good sections across them. In this portion of the area discussed the extension of even these higher parts of the formation can not, however, be followed farther east than the Michigamme River. That the Crystal Falls synclinal basin is not simple, but has minor rolls, is shown by the way in which the Upper Huronian series indents the Lower Huronian at the eastern end. Also the close and complicated folding is shown by mining work, and can be nicely seen in the open pits of the Columbia and Crystal Falls mines, in the exposures in the railroad cut near the Crystal Falls mine, and also along both banks of the Paint River near the town of Crystal Falls. Pl. XIV shows the general character of this syncline. The folding has produced extensive “reibungs-breccias.” Near Crystal Falls, along the river bank, about one-fourth mile south of the rail- road bridge, may be seen such a breceia, which has been formed at the junction of a chert with the slates. TIME OF FOLDING OF THE UPPER HURONIAN. The latest folding to which the rock of the Crystal Falls district has been subjected is that which affected the Upper Huronian and likewise involved the underlying Archean and Lower Huronian rocks. Therefore the determination of this period of folding is of especial interest, as mark- ing the close of orogenic movements in this district. Overlying the Upper Huronian is the Potsdam Cambrian, or Lake Superior sandstone. The beds of this formation are horizontal, or else show a very slight tilting, following the general inclination of the district, which perhaps to a great extent may be explained by the initial dips of the beds. ‘They overlie with strong unconformity the upturned and strongly plicated beds of the Upper Huronian. This unconformity marks a lapse of time represented in other districts by the following events: (1) A period of upheaval and denudation of the Upper Huronian; (2) the subsidence and deposition upon the truncated Upper Huronian sediments of the hetero- MON XXXVI 11 162 THE CRYSTAL FALLS IRON-BEARING DISTRICT. geneous volcanic and sedimentary Keweenawan series; (3) the upheaval and truneation of the Keweenawan, in which movement of course the Upper Huronian in the Keweenawan areas was likewise involved. Subsidence of the land areas and the transgression of the Cambrian sea followed, with deposition of the horizontal Lake Superior sandstone upon the inclined Keweenawan and Upper Huronian rocks. The Upper Huronian of the Crystal Falls district may have been involved in one or both of the foldings which took place prior and subsequent to the Keweenawan; or, second, since no Keweenawan deposits are known in the Crystal Falls district, it may be that it suffered an early period of powerful orogenic movement, which raised the rocks above the sea, and was synchronous with the pre- Keweenawan upheaval. i { “ . E: } ‘ | ; in 0 + 4 6 Ce c ORE DEPOSITS OF UPPER HURONIAN. Wee 87°. The severe deformation is clearly shown by the plication of the beds, and by faults whose extent can not be determined, but which are accom- panied by rather extensive reibungsbreccias. The breccias are cemented by iron oxide. THE AMASA AREA. The Amasa deposits must of necessity be very briefly described, as I have been unable to obtain much information concerning the relations of the rocks as shown in the closed mine. In the early days of the mine it was thought by the mine captain that the WH ¢ voleanics formed the TIRING ARN WY foot wall of the ore, S NS « XY xy < eR ; XS an and on his authority Ay) Yc NN ’, Van Hise says, ‘The Wine) Kr ) YM Syn s Ley AN) ore of the Hemlock yy Jur: Li, LE» 4, MN PVE KEIN KB IXRN RNR mine rests upon a A rranne wie Wi nN Stratum consisting of Q surface volcanic mate- rial.” il Fic. 11.—Profile section illustrating results of diamond-drill work. Probably this is a mistake, for the section from west to east (fig. 11), i. e., from the higher to the lower beds, obtained in two drill holes, is as follows: Feet Gran; gemicnine SHH, GlsCOlOneG! Wy; WR 6555 666650 cas0es coseds sopace ose s6s ceoeSoSeccHs soon oses 115 Cierny amine ie dased eaccad a aue dee Sanndeesee Peer OMe ene setasona sdtobsecon cacHeO Seca meSH BeAeae 59 Pyritiferous black slate and quartzite .--..----. 2-2 n= ae nn nn eee wane ws wanes 180 (OMAR NC Baas ppSe ep oIoa aa CORE Lacie COCA PIE SEA Coe Clone a Sein hone. aa eeeeece Seesaceecce 3. 62 +3. 26 +12. 34 52, PsOm os2eh2 sae Bet eee ce ee eos - 03 oll) . OL - 08 COs as escsn eee aan es Be eee sce MODES None. None. .09 Stand’ SOs eacee ase easter Hee ee peta=| eNOne. IN@B@) ss sce ss 58 QILE Sok sh Sees ee RS Sat sel eels | SN ONO INGOT Gea Beeee eas | ee eee Oe toe Seaasl(taeecmcens Trace. None. \E oan ere Total 24 Seek ee eee eee SEE ST 99.72 | 99.76 | 100.76 >xH.O at 110°. -++H:0 above 110°. No. 1=Clay slate (Sp. 32497). Sec. 17, T. 43 N., R. 31 W., 450 N., 1620 W.; George Steiger. No. 2=Spilosite (Sp. 32861). Sec. 7, T. 43 N., R.31 W., 750 N., 1380 W.; H. N. Stokes. No. 3=Spilosite (Sp. 32827). Sec. 7, T. 43 N., R. 31 W., 250 N., 325 W.; H.N. Stokes. No. 4= Adinole (Sp. 32465). Sec. 8, T. 43 N., R. 31 W.,500 N., 475 W.; George Steiger. In these analyses the usual increase of silica as the dolerite is approached is at once noticeable, and hand in hand with it goes the diminu- tion in percentage of alumina and iron oxides. The content of water and carbonaceous matter also suffers a diminution. ‘The most noteworthy differ- ence between the clay slate and the contact rocks is shown in the relations of potassa and soda. This is well brought out m an examination of analyses Nos. 1 and 2. It will be seen that there is only about one-eighth as much potassa in the contact rocks as in the normal clay slate; while, on the con- CONTACT METAMORPHISM BY INTRUSIVES. Piri trary, about 12 times as much soda as there was in the slate has been added to the contact rock. This causes a reversal of the relations of the soda and potassa, so that, whereas in the clay slate there is present 10 times as much potassa as soda, we find in the contact rock taken as a sample very nearly 10 times as much soda as potassa. The very considerable change in chemi- cal composition, especially in the amount of silica and soda, seems to lend great weight to the supposition that in such contacts an actual transfer of material (soda-silicate) takes place from the basic intrusive to the slate. This idea is upheld by Roth,’ Zirkel,’ and others. W. Maynard Hutchings?’ advocates this view, and has described some interesting products as a result of the contact of the Whin Sill which still further support it. NO ENDOMORPHIC EFFECTS OF DOLERITE INTRUSION. Although the exomorphie contact effects of the dolerite intrusion were so obvious, no evidence is found that the dolerite itself suffered any change consequent upon its intrusion. METABASALT. Basalt has been described at length under the volcanics, where it plays an exceedingly important role. Basalt as a dike has been found in only two places, and therefore very little remains to be added. The two basalt dikes oceur within a very short distance of each other, in sees. 15 and 16, T. 42 N., R. 31 W., and are found penetrating the erystal- line schists of the Upper Huronian. Their relations to the other intrusive rocks of the same region are not known. ‘They are probably of the same age as the dolerites, of which they should most likely be considered offshoots. These dikes are a porphyritic basalt. The phenocrysts were of augite, olivine, and labradorite. They were in a very fine groundmass of feldspar, augite, and iron oxide. However, the former existence of the augite and olivine phenocrysts is determinable only by means of their outlines. They are in very small quantity and are entirely altered to pilite. The feldspar phenocrysts are in coarse, heavy crystals and are remarkably fresh. The groundmass is very fine grained, and ranges from an exceedingly fine micro- 1Chem. Geol., by J. Roth, Berlin, 1890, Vol. III, p. 145. 2 Lehrbuch der Petrographie, by IF. Zirkel, Vol. II, 1894, p. 722. 3 Notes on the composition of clay slates, etc., and on some points in their contact metamorphism, by W. Maynard Hutchings: Geol. Mag., Vol. I, Dec. 4, 1894, p.75. Chem. Geol., Vol. III, p. 145. An interesting contact rock, with notes on contact metamorphism, by W. Maynard Hutchings: Geol. Mag., Vol. II, 1895, pp. 122-131, 163-169. 212 THE CRYSTAL FALLS IRON-BEARING DISTRICT. ophitic texture to the pilotaxitic texture. The feldspars in it are in small lath- shaped individuals, and, like the phenocrysts, are fresh. The augite of the groundmass is to a great extent altered to uralite, and the iron ores to sphene. One of the dikes is about 5 feet wide. In the center it is a moderately fine-grained rock; on the edges it is a dense aphanitic basalt. Even in thin section the gradation from the rock with microophitic groundmass to the- one with a dense pilotaxitic groundmass is well shown. A dike of larger size might readily have cooled sufficiently slowly to have crystallized at its center as a dolerite. ULTRA-BASIC INTRUSIVES. ‘Under this head are the descriptions of the picrite-porphyries (porphy- ritic limburgites). PICRITE-PORPHYRY (PORPHYRITIC LIMBURGITE). GEOGRAPHICAL DISTRIBUTION AND EXPOSURES. The picrite-porphyries occur in isolated outcrops of comparatively small size in secs. 9, 22, and 27, T. 44 N., R. 32 W., in.the area supposed to be underlain by the Lower Huronian Hemlock voleanics. They are surrounded by outcrops of the altered poikilitic dolerites, but the exposures are not such as to allow their relations to he determined. Their occurrence points to an intrusive character. It is on account of their field occur- rence alone that we feel justified in describing them here under the general ? heading for this chapter, ‘‘Intrusives,” instead of under the volcanics with the basalts, their proper place from a strict petrographical standpoint. PETROGRAPHICAL CHARACTERS. The picrite-porphyries are medium-grained rocks, which vary in color from gray to dark green and almost black. In general they have a por- phyritic character. This is, however, not so well marked in the gray as in the darker-colored rocks. The gray ones have a spotted appearance. The spots are gray in color, fibrous, very rarely larger than 3 or 4 millimeters in length, and lie in a finely fibrous, dark-green matrix. In the dark rocks the porphyritic crystals reach a length of 1 centimeter, and are bluish to black, with silky luster. They lie in a fine-grained, more or less fibrous, green groundmass. In one of the dark rocks the magnetite is very notice- able. The crystals project from the weathered surface and the rock is strongly polar-magnetie. = ULTRA-BASIC INTRUSIVES. 213 The rocks originally consisted largely of olivine, pyroxene, hornblende, biotite, magnetite, and ilmenite. They now contain also, in considerable quantity, a chloritic product which seems to have been derived from the alteration of an original vitreous base. All of the specimens are exceed- ingly altered. The original mineral constituents have to a great extent been determined from their form, which im some cases has been preserved by the products of alteration, and by certain structures in the pseudomorphs. The minerals now composing the rock are original hornblende, biotite, apatite, magnetite, and ilmenite, with secondary amphibole, serpentine, chlorite, calcite, sphene, and rutile. The two kinds of rocks, the gray and the dark-colored ones, were evidently derived from rocks of essentially the same composition. They have undergone different processes of alteration, and upon this depends the difference in color. As the study of these picrites is chiefly one of the alteration products of the minerals which composed them, it seems best to describe separately the two rocks showing the different products of alteration. GRAY TREMOLITIZED PICRITE-PORPHYRY. In the gray rocks the spots which are macroscopically observed are found under the microscope to consist of an aggregate of minerals. Hxami- nation of these aggregates shows them to be entirely secondary. A careful study of these ageregates shows them to consist of amphibole, magnetite, ilmenite, and serpentine, the first being predominant. No trace of the origi- nal minerals remains. The aggregates are the same in all of the crystals, and the only clue to the original mineral is the form of the pseudomorphs and certain structures in the aggregates. By means of the form the pheno- crysts are readily divisible into three kinds. The first kind has a long prismatic habit, with pyramidal faces meeting at rather an acute angle. The iron oxide is arranged along certain lines, giving the characteristic mesh structure of serpentinized olivine. The second kind is a short, thick prism, for the most part with rounded ends, in some cases the pyramidal faces meeting in a rather obtuse angle. The iron oxide in some of these cases marks an imperfect parting perpendicular to the long direction of the prism. These are supposed to be pseudomorphs after a pyroxene. The third kind consists of round and irregular grains or plates, some of which may be referred to pyroxene, others to olivine. 214 THE CRYSTAL FALLS IRON-BEARING DISTRICT, The amphibole is in small needles. It has a very faint greenish tinge. In cross section it shows marked prismatic development. The character of the needles is plus in the long direction. The maximum angle found between ¢:¢ is 18 degrees. The needles appear to be tremolite containing some iron, and thus approaching actinolite in composition. Usually the needles have no regular arrangement, but in some of the pseudomorphs with rectangular outlines there is a parallel arrangement of such a number of the needles parallel to the long axis of the pseudomorphs as to give to the pseudomorph a distinctly uniform polarization effect. Isolated crystals of magnetite and brownish transparent plates of ilmenite are scattered among the actinolite needles. By far the greater part of the iron oxide is collected in aggregates of small crystals and irregular grains. The formation and arrangement of these aggregates has in some cases taken place along fracture and cleavage planes of the original minerals, and thus in the pseudomorphs we see the mesh structure of olivine and transverse parting of pyroxene clearly brought out. In other cases the iron oxide is in irregular masses collected at the center or outlining the periphery of the pseudomorphs or scattered in small masses through them. Between the tremolite needles and the iron oxide is a small quantity of minute fibers. They have a greenish tinge and low double refraction. Their extinction is parallel to the long ¢ axis, which is also the axis of least elasticity. They are believed to be serpentine fibers. No definite arrange- ment of these fibers could be discerned over the greater part of the pseudo- morphs, but in one crystal, on the edge of the section, where it is especially thin, the arrangement of these needles perpendicular to the long direction of the iron aggregates outlining the meshes is unmistakable. Calcite is in considerable quantity in some of the pseudomorphs. It is highly probable that it owes its origin to the alteration of the original mineral, though some of the calcium went into the amphibole. Besides the above-described pseudomorphs after olivine and pyroxene, a few large prismatic and irregularly bounded areas were found among the phenocrysts, which now consist chiefly of chlorite, with grains of calcite, titanite, magnetite, and minute plates of ilmenite scattered through them. It is clear that the chlorite is derived from a hornblende, as shown by the presence of ragged remnants of hornblende which possesses uniform orien- tation throughout each area. This hornblende shows weak pleochroism in ULTRA-BASIC INTRUSIVES. 215 the light-yellow to greenish tones of actinolite, although its character is more that of the compact hornblende. In one case its secondary nature was shown by the presence of a small irregular area of brown hornblende lying in a mass of the green. The two have the same orientation. In this case the chlorite is apparently a tertiary product, the original mineral being the brown hornblende, from which was formed the light-green variety, which in its turn alters to the chlorite. Between the various pseudomorphs are irregular plates of compact, dark-brown hornblende, plates of biotite, large crystals of magnetite, and rough branching aggregates of ilmenite. These, while molded on the phenocrysts, themselves le in the chloritic mass already mentioned, which also often completely surrounds the phenocrysts, and which is probably an altered vitreous base. The pieces of brown hornblende which remain unaltered show moder- ately strong pleochroism, reddish brown for ¢ and % and light brownish yellow fora. c=b>a. This hornblende contains inclusions of iron oxide and has all the appearance of an original mineral. By alteration it passes through a compact greenish amphibole to a much lighter colored, reedy, actinolitic variety of amphibole. In the secondary amphibole occur certain golden-brown grains with high single and double refractién, which are supposed to be rutile formed from the hornblende, and also some brown transparent plates of ilmenite. The orientation of the secondary horn- blende is the same as the original. No further alteration of this amphibole was observed, but it is believed that the prismatic crystals altered to chlorite, calcite, and magnetite, as described above, are the extreme cases of alteration of an automorphic form of a brown hornblende very similar to the part described. The biotite between the phenocrysts is in ragged areas either surround- ing iron oxide or associated with it or with the hornblende. It is very pleochroic, the absorption parallel to the basal cleavage being so strong as to render the section opaque. Perpendicular thereto the color is a dark chocolate brown. The mica does not show its usual bright polarization colors in sections cut parallel to crystallographic c. This may be due in some measure to the very strong absorption. In some cases the biotite is seen to have a strong blue to violet metallic luster in incident light. The biotite has partly altered to chlorite. The alteration proceeds along the basal 216 THE CRYSTAL FALLS IRON-BEARING DISTRICT. cleavage. As this alteration progresses there is a lightening of the color of the biotite, and, as a consequence of this the whole cause of the metallic luster and the partial cause of the color of the biotite is disclosed. In the lighter biotite one by careful examination can see innumerable small plates of a brown or smoky color. At first sight they remind one strongly of the inclusions so common in many hypersthenes. Closer examination only emphasizes this resemblance, and they are believed to be micaceous ilmenite plates. These inclusions were studied by means of an oil immersion objective giving a magnification of about 1,250, and were found to have mainly a roundish or hexagonal outline. In addition to these, some plates of long, pleochroic. These minute plates lie parallel to the biotite lamellz. The consequence of this is that in sections parallel to ¢ one sees, for the most irregular form were observed. ‘These are all isotropic and non- part, only short black streaks—the edges of the plates—whereas in the basal sections of the biotite one can determine the irregular or rounded contours of the plates. The plates are too small to allow the metallic luster to be seen on an isolated one. En masse they produce a very decided blue metallic shimmer, as seen in some of the biotite fragments. Numerous apatite crystals occur. They are usually clear white, but one crystal was seen showing a dichroism from faintest brownish for rays perpendicular to crystallographic ¢ to light smoky brown for rays parallel thereto. This crystal contains a core of brown glass. Some of the iron oxide is in roughly rectangular masses, and appears to be magnetite. This is associated with an iron oxide, which occurs in opaque, ragged masses formed of long, irregular, and knotty stringers. These at places are parallel to one another and at other places cut one another at various angles, and at still other places meet at a common cen- ter, forming an opaque mass of varying dimensions, but usually small. Now and then one of the large magnetite masses constitutes a center from which extend the knotty, irregular stringers. The general appearance of these ragged masses is that described by German petrographers as zerhackt. When these stringers pierce the section at an oblique angle, the ends are translucent, with a brown color, becoming more opaque as the section gets thicker. Such masses have all the appearance of ilmenite, and are believed to be that mineral. Similar ilmenite stringers are included in the chlorite, which results from the alteration of the biotite ULTRA-BASIC INTRUSIVES. Pall The chlorite in the paramorphs after biotite shows extremely low blue polarization color, and the characteristic pleochroism—yellowish, tinged with red, when the rays vibrate perpendicular to the cleavage, and green when parallel thereto. Apatite needles included in the biotite are unaltered in the secondary chlorite. : Some of the minute octahedral crystals in the amphibole pseudomorphs after olivine appear to be slightly pellucid, with a brown color. If so, they might be referred to picotite, but there is doubt of the correctness of the observation, in view of the high power used, the oil immersion lens, and the fact that the search was for picotite. Close search was also made for perovskite, but none could be found, unless the transparent crystals very doubtfully referred to picotite are really perovskite. ; Forming the matrix in which the pseudomorphs after hornblende, olivine, and pyroxene of these rocks ‘lie, and frequently surrounding isolated crystals, one sees an aggregate composed chiefly of a fine felt of chlorite fibers. This alteration product contains a few apparently original apatite needles, some secondary grains of magnetite, and crystals of amphi- bole which are colorless or else show but the faintest tinge of green, and are larger than the amphibole crystals in the pseudomorphs. It is a secondary amphibole very poor in iron, probably highly calcareous, and approaching tremolite in composition. This chlorite aggregate shows no indication whatever of crystal forms. It seems to be the product of a homogeneous mass, such as would result from the decomposition of a vitreous base. Such a base the aggregate is presumed to represent, although no trace whatever of the glass has been observed in the rock, nor in view of the altered con- dition of the rock could such a glass be reasonably expected to still remain. DARK SERPENTINIZED PICRITE-PORPHYRY. The second variety of the picrite-porphyries is very dark greenish- black, and represents the results of a slightly different process of alteration from that by which the gray forms just described were produced. These dark picrite-porphyries show a very much better developed porphyritic structure than do the gray ones. This is due to the fact that the olivines in these rocks were well developed and reached a length of a centimeter. The olivines are completely altered, serpentine, pilite, and magnetite, being the products which form the pseudomorphs. The characteristic mesh structure 218 THE CRYSTAL FALLS IRON-BEARING DISTRICT. of altered olivine is well brought out by the serpentine and iron ore. In the centers of the meshes there remain small masses of a felt of tremolite needles (pilite). This alteration of the olivine corresponds to that first described by Lewis,’ and more recently by Professor Bonney and Miss Raisin,” from a rock—kimberlite—yery similar to the picrite-porphyries here described. He writes as follows: ‘It frequently happens that while serpentinization begins at the outside of a crystal, fibrous tremolite begins growing within, finally forming a mass of asbestiform fibers surrounded by a zone of green serpentine.” The minerals which composed these black picrite-porphyries were the same as those constituting the gray ones. These minerals were olivine, pyroxene, hornblende, biotite, magnetite, and ilmenite. They were cemented by a glass matrix. The glass is completely altered. All of the minerals are represented by pseudomorphs. Remnants of the original hornblende and biotite alone are preserved. The contours of the original pyroxene crystals are filled with pilite, serpentine, and magnetite. The serpentine is present in greater quantity in these pyroxene pseudomorphs than it was in the pyroxene pseudomorphs in the gray picrite-porphyries. The alteration of the hornblende results in the production of an aggregate of chlorite mclosing grains of calcite, some sphene, and iron oxide, similar to that in the gray picrite-porphyries. The biotite, magnetite, and ilmenite also show those characters which have been described for the same minerals in the first-described picrite-porphyries. Between all of the foregoing minerals we find a fine felty chlorite mass containing grains and dendritic masses of iron ore and a few needles of tremolite. This corresponds to the material forming the cement for the minerals in the gray porphyries, and, like that, is believed to represent an original vitreous matrix. In one of the dark picrite-porphyries the magnetite is present in large quantity and is very noticeable, crystals of it standing out upon the weath- ered surface. This rock did not affect the magnetic needle very powerfully, though it was expected that it would do so. However, another one of 1 On a diamondiferous peridotite and the genesis of the diamond, by H. C. Lewis: Geol. Mag., 3d ser., Vol. IV, 1887, p. 22. Papers and notes on the genesis and matrix of the diamond, by the late Henry Carvill Lewis, edited by Prof. T. G. Bonney, London, 1897, p. 14. 2 Notes on the diamond-hearing rock of Kimberly, South Africa, Part II, by Prof. T. G. Bonney and Miss C. A. Raisin: Geol. Mag., 4th series, Vol. II, 1895, p. 496. ULTRA-BASIC INTRUSIVES. 219 these porphyries, in which, by the way, the iron content is relatively low, is unique, in that it is very strongly polar magnetic, and in this, as well as its probable original mineralogical composition, may be compared with the polar magnetic wehrlite from the Frankenstein, Hesse-Darmstadt, Germany.' The German rock shows tremolite scattered through the serpentine result- ing from the olivine. It is a coarse, evenly granular rock, differing in this respect from the Crystal Falls rocks which are porphyritic. An analysis (No. 1) of the polar magnetic serpentinized picrite- porphyry, m which great abundance of olivine was originally present, is here given, and there is placed with it for comparison an analysis (No. 2) of a very similar rock described by Darton and Kemp,’ from New: York. Both analyses were made by Dr. H. N. Stokes, United States Geological Survey. In No. 1 Ba, Sr, Li, Cl,'S, and SO? were not looked for. Analyses of picrite-porphyry. 1 2. SI Opies Sotto se ee Sey 37. 36 36. 80 MUO eeaa ee hea aued aoe BHCC orecse Sac 79 1. 26 INIGO)S5 Sate Gae Ease rericte Seer 4.76 4.16 Cr,03 roessoesesscocceses sh06 Ssp00 5600 62 . 20 JGH Ol ec enn aeRO CeO ea BOee ered GH6L, Ras! Susse. LO) o a8 GOta Bea eet Serer ae eee 6. 12 8. 33 Mira ©) Ree es meas eae as alee ah: fe Trace. 13 INE Oe eciesiete seve ae ae cee te oe eee 04 09 (CEO Bona Saaesaa Crees Saar Teen 1.19 8. 63 BAO Sistas ceacnee se 2 SAS Seater eee a ne| oi) STO Marte ryps arctate oye ic rs anise ose ate ere ae eile eters Trace. BESO) See aC eae ee Sens rsccuse Set seers | Sil, wal 25. 98 iGO) eg seteneepeoodeeeESoEEeBHoes il TAGE), f 2.48 INGO) san SoS eee ae ABEEe coee ee ner Eas | | ol EY Oe Scan Sena case BE EEE a See cInsasosss . 06 47 COM aera cin siaie eran oka same eeereeee None. 2.95 SO setae ae aceite Soe) acne Seen aoe . 06 iS) Sesous re Saue BBR OSE eoaE eB sesosrorelasoromesas . 95 Hs OpartliQ ere e os 2 aa Sce eet ee AS - pil 18LO) Aloo WOE coop cecocoedocbe Seoc 10. 37 6.93 | 100, 22 MeSS86O——Soe ts jeaysk bec Sete eel Ree eee 47 otal ees cece cent eee 99. 68 99.75 1 Der magnetstein von Frankenstein an der Bergstrasse, by Andreae und Konig: Abhandl. der Senkenberg. naturf. Gesell., Frankfort a Main, 1888, pp. 59-79. Cf. Above article, p. 66, footnote, for references to other occurrences of tremolite associated with serpentine. : * Newly discovered dike at Dewitt, near Syracuse, New York, by N. H. Darton and J. F. Kemp: Am. Jour. Sei., Vol. XLIX, 1895, p. 461. 220 THE CRYSTAL FALLS IRON-BEARING DISTRICT. CLASSIFICATION. These rocks just described, from their mineralogical composition, if we admit the presence of a vitreous base, would belong with the picrite- porphyrites of Rosenbusch.* This designation does not seem, however, to be appropriate, as he states? that he uses the term ‘‘ PPro} ) porphyrite” only for certain textural phases of rocks containing lime-soda feldspar. He has evidently extended that definition so as to be able to use it for these picrites, considering that the glass possesses the necessary ingredients for the formation of such lime-soda feldspar, provided the conditions under which it cooled had been favorable for the feldspar development.. The porphyritic texture of these Crystal Falls picrites and the presence of a vitreous base® show them to be closely related to rocks of effusive char- acter. Those which they most closely resemble among the younger basaltic lavas are the porphyritic forms of the limburgites Qnagma basalts). One of the best-known rocks with which this may be closely compared, as far as association is concerned, is the rock first described by H. Carville Lewis as a saxonite-porphyry,* later called kimberlite. This was described by him as voleanic, and as associated with dolerites and melaphyres. He described it as a basic lava.? Other occurrences of very closely related basic rocks having a vitreous base have been described from the United States by Diller, Williams, Merrill, Branner and Brackett, Kemp, and Darton and Kemp.° 1 Microscopische Physiographie, by H. Rosenbusch: 3d ed., Stuttgart, Vol. II, 1896, p. 1191. 2Op. cit., p. 436. ; 5Should the vitreous base be considered as not having been present and the rocks be put among the peridotites, then they would correspond very closely to the wehrlite described on p. 254. 4 Papers and notes, cit., p. 50. »The genesis of the diamond, by H.C. Lewis: Science, Vol. VIII, 1886, p. 345. On a diamondiferous peridotite and the genesis of the diamond, by H. C. Lewis: Geol. Mag., 3d series, Vol. IV, 1887, p. 22. 'Dikes of peridotite cutting the carboniferous rocks of Kentucky, by 2.8. Diller: Science, 1885, p. 65; Notes on the peridotite of Elliot County, Kentucky, by J.S. Diller: Am. Jour. Sci., Vol. XXXII, 1886, p. 188; Bull. U.S. Geol. Survey, No. 38, 1887. The serpentine (peridotite) occurring in the Onondaga salt group, at Syracuse, New York, by G. H. Williams: Am. Jour. Sci., Vol. XXXIV, 1887, p. 187; Proc. Geol. Soc. Am., Vol. I, 1889, p. 533; Perowskit in serpentin von Syracuse, New York, by G. H. Williams: Neues Jahrb. Vol. II, 1887, p. 263. On a peridotite from Little Deer Isle, in Penobscot Bay, Maine, by G. P. Merrill: Proc. U.S. Nat. Mus., 1888, p. 191. The peridotite of Pike County, Arkansas, by J.C. Brauner and R.N Brackett: Am. Jour. Sci.. Vol. XX XVIII, 1889, p. 50. Peridotite dikes in the Portage sandstone of Ithaca, New York, by J. F. Kemp: Am. Jour. Sci., Vol. XLII, 1891, p. 410. A newly-discovered dike at Dewitt, near Syracuse, New York, by N. H. Darton and J. F. Kemp: Am. Jour. Sei., Vol. XLIX, 1895, p. 456. lhe roek deseribed by F. L. Ransome as a fourchite should perhaps also be compared with these ULTRA-BASIC INTRUSIVES. 221 Hatch" has also described a very similar pre-Tertiary rock from Eng- land as a limburgite. Kemp* emphasizes the resemblance of the Dewitt dike to limburgite, and states that it should be called limburgite.? If we attempt to extend the use of the term “‘limburgite” to include the pre-Tertiary vitreous basalts, we shall have to include under it the rocks heretofore desig- nated as picrite and picrite-porphyrite. -Rosenbusch has now put the picrites and picrite-porphyrites with the effusive rocks, and if of these two sets of terms there is one to be discarded, it should be the name ‘“‘limburgite.” It seems preferable under the rules of priority to retain the name “‘picrite.” It would then seem very suitable to apply to these pre-Tertiary porphyritic b) limburgites Hussak’s old term, ‘“picrite-porphyry,” using the term ‘por- phyry” simply with a textural significance.* SECTION II.—A STUDY OF A ROCK SERIES RANGING FROM ROCKS OF INTERMEDIATE ACIDITY THROUGH THOSE OF BASIC COMPOSITION TO ULTRA-BASIC KINDS. , Beginning near the town of Crystal Falls, in isolated knobs, and extending southeast toward the Michigamme River, where the exposures are larger and better connected, there is found a series of rocks wnose charac- ters are of such interest petrogenetically as to warrant a detail description of them. These rocks are all intrusive in character, with few exceptions are medium to coarse grained, and, while the granitic texture is predominant, there are certain facies in which the texture is porphyritic and even parallel. They have been only slightly affected by dynamic action, and these cases are purely local. Analyses show them to vary in chemical composition from those of intermediate acidity to those of ultra-basic character. The prevailing rocks are, on the one hand, diorites of intermediate acidity ranging to more acid rocks, tonalites, quartz-mica-diorites, and rocks, representing as it probably does the olivine-free form of the limburgite (augitite). Geology of Angel Island, by F. L. Ransome: Bull. Geol. Dept. Univ. of California, Vol. I, 1894, p. 200. ‘The Lower Carboniferous volcanic rocks of East Lowthian (Carlton Hills), by F. Hatch: Trans. Royal Acad. Edinburgh, Vol. XXXVII, 1892, p. 115. 2Op. cit., p. 460. ‘««Taking plutonic rocks as practically the granitoid, and volcanic as the porphyritic, the Dewitt rock is a basaltic dike of the same composition and texture as limburgite, and should be cailed limburgite, even if it is not a surface flow.” (Loe. cit., p. 460.) 4T believe E. Hussak was the first to use this term for asomewhat similarrock, Pikrit-phorphyr yon Steierdorf im Banat, by E. Hussak: Verhandl. K.-k. geol. Reichsanstalt, 1881, pp. 258-262. 222 THE CRYSTAL FALLS IRON-BEARING DISTRICT. granite (plagioclastic), and, on the other, hornblende gabbros, gabbros, norites, and, lastly, peridotites of varying mineralogical character. These rocks thus resemble in their variations those Scottish plutonic rocks so well described by Messrs. Dakyns and Teall.’ The rapid changes in mineralogical composition and texture m a single rock, and the changes from one kind to another through intermediate facies, show very clearly the intimate relationship of these rocks to one another, and warrant the assumption that they all belong to a geologic unit, a con- clusion reached a number of years since by Williams for a somewhat sim- ilar series, the Cortlandt series, from New York. Granite is present as a local facies of the diorite. However, it is very subordinate in quantity and not altogether typical, and as no analysis has been obtained, its position is still more or less doubtful. In the following pages only those kinds of rocks of which analyses have been obtained will be included in the final discussion. Others will be described in detail or merely mentioned, as representing facies of the main types, according to their petrological interest. The rock types of which analyses have been made are as follows: Diorite, gabbro, norite, and peridotite. DIORITE. NOMENCLATURE. Diorite, according to the generally accepted definition, is a granular rock consisting essentially of hornblende, which must be primary, and a soda-lime feldspar.” The term has been used in a different sense by many writers on the Lake Superior and other regions. It has, been used to com- prise rocks which contain hornblende and plagioclase as preponderating constituents, it is true, but in which the hornblende is secondary, therein differing from a true diorite. These so-called diorites have been regarded as derived from an original dolerite (diabase) by uralitization of the pyrox- ene. By some writers these rocks have been classed with the epidiorites, thus recognizing their secondary nature, but by this name, epidiorite, unfortunately implying a false relationship. In this paper, following Brégger, I restrict the name to the granitic 'On the plutonic rocks of Garabal Hill and Meall Breae, by J. R. Dalkyns, esq., M. A., ana J. J. H. Teall, esq., M. A., F.R.S., F.G.S.: Quart. Jour. Geol. Soc., Vol. XLVIII, 1892, pp. 104-121. * Lehrbuch der Petrographic, by F. Zirkel, Leipzig, Vol. II, 1894, p. 465. DIORITE INTRUSIVES. 223 rocks of intermediate acidity, in which the feldspar is plagioclase and the bisilicate constituent is mica or primary hornblende. The feldspar is a lime- soda plagioclase.* DISTRIBUTION AND EXPOSURES. The distribution of the diorite is limited to a few localities, all of which are in the area underlain by Upper Huronian sedimentaries. The most typical occurrences, and those showing greatest variations, form knobs beginning near Crystal Falls and continuing to the south and south- east. Especially large outcrops form the hills in sees. 19 and 20, T. 43 N., R. 31 W. The smaller occurrences are not indicated on the map. These diorite exposures are always good, so far as getting fairly fresh specimens are concerned, but their contacts with other rocks are almost invariably deeply covered with drift; hence their relations in many cases are doubtful. PETROGRAPHICAL CHARACTERS. The diorites are holecrystalline rocks of medium to coarse grain. In texture they show some variation from those which are granitic to those in which the texture is imperfectly ophitic. The color is, on the whole, moderately light gray or reddish, but at times when the dark minerals become more prominent in the basic facies, especially where we get basic schlieren, the rock is very dark gray or greenish brown. The important mineral constituents are feldspar, quartz, biotite, and hornblende. The accessory minerals are epidote, apatite, zircon, sphene, and iron oxides. The secondary minerals, white and brown mica, chlorite, biotite, epidote-zoisite, calcite, and rutile are also present. Feldspar— Plagioclase feldspar, orthoclase, and microcline oecur together. The plagioclase is found in individuals which are fairly automorphic. In the ophitic textured diorites, the plagioclase is the best developed of all the essential constituents. In the granular rocks the degree of automorphism is highest where orthoclase and quartz are present in the largest quantity, and diminishes as these diminish, when the plagioclase individuals begin to interfere with one another’s development. For the most part the plagioclase ‘Die Eruptivgesteine des Kristianiagebietes. 1. Die Gesteine der Grorudit-Tinguait-Serie, by W. C. Brégger, 1894, No. 4, p. 93. II. Die Eruptionsfolge der triadischen Eruptivgesteine bei Predazzo in Siidtyrol, 1895, No.7, p.35. Videnskabsselskabets Skrifter, I Mathematisknatury. Klasse. 224 THE CRYSTAL FALLS IRON-BEARING DISTRICT. gives rather thin sections, though they can hardly be correctly called lath- shaped. No other form of plagioclase, showing a uniformly better or poorer development, or any other difference in character indicating the presence of two kinds of lime-soda feldspar, was observed. The plagioclase sections almost invariably show polysynthetic twinning according to the albite law, with twinning lamelle which vary from very thin to moderately thick plates, the thinner being the more common form. Very common is the combination of the albite and Carlsbad twinning laws in one individual. Less commonly we find individuals twinned according to the pericline as well as the albite law, and sometimes a Carlsbad twin is made up of individuals twinned according to the albite and pericline laws. In determining the character of the feldspar, the Lévy method was followed.' A great number of measurements made on the zone perpen- dicular to 010 gave equal extinction angles, varying chiefly around 15 degrees, but running as a maximum to 19 degrees. From this it appears that the plagioclase is andesine, probably a somewhat basic kind. That these andesines vary slightly in composition is shown by a very slight but noticeable zonal structure, the more basic character of the center of the individuals being most admirably brought out by the more advanced con- dition of alteration of the center as compared with the periphery. The andesine is for the most part very much altered, to such an extent that in many sections the boundaries of the twinning lamellee are so blurred that measurements are rendered impossible. Muscovite, which appears in minute rectangular sections showing good cleavage, is the chief secondary product from the feldspar, with epidote-zoisite next in importance. Calcite and biotite are present, but in comparatively small quantities. In some cases muscovite almost replaces the feldspar; in others epidote-zoisite does so. In such a case one sees in the center of the feldspar only a mass of secondary mineral. As the examination is carried from the center toward the outside, the original feldspar material is distinguished as a thin film between the secondary minerals. This increases in mass until we reach the outside narrow rim of practically unaltered feldspar. Orthoclase—This is present in large quantity in irregular plates which form a part of the mesostasis for the plagioclase and bisilicates. Less com- monly we find it in micropegmatitic intergrowth with the quartz. It is ‘Etude sur la détermination des feldspaths, by A. Michel Lévy, Paris, 1894. DIORITE INTRUSIVES. 225 invariably more or less decomposed, and shows innumerable minute dark specks scattered through it. The quantity of orthoclase varies in these dioritic rocks considerably; at times it almost equals or even exceeds the plagioclase, when the rocks approach the granites, and at times it sinks to a few large plates in each section, when the rocks are a normal diorite. Microcline—T his mineral is not abundant. It is in individuals which frequently, though not in all cases, are automorphic with respect to the orthoclase and quartz. It is remarkably fresh. Quartz—()uartz, at times, is an essential constituent, and again it dimin- ishes in amount until it is present only in a few grains, or even disappears altogether. Like the orthoclase, it is completely xenomorphic, and with the orthoclase constitutes the mesostasis. Undulatory extinction in the quartz gives indication of slight pressure. Biotite—The original biotite in the granular dioritic rocks is automorphic with respect to all minerals but the hornblende. In the ophitie forms it has a development about equal to that of the hornblende. It shows a dark rich chocolate-brown or greenish-brown color for and c, and a light yellowish-brown for a. The biotite includes small epidote crystals with pleochroic courts and some grains of sphene. Both of these are original. The biotite is almost invariably more or less altered, bleaching in some cases to a very light colored mica with exceedingly high polarization colors. This bleaching follows along the laminz of the biotite and results in giving sections parallel to the vertical axis a banded appearance resembling parallel intergrowths of muscovite and biotite laminz. More commonly it alters to chlorite, rutile (often present as sagenite), sphene, epidote-zoisite, and calcite. There is also a distinct banding of the biotite and the chlorite in places. In the alteration of the biotite we very commonly find lenses of calcite produced between the laminz. In some cases the epidote-zoisite is clearly a product of the alteration of the biotite, for in many cases it is found in the rectangular shape of the biotite section, and in other instances in spaces between the feldspars in the ophitic rocks, which in fresher specimens are found to be occupied by the biotite. Moreover, in the epidote-zoisite are minute grains of sphene similar to those contained in the original biotite. Where it is present as a secondary product, it occurs with the musco- vite and is xenomorphic with respect to it. The green tone is absent from the secondary biotite. MON XxxvI——15 226 THE CRYSTAL FALLS IRON-BEARING DISTRICT. Hornblende—T he hornblende in the diorites shows a most excellent devel- opment in the prism zone; very much less well developed are the termi- nating planes. The color varies from dirty green to a reddish-brown. The brown hornblende occupies the center of the crystals, while the green occu- pies the outside, the green agreeing perfectly, optically, with the brown. A “zonal structure is indicated by the difference in the character of the horn- blende, though the zones are not sharply delimited, but grade imte one another. In a few cases the greenish hornblende grades into one whieh is almost colorless. The pleochroism is as follows: Brown hornblende: @, light yellow or light reddish-yellow; %, light reddish-brown; c, darker reddish- brown. Green hornblende: a, light yellow; %, bright green; c, dull or olive green. This green hornblende is clearly original and not to be con- sidered as a secondary product after the brown hornblende. Both kinds are free from inclusions. Azcessory minerals—The epidote is observed very frequently inclosed in the altered biotite, and is surrounded hy pleochroic halos. In such cases it is considered a primary constituent. The accessory minerals, apatite, sphene, and zircon, show none other than their usual characters. itaniferous magnetite is present in the diorites in very small quantity. According to the relative proportion of the important minerals just described—plagioclase, orthoclase, quartz, hornblende, and biotite—com- posing the diorites, we get the following varieties: Mica-diorite, quartz- mica-diorite, quartz-diorite, and tonalite. These grade into one another, as stated above; and, as will be shown later, certain of them grade into granites. On account of these variations these dioritic rocks are especially interesting. DESCRIPTION OF INTERESTING VARIATIONS. sEC. 15, 1. 42 N., R. 31 Ww. A dike of rock 4 feet wide, occurring at 425 paces N., 1050 W., see. 15, T. 42 N, R. 31 W., near Norway portage, shows the following min- eralogical variation. A specimen taken from the center of the dike shows the rock to be there a typical fine-grained granitite with little or no plagioclase. (Photomicrograph, fig. 4, Pl. XX XIX.) Along the sides the dike rock is a mica-diorite consisting of mica and plagioclase without any quartz. Measurements on zone perpendicular to 010 gave equal extinction angles —_—— ee ee DIORITE INTRUSIVES. 227 with a maximum of 15 degrees. Only one kind of plagioclase is distin- guishable by its mode of development, and this is rich in CaO, as shown by its alteration products. The feldspar ranges at most from albite to andesine. No chemical analysis has been obtained of either the granitite or the mica- diorite phase, but the mineralogical composition is sufficiently marked to show conclusively that we have here a gradation from a granitic to a dioritic- rock rich in CaO. The idea has been suggested by Johnston-Lavis’ that im some cases the variation in chemical composition of intrusive rocks, especially where this variation is one between the center and the periphery of an intrusive mass, may be due to resorption by the intrusive of parts of the rock intruded. The sharp line of demarcation which exists between the dike and the intruded hornblende-gabbro in the occurrence described above seems to preclude the possibility of a fusion and mingling of the two rocks. ACROSS RIVER FROM CRYSTAL FALLS. Near Crystal Falls, just across the river from the town, are a number of small knobs of granite grading into quartz-mica-diorite. They are medium-grained rocks, reddish to gray in color. hey take a very fine polish and are well adapted to ornamental stonework, as is shown by the columns made from them which are used in the court-house at Crystal Falls. When examined under the microscope, the rocks are found to con- sist of automorphic biotite and plagioclase, with xenomorphic orthoclase and quartz, these last forming the cement. Some of the slides show beautiful micropegmatitic intergrowths of quartz and feldspar. The amount of quartz, plagioclase, and orthoclase varies so that, depending upon the specimen examined, one would call the rocks forming the knobs granite or quartz-mica-diorite. Most commonly the rock is a plagioclase-bearing granite. No analysis has been obtained of the granite, but it is confidently believed that the chemical composition would sustain the microscopical diagnosis. Within the granite there are found lenticular schlieren of considerably darker color than the main mass, in which the plagioclase is the preponderant feldspathic constituent. The rock of these lenses is essentially a quartz-mica-diorite. 'The basic eruptive rocks of Gran (Norway) and their interpretation; a criticism by H. J. Johnston-Lavis: Geol. Mag., 4th ser., Vol. I, 1894, p. 252. The causes of variation in the composition of igneous rocks, by H. J. Johnston-Layis: Natural Science, Vol. IV, 1894, pp. 134-140. 228 THE CRYSTAL FALLS IRON-BEARING DISTRICT. These knobs are cut by a number of small dikes from a fraction of an inch to 3 inches in width. In all of these dikes the rock shows the same characters. It is very light gray to pink in color, and aphanitie. An examination under the microscope enables the separation of each dike into a very compact fine-grained saalband and a somewhat coarser- erained porphyritic central portion. In the central part of the dike pheno- crysts of quartz, feldspar, and biotite le m a very fine groundmass of quartz and feldspar. The texture of this groundmass is microgranitic. The saalband is composed of the microgranitic groundmass without the phenocrysts. The quartz phenocrysts show the usual characters. The feldspar phenocrysts are in most cases so completely replaced by a musco- vite ageregate as to preclude any exact determination of their original character. In some cases indistinct remains of polysynthetic twinning are seen. Hyen when the main mass of the feldspar phenocrysts is entirely altered, there is a narrow zone of very fresh feldspar material surrounding it. Twinning in the center is also continuous through this zone. More- over, this zone itself shows a very noticeable zonal structure by the change in extinction angle observed in passing from the inner to the outer portion. This less altered zone of feldspar contains numerous inclusions of quartz from the groundmass. The character of the feldspar phenocrysts could not be determined, but the presumption is that they are of the same character as the feldspar in the coarser main mass—that is, andesine—with a more acid feldspar, possibly oligoclase, surrounding them. The further presump- tion is then warranted that the feldspar of the groundmass agrees with this outer feldspar zone in character—that it is also oligoclase, or at least is more acid than the phenocrysts. Automorphic biotite plates are now repre- sented by chlorite pseudomorphs, with here and there some secondary epidote. The eroundmass consists chiefly of quartz and feldspar, but contains disseminated through it many minute plates of white mica and a few erystals of zircon. The feldspar of the groundmass is too small to permit of its accurate determination. A plagioclase feldspar in sections indicating an approach to automorphism was observed. Its character as oligoclase (?), or at least a feldspar of a more acid character than that of the centers of the phenocrysts, is surmised for reasons given above. Microcline in see- tions showing characteristic twinning and in more or less rectangular out- lines was observed in considerable quantity. An untwinned feldspar was ee DIORITE INTRUSIVES. 229 determined as orthoclase by the difference shown by its refractive index and that of the twinned plagioclase. Quartz was also recognized in this way in the groundmass. “The quartz and orthoclase form the cement for the other constituents. The muscovite in the groundmass is presumably secondary, as is that in the phenocrysts. (Figs. 4 and b, Pl. XL.) The rock is here inserted as showing an exceedingly fine grained por- phyritic form of the quartz-mica-diorite. It may compare to this mica- diorite as does the tonalite-porphyrite of Becke’ to the tonalite described by him, and one might eall it a quartz-mica-diorite-porphyry. No analysis of this rock has thus far been obtainable. Possibly its chemical composition may indicate it to be more closely allied to the true granites than is believed to be the case, judging from its mineralogical composition and its association with the rocks on the border line between granites and diorites. SOUTHEAST OF CRYSTAL FALLS. Southeast of Crystal Falls, in sec. 16, northwest of Lake Tobin, and extending southwest into sec. 28, T. 42 N., R. 32 W., is a range of hills upon which are numerous exposures of a uniformly medium-grained rock. The main mass of the knobs is of tonalite, which shows several facies. A miarolitic texture is very common in this massif. The cavities are now filled with calcite, quartz, and epidote-zoisite alone or together. This last mineral occurs in single large individuals or in tufts of mdividuals, which radiate from one side of the cavity. In one case a. cavity incompletely filled by such a tuft has been completely filled by a later infiltration of quartz. The color of the rock varies from light pink to very dark greenish eray. The areas of the lighter-colored rocks may be seen extending in finger-like projections into the darker-colored phases. There are, however, no sharp lines between these varieties, but a gradual passage from the lighter to the darker rock. These different phases evidently belong to a single rock mass. Under the microscope, however, important variations in the textural and mineralogical character of the rock masses are seen. The main mass of the rock is granular tonalite. The essential constituents are plagioclase, orthoclase, quartz, hornblende, and mica. The most common association of minerals is hornblende and mica in automorphic crystals, 'Petrographische studien am Tonalit der Rieserferner, by F. Becke: Tschermaks mineral. Mittheil., Vol. XIII, 1892, p. 435. 230 THE CRYSTAL FALLS IRON-BEARING DISTRICT. with plagioclase somewhat less well developed. Between these there is found the quartz, with some accessory orthoclase, and microcline as the last products of crystallization. In some cases these two minerals are present in micropegmatitic intergrowths. A textural variation, which the facies mentioned below also undergo, is from a granular to an imperfectly ophitic texture. In such cases the order of crystallization of the hornblende- mica and plagioclase is reversed, the plagioclase being most automorphic in the ophitic varieties. The rock resembles the tonalite described by Becke from the Rieser- ferner.' Italso closely resembles some slides of the typical Adamello tonalite with which I have been able to compare it. The chief difference between them is that the plagioclase and hornblende have a better crystallographic development in the Crystal Falls rocks than in the Adamello tonalite, and that the accessory allanite of the Adamello rock is wanting in the Crystal Falls tonalite, though the normal epidote may represent it. The horn- blende also differs slightly from that of the Adamello rock in that it is not throughout reddish brown. The central portion of some of the crystals shows this color, but the outer portion is a dirty green, even grading into an almost white hornblende. The tonalite grades, by diminution of biotite, with corresponding increase of hornblende, into a quartz-diorite, and by diminution or disap- pearance of the hornblende andincrease of the biotite into a quartz-mica-diorite. Hornblende never occurs alone in the rocks, whereas biotite may occur as the only bisilicate constituent. It is a very common thing to find in the diorites rounded basic segregations consisting chiefly of mica with hornblende subordinate and just a little accessory feldspar. When the orthoclase and quartz diminish, we get the mica-diorites. Orthoclase is always present in all of these dioritic rocks. In certain facies orthoclase and quartz are very abundant and the plagioclase is correspondingly dimin- ished. Such rocks are clearly plagioclase-bearing granites, and represent gradations between the ordinary tonalite and granite, and point to close relationship of this occurrence with the occurrences nearer Crystal Falls already described, in which the granitic facies predominates and the dioritic facies is subordinate. ! Petrographische studien am Tonalit der Rieserferner, by F. Becke: Tschermaks mineral. Mitt- heil., Vol. XIII, 1892, pp. 364-379. DIORITE INTRUSIVES. Zoi Similar gradations have been noted by Becke in the tonalite from the Rieserferner." The diorite massif of the Crystal Falls district seems to cor- respond very closely to the granodiorite masses of Becke, Turner, and Lind- gren,” which on the one hand grade into the granitites and on the other into the diorites. ANALYSIS OF DIORITE. Tt has not been found possible thus far to obtain analyses of all these varieties. The more acid facies of the diorites seem in their mineralogical composition to show very clearly their gradations toward the tonalites and granites. This bemg the case, it was deemed of more importance to study the relations of the more basic dioritic facies in order to determine the rela- tionship of these rocks to those of the more basic gabbro and peridotite families which are found in association with them. To this end an analysis of one of the mica-diorites was obtained. This rock contams the dark constituents, biotite and hornblende, in large quantity, and of these the mica predominates. Plagioclase predomi- nates among the white silicates, with orthoclase and quartz very subordinate. The mica is considerably altered, but on the whole the rock is fairly fresh. Pig. B, Pl. XXXIX, is a photomicrograph of the rock and shows its general characters. The following analysis was made by Dr. H. N. Stokes, in the laboratory of the United States Geological Survey: Analysis of diorite. | Per cent. | Per cent. | SiO paacace sere a sccisesce 58. 51 Ky Qo es aeeeeee ee eee 4. 08 | AN Op. eesa SESE RE SAE ere - 12 NaOLGst ics 5 see saameneece 3.11 WIE Ostebeeasceeeeee got | 16.32 Ee Ojai 00 See ee eae .23 Wes Ogpescee = ste eee Gccee se 2.11 H30 above 100°. -.--..---. | 2.00 HCO nasties vers syseiseo 5 3 =e 4. 43 120 eseecan sass sescaboss 30 MON) 23 Sescltcaseee aaa aeeeee Trace. Co, Fee ttec tees eee ee eens] None. CHO) ga | 3.92 Totephees Rua con IMG OMe ten sno. ace tse 3.73 | ' Petrographische studien am Tonalit der Rieserferner, by F. Beeke: Tschermaks mineral. Mitt- heil., Vol. XIII, 1892, pp. 379-464. >The granodiorite of California appears from Lindgren’s description (Granitie rocks of Cali- fornia, by W. Lindgren: Am. Jour. Sci., 4th series, Vol. III, 1897, p. 308; where can be found references to mention and descriptions of granodiorite) to correspond very closely to tonalite, though Turner uses the name as synonymous with quartz-mica-diorite (Geology of the Sierra Nevada, by H.W. Turner: Seventeenth Ann. Rept. U. S. Geol. Survey, Part I, 1896, pp. 636, 724). es THE CRYSTAL FALLS IRON-BEARING DISTRICT. The altered character of the rock is readily seen in the large content of water. It is, nevertheless, not so marked as to render the analysis use- less for purposes of determination. ; The character of the plagioclase feldspar is clearly indicated by the relatively high percentage of lime. This high content in lime and the large amount of alkalies present, 7.18 per cent, clearly show its relationship to the diorite family. The content in potash feldspar and the possible derivation of the rock from a granitic magma is shown by the high content in potash. Possibly a considerable amount of the potash, with the greater part of the magnesia, should be deducted for the biotite which is so abundant. This rock is one which it is somewhat difficult to place definitely in’ the existing division of rock families. The large amount of lime and the relatively low percentage of alkalies prevent the placing of the rock with the syenites. On the whole it approaches close to the monzonite group according to the chemical composition of the group given by Brégger. But it differs from this in that the lime (8.92 per cent) is too low to bring the rock within his limits.(4.52 to 10.12 per cent).’ However, if we con- sider the total of the alkaline earths (7.65 per cent) in the rock under dis- cussion, we find that it comes well within Brogger’s range (6.05 to 17.52 per cent) for a total of magnesia and lime. Moreover, the alkali total (7.19 per cent) is about right to warrant its classification in the monzonite group as a representative of the type of biotite-monzonite. On comparing the analysis with that of true normal diorites. we find that the relative proportions of the alkalies are abnormal. The lime con- tent is also too low for rocks of this character, and the magnesia is too high. The above considerations seem to make clear the relationship of the rock to the monzonites and diorites. However, jt is so intimately asso- ciated with and so evidently but a facies of the tonalite which is the domi- nant type where this rock occurs, that it is considered to be more closely related to the lime-soda feldspar rocks, in which the orthoclase is but acces- sory, than to the monzonite family of orthoclase-plagioclase rocks. It is therefore considered to be a mica-diorite. 1 Op. cit., Part II, p. 51. GABBRO AND NORITE INTRUSIVES. 233 GABBRO AND NORITRE. PETROGRAPHICAL CHARACTERS. The gabbros and norites are holocrystalline rocks of moderately fine to coarse grain. They show a considerable variation in texture. Some, the finest-grained forms, possess a very good parallel texture (Pl. XLII, figs. A and &); others are noticeably porphyritic. A few have poikilitic tex- tures (Pl. XLI, figs. 4 and &); less common is an approach to the ophitic texture of the dolerites. Most common of all the rocks are the hypidio- morphic granular ones (PI. XLIV, fig. A, and Pl. XLIII, fig. A). The rocks vary from a light grayish-green color for some of the coarse- grained ones, through darker greenish colored rocks to those of a dark brownish or greenish-black color for the finest-grained forms. The important original mineral constituents are feldspar, mica, horn- blende, pyroxene, and olivine. Apatite, sphene, zircon, rutile, octahedrite (anatase), brookite (?), and iron oxide occur as accessory minerals. White and brown’ mica, chlorite, hornblende, tale, serpentine, sphene, rutile, and calcite occur as secondary minerals. Feldspar—Both plagioclase and orthoclase are present in the gabbros and norites. Plagioclase is by far the most important. It occurs normally im the coarse-grained kinds of rock as broad tabular individuals. In the finer- grained, especially the porphyritic and poikilitic facies, the feldspar sections assume a broad, lath-shaped character. The sections show the character- istic polysynthetic twinning. ‘Twins, according to the albite, pericline, and Carlsbad laws, are present, usually the albite and Carlsbad or the albite and pericline being combined; in some cases all three occur together. Twin- ning lamelle vary in breadth, but on the whole are moderately narrow. Measurements made on the zone normal to 010 give equal extinction angles against the twining plane, which reach a maximum of 34 degrees. The feldspar is evidently labradorite. A zonal structure is noticeable, and is especially shown by the alteration being more advanced in the centers of the individuals. It is possible that the labradorite is accompanied by a small amount of more acid feldspar, andesine in zonal growth with it. The alteration of the feldspar results in the production of the same sec- ondary products formed from the slightly more acid feldspar of the diorites, 934 THE CRYSTAL FALLS IRON-BEARING DISTRICT. mica (both muscovite and biotite), epidote-zoisite, and calcite. The plagio- ‘lase shows very beautifully the effects of dynamic action in local granu- lation of the peripheries of the individual. Such lines of granulated feldspar can be followed through the sections, probably indicating shearing planes. Inclusions are common. Some stout rutile crystals were observed in the feldspar. In some cases minute hair-like needles, which m a few instances were of a size sufficient to admit of their ready determination as rutile, were also found penetrating the plagioclase. Crystals of apatite and iron ores are also commonly included in it. There have also been found in a few cases minute hexagonal plates, which are translucent, with brown color, and are presumably micaceous ilmenite. Mention of the presence of orthoclase in these rocks is made with con- siderable doubt. Here and there a few plates of untwinned feldspar, showing a character somewhat different from that of the plagioclase plates, were observed. As will be seen, the possibility of its presence is indicated by the potash shown in the analysis. Biotite—The biotite in the coarsely granular rock is in irregular plates. They are frequently included in and attached to the outside of the horn- blende. Its period of crystallization thus overlaps that of the hornblende, though on the whole being contemporaneous with it. In the fine-grained rocks biotite is better developed than the hornblende, and is apparently for the most part older than it. In color it varies from a rich reddish brown for rays vibrating parallel to the cleavage to a light yellow for those per- pendicular thereto. It includes crystals of zircon and apatite, which are surrounded by pleochroie halos. Hornblende—In most of the sections hornblende is the most striking com- ponent. It is present in the gabbros in three different varieties. The most prominent kind is a reddish-brown hornblende, which has a dirty green hornblende commonly associated with it and frequently intergrown with it zonally. This hornblende occurs without the green, but the green is invariably associated with the brown. The two are optically continuous in the intergrowths. It is possible, though not susceptible of proof, that the green is the result of the incipient alteration of the brown. The second kind is compact, strongly pleochroic, common green hornblende, and the third is a noncompact, reedy variety of light-green hornblende. The first ee, ee GABBRO AND NORITE INTRUSIVES. 235 two kinds of hornblende are presumed to be primary. The third variety is secondary, but secondary after the original hornblende, thus not affecting the character of the rock. The first variety, the reddish-brown hornblende, occurs in the gabbros in anhedra. A zonal structure is marked by brown hornblende oceupying the centers of the crystals and by dull green hornblende, which agrees optically with the brown, occupying the outsides. The brown hornblende is somewhat lighter colored than basaltic hornblende. The pleochroism is strong in the following colors: Brown hornblende: a, light yellow or red, with tinge of green; , red brown; £, same or darker red brown, excep- tionally yellowish-brown; cSb>a. Green hornblende: a, ereenish- yellow; &, yellowish- or brownish-green; c, dull olive green, frequently with bluish tinge; c>b>a. This hornblende, with respect to its rather exceptional pleochroism and its general characters, seems to agree very well with that described by van Horn from very similar rocks from Italy, and, like that, is probably a very basic hornblende.’ Twinning parallel to 100, co P o, is very common. An imperfect parting parallel to the orthopinacoid 100, 2% Po, was also observed in some cases. It is also indicated by the platy inclusions which lie in this plane In some of the sections where the green hornblende is not intergrown with the brown the green kind shows very commonly a system of fine striations parallel to the positive orthodome 101,P o. In rare cases the brown hornblende is intergrown with almost colorless hornblende, one end of a crystal being brown, the other faintly yellowish. Irregular mottled intergrowths of the two were also found. The normal brown-green hornblende is rendered poikilitic in some specimens by a few rounded grains of perfectly fresh pyroxene, and also by plagioclase crystals which it includes. This same kind of hornblende is frequently rendered very dark by the number of exceedingly small inclu- sions which it contains, and in this, and also in its reddish-brown color, resembles so strongly many hypersthenes as to be readily mistaken for them upon cursory examination. These inclusions are of several kinds, all dis- tributed throughout the same individuals. It is impossible in studying them ' Petrographische Untersuchungen iiber die noritischen Gesteine der Umgegend von Ivrea in Oberitalien, by F. R. van Horn: Tschermaks mineral Mittheil., Vol. XVII, 1897, Part V, p. 09. 236 THE CRYSTAL FALLS IRON-BEARING DISTRICT. to get any optical tests, except that of extinction, owing to the minute size of the inclusions and to the fact that where large enough for examination the tests were vitiated by the presence of the hornblende. Of these inclusions some are readily distinguishable as rutile. Some of the larger of the crystals reach a length of 0.045 mm. and a thickness of 0.0125 mm. Numbers of them show the characteristic heart-shaped and geniculated twins of rutile, so that there is no doubt as to the determi- nation. Associated with the rutile are other crystals 0.019 mm. long, which show the typical pointed pyramidal development of octahedrite (anatase). Still others show a flat tabular development somewhat similar to that of brookite, though these could not be positively determined as that mineral. The hexagonal plates of clove-brown color so frequent in hornblende and also in hypersthene occur also in this hornblende. They are believed to be micaceous ilmenite. The thin plates are translucent, thicker ones are less so, and those which are still thicker are opaque and metallic. The thin plates appear when on edge as fine, hairlike streaks. The thick ones appear in the same position as more or less rectangular bars or rods. Often these small plates are associated with masses of iron oxide, also included in the hornblende. This iron ore occurs in the plates and bars characteristic for ilmenite. These ilmenite masses are translucent only on the edges, where the slide has cut the mass in such a manner as {o give an exceedingly thin section of the ore. At such places the ore is translucent with the same brown color as the thin plates. Another rare variety of the inclu- sions occurs in round grains of rich green color, and may possibly be a ‘spinel. In those sections in which both original brown and original green hornblende occur the inclusions are confined to the brown kind. Where the brown kind is surrounded by the green hornblende the inclusions grad- ually diminish in quantity as we approach the green zone. With this goes also, hand in hand, a lightening of the color of the including mineral (brown hornblende), and there is thus an imperceptible change from the brown to the green hornblende. Where the green hornblende occurs alone it is frequently as full of inclusions as is the brown hornblende of other sections. Individuals of the same sections differ from one another with respect to the quantity of the inclusions, some being crowded with them, while others are practically free from them. GABBRO AND NORITE INTRUSIVES. Den This brown hornblende, on alteration, breaks up into aggregates of epidote-zoisite and light-green chlorite. The second kind of hornblende is the perfectly fresh, compact, com- mon dark-green kind, with pleochroism varying from yellow for a, to yellowish-green for %, and to bluish-green for c; c>b>~a. This is found in very few cases. It appears in every instance to be a primary constituent. The third kind of hornblende may be primary, although the evidence obtainable points to its secondary nature. It has a light-green color, and when examined for pleochroism exhibits a scarcely noticeable change. This hornblende differs very much from the other two hornblendes described, in that it is not compact, but occurs in aggregates of coarse reed- like (schilfaehnliche) individuals. Such aggregates do not at all resemble uralite. The individuals are far too coarse and wedge out at short distances within the aggregates. The aggregates occupy irregularly shaped areas. The aggregates consequently have a coarse patchy polarization. They are frequently surrounded by ragged pieces of biotite, just as are the plates of compact hornblende. Moreover, they occur in rocks which show pressure effects, and are best developed in those in which such effects are most marked. The aggregates rather frequently occur with irregular pieces of the greenish or brown compact interposition-bearing hornblende bordering the aggregates or in the midst of them. The light-green reedy hornblende never contains such interpositions, but does have associated with it fairly large grains of rutile, which may perhaps be considered as having been derived from the various titanium-bearing microlites in the original brown hornblende. The general appearance of these aggregates and their asso- ciation with the origimal hornblende seem to point toward their secondary origin from the latter through the effects of pressure. Pyroxene— The pyroxene comprises both a monoclinic and an ortho- rhombic kind. These are the first of the bisilicates to crystallize in these gabbros. ‘The monoclinic kind is of two varieties. The first is in colorless to faint-pink grains included in large plates of original brown hornblende. These grains have a well-developed prismatic cleavage. One basal section shows very nicely the characteristic pyroxene cleavage. The extinction measured against the prismatic cleavage reached as high as 50 degrees. This pyroxene is presumed to be augite. It never shows diallagic parting. In other sections the monoclinic pyroxene is a clear white to faintest 238 THE CRYSTAL FALLS IRON-BEARING DISTRICT. green malacolite or diopside. This is in roundish grains included in the original green hornblende, which it equals in quantity. The orthorhombic pyroxene occurs in individuals which show fairly good prismatic development, but with rounded terminal faces. The pris- matic cleavage is very well developed. A transverse parting is also com. mon. The pyroxene is usually colorless, but in some cases a scarcely noticeable pleochroism was observed, varying from a faint-greenish tinge for rays vibrating parallel to c, to a yellow for those parallel to a and b. It contains small, dark, streak-like intrusions, some of which under exceptionally favorable conditions are transparent, with a faint-greenish tinge The exit of the bisectrix in basal sections, as well as the parallel extinction, renders it easily distinguishable from the monoclinic pyroxene. The optical angle could not be measured, but was clearly very large, as the hyperbolas passed completely out of the field of view. The orthorhombic pyroxene is evidently enstatite or bronzite, and the pleochroism clearly points to its position near bronzite. A few crystals of rutile and also some of the ilmenite plates, so common in hypersthene, were found occurring im the bronzite. The ilmenite plates are in rather rare irregular patches in the crystals. In many cases along the cleavage lines or around the edges of the crystals or along the transverse parting planes are narrow zones of a sec- ondary yellowish-green, finely fibrous, serpentinous mineral. Beyond these zones is a pure white aggregate of secondary tale scales (Fig. B, Pl. XXXVIII). Among these scales are a few minute rutile crystals, and also a few black ferruginous specks, these products being probably derived from the inclusions in the bronzite, and the ferruginous material possibly to some extent from the mineral itself. In some cases, instead of the intermediate serpentine zone, the rather rare occurrence is observed of the passage of the bronzite directly into the tale aggregate. otivine—The determination of the original presence of olivine in the gabbroic rocks is based upon very slight evidence. In some of the sections containing augite almost every individual of this augite has near its center a rounded, very rarely irregular area of yellowish-green fibrous serpentine. These areas are sharply delimited from the surrounding pyroxene, and the conclusion seems warranted that it resulted from the alteration of some mineral included in the pyroxene. The only bisilicate found in the rocks a GABBRO AND NORITE INTRUSIVES. 239 of the district which crystallized before the pyroxene is olivine. In the peridotite, to be described in the next section, this is usually surrounded by monoclinic or orthorhombic pyroxene. This altered mineral is not important in quantity. Iron oxide—I]menite and titaniferous magnetite occur in some of the rocks in considerable quantity. Both alter to sphene and rutile. Apatite——Among the accessory minerals apatite is perhaps the most common, and, as usual, one of the very earliest minerals to crystallize. It is contained in all the essential constituents, and in biotite is surrounded by pleochroic halos. In some cases it has even crystallized before sphene. It is noticeable in some sections that great numbers of apatite crystals are arranged along lines representing sections of planes between the plagioclase plates, thus practically outlming the feldspar individuals. Sphene—In many cases in these gabbros sphene is found contained in some of the freshest rocks as an original accessory constituent. It is present in largest quantity in the very finest-grained gabbros, which show a parallel texture. In these rocks sphene in some cases surrounds an iron ore, which, to judge from the rod-like sections which are so common, is ilmenite. One might be led to think that the sphene was secondary in such cases, but the iron ore is perfectly fresh, and, considering that in the same thin section crystals of apatite are also surrounded by sphene, it seems clear that we may consider such sphene as original. It thus appears that a portion of the titanium oxide combined with the iron oxide first, forming the titanic iron ore. This was followed by the crystallization of the calcium-titanium compound, thus giving the sphene. In these rocks sphene is not in crystals, but in grains. |These grains are arranged in long chains lying between the other mineral constituents and with the long direction of the individual erains, as well as of the lines of grains, parallel to the long directions of the other constituents of the rock. Zircon and rutile—Zircon is in very small quantity. Rutile shows its usual characters. It is most commonly associated with the octahedrite (anatase) and brookite (2) as inclusion in the hornblende. The iron oxide is chiefly present as ilmenite, with some titanic magnetite. The secondary minerals have already been mentioned and their char- acters described under the description of the minerals from which they are derived. 240 THE CRYSTAL FALLS IRON-BEARING DISTRICT. DESCRIPTION OF INTERESTING KINDS OF GABBRO. The minerals described above as the leading essential constituents of the rocks to be described may be combined in varying quantities. Accord- ing to these combinations a number of different mineralogical types of rock may be produced. The wide range in mineral composition of the gabbroic rocks is equaled, if not surpassed, by similar variations noted by Fairbanks in certain rocks from Point Morrito, California." It may cause the further description to be more readily understood if we preface it by the statement that all of these types, however, are simple facies of a single magma. The important phases which will be described are, in the order of their impor- tance, hornblende-gabbro, consisting essentially of hornblende and lab- radorite; gabbro, consisting of monoclinic pyroxene and labradorite, and bronzite-norite, consisting essentially of bronzite and labradorite. The various mineralogical types exhibit very interesting ranges in texture in certain cases, to which attention will be called. HORNBLENDE-GABBRO IN SEC, 15, T. 42 N., R. 31 Ww. A hornblende-gabbro forms a large knob in sec. 15, T. 42 N., R. 31 W., just at the foot of the Norway Rapids, on the west bank of the Michigamme River. This exposure shows very prettily a change in texture. The change in texture is also accompanied by a slight mineralogical change. The knob ‘is composed partly of a fine-grained granular, but more largely of a coarse- erained porphyritic, gabbro. The fine-grained portion is a pure gabbro composed of plagioclase and brown hornblende, with very little brown mica. No quartz was observed, nor was any orthoclase definitely deter- mined. The plagioclase is in fairly well-developed autcmorphic plates. The hornblende is the brown variety, with numerous minute inclusions, which has already been described, and is not always so well developed as is the plagioclase. In places it plays rather more the réle of a cement. This relation of the two minerals results in formmg an imperfect ophitic texture in places, though on the whole the two minerals are about equally developed, and produce a granular structure. (Fig. 4, Pl. XLIV.). 'The geology of Point Sal, by H. W. Fairbanks: Bull. Dept. Geol., Univ. California, Vol. II, 1896, p. 56 et seq. GABBRO AND NORITE INTRUSIVES. 241 The coarse-grained porphyritic gabbro forming the greater part of the knob consists of plagioclase, hornblende, biotite, and iron oxide, with a very small amount of pyroxene. The hornblende occurs in phenocrysts which have irregular rounded shapes instead of being well crystallized. Some of the largest phenocrysts have a diameter of slightly less than 1 centimeter. They are poikilitic, rendered so by inclusions of lath-shaped plagioclase and rounded grains of pyroxene. (Photomicrograph, fig. A, Pl. XLI.) This por- phyritic hornblende is a dark reddish-brown variety containing such great numbers of minute inclusions as to be opaque in many places, which grades over into, and is in many places in optical continuity with, a dirty green hornblende. This green hornblende is in anhedra and forms the cement for the feldspar, and the two together the groundmass for the brown horn- blende phenocrysts. The plagioclase is most commonly in broad, well- developed crystals, which frequently give quadratic sections. Some few grains of a pink monoclinic pyroxene are included by the hornblende. SECS. 15, 22, 25, AND 29, T. 42 N., R. 31 W. Exposures of a hornblende-gabbro with interesting facies associated with it occur in the southeastern corner of sec. 15, at the southeastern corner of sec. 22, extending east and west through the northern part of sec. 28, at the southeastern corner of sec. 28, and on the west bank of the Michigamme River in sec. 29, T. 42 N., R. 31 W., at the location N. 100, W. 1,250 paces. This is medium to coarse grained and of a gray color from a short distance. Examined at moderately close quarters, one distinguishes very readily a milky white feldspar and a black or dark-green hornblende in about equal quantities. The microscopical examination adds to these two minerals in a very subordinate quantity biotite, pyroxene, and orthoclase. The labra- dorite plagioclase is in medium-broad, irregular plates, though at times approaching a very distinctly lath-shaped form. The orthoclase is present in a few rare individuals in the form of irregular plates. The hornblende constituent is in irregular plates and varies in character. It may be the brown or the green variety already described, or the two together in sepa- rate individuals, or even the brown grading into the green. This green is original and not the alteration product of the brown. Biotite is the normal reddish-brown kind in irregular plates. The pyroxene is usually absent MON XXXvI——16 949 THE CRYSTAL FALLS IRON-BEARING DISTRICT. from the sections of these rocks, but when present it is very rare, and occurs in small irregular grains not uncommonly intergrown with the hornblende and evidently older than the hornblende. It is light green in color, with a scarcely noticeable pleochroism. Its monoclinic character was readily deter- minable; but a more exact determination was not made. It does not, how- ever, show diallagic parting, and is diagnosed as possibly diopside. The feldspar shows the best development of the accessory minerals. It can rarely, however, be said to be automorphic. The texture is, on the whole, granular. From a mineralogical study of the rocks alone, one would unhesitat- ingly place them with the diorites, especially if those facies were seen in which the pyroxenic constituent was wanting. The following analysis (Sp. 23354), obtained from Mr. George Steiger, of the United States Geological Survey, shows the chemical composition of one of these rocks: Analysis of hornblende-gabbro. Per cent. ' Per cent. SHO Sedusoncoceds sneacacT 49, 80 KO} aes he eae seeece . 61 INO)y saccone cose senosazsce +19 INE Ops osa qaeuessaonsscoos 2.22 INOS cosbeacdcosectosbceos 19. 96 H5O 100° 25-2 see ee= Hes Osi cen estes sreceetsecis 6.32 Es O 1000 eee anneeieae ae 1.71 BeO 2 seis ase seeeacicees 49 PeOgi cede. ee seas cecerseeese .07 Mn Op Seat c arta reretatare ween eee CO; wee eee ee eee eee 15 CRO Seer yeas 11.33 aN a ate Nel ahaa TaGnGa | MgO c>a. Patton’ has already called attention to the pleochroism of the hornblende, which ‘is exceptional, inasmuch as the brownest color is that of rays of light vibrating parallel to the orthodiagonal axis.” The brown hornblende is accompanied by a very small quantity of green hornblende. Moreover, the brown hornblende grades over into a light green, the two being in perfect crystalline conti- nuity. In rare cases this brown hornblende is also intergrown with a light- green pyroxene in such way as to give a mottled polarization effect. The pinacoidal cleavage of the hornblende continues through the pyroxene, and the extinction angle of the hornblende against this cleavage is 19 degrees, while that of the pyroxene runs up to 34 degrees. The hornblende includes numerous anhedra of pyroxene, somewhat fewer grains ot olivine, and, less commonly, ragged pieces of biotite, giving it a poikilitic character. It is also very full of opaque metallic or brownish translucent plates of ilmenite. In some individuals minute clear microlites, similar to those described above in the hornblende of certain gabbros, are noticed in small quantity. These are irregularly distributed in the hornblende, giving the crystals a patchy appearance. In respect to its color and these inclusions, this horn- blende, as noted by Patton, bears a rather striking resemblance to hyper- sthene on superficial examination.” Iron ore in large masses is fairly frequent as an inclusion, and it is very commonly noticeable that where such inclusions occur the zone of hornblende immediately surrounding them is free from the platy inclusions mentioned above. Such a clear zone is also observed at times surrounding the inclusions of biotite and olivine, but never in case of pyroxene. Where these clear zones surround the ' Microscopic study of some Michigan rocks, by H. V. Patton: Rept. State Board of Geol. Sury. for 1891-92, 1893, p. 186. 2Loe. cit., p. 186. 252 THE ChYSTAL FALLS IRON-BEARING DISTRICT. included biotite or olivine, these two minerals have associated with them numerous small grains of ore, which probably represent the iron that would have been incorporated in the surrounding hornblende but for some selective influence exerted by the olivine and biotite. Biotite —This mineral is present in flakes of very irregular outline. The pleochroism varies from cream color to yellowish red or brown. Although one of the last minerals to crystallize, its crystallization began before that of the pyroxene or hornblende had entirely finished. Hence we find flakes of it included in these minerals, but near the edges of the crystals. The biotite itself is almost free from inclusions, containing only a little hematite and mag- netite. It alters to a brilliant green, strongly pleochroic, chloritic mineral. Feldspar— This is present in specimens from two outcrops, and in these hardly reaches the rank of an essential constituent. It was the last mineral to crystallize, and is consequently in anhedra, forming the mesostasis. All of the feldspar sections were tested, but no determinative measurements could be made. It is probably very basic. Apatite—Apatite is present in small quantity. It exhibits its usual characters. Spine— There is a spinel found in round grains which are meluded in the olivine (serpentine). It is green in color, and is possibly pleonaste. A second spinel, probably picotite, occurs in small brown grains and octahedra in the olivine. catcite —This mineral, derived partly, if not wholly, from the altering minerals, is found in lenses between the biotite lamella and in minute veins which traverse the slide. Iron ores—Iron ore is represented by hematite, magnetite, and pyrite. The hematite is in blood-red transparent flakes inclosed in the biotite. Magnetite is included by all of the chief minerals, and is in irregular masses without good crystal development. The iron pyrite is found in good erys- tals, though not in large quantity, and is scattered here and there through the slides. PERIDOTITE VARIETIES. The relative proportions of the minerals described above differ very much, and we have different kinds of rocks corresponding to these min- eralogical variations. These kinds are not sharply separated, but are seen under the microscope to grade into one another. ere PERIDOTITE INTRUSIVES. 253 The purest form of peridotite is wehrlite, which is composed essentially of olivine and augite. When, besides these minerals, hornblende is present in large quantities, the rocks belong to the amphibole-peridotite type. In some specimens biotite is almost in sufficient abundance to warrant the naming of them biotite-peridotite. Again, in other specimens feldspar is present in considerable quantity and the rock approaches an olivine-gabbro or olivine-hornblende-gabbro. WEHRLITE. This is represented by a coarse-grained rock which is mottled and has a dark-green color. (Specimen 23763, from sec. 22, T. 42 N., R. 31 W., N. 1,500, W. 900 paces.) Under the microscope the mottling is seen to be due to the association of very dark greenish-black serpentine pseudo- morphs after olivine with a light-colored augite. Olivine and augite were present in about equal quantities. They are in anhedra, and therefore must have crystallized at about the same time. The olivine is, with very few exceptions, completely altered to serpentine. Augite has a very poor development. Between the olivine and augite are small quantities of irregular plates of biotite. A few small irregular pieces of a very light colored greenish hornblende were observed. ‘They are intergrown with the pyroxene and give it an imperfect poikilitic texture. This wehrlite is unquestionably the same as specimen 1247 of the Geological Survey of Wisconsin, described by Dr. A. Wichmann as serpen- tine, consisting chiefly’ of serpentine with some unaltered olivine and augite. AMPHIBOLE-PERIDOTITE. This variety of peridotite was obtained from the outcrop N. 1,260, W. 200 paces from the southeast corner of sec. 29, T. 42 N., R. 31 W., on the east bank of the Michigamme River. The rock is very coarse grained, and possesses poikilitic texture. It is composed of hornblende, pyroxene, olivine, biotite, and iron oxide. The hornblende equals in quantity all of the other constituents. Some of the hornblende individuals measure 3 cm. in length, and include all of the other constituents except the biotite. The pyroxene and olivine seem to have crystallized at about the same time, as 1 Microscopical observations of the iron bearing (Huronian) rocks from the region south of Lake Superior, by Dr. Arthur Wichmann, Leipzig, 1876: Geol. of Wisconsin, Vol. III, 1880, p. 619. 254 THE CRYSTAL FALLS IRON-BEARING DISTRICT. they never include each other. They are both, however, included in the hornblende, which with the biotite forms, as it were, the mesostasis. Biotite is present in this specimen in very small quantity, and is essentially the same kind as that above described (p. 252), except that it shows a trifle higher absorption parallel to the cleavage and becomes a yellowish-red. The rock is very fresh and shows scarcely any traces of alteration. This is partly due to the erosive action of the Michigamme River having removed the weathered crust, thus making fresh specimens obtainable. This rock, from the description just given, would be classified as an amphibole-peridotite, with accessory diallage, bronzite, and biotite. It approaches Williams's cortlandtite. In some specimens the biotite is pres- ent in very large quantity, though hardly in sufhcient quantity to warrant the designation of any of the rocks as biotite-peridotite. GRADATIONS OF AMPHIBOLE-PERIDOTITE TO WEHRLITE AND OLIVINE-GABBRO. There were taken from the same exposure whence the above-described amphibole-peridotite came some specimens which macroscopically can not be distinguished from those of the amphibole-peridotite except in that they are a trifle fer grained. Examined under the microscope, however, we . find differences. In some the hornblende is very much reduced in quantity, and varies from the brown kind just described to a light-greenish color, the two being in optical continuity, and the augite and olivine are increased in quantity. These are good types of a wehrlite. In some of the wehrlites there is a variable percentage of feldspar. In certain cases it reaches an amount which would almost warrant the classing of the rock as an olivine- gabbro. Patton described a rock from the same outcrop in which the horn- blende still predominated, but in which there was also a certain amount of plagioclase’ He called it a hornblende-picrite.” According to the ter- minology here used, if the plagioclase is to be neglected, it would be an amphibole-peridotite. The thin sections of the feldspathic phase of this rock seem to show that it approaches more closely to a gabbro—that is, to be more feld- spathic than the one described by Patton. They certainly contain far less hornblende than his, judging from his description, and more feldspar. The | Mikroscopische Physiographie, by H. Rosenbusch; 31 ed., Stuttgart, Vol. II, 1896, p. 352. 2 Op. cit., p. 186. PERIDOTITE INTRUSIVES. 255 constituents are the same in the two rocks, and with some few modifications his deseription would answer. Augite is the chief constituent, and following it, in order of importance, come olivine, hornblende, biotite, and feldspar. The diallagic augite is more automorphic (see fig. B, Pl. XLY) as the feldspar increases in quantity. It is the only one of the minerals which shows any marked degree of automorphism. The augite present in the sections which I have studied has a light-brownish color, differing from that described by Patton, which is green to colorless. The augite contains the inclusions occurring in hyper- sthene, as well as the green (hornblende?) ones already described. It is invariably surrounded by a narrow rim of light-brown hornblende, and includes in places on the edges irregular patches of the same brownish hornblende. J. Romberg describes the augite in Argentinian gabbros,! both with and without olivine, as beg almost always surrounded by a rim of green hornblende. In one case, however—that of the olivine gabbro from the island of Martin Garcia, in the La Plata River*—both brown and green hornblende is present around the augite. The brown hornblende forms part of the periphery of a crystal; the green the remaining portion. Of the green hornblende some is fibrous, and is considered by Romberg to be certainly ‘secondary. The olivine possesses its usual properties. It is in annedra, with the exception of three or four individuals, which show a fair degree of auto- morphism. The olivine includes rounded grains of a brown spinel, and is traversed by anastomosing veins of the iron oxide. It shows the usual alter- ation to serpentine, and the iron oxide is the result of this serpentinization. The olivine is of exceptional interest on account of the fact that it is sur- rounded by certain zones where it is close to the feldspar (figs. A and B, Pl. XLVI). The characters of zones observed in sections from this same locality, and which are almost, if not quite, identical with these which I shall proceed to describe, have already been described by Patton.* There are two of these zones. An inner one is composed of a mineral which is probably an orthorhombic pyroxene. It was so determined by Patton in 1Untersuchungen an Diorit-Gabbro-und Amphibolitgesteinen aus dem Gebiete der Argentini- scnen Republik, by J. Romberg: Neues Jahrbuch fiir Mineral., BB. IX, 1894, pp. 320-321. 2 Op. cit., p. 322. 3 Op. cit., p. 168. 256 THE CRYSTAL FALLS [RON-BEARING DISTRICT. the specimens collected and studied by him. I can obtain no positive proof for or against this statement. If it is an orthorhombic pyroxene, it agrees with the inner zones of related occurrences which have been described by Témebohm, G. H. Williams, Adams, Romberg,’ and others. This zone is at any rate composed of a colorless, compact mineral, with high smgle and moderately high double refraction. Its single refraction is nearly equal to that of olivine. The mineral, as a rule, extinguishes parallel to the lines of cleavage. In a few instances the line of extinction made a scarcely noticeable angle with the cleavage. It is separated from the olivine by a sharp line. At times this inner zone seems to disappear, and at others becomes considerably broader than the average. The width is usually about 0.02 mm., though it becomes at times 0.08 mm. Outside of this pyroxene zone there is a very much broader zone of light-green hornblende. This is compact, and is in optical continuity with the ordinary brown hornblende, which is the dominant hornblende in the rock. This, in its compact nature and in its relation to the compact brown hornblende of the rest of the slide, differs from the short fibrous actinolite zone ordinarily described as taking part in such “reaction rims.” ‘This hornblende zone reaches an extreme width of 0.15mm. The outer edge of this zone is penetrated by tubular ramifying growths of a colorless mineral, which usually extend inward, perpendicular to the periphery, and which appear to be continuous with the feldspar. This portion of the hornblende rim is about 0.05 mm. wide No such intergrowth of feldspar with the brown hornblende was found, nor have I been able to find elsewhere any description of such an outside zone.* However, Romberg describes the interesting occurrence in an olivine-gabbro from the Argentine Republic of zones around the hornblende which are very much like those above described, except that the pseudopodia-like growths, as he describes them, consist of a dark-green spinel instead of a clear white feldspar, as in the Michigan rock. In some cases, where the olivine and augite are in juxtaposition, the inner orthorhombic pyroxene zone completely surrounds the olivine. The outer hornblende zone, however, surrounds both the augite and the olivine !Uber das Norian oder Ober-Laurentian von Canada, by F. D. Adams: Neues, Jahrbuch fiir Mineral, BB. VIII, 1893, p. 466, where references to observations made previous to 1898 may be found. 2 Op. cit., p. 322. 3 Op. cit., p. 323. PERIDOTITEH INTRUSIVES. 257 with its orthorhombic pyroxene zone. Where it is in contact with the augite it is the brown variety of hornblende, but is in optical continuity with the green, which is the kind around the olivine and the orthorhombic pyroxene. . Of the remaining mineral constituents brown hornblende is the next one in importance. It has in it patches of inclusions, previously described as occurring in the hornblende of these ultrabasic rocks. It includes also the augite and olivine. This brown hornblende is comparatively rarely found in large plates, but usually as a rim of varying width around the augite and olivine, as already described. Where it occurs in large plates it is in that part of the section which is free from feldspar, and more closely resembles the amphibole-peridotite phase. The biotite has a cream to light yellowish-brown color, and occurs in irregular plates. The plagioclase feldspar is in irregular broad plates, and forms the mesostasis. The feldspar contains, in not very large quantity, small microlites, which by very high power are translucent and show a ereenish tinge. ‘They are supposed to be hornblende microlites. PROCESS OF CRYSTALLIZATION. From the relations described as existing between the various minerals it seems that the following stages may be outlined in the progress of the consolidation of this rock. From the coarse even-grained character, and from the fact that neither a fine-grained groundmass nor glass is present, the conclusion seems to be warranted that it crystallized under high pressure and must have, of course, at some time been under very high temperature also. The augite and olivine were the first and chief silicate constituents to form, and erystallized out of the magma at approximately the same time. The magma soon reached a condition unfavorable for further production of olivine, probably on account of increasing acidity. Immediately around the olivine there was formed, at this stage for a short while, the orthorhombic pyroxene. The monoclinic pyroxene continued to grow during the forma- tion of this orthorhombic variety. Finally, however, the condition was reached when, in place of the monoclinic and orthorhombic pyroxenes, the -erystallization of hornblende began: It is not known what the conditions were which caused the formation MON XXXVI——17 258 THE ORYSTAL FALLS IRON-BEARING DISTRICT. of the hornblende subsequent to and in such intimate association with the pyroxene which it surrounds in zonal growth. An explanation of such occurrences has been attempted by Becke in a recent article,* in which the conclusion is reached that the formation of the hornblende and pyroxene depends upon changes in temperature and pressure. His explanation is based upon the facts of occurrence of pyroxene and hornblende in plutonic and effusive rocks, and also upon the well-known fact that under high temperature and atmospheric pressure they can not exist, but when fused recrystallize as pyroxene; and in addition to this, upon the experiments of Von Chrustschoff,? who has obtained hornblende at a temperature of 550 C. with the presence of water, under which conditions a high pressure must be developed. However, attention should be called to the fact that his explana- tion does not take into account other important factors which certainly influence the crystallization of minerals—for example, the chemical composi- tion of the magma and the fusing point and specific gravity of the minerals. Whatever the factors are which determine its crystallization, the fact is that hornblende began to crystallize from this peridotite magma in the place of pyroxene. The biotite appears to have been formed at the same time with the hornblende. The production of these two minerals, hornblende and biotite, then continued until the remaining magma had reached the composition of basic feldspar, which then crystallized and now forms the mesostasis. A zone of orthorhombic pyroxene, succeeded by one of hornblende, has been described as surrounding the olivine in this peridotite. The term reaction rim has been applied to similar zones by various observers, but it seems to me that this term is applicable to such zones. It is not probable in such a case as this that there is a reaction between the magma and the olivine. Moreover, the zones should not be compared to the resorption rims found so commonly in certain effusive rocks, where from the fusion of the hornblende crystals pyroxene has been produced. Such a zonal growth around the olivine seems to me comparable to such a case as that described by Washington,’ where colorless diopside 'Gesteine der Columbretes ; Anhang: Hiniges iiber die Beziehung von Pyroxen und Amphibol in Gesteinen, by F. Becke: Tschermaks mineral. Mittheil., Vol. XVI, 1896, pp. 327-336. 2Bull. Acad. imp. sci. St.-Pétersbourg, 1890, p13. Cf. Becke, Op. cit., p. 337. 3 Italian petrological sketches; 4. The Rocca Monfina region, by H.S. Washington: Jour. Geol., Vol. V, 1897, p. 254. PERIDOTITE INTRUSIVES. 259 phenocrysts are surrounded by a narrow border of yellowish-green augite, which corresponds to the small augites in the groundmass, or to those cases which are so common in plutonic rocks—even in this rock deseribed—where hornblende is found surrounding the pyroxene. A general explanation which would account for the successive crys- tallization of hornblende and pyroxene in this rock should be applicable to such a zonal growth as occurs around the olivine, taking into consideration, of course, the probability that a factor of sight importance in the one case may be the controlling factor in the other. Such occurrences seem clearly to indicate a change in the chemical composition of the magma as the chief factor in the crystallization of the different minerals, in the pressure, in the temperature, and also in other factors, either one alone or more of these combined. ANALYSIS OF PERIDOTITE. The peridotite just described was analyzed by Dr. H. N. Stokes of the United States Geological Survey, and his results are here given (No. 1): Analysis of peridotite. | 1 (23353). | 2 (22981). AKO 2 ok seule aaa eens Cotes I Anes | Be | Thi Osgennis Sete FOR Lala QAI Wi ude 97 | 7 | INTL ORS Gin oUt ene acer wee 5.91 4.76 Cri ONeee ee tr a ene we | 125 62 TELE OH e t arean ega SC LL ee 3. 42 6.61 | TOs SaaS etree cis Se 8. 30 6.12 Min @ wee sama soos tee he eee Trace. Trace. TON ee rR NB eae aE es SSE 04 CAO eres een se oe 8.79 1.19 MOM era Jena ee ae une manele 21. 02 Sil, ii TGA Oe tee we a Ch NOR Le 74 Ie Oa erst eed age ane | 91 urate: sO fat Loss essen tere | 63 | 65. | LO Hivore WMS... 22 sees esas | 3.19 10. 37 EH OMNI eae ate Rea Sali a | 05 .06 CORSE a eee Bees Trace (?). | -—-None. | Notaleeesee renee eee ley, Ie 99. 68 260 THE CRYSTAL FALLS IRON-BEARING DISTRICT. It will be seen from the analysis that the silica is somewhat too high for the typical peridotites. This same fact is also emphasized by the tendency manifested in some facies of the peridotite for feldspar to’develop, and thus for transitions to norite and gabbro to be produced. With this analysis of the peridotite there is placed for comparison the analysis (No. 2) by Dr. H. N. Stokes of the picrite-porphyry already described. The close resemblance chemically becomes at once manifest, although the latter is more nearly a typical peridotite in composition. It can not be denied that possibly this picrite is but a further differentiation product of the same magma to which the peridotites belong, although its occurrence is so remote from these that it is impossible to connect them in the field. PERIDOTITE FROM SEC. 22, 1. 42 N., R. 31 w., N. 1,990, w. 150. Just west of the northeastern corner of sec. 22, T. 42 N., R. 31 W,, there is a bold outcrop of hornblende gabbro, which is cut by a dike, about 10 feet wide, of a very massive, coarse, granular black peridotite. Macro- scopically one can readily distinguish in the peridotite flakes of biotite, poikilitic plates of hornblende, and a smaller amount of white feldspar. Under the microscope the constituents are, in order of importance: Horn- blende, augite, feldspar, biotite, bronzite, olivine, magnetite, and quartz.’ Hornblende—This is the rich brown kind, full of inclusions, grading into the green variety which was described on p. 234 as occurring in the gabbros of this district. It is present in anhedra inclosing biotite, pyroxene, and olivine. Pyroxene—T his is represented by monoclinic and orthorhombic varieties. The monoclinic pyroxene, augite, is most abundant, and is in light-yellow to pink-colored anhedra, except where it touches the feldspar; there the augite is automorphic, and is surrounded by a narrow border of light- brown hornblende. The orthorhombic pyroxene is present in a few anhedra, which are colorless or have a faint cream tint. It is presumed to be bronzite. Feldspar.—This fills the interspaces between the other constituents, and occurs in grains which are polysynthetically twinned after the albite law. ! Only one section has been prepared from this specimen, and it may not give a correct idea of the true proportion of these minerals in the rock mass. In the macroscopical examination of the hand specimen the biotite seemed to be subordinate only to the hornblende. PERIDOTITE INTRUSIVES. 261 Measurements gave a symmetrical extinction of 32 each side of the twinning plane on zone |.010. I therefo.; conclude the feldspar to be labradorite. Biotite—-This is the ordinary yellow to brownish kind, and is in irregu- lar plates. It shows its usual characters and is included in the hornblende. Magnetite—This mineral occurs in crystals and grains, included in all the other constituents. Quartz—A few grains of quartz were found associated with the feldspar. The presence of dihexahedral liquid inclusions easily gave a clew to the orientation of the grains. The rock composed of the above-described minerals offers a good illus- tration of that gradation which is one of the fundamental laws of nature and is nowhere better exemplited than in the rocks. On the one hand, from its texture and from the presence of the dominant hornblende, with the small quantity of quartz, this rock may perhaps be considered to be closely related to the diorites. On the other hand, the presence of the pyroxene and olivine seems to point toward its connection with a gabbro. Its geological occurrence points most satisfactorily toward its corre- spondence in age and its intimate relationship to the peridotites of the dis- trict. The predominance of the bisilicates indicates it to be of very basic character, and for these reasons I have called it ‘“peridotite,” although I have not succeeded in getting an analysis to prove its ultrabasic nature. RELATIONS OF PERIDOTITES TO OTHER ROCKS. The peridotites occur in such small quantity that general conclusions concerning their relations to other rocks occurring in their vicinity are scarcely warranted. However, from the fact that they are so intimately associated with the gabbro cutting it in two cases where the contact was observed—and from the fact that among the peridotites themselves certain phases approach in mineralogical composition certain of the gabbros (see p- 254), it seems advisable to conclude that they represent ultrabasic difter- entiation products of the same magma from which the gabbro types were derived. The inappreciable differences in grain between the portion of the rock nearest the contact between these basic rocks and the gabbros and those farther away can be explained by supposing their intrusion to have taken place while the main mass of the gabbro retained considerable heat and thus prevented their rapid cooling. 262 THE CRYSTAL FALLS IRON-BEARING DISTRICT. AGE OF PERIDOTITES. The only statement which can be made concerning the age of the peri- dotite dikes is that they are younger than some of the gabbros, and that,’ not having suffered the deformation of the pre- -Keweenawan orogenic move- ments, they are Keweenawan or post-Keweenawan. GENERAL OBSERVATIONS ON THE ABOVE SERIES. TEXTURAL CHARACTERS OF THE SERIES. There are represented in the above series rocks with moderately fine erain as well as those of very coarse grain. They vary from those with parallel texture, through those with porphyritic, poikilitic, and ophitic texture, to those with granular texture. There is, however, throughout a clear preponderance of the medium to coarse granular rocks. The rocks are evidently not of effusive character, though some possess the textures prey- alent in effusive rocks. The order of crystallization of the minerals in the rocks of granular texture, excluding the iron ores and the accessory minerals, is as follows. The order in the imperfectly ophitic and porphyritic rocks is not considered, as those are rather exceptional occurrences. In the lists those minerals are hyphenated of which it has not been possible to determine accurately the order of crystallization. It seems that either they were formed at the same time or, in some cases, their formation has overlapped. In such cases the one placed first is the one presumed to have begun its erystallization first. DIORITE. GABBRO AND HORN- BRONZITE-NORITE. PERIDOTITE. BLENDE-GABBRO. Hornblende. Olivine. Bronzite. Olivine. Biotite. Monoclinic pyroxene. Monoclinic pyroxene. Orthorhombic pyroxene. Plagioclase. Biotite-hornblende. Biotite-hornblende. Monoclinic pyroxene. Microcline. Plagioclase. Plagioclase. Biotite-hornblende. Orthoclase-quartz. Plagioclase. For the entire series the order may be arranged as follows: Olivine, bronzite, monoclinic pyroxene, mica-hornblende, plagioclase, orthoclase, quartz. This is the same order that is exhibited by the most basic rock represented in the series, the peridotite, so far as this rock contains the minerals. SERIES OF INTRUSIVES. 263 The order of crystallization of the minerals throughout the series is due to their relative solubility in the eruptive magma. Among various factors affecting solubility the fusion point of the chemical compounds constituting the different minerals, the temperature of the magmas, and the pressure under which the minerals crystallized, are important. The porphyritic and the ophitic textured rock facies, having crystallized under different condi- tions of pressure and of temperature from those under which the granular rocks were formed, show, as is to be expected, a different order of crystal- lization of minerals. CHEMICAL COMPOSITION OF THE SERIES. In the following tables there are reproduced the analyses which have been obtained of the various types. They are arranged according to dimin- ishing acidity. Nos. 1 and 4 were analyzed by Dr. H. N. Stokes, Nos. 2 and 3 by Mr. George Steiger, both of the United States Geological Survey: TabLE I.—Analyses of Crystal Falls rocks. 1 (26023). 2 (23354). | 3 (23755). 4 (23358). | Si Osea reyes feel yaa 58. 51 49.80 | 48,28 44,99 MMO); Soeoe a 12 = 78) 1. 00 0 Sl AlsOsmeerencasnesewsceas= 16. 32 19. 96 18. 26 5), Shik CTO Re ees ee cee iens Nones wii sees 244 | wescieoseeces .25 Mes Ose aes estee eee 2.11 6. 32 1,26 3. 42 Me Oe ears ete rss ochre 4,43 . 49 6.10 8.30 Mn OM sees sete at seiserc ets MET ACO. ylleyee eerste ocala erat veers Trace. INI ONS See See sist cca: Noneiid| senso cee. Gace ee ee None. CaO. = 3. 92 11. 33 9.39 8.79 NIKO) Baap sep Ee aateameere 3.73 7.05 | 10. 84 21. 02 iGO s sendcboosetoceees Bees 4.08 .61 573} 74 IN is Oye see seers cise cic oieecis 3.11 2.22 1. 34 oil 1860) eup NOS Sass seoc eoecoe -23 |100°— .13 |100°— .26 |110° .638 H20 above 110°. .-.-- 2.00 |100°-+-1.71 |100°+2.00 |110°-13. 19 12(0)5- 3 Doemaaaeece noose eee . 30 07 07 . 05 (COsS5 acon Saee eee eae neee None. ~15 - 43 Trace. (?) Ton seeeneeecneees 99.46 100. 63 99.91 99.17 (1) Mica-diorite (quartzitic); (2) Hornblende-gabbro; (3) Noritie; (4) Peridotite (wehrlite). 264 THE ORYSTAL FALLS IRON-BEARING DISTRICT. TABLE II.—Percentages of chief oxides reduced to 100. 1 P 3. 4 SiOseee et eee | 60. 36 50. 52 49, 64 47. 33 TOs eee eee eee 15 . 80 1.08 “1.02 INTAO} bac te ks ete 16. 83 20, 25 18.79 6.22 Wie,O; < delete asso ie |e LE 6.41 1.30 3.60 PEOe oosc base eee 4.57 .50 6.28 8.73 CaO Se oe eee 4, 04 11.50 9. 67 9. 25 INPRO eRe peer abe Sess 3.85 7.15 11. 16 22.11 Re Ol ay ees, ee 4,21 . 62 75 78 Nason et te panna enc 3.21 2.25 1.38 | 96 | TABLE III.—Atomic proportions of metals. Siocieesekccosese se teceeee 55. 85 46.53 45,27 42.48 UE Poem oaamiacaa c6bS boeeano9 53 - 56 -71 .70 IN Waa eta seeenoacesostSooG 18. 41 22. 03 29, 26 6. 60 Pee cee ar te eee ric ele 5. 08 . 834 5.70 9. 02 (CH cosccascesossdesscocess 4, 04 11. 42 9.51 8.98 Mast S35. some saeeee 5. 32 9. 85 15. 22 29. 67 Re eee ke assesses eaee 4.99 74 Sen eee yl Nad Ga Se aes sae 5. 78 4, 04 2.45 1. 67 The analyses show that all of the rocks contain a moderately large amount of water. Nevertheless, they are sufficiently well preserved to warrant a discussion of their analyses for classification purposes. This is especially true of No. 4, which is remarkably fresh for so basic a rock. The chief rock-making oxides in the above analyses appear in Table II reduced to 100. The molecular proportion of these oxides was then obtained. From these data the atomic proportions of the metals were derived, and are given in Table III. These calculations were kindly made for me by Mr. V. H. Bassett, assistant in the chemical laboratory of the University of Wisconsin. If we examine Table II we see that, in passing from the more acid to the basic end of the series, in correspondence with this decrease in silica the alumina increases rapidly, then decreases until it reaches the extreme basic rock, when it drops suddenly to 6.22 per cent. The analyses also show an increase in iron, which is best brought out in Table III. The alkalies decrease with diminishing silica, whereas the MgO, which for rocks of this character is very characteristic, shows a decided increase. Within the gabbro-norite-peridotite series (Nos. 2, 3, and 4) the lime shows SERIES OF INTRUSIVES. 265: a constant diminution corresponding to the increasing magnesian character of the rocks. The potash increases as the soda shows a decrease. The rocks represented by the analyses are believed to belong to a series ranging from a diorite on the one hand, through hornblende-gabbro and norite, to peridotite on the other. It should be borne in mind that the diorite is somewhat exceptional, representing a gradation toward the ortho- clase rocks. On the acid side of the series the microscope also shows variations to tonalitic and even granitic rocks very rich in quartz and ortho- clase, consequently much more acid in character than the diorite repre- sented in the analysis. ° It is a difficult matter to estimate quantitatively the amount of the one or the other kind of rock present in the Crystal Falls district. We are thus prevented from drawing from the predominance of the one kind or the other the conclusion that those represented in the minority are the results of the differentiation of a magma most nearly resembling in its original ‘constitution that which predominates. Moreover, since the analyzed rock types were not selected as representatives of the extremes of the process of differentiation, it would not be wise to endeavor to give the mean composi- tion of the parent magma from the analyses of the differentiation products which have been presented. The main thesis, however, is established that the separation of a magma into the various products described has taken place, as is indicated by the relations im the field, and as has been shown by the microscopical and chemical analyses. RELATIVE AGES OF ROCKS OF THE SERIES. Study of the relative periods of eruption of the various rocks results in the determination of the hornblende-gabbro as the rock which first reached its present position. It was followed in the acid part of the series by the diorite, which in one place cuts it. The diorite is cut by the diorite-porphyry Along the basic series the order has been determined as hornblende- gabbro, gabbro, bronzite-norite, peridotite. In general the forces of differentiation seem to have been active in two directions, tending toward increasing acidity and increasing basicity of the products of differentiation, thus agreeing with the law of succession of igneous rocks as propounded by Iddings. ‘The origin of igneous rocks, by J. P. Iddings: Bull. Philos. Soc. Wash., Vol. XII, 1892, p. 195. Pu Th cee © e 267 PAGACIN Eee Fie. 4. (Sp. No. 32756. Without analyzer, x 90.) Photomicrograph of fractured quartz phenocryst from a rhyolite-porphyry. It includes num- berless liquid inclusions, which diminish in quantity as the distance from the plane of fracture is increased, thus indicating their close connection with the fracturing of the quartz. The fracture in the quartz phenocryst continued into the groundmass, as may be seen on the left-hand side of the figure. It has been healed with secondary quartz. (Described, p. 82.) Fie. B. (Sp. No. 32914. With analyzer, x 47.) Photomicrograph of a section of rhyolite-porphyry, designed to show the rhombohedral part- ing, which is very common in many of the quartz phenocrysts. (Described, p. 82.) 268 U. S. GEOLOGICAL SURVEY MONOGRAPH XXXVI PL. XIX (A) INCLUSIONS IN A FRACTURED QUARTZ PHENOCRYST. (B) RHOMBOHEDRAL PARTING IN A QUARTZ PHENOCRYST. THE MERIDEN GRAVURE CO. IP Jy AMP, OX OX Fie. A. (Sp. No. 32119. With analyzer, x 90.) Micropoikilitic rhyolite-porphyry, showing the peculiar texture of the zones which invariably surround the quartz phenocrysts in sections in which the texture occurs. The same texture prevails in the groundmass. The irregular white areas which are continuous with the quartz phenocrysts and are connected with each other represent quartz. Disconnected dark and light areas between the quartz stringers are feldspar grains. These do not possess uniform orientation; hence the texture is not micropegmatitic. (Described, p. 84.) Fie. B. (Sp. No. 32187. With analyzer, x 90.) Photomicrograph of micropoikilitic rhyolite-porphyry. In this rhyolite-porphyry the micropo- ikilitie texture is much finer than that represented in Fig. 4, and the quartz in the zones shows a tendency toward spherulitic development. Owing to the extreme fineness of grain it is difficult to distinguish the quartz and feldspar in many cases. The greater part of the light areas shown in the photomicrograph are quartz. The dark areas between the quartz, and also some of the lighter areas, represent irregular pieces of feldspar. (Described, p. 84.) 270 U. S. GEOLOGICAL SURVEY MONOGRAPH XXXVI PL. XX MICROPOIKILITIC RHYOLITE-PORPHYRY. THE MERIDEN GRAVURE CO. 271 TP AIR 1B) CO TL Fig. A. (Sp. No. 32136. Without analyzer, x 90.) Rhyolite-porphyry with aureoled phenocrysts. The finest-grained type of micropoikilitic texture is here represented. The groundmass of this porphyry consists of rounded areas of material (‘quartz épongeuse”), corresponding to that forming the zones around the phenocrysts. Between these areas there may be found in places small feldspars. These photomicrographs, represented in figs. 4d and 5, Pl. XX, and in this figure, show every gradation in the micropoikilitic texture, from that which is with difficulty distinguishable as such to the coarser-grained unmistakable variety. (Described, p. 85.) Fic. B. (Sp. No. 32136. With analyzer, x 90.) Rhyolite-porphyry with aureoled phenocrysts. This is the same section as is represented above when viewed between crossed nichols. The texture of the groundmass is brought out somewhat better. The feldspars especially become more noticeable. For instance, one Carlsbad twin may be seen at the lower right-hand corner of the phenocryst partly indenting the aureole. Other feldspars may be noticed through the groundmass. In other portions of the section from which this photo- micrograph is taken the quartz phenocrysts have no aureoles and the groundmass possesses an imper- fect microgranitic texture. This figure brings out clearly. the gradation toward that texture. (Described, p. 85.) : 272 U.S. GEOLOGICAL SURVEY MONOGRAPH XXXVI PL. XXI MICROPOIKILITIC RHYOLITE-PORPHYRY. THE MERIDEN GRAVURE CO. } ag! t : baad j b 4 i Te t y a . f I ir i 1 f . i “ 7 i - ; a l fl ‘ . | “PLATH XXII ae ACE, Ne eal Fie. A. - (Sp. No. 32732. Without analyzer, x 18.) Aporhyolite showing beautifully developed perlitic parting. The perlitic cracks are brought out clearly by the chlorite which has accumulated in them. (Described, p. 87.) Fie. B. (Sp. No. 32732. With analyzer, x 18.) Aporhyolite showing perlitic parting, when viewed between crossed nicols. The groundmass resolves itself into a fine-grained mosaic of quartz and feldspar, showing microgranitic characters. The perlitic parting is thereby almost completely obscured. (Described, p. 87.} 274 U. S, GEOLOGICAL SURVEY MONOGRAPH XXXVI PL. XXII (B) (A) PERLITIC PARTING IN APORHYOLITE. (B) PERLITIC PARTING IN APORHYOLITE BETWEEN CROSSED NICOLS. THE MERIDEN GRAVURE CO. 4 : ; i Ne ith | i alee ‘ie wn t ay re I ’ h Lf r f ¥ . Fy 34 uy F ’ , ial i, J a, f ‘ a t | ab il pe ; ; ; be yY Go: 7 | Ah | Fad ~ 895 IPI AVP IBS OX OSCILLA Fig. A. (Sp. No. 22953. Without analyzer, x 38.) Rhyolite-porphyry rendered schistose by crushing. Granulation of the feldspars and the result- ing production of schistose aggregates of secondary quartz, feldspar, and sericite is here shown. The two large areas shown near the center of the figure were formerly occupied entirely by feldspar. The greater portion of this has now become altered, mere remnants of the original remaining. This secondary aggregate has especially well-developed parallelism. (Described, p. 93.) Fie. B. (Sp. No. 32726. With analyzer, x 18.) Photomicrograph of aporhyolite-porphyry breccia showing the fractured character of the quartz and feldspar. Certain portions of the section show the perlitic parting, with accumulations in these areas of chlorite. (Described, p. 93.) 276 PL. XXIII MONOGRAPH XXXVI U.S. GEOLOGICAL SURVEY (A) SCHISTOSE RHYOLITE PORPHYRY. (B) APORHYOLITE BRECCIA. fe) 3) w 5 > < C) z re a i w = x i Fn 7 ina i age Oa err ARN EN: Se ‘ i fuss “¥ ort i PLATE Xx. isis bk a JEG WEC AD A) OOS TENE Fie. A. (Sp. No. 22968. Without analyzer, x 18.) Schistose rhyolite-porphyry with well-developed flowage structure. A feldspar phenocryst which has been more or. less rounded by crushing, occupies the center of the figure. There is also shown in the upper left-hand quadrant of the figure a small crushed quartz phenocryst. (Described, p. 93.) Fie. B. (Sp. No. 22968. With analyzer, x 18.) When the section represented in fig. 4 is viewed between crossed nicols, the crushed character of the feldspar phenocrysts is thereby well brought out. The minutely granular character of the groundmass is also well shown. (Described, p. 93.) 278 U.S. GEOLOGICAL SURVEY MONOGRAPH XXXVI PL. XXIV (A) SCHISTOSE RHYOLITE PORPHYRY. (B) SCHISTOSE RHYOLITE PORPHYRY BETWEEN CROSSED NICOLS. THE MERIDEN GRAVURE CO. Wie elt A ‘ , 4 ‘ ; ; F . ez nN PN SOON, pager? s Ri - 7 % * ez Ic RAW ee Nea: Fig. A. (Sp. No. 32116.) Photograph, with very slight enlargement, of the polished surface of a very fine grained but very amygdaloidal basalt. The amygdules are of irregular shape, but in general with a rounded or tubular character. The original cavities have been filled with chlorite and quartz. The chlo- titic amygdules are the most common. A few of the white quartz amygdules may be seen on the left-hand side of the figure. It should be noted that owing to the softness of the chlorite some of the amyegdnles have become impregnated with the powder used in polishing the specimen. This could not be remoyed, and in many cases may be seen filling as well as ontlining the chlorite amygdules. (Described, p. 95.) Fig. B. (Sp. No. 32903. Without analyzer, x 18.) Photomicrograph of a section of the fine grained, possibly vitreous, ainygdaloidal basalt repre- sented in fig. 4 of Pl. XXVII. The amygdules consist of chlorite, quartz, and feldspar. The greatest interest centers in the groundmass. This consists of a fine felt of chlorite with minute epidote grains. Traversing this, one sees in places delicate flowage lines. This is believed te represent a once vitreous basalt. (Deseribed, p. 102.) 280 U. S. GEOLOGICAL SURVEY 2 MONOGRAPH XXXVI PL. XXV (A) AMYGDALOIDAL BASALT. (B) AMYGDALOIDAL BASALT. THE MERIDEN GRAVURE CO. 7, Meeereils 2 SX XO atta : (ehel ; Je aa DONTE, Fie. A. (Sp. No. 32541. Without analyzer, x 18.) Fine-grained amygdaloidal basalt. The only recognizable original constituent in the ground- mass is the feldspar in microlites which most commonly fringe out at the ends. They are not infre- quentiy arranged in sheaf-like aggregates. These are best seen with high-power objectives. The major portion of the groundmass consists of a fine felt of chlorite, with minute grains of epidote. It is considered to have resulted from the alteration of a vitreous base. The amygdules consist of calcite. (Described, p. 99.) Fie. B. (Sp. No. 32541. Without analyzer, x 35.) Portion of section from which fig. 4 was taken viewed with ahigh power. In this the sheaf-like aggregates of feldspar can be seen. (Described, p. 99.) 282 U.S. GEOLOGICAL SURVEY MONOGRAPH XXXVI PARTI PL. XXVI 7 (A) AMYCDALOIDAL BASALT. (B) SHEAF-LIKE FELDSPAR AGGREGATES. "a THE MERIDEN GRAVURE CO. my y * . m - < me ; St gis ‘ ‘ f . p16 4 yi woe * o™ PPK Oe i , ee - é A 1 ' fi \ a } f . > f pet Aon \ Yh . ‘ 5 i * * a ~ ; “y ATE Sy il . IPIGVA ME IB, JX SOW ILI Vie. A. (Sp. No. 32903. Natural size.) Reproduction of a very fine grained, possibly vitreous, amydaloidal basalt. The amygdaloidal cavities show very little contortion. The amygdules consist of white quartz, pink feldspar, and dark-green chlorite. Compare this figure with photomicrograph fig. B, Pl. XXV. (Described, p. 102.) Tia. B. (Sp. No. 32910. Natural size.) Colored reproduction of the pseudoamygdaloidal phase of the siderite-quartz matrix which occurs in places between the ellipsoids. The original matrix was first replaced in the zone of weath- ering by siderite. Deep burial of the rock resulted in the mashing of the siderite and the subsequent replacement of a great portion of it by silica, leaving a few oval areas unsilicified. Brought into the zone of weathering again by erosion, these siderite areas are removed on the weathered surface giving a scoriaceous appearance to it, as may be seen on the figure. (Described, p. 135.) Fie. C. (Sp. No. 33507. Natural size.) Fine-grained voleanic clastic. The water-deposited character of this specimen is unquestion- able. It can be traced in the field down into a coarse bowlder conglomerate. ‘The fragmental nature can only be seen under the microscope in the coarser-grained portions. The finer-grained material represents apparently the excessively fine-grained mud derive from the trituration of the coarser fragments. The eolian-deposited sands and dust have essentially the same appearance as the specimen here represented. The microscope, however, shows the very angular character of the fragments in the coarser-grained portions. ‘The very fine eolian-deposited dust can not he distinguished from that which has been deposited through water. It is sometimes very difficult to determine the nature of rocks of this fine-grained character. (Described, p. 144.) 284 U.S. GEOLOGICAL SURVEY MONOGRAPH XXXVI PL. XXVII JULIUS BIEN &CO LITHNY A. AMYGDALOIDAL BASALT, B. PSEUDO -AMYGDALOIDAL MATRIX OF ELLIPSOIDAL BASALT C. VOLCANIC CLASTIC. Pas ae. eat a ey oe ~ iat apie ; aes 4 wil Dir at a - = 0) z oye Re, J el sc JEG NAN) oO IIE Fie. A. (Sp. No. 32909c. Without analyzer, x 35.) Photomicrograph of section of fine-grained basalt with well-developed igneous texture. The feldspar outlines are now occupied chiefly by flakes of muscovite and grains of zoisite. (Described, p. 127.) Fie. B. (Sp. No. 32909c. With analyzer, x 35.) Photomicrograph of the same section when viewed between crossed nicols, showing the obliter- ation of the igneous texture. The lath-shaped mineral is muscovite. (Described, p. 127.) 286 U. S, GEOLOGICAL SURVEY MONOGRAPH XXXVI PART! PL XXVIII (A) BASALT SHOWING CHARACTERISTIC TEXTURE. (B) BASALT SHOWING OBLITERATION OF TEXTURE BETWEEN CROSSED NICOLS. THE MERIDEN GRAVURE CO. 287 PAE SO Xe eNe: Fie. A. (Sp. No. 32582. Without analyzer, x 38.) Photomicrograph showing the normal igneous texture of a basalt. The area formerly occupied by the feldspar substance is now occupied by a granular aggregate of various minerals. (Described, p. 127.) ; Iie. B. (Sp. No. 32582. With analyzer, x 38.) The same section of basalt when viewed between crossed nicols. The only indication of an igneous texture is shown by the amygdules present. The igneous texture of the rock is completely concealed as soon as the ageregates occupying the feldspar areas break up into their constituent grains. (Described, p. 127.) 288 U. S. GEOLOGICAL SURVEY MONOGRAPH XXXVI PARTI! PL. XXIX (A) BASALT SHOWING CHARACTERISTIC TEXTURE. (B) BASALT SHOWING OBLITERATION OF TEXTURE BETWEEN CROSSED NICOLS. THE MERIDEN GRAVURE CO. MON XXXVI——19 289 IP IN ID) 2 OC OK Fie. A. (Sp. No. 33313. Without analyzer, x 38.) Photomicrograph of a section of basalt from a pyroclastic showing in ordinary light a distinctly amygdaloidal character. The alteration of the basalt has, however, reached such a stage that the groundmass materials have for the most part been completely altered, with the production of porphy- ritic rhombohedra of calcite and plates of muscovite, which may be seen in abundance in the lower right-hand side of the figure. (Described, pp. 129, 145.) Fig. B. (Sp. No. 33313. With analyzer, x 38.) Photomicrograph of the same section of basalt, showing in ordinary light a distinctly amygda- loidal character when viewed between crossed nicols. The igneous texture is almost completely oblit- erated by secondary products. The secondary calcite and muscovite stand out very sharply from the very fine grained groundmass. (Described, p. 129.) 290 U.S. GEOLOGICAL SURVEY MONOGRAPH XXXVI PARTI PL. XXX (A) BASALT FRAGMENT IN A PYROCLASTIC SHOWING AMYGDALOIDAL TEXTURE. (B) BASALT FRAGMENT SHOWING OBLITERATION OF THIS TEXTURE BETWEEN CROSSED NICOLS. THE MERIDEN GRAVURE CO. aT - 291 TEMG GAVE DCOONC TE Fic. A. (Sp. No. 32909c. Without analyzer, x 35.) Photomicrograph illustrating the igneous texture of the basalt from the center of an ellipsoid. This has already undergone advanced alteration, and the feldspars have been replaced by a granular aggregate of calcite, sericite, chlorite, epidote, quartz, and albite. With advancing alteration the igneous texture is destroyed, and infiltrated calcite becomes more prominent. The photomicrograph shows in the lower left-hand corner a portion of the section in which a large quantity of calcite is present. (Described, p. 131.) Tie. B. (Sp. No. 32909c. With analyzer, x 35.) Photomicrograph illustrating the igneous texture of the basalt from the center of an ellipsoid, when viewed between crossed nicols. The igneous texture is almost completely destroyed by the breaking up of the secondary aggregates. (Described, p. 131.) 292 U.S. GEOLOGICAL SURVEY MONOGRAPH XXXVI PART! PL. XXXI (A) BASALT. (B) BASALT SEEN BETWEEN CROSSED NICOLS. THE MERIDEN GRAVURE CO ri ra, PIG ANE Ti. XC OK OC ICIL, Fic. A. (Sp. No. 23645. Without analyzer, x 35.) Perlitic parting in a fragment from a basalt tuff. The perlitic cracks are marked by accumu- lations of epidote grains. The remainder of the fragment consists of an exceedingly fine chlorite felt, with here and there a small feldspar microlite embedded in it. This was probably a fragment of basalt glass. (Described, p. 138.) Fie. B. (Sp. No. 23646. Without analyzer, x 35.) Photomicrograph of a section of basalt tuff. The illustration shows the sickle-shaped bodies which are characteristic for the fine eolian-deposited volcanic ejectamenta. (Described, p. 142.) 294 PL. XXXII PART | MONOGRAPH XXXVI U. S, GEOLOGICAL SURVEY (A) PERLITIC PARTING IN BASALT (GLASS?). (B) TUFF. THE MERIDEN GRAVURE CO. + %, wa ae . SANS MOOOEM ys. oe 295 IP IL AP) OK OX TEI Fic. A. (Sp. No. 32713. Without analyzer, x 35.) Water-deposited sand. This volcanic sand consists chiefly of feldspar and hornblende. The rounded nature of the feldspar grains is readily seen. Under the microscope seme of these show secondary enlargements. The hornblende is in irreeular areas, and is presumed to be secondary after fragments of augite. (Described, p. 144.) Fie. B. (Sp. No. 32711. Without analyzer, x 17.) Water-deposited voleanic sediment. This illustration shows very clearly the gradation from the medium-grained volcanic sand in the lower portion of the section, through the finer-grained mate- rial, to the very fine grained dust. ‘The very fine grained material was deposited following the con- tours of a large pebble, which is shown in the upper portion of the figure. The fragments of this sand consist chiefly of basalt. There are some which were probably feldspar. (Described, p. 144.) 296 ee ee U. S, GEOLOGICAL SURVEY MONOGRAPH XXXVI PARTI PL. XXXIII (A) WATER DEPOSITED SAND. (B) GRADATION IN WATER DEPOSITED CLASTIC. THE MERIDEN GRAVURE CO. yi 4-0 Non +e ; ; ; ‘ kan ey Mune ty yi iP ope ’ af ye Mom we? e f : : { ie ‘ \ a } fs ' { ° / IPAS, ROO ID. Fig. A. (Sp. No. 23746. Without analyzer, x17.) Contact product of a granite. This muscovite-biotite-gneiss (?) is the result of contact action of a granite upon a graywacke. Complete recrystallization of the sedimentary rock has taken place, with the production of a porphyritic schistose structure. The large porphyritic muscovite plates seen as white areas in the photomicrograph evidently represent the last products of recrystallization, as they include all of the minerals which have been previously formed. They are possibly to be looked upon as the product of mineralizers, to whose action may also be referred the presence of tourmaline, which occurs in the section. The irregular white areas represent quartz and feldspar. Dark greenish-brown biotite is included in the muscovite, and with iron oxides occupies the areas which in the figure are dark. (Described, p. 197.) Fie. B. (Sp. No. 23226. Without analyzer, x17.) Photomicrograph showing the brecciated character of the matrix which is at times found between the ellipsoids in the ellipsoidal basalts. In this specimen the schistose character is not so marked as itis attimes. (Described, p. 117.) 298 U.S. GEOLOGICAL SURVEY MONOGRAPH XXXVI PL. XXXIV (A) CONTACT PRODUCT OF GRANITE. (B) BRECCIATED MATRIX BETWEEN ELLIPSOIDS. THE MERIDEN GRAVURE CO. ey Ns SOOO, 6 BS, 299 PLATE XXXV. Fie. A. (Sp. No. 26059. Without analyzer, x 18.) Photomicrograph showing an eruptive contact between granite and metamorphosed sedimentary tock. Asa result of this contact the elements of the granite have become partly automorphic. The center of the figure is occupied by a quartz phenocryst which is partly surrounded by the schistose metamorphic product. 1t contains, near the edge, grains of feldspar and flakes of mica, and thus an imperfect poikilitic zone is produced. The mineral constituents of the metamorphosed sedimentary are arranged parallel to the contours of the phenocrysts. (Described, p. 198.) Fic. B. (Sp. No. 26059. With analyzer, x 18.) The same section as the above, viewed between crossed nicols. (Described, p. 198.) 300 U.S. GEOLOGICAL SURVEY MONOGRAPH XXXVI PARTI! PL. XXXV (A) CONTACT OF GRANITE AND SEDIMENTARY. (B) CONTACT OF GRANITE AND SEDIMENTARY SEEN BETWEEN CROSSED NICOLS. THE MERIDEN GRAVURE CO. 2D, ONT, 301 : 12 bE 1 GOOG. Fig. A. (Sp. No. 32827. Without analyzer, x 38.) Photomicrograph illustrating a rather exceptional form of spilosite with white spots lying in the fine-grained groundmass. Albite, with a very small amount of chlorite and epidote, forms the spots. (Described, p. 206.) Fie. B. (Sp. No. 32827. With analyzer, x 38.) This is the same section as above, viewed between crossed nicols, showing the aggregate char- acter of the spots of this spilosite. (Described, p. 206.) 302 ' U.S. GEOLOGICAL SURVEY MONOGRAPH XXXVI PARTI PL. XXXVI (A) SPILOSITE. (B) SPILOSITE SEEN BETWEEN CROSS NICOLS. THe MERIDEN GRAVURE Co. ¥ Want Ay oe wi n , | PS ye = “ x : = IP Ey OS COORG WEIL IES Fie. A. (Sp. No. 32958. Without analyzer, x 18.) Normal spilosite. Oval spots of macroscopical size, in which chlorite is the predominant con- stituent, with some quartz, feldspar, rutile, and muscovite, are sharply defined from the surrounding fine-grained groundmass, consisting of the same constituents, but with the muscovite in large quantity and the chlorite very subordinate. (Described, p. 206.) Fic. B. (Sp. No. 32861. Without analyzer, x 38.) Spilosite in which the spots are of microscopical size and consist predominantly of chlorite aggregates lying in the fine-grained quartz-albite groundmass. (Described, p. 207.) 304 U. S. GEOLOGICAL SURVEY MONOGRAPH XXXVI PARTI PL. XXXVII (A) SPILOSITE. (B) SPILOSITE. THE MERIDEN GRAVURE CO on i n . ye "3 pe ry : * = 7 , } if : ; . " ih hs d “ Ee OO Wu | MON XXXVI 20 ; a a ARTE ee ONG Vell Teale Fig. A. (Sp. No. 32826. Withovt analyzer, x 38.) Photomicrograph illustrating the passage of spilosite to a desmosite. In the upper portion, especially in the upper left-hand corner, of the figure, chlorite aggregates similar to those illustrated in fig. B, Pl. XXXVII, are seen. These become united, and thus there is a passage into the banded product. This banded character is well shown in the lower half of the photomicrograph. (Described, p- 207.) Fig. B.- (Sp. No. 23755. Without analyzer, x 90.) Occurrence and alteration of bronzite in bronzite-norite. This illustration shows the way in which the bronzite occurs in the bronzite-norite. It is frequently included in the hornblende. The bronzite alters around the edges and along the cracks toa yellowish-green fibrous serpentine mineral, which is represented in the section by the dark fibrous material next to the unaltered bronzite. This secondary mineral then alters to a scaly aggregate of tale. These two secondary products can be seen bordering the bronzite, especially well where it is traversed by a crack. (Described, p. 238.) 306 U.S. GEOLOGICAL SURVEY MONOGRAPH XXXVI PL. XXXVIII (A) PASSAGE OF SPILOSITE INTO DEMSOSITE. (B) ALTERATION OF BRONZITE IN BRONZITE NORITE. THE MERIDEN GRAVURE CO. PLATE XX XTX. AGW AVAIL pe NO NaNO NG: Fig. A. (Sp. No. 23321. With analyzer, x 35.) Photomicrograph of a section of biotite-granite from the center of a dike 5 feet wide. On the borders of the dike the magma has crystallized as a normal mica-diorite without quartz and orthoclase. (Described, p. 226.) : ; Fic. B. (Sp. No. 26023. With analyzer, x 35.) t Mica-diorite showing tendency toward an ophitic texture. Plagioclase is the most automorphic mineral. Biotite is next, but it is poorly developed. Orthoclase and quartz fill irregular areas between the plagioclase. (Described, p. 231.) 5 308 U. S, GEOLOGICAL SURVEY MONOGRAPH XXXVI PL. XXXIX (A) BIOTITE GRANITE. (B) MICA DIORITE. THE MERIDEN GRAVURE CO. | ‘ae os PEATER Sol ane Je JON AU Jay CJ Die Fig. A. (Sp. No. 32648. Without analyzer, x 35.) Quartz-mica-diorite-porphyry. The phenocrysts of feldspar and quartz stand out clearly from the fine microgranitic groundmass. Mica phenocrysts are not seen in this figure, which is intended chiefly to illustrate the character of the feldspar phenocrysts. Muscovite has resulted from their alteration. The zone which surrounds the altered center is very fresh, and is rendered poikilitic by inclusions of minute grains of quartz. (Described, p. 229.) Fie. B. (Sp. No. 32643. With analyzer, x 35.) Quartz-mica-diorite-porphyry. The same section, viewed between crossed nicols, The micro- granitic texture of the groundmass is well shown. (Described, p. 229.) 310 U.S. GEOLOGICAL SURVEY MONOGRAPH XXXVI PL. XL (A) MICA DIORITE PORPHYRITE. (B) MICA DIORITE PORPHYRITE SEEN BETWEEN CROSSED NICOLS. THE MERIDEN GRAVURE CO. i * “= ’ ; 2 : ;. 5 Le = 311 IPI AMP IS, OX Ib IL, Fie. A. ' ! (Sp. No. 23320. Without analyzer, x 18.) Porphyritic poikilitic hornblende-gabbro. The brown hornblende occupying the center of the phenocryst grades over into a dull-green hornblende. Feldspar and pyroxene are included in the hornblende. (Described, p. 241.) : Fie. B. (Sp. No. 23344. With analyzer, x 18.) Hornblende-gabbro showing a poikilitic texture. The tendency of the feldspars toward a lath- shaped development is very evident. Were they lath-shaped the texture would agree with the Lévy definition of the ophitic texture. (Described, p. 233.) 312 U.S. GEOLOGICAL SURVEY MONOGRAPH XXXVI PL. XLI (A) PORPHYRITIC POIKILITIC HORNBLENDE GABBRO. (B) POIKILITIC HORNBLENDE. THE MERIDEN GRAVURE CO. Teas Or ie . ¥ : i y ’ + x ‘ 5 + i 5 a . ‘ t . . , 2 ) ay IX ‘ PLATE Sci ” . ~ ay | : 1 5 7 1 PIG ATE, XIII. Fie. A. (Sp. No. 23754. Without analyzer, x 38.) A moderately fine grained hornblende-gabbro showing parallel texture. This hornblende- gabbro occurs in dikes cutting the coarse forms of hornblende-gabbro. The specimen shows very clearly the parallel texture rather commonly found in sections from these dike rocks. This texture is best developed nearest the contact, and is presumed to be a flow texture. The chief mineral constituents—plagioclase, hornblende, and mica—can be readily distinguished in the section. (Described, p. 244.) Fie. B. (Sp. No. 23754. With analyzer, x 38.) The parallel texture in the hornblende-gabbro is brought out somewhat better when viewed ‘between crossed nicols. (Described, p. 244.) 314 U. S. GEOLOGICAL SURVEY MONOGRAPH XXXVI PL. XLII (A) HORNBLENDE GABBRO. (B) HORNBLENDE GABBRO SEEN BETWEEN CROSSED NICOLS. T=E MERIDEN GRAVURE CO. . 4 : ; ; ae * ; < ‘, 4 t “ x ; ‘i "a : : . 4 Bier Y Z s “ = - " r RL pew ae | PLATH XLII: Fie. A. > (Sp. No. 26070. Without analyzer, x 17.) e Normal granular hornblende-gabbro. This illustrates the normal medium-grained hornblende- gabbro which occurs in this district. The mineral constituents hornblende, feldspar, and iron oxide can be readily distinguished. (Described, p. 248.) Fig. B. (Sp. No. 26069. With analyzer, x 18.) Schistose hornblende-gabbro. This illustrates a crushed specimen of a normal hornblende- gabbro such as is represented above in fig. 4. In this the feldspar has been partly granulated, and new feldspar and quartz has resulted therefrom. The hornblende has also to a considerable extent been recrystallized as light-green secondary hornblende. The mica has become chloritized. These are the important secondary products. A very fair schistose structure has resulted from the crushing. Carried to its full extent, metamorphism of this rock would result in producing an amphibolite or a hornblende-gneiss. (Described, p. 248.) d , 316 U. S, GEOLOGICAL SURVEY MONOGRAPH XXXVI PL. XLIII (A) COARSE HORNBLENDE GABBRO. (B) SCHISTOSE HORNBLENDE GABBRO. THE MERIDEN GRAVURE CO. Iba, LN IPL AIO, OX IL LW Wie. A. (Sp. No. 23319. Without analyzer, x 35.) Moderately fine grained hornblende-gabbro. The constituents hornblende, plagioclase, and iron oxide can be readily recognized. Many of the hornblende individuals contain a great number of minute inclusions. (Described, p. 240.) Fie. B. (Sp. No. 23755. Without analyzer, x 35.) Bronzite-no1ite. The rock consists of bronzite, hornblende, and plagioclase. The bronzite individuals show a tinge of green and pink, and are slightly fibrous, due to beginning alteration. Cracks, filled with alteration products, cross them. The bronzite is frequently surrounded by a rim of brown hornblende. In some parts of the rock it is partly automorphic. The hornblende can be readily recognized by its strong yellowish and reddish-brown color. Itisinanhedra. The plagioclase is the white mineral including numerous minute dark specks. It, like the bronzite, is xenomorphic. (Described, p. 244. ) 318 U.S.GEOLOGIGAL SURVEY MONOGRAPH XXXVI PL. XLIV F. Kk Denniston, del. JULIUS BIEN & CO.LITH, Ni¥ A. HORNBLENDE- GABBRO. B. BRONZITE-NORITE. ere - : * * PLATE XLV. 7 e. Parga RATE 1h SOLA WW Fie. A. (Sp. No. 23356. Without analyzer, x 35.) ; Bronzite-norite-porphyry. The phenocrysts of bronzite lie in a groundmass of bronzite, mono- -clinic pyroxene, hornblende, and feldspar. In the drawing the bronzite and monoclinic pyroxene of the groundmass can not be distinguished. (Described, p. 247.) Fic. B. (Sp. No. 23353. Without analyzer, x 18.) Feldspathic wehrlite. One can readily distinguish the constituents—augite, hornblende, olivine and feldspar—in the illustration. The augite is automorphic where it is near the feldspar. It is surrounded by a brown hornblende zone. Between the olivine and the feldspar there are always two zones. One, the inner one, best seen between crossed nicols, is orthorhombic pyroxene; the other, the outer one, is compact green hornblende. This green hornblende is in optical continuity with the brown hornblende which surrounds the augite. (Described, p. 255.) 320 U.S.GEOLOGIGAL SURVEY MONOGRAPH XXXVI PL. XLV F. K Denn/ston, del. JULIUS BIEN &CO,LITH N.Y. A. BRONZITE- NORITE - PORPHYRY. B. FELDSPATHIC WEHRUITE- « a ° = ~ -: « ¥ 6 oe u ork Tat ‘ i . “ a “0 1 ° - Dees Lay pe Ee ee Mel TF Wate pe PUR PAPAS a evil Fie. A. (Sp. No. 23353. With analyzer, x 90.) Wehrlite. There are shown in this illustration the zones of orthorhombic pyroxene and horn- blende which he between the olivine and feldspar. (Described, p. 255.) Fig. B. (Sp. No. 23353. Without analyzer, x 90.) Wehrlite. The ramifying feldspar growths in the hornblende can be seen better when the section is\viewed with the analyzer in, asin fig. 4. (Described, p. 255.) 322 U.S. GEOLOGICAL SURVEY MONOGRAPH XXXVI PL. XLVI (A) WEHRLITE VIEWED BETWEEN CROSSED NICOLS. (B) WEHRLITE. THE MERIDEN GRAVURE CO. THE CRYSTAL FALLS IRON-BEARING DISTRICT OF MICHIGAN Part lk—iiLtk, HASPEIRN PART OR Eis DilsmElCas, INCLUDING THE FELCH MOUNTAIN RANGE By HENRY LLOYD SMY TE WITH A CHAPTER ON THE STURGEON RIVER TONGUE By WILLIAM SHIRLEY BAYLEY CHAPTER I.—GEOGRAPHICAL LIMITS AND PHYSIOGRAPHY CHAPTER IJ.—MAGNETIC OBSERVATIONS IN GEOLOGICAL MAPPING COME E NaS IMtROM WE 1 O nuestra aane eis See ce eae SBE eas Sra are OTE SIPS Ta nee ee eae a Preliminary sketch of geology Character of surface Drainage Section IIJ.—Distribution of magnetism in the magnetic rocks SewOn MW insie ME) NG MAGN) OF WOM! | socdeosa ses ccesoo cond Seocesaeen seosee cone Section V.— Facts of observation and general principles .........,..........---.---------- (1) Observed deflections when the strike is north and south and the dip vertical (2) Deflections of the horizontal needle @) Dailecwions OF Wie chin MECC cosas S6c6o6=s5e\s00n sdaq dose soo Sasso0 ose cOO S50 conaas (4) Horizontal and vertical components when the magnetic rock dips vertically ....--. _ (5) Horizontal and vertical components when the magnetic rock dips at any angle. -... (©) Wevemimimeawiom OF GEDWN ooscac cceson sacdse aesear HoSos0 cones doasec caeesooeeese SHuC GOV (SITE Ayo ceseqoicooasd Hoos dane aEeerne Saatee SecA re recaacecssose oduoabba teseceouE sae Sectionyvil.— Applications) topspecialicasess sees -= sss) eae eee eeeer eee ee eee ee nn eenee (1) The magnetic rock strikes east or west of north and dips vertically..........-----. (2) The magnetic rock strikes east and west .........- 2-2-2222 cece eee wees wane wee nee @)eiwompanallelamaenetichormahlons\=-es ee eeee skeen ae eee e eee eee eet eee eeten Section VII.—The interpretation of more complex structures.....-........--..-...-------- (il) JEinaning? EyNOlinesoc lees sues cenbesbeen cobsbo NeseoD ca50 SoCode pSocoaeced cpcRdd Beqebe (Z)) ihielavinys: Qantas 25 cso acne son code coed samen Onod Sosoce Homeée boseeaaSSeCOSSsoS (6) PRlormationsispliti bys trusives)-seeresse cla see las eaee ee eee eee eee eeeer eee aaaee (2b) STRUM. cos seoteess socaHe sdses S50506 DONSOD OSadSODNSS aca OsHO Mees Soom SaSSOoceoS CHAPTER DI Laney Enema MOUNTAIN IRAN GER = s-cclsaccloeleces elec reece ae toee eee eee eee Eee Section: I-—Position;, extent, and previous) work... 5220s. emeseeesiees see ceceoe once ee ene eee Nection Me—Generalisketch of the geologyens---c--ela= = -2)-e eee eee eee tnoe ee eeeeeeeeee SOCHOR IL —1ne AN@MNERI 6 cos So00s0 6500G0 685086. cb00 B60550 ando05 cogGue HenU Sano sass Caen SeKe TNODOSTADM occ con aseens os0008 Conese oNomas OboRs EoOSSS OSooES seS9 Cob Seno cesd eseceneces IP ROG MIO! OMAR Sado sccons coses0 noes asaSs eSeene seus Poses essocsso esos essose SECTOR IN lng) SATO CWENATAN no 5o5 eoncs6 soso S46 namoSH emo asa Sonode HUSSeDee ceo beSOEE ' Distribution, exposures, and 10) NOSE hy scoe eos GdoooL OGdSnd Hotnon Good Guo SoD Saneeadoas INoldltings ama) WHC AGS 55050 s2c00s soce se c6b552.0508 Coss se nosona Dobeds Soon Hadcbo sess aces IPGHRC RAN MCA CHERACIENS scoses seaece cosnso SoonaD cusecs coarse socd cHedoasess seeaescoes Speitiom Wo ine iaingh nil @@lonihts Shsscccsen ccocco nc056n soS5ce0 gaccod sSceaa eons cde sae mone IDS TED UNNOMN, GsxpOsMbHes, Bal UO OSEAN thy ecogeseens concede ce69 soso6e sone Soeoeu -ese Reese Perma NNea CATCHES 5—s255 cosacs saga cesses esones ces Sons Or esse ceeess sazeeseeooss Sechion WAl_—Na® ME MaTOIG! BONIS copes ooe0 santos anssos S000 cancooseecus coeneS conse SoosEs DiSiribuGlonwe xposures; andstopoeraphyese= === ess esses eee ee eer eee ree eee eeeeeaee IPL AROSE OVA CHENEY OUEME) socio osaase aneoco Medco seoees cee scu bee eeo ceeeer ance coeo cane 401 326 CONTENTS. CuarrerR II].—Tue Fetch MounTAIN Rance—Continued. Page. Section VII.—The Groveland formation. ...--.-.---.-.---.-------- ---------- -------------- 415 Distribution, exposures, and topography .----.---------------------------+-+-+-------- 415 Petrographical characters ..---..---------------+------- «+2222 2-222 2222 n 2 oes 417 Section VIII.—The mica-schists and quartzites of the Upper Huronian series...--.--..---. 423 Petrographical characters -..----------------- -----+ e+ 222+ 22-22 22 = see ene 22-2 20s 425 Section IX.—The intrusives .----. .----. ------ ------ +--+ + 2222s ene eee one ee eee eee 426 CuaprerR IV.—THE MICHIGAMME MOUNTAIN AND FENCE RIVER AREAS..--.-.---------------- 427 Section I.—The Archean...-.. .----- .----- --- +--+ eee nee ene ee nn nnn eens teen eee 428 Section IJ.—The Sturgeon formation......-.---.------.----. ------------------ ------ -----~ 430 Section IJI.—The Randville dolomite ...--.....--.----.----------. ---- +------------------- 431 Distribution and exposures ...-.-. .----.------ ------ == 2+ 22 nn en ene nee 431 Folding and thickness...... .----------- ------ ----- + --- 20+ 23-200 e222 nee 2222 222-222 432 Petrographical characters ....-...------------ --- 22+ «2-2-2 228 een enn een eee ine 434 Section IV.—The Mansfield formation ..-........-..---------- ---------- ---+ -------------= 437 Distribution, exposures, and topography .-----.--------------------------------------- 438 Folding and thickness..--.. ---- -------- 2--+ ------ +--+ ¢-- 200 0-02 2222 eee eee ones eros 438 Petrographical characters ---------------.--- Soadees Scoo cand oDee soon esas CoSSoS ESET ODOC 439 Section V.—The Hemlock formation .-..-... ...--.---------------------- -----------------= 440 Distribution, exposures, and topography -.-...----..---- .-----.------------------------- 440 Folding and thickness......-.-.------ ---------- ---2 +++ 22 2-220 2 222 0 e eee een een ee 441 Petrographical characters .....-.----------- ---------- <== +--+ -- 22 0-20 eee renee 442 Section VI.—The Groveland formation.........----..-.--------------------+ -------------= 446 Distribution, exposures, and topography ......---..----.------------------ -------+«--- 446 Folding and thickness......-----.------ ---- --- === -- += s22 200 e022 eee eee eee ee nee 448 Petrographical characters ..---..-.---.----------------------- ewe eee eels eee eiee ares eee 448 CHAPTER VY.—THE NORTHEASTERN AREA AND THE RELATIONS BETWEEN THE LOWER MaR- QUETTE AND THE LOWER MENOMINEE SERIES ..--...---..------------------- 451 CuaPrerR VI.—THE STURGEON RIVER TONGUE, BY WILLIAM SHIRLEY BAYLEY.----.---.------ 458 Description and boundary of area ...--..----------- ------ ++ --0+ een ene ooo ee ee eee 458 Literature ..... ---- ---- -- oe cee nee wn nn ee ne ne ne en en nnn tne ene eee noes 459 Relations between the sedimentary rocks and the granite-schist complex .-..-..----------- 461 The Basement Complex -..- ---- ---- ------ ---- -- 2-222 22 oe ee nee nn ee nee ee ene 463 The gneissoid granites -..-..----. ---- e----- 2-2 2-2 eens enn enn een ee eee 463 The amphibole-schists......---..----- ---- ------ ------ +2 -20+ 22-202 222 ee e222 eee ee 465 Origin of the amphibole-schists --..---.------------------------------------------ 466 TING MOWURDAO INI coco ces cose cah0 sadses Sa0s Seo bens csSs soos sooo ooSS Socase SoessoSse0 467 Mheintrusye LOCKS poeta sale teeter sea ale mae ad ll ell lal la alta 469 Comparison of the Sturgeon River and the Marquette crystalline series...--.----.----. 470 The Algonkian trough.......--.---- ---- -------- ------ 2-22-22 one eee rene eee eee eee 471 Relations between the conglomerate and the dolomite series.....---.---.-------------- 472 Relations between the dolomites and conglomerates and the overlying sandstones.. --.- 473 The conglomerate formation...... .--------- +--+ +--+ 0-22-22 == een ee ee eee ee 473 Important exposures ....-..--------- +--+ + = 2-22 eee en we wee een wee 474. Petrographical descriptions ---.-..----..-----.-----------+----,---------- Ue narra ATT Nang Gallowarine TOMTOM oc 525 obooee Sha sa06 code SbS0 Cond sens coSSEy Soe zoo EseSeSeoD Seoe 479 Important exposures - .....----- ------ ---- ---- 2-2 22 ee = = ae wa oe one wee 480 Petrographical description --..-.-..- Bhbatcan. dco. ceds couDIEduS Coce.cous abdcedaeoe HONE 481 Slates and sandstones on the Sturgeon River..----...-..--..-------.-------------- 481 The igneous rocks.....----.------ -- ---- 0-22 e 2 eee een nn nnn en ee ee eee 482 The intrusive greenstones ..-.-...-.-. ---.------ ---- ---- --- = 22 = ee en eee 482 Petrographical description .--..-.-.---..----- ---- ------ ------ ---- ---- +--+ ==> 482 The banded greenstones ....---- -...---. ---- ---- <= - ---- e nen ene - ne eee 485 Petrographical description .........-------.---------------------------------- 486 MLL US TRATIONS. ' Page. PLATE XLVII. Relations of magnetic beds to variation and dip...-....----....... panes aDaqKC 352 XLVIII. Relations of magnetic beds to variation and dip...-.....---..----..-------- eSbe) XLIX. Geological map of the Felch Mountain range ...-.....-....-----.--------------- 374 L. Geological map of a portion of the Crystal Falls district ....-.-------.----.--- 450 LI. Geological map of the Sturgeon River tongue.........--...---.----------~----- 458 LII. Map of exposures in sec. 7 and portions of secs. 8, 17, and 18, T.42 N., R.28 W.... 474 LIII. Schist conglomerate from dam of Sturgeon River ....-...---..----------------- 476 BrGs 15.) Macnetic cross section\ in) 24 45)Nin, Ri. Sill Winee-sose-e see aeeenececernene as cease eee ee=e 345 16 CinclesrotabMmactlonptrsajs sclera) csselce ace seisesils Ce ee se see ane aie SES SECEE 346 If, IAS TOROS MOH? OM WINS ClifD MERCI sade gace Coco pads soem sace Sone Sabo sosascesncossess 347 18. Curves showing the relations between the horizontal components at the points of maxi- mum deflection, for rocks dipping at various angles and buried to various depths.... 354 19. Truncated anticlinal fold with gently dipping limbs....-..--....-.....--.------------ 364 20. Truncated anticlinal fold with steeply dipping limbs ...-...----...---..-----.------ ee oGD) 21. Plan and cross sections of a pitching syncline .-.-......-...---..----..----.------~--- 367 22. Magnetic map of the Groveland Basin .......-.--.------ ------ ----- 2 2<-- +n nee sone 370 23. Plan and cross sections of a pitching anticline.........-.....--.------.----- secessseacc 371 24, Magnetic map of a single formation split by an intruded sheet.............-..---- wea, ote THE CRYSTAL FALLS ITRON-BEARING DISTRICT OF MICHIGAN. PART Il. THE EASTERN PART OF THE DISTRICT, INCLUDING THE FELCH MOUNTAIN RANGE. By Henry LiLoyp SmyrnH. WITH A CHAPTER ON THE STURGEON RIVER TONGUE, BY WILLIAM SHIRLEY BAYLEY. CHAPTER I. GEOGRAPHICAL LIMITS AND PHYSIOGRAPHY. INTRODUCTION. The territory to be described in this and the four following chapters is situated in the Upper Peninsula of Michigan, between the Marquette and Menominee iron ranges, and is all embraced within T. 42 N., Rs. 28-30 W., and T's. 42-47 N., Rs. 30-31 W. The area of about 300 square miles included within these townships had for the most part been covered hastily by previous reconnaissances of the Lake Superior Division of the United States Geological Survey, the results of which were placed at my disposal. Our task was to go over with especial care those portions im which outcrops had been found by our predecessors, or which seemed likely to contain the iron-bearing formations. At the same time much of the rest was examined more hurriedly. The tract surveyed in detail comprises a continuous belt about 30 miles in length, and of width varying from 2 to 5 miles, lying wholly within the drainage basin of the Michigamme River and its principal upper tributary, the Fence River, and extending southward from the northern end of the Republic tongue, where it was connected with rocks of well- determined Marquette types, as far as the south line of T. 43 N., R. 31 W. From this line we passed southeast (leaving a gap of 5 miles) across the low 329 330 THE CRYSTAL FALLS IRON-BEARING DISTRICT. divide between the Michigamme and the headwaters of the Sturgeon, to the Felch Mountain range, which was then carefully studied for a distance extending 13 miles to the east. Until within the last few years the larger part of this area had been very difficult of access, and much of it is difficult still. The rock surface is almost wholly concealed by a cover of glacial deposits of various kinds; dense forest and great swamps also obscure the rocks, and make traveling difficult and slow. - It is therefore not a field to invite geological study. While exploration for iron ore has here and there passed the frontiers of the productive ranges on either side, the general ill-success which attended the early enterprises has discouraged the active search that would at least have resulted in important additions to geological knowledge. For these reasons the area as a whole, with the exception of the Felch Mountain range, has remained almost unknown geologically, until our work in 1892. The refer- ences to it in geological literature are consequently but few in number, and are for the most part merely the records of the unrelated observations of casual visitors. . The district, nevertheless, deserves attention from both the economic and the geological standpoint. The iron-bearing formations of the Mar- quette range extend into it from the north, those of the Menominee range from the south. On the west the ore deposits of the Crystal Falls area are connected geographically at least with the western extension of the Menom- inee range. Between these boundaries the area stands as the largest one remaining in Michigan in which iron-bearing formations are known to occur, but as yet not yielding important bodies of ore. Here, too, if anywhere, the questions of the equivalence or nonequivalence of the individual for- ‘mations of the Marquette and Menominee iron-bearing series are to be answered. It is proper to state that the field study, in consequence of the condi- tions under which this work was done, was almost wholly directed to eco- nomic questions, and that it was not originally anticipated that the results were to be published as a monograph on the district. This will explain the very brief space devoted to the Archean in the following pages. The field work was begun and ended in 1892. Since that time there has been no opportunity to revisit localities, and the conclusions now stand essentially as they were reached in the field. Considering both the obscurity and com- INTRODUCTION. 331 plexity of the area, it is very probable that further study of important local- ities would clear away many of the difficulties, as well as modify certain of the opinions now held. The writer was efficiently aided in the field work by Messrs. Samuel Sanford and Charles N. Fairchild for nearly the whole period, and E. B. Mathews and H. F. Phillips for part of it, as assistant geologists, and by Messrs. Lewis and Forbes as skilled woodsmen. PRELIMINARY SKETCH OF THE GEOLOGY. The rocks of the Michigamme and Felch Mountain areas range in age from Archean to early Paleozoic. North and west of the Michigamme River, where geological boundaries are most susceptible of determination, the granites and gneisses of the Archean come to the surface in three oval areas of great regularity of outline, from 10 to 12 miles long by 2 to 6 miles wide, while the intervals between the Archean ovals are occupied by highly tilted sedimentary and igneous rocks of Algonkian age. The lower member of the Algonkian has derived its materials from the wasting of rocks lithologically similar to the underlying granites and gneisses. In the southern and eastern portions of the district the edges of the tilted older rocks are partially covered by a blanket of gently dipping sandstones of Cambrian age, very soft and easily disintegrating. These rocks first appear near the Michigamme River as detached outliers. In going south and east from that river the separated patches become larger and more abundant, until finally a few miles beyond the eastern limit of our work in the Felch Mountain range they unite and entirely cover the pre-Cambrian formations. CHARACTER OF THE SURFACE. In its most general aspect the surface throughout this area is a plaim, somewhat rolling indeed, which slopes gently upward from the southeast toward the northwest. The surface is formed partly by the soft and gently inclined Upper Cambrian sandstones and partly by the much harder and highly tilted pre-Cambrian rocks of diverse physical and mineralogical characters, and yet over all it maintains a very uniform slope. On the southeast, in the Felch Mountain range, the plain has an average elevation above the sea of 1,200 to 1,300 feet. In the northwest, in the southern Oe, THE CRYSTAL FALLS IRON-BEARING DISTRICT. sections of T. 47 N., R. 31 W., the average elevation is 1,800 to 1,906 feet. Since the intervening distance is somewhat more than 30 miles, the gen- eral slope is therefore less than 20 feet to the mile. The minor topographical features based upon this plain are multitudi- nous in variety and detail, but generally quite insignificant in relief. The maximum difference of elevation between the top of the highest hill and the bottom of the neighboring valley is less than 300 feet, and this is reached in but two cases. The country possesses no commanding emi- nences, and in the widest panoramas now and then obtainable from the summits of glaciated knobs the background is restricted to a radius of a few miles. In these the general evenness of the sky line is usually broken only by the remnants of the old forest, which have not yet succumbed to fire and the lumberman. These lesser features have been shaped mainly by the work of the conti- nental ice-sheet, both through the materials which it brought in and through those which it carried away. In the areas underlain by relatively massive rocks, particularly the Archean crystallines, the surface has been left mam- millated with rocky knobs, which doubtless were the unattacked cores rising into the pre-Glacial zone of disintegration. These are separated by the similar inverse forms, now for the most part occupied by swamps. In the Archean borders of the Felch Mountain area, where the glacial cover was originally thin, the periodical fires that have followed lumbering operations have burned out the organic matter from the soil and so loosened it that, on the steeper slopes, it has been entirely washed away and the rock surface laid bare. The hummocks and bowls are generally elongated east and west, which is the direction both of the gneissic foliation and of the ice movement. The elevations rise, often with steep, smooth walls, for 5, 10, 20, or even in some cases 60 feet, above the intervening depressions. The latter hold muskeg to the rims. In the wet season they fill with water, which overflows to the next bowl below, but permanent lines of minor drainage, here as elsewhere in the Archean areas, are very infrequent. Over most of the area, however, the ice has spread a sheet of till, and has here and there deposited the materials swept along in the subglacial streams in characteristic complexity of form and grouping. The more prominent elevations are, in fact, deposits of modified drift, although occasionally TOPOGRAPHY. 3i3y5) small rock masses like Michigamme Mountain, which is composed of mate- rial that offers a most stubborn resistance to all degrading agents, reach an elevation of 100 to 200 feet above the general level of the surrounding country. The fact that the name “mountain” has been applied to hillocks of this order by the surveyors and woodsmen, who have the widest knowl- edge of the Upper Peninsula, conveys perhaps the clearest idea of the generally level character of the surface. While the details of the topography are thus mainly glacial in origin, the broader features of the next order of importance have often clearly been determined by the presence of the more resistant rocks. The large structural domes of the Archean, which are such characteristic geological features, are also indicated by a general upward swell of the surface of the areas which they occupy. The topographical transitions at the margins of these swells are frequently abrupt, and sometimes for considerable distances are marked by scarp-like slopes in the granites, caused by the almost ver- tical contacts with the softer Algonkian formations. Considerable portions of all three of the Archean ovals in the northern part of the district display this slight topographical prominence. Marginal scarps are particularly well shown in the oval west of Republic, in sees. 19 and 30, T. 47 N., R. 30 W., and along the south side of the oval which lies between the Fence and Deer rivers, near their junctions with the Michigamme. The more impor- tant bodies of greenstone also are generally expressed by a noticeable degree of elevation. Thus the great intruded sheets folded in with the Lower Mar- quette series in secs. 24, 25, and 36, T. 47 N., R. 31 W., give rise to long broad ridges that closely follow the changes in the strike. But in all these cases the topographical emphasis is very slight, and the plain as a whole may truly be said to maintain its general slope with practical indifference to the weather-resisting differences in the underlying rocks. These broader swells of the harder rocks are separated by broad, slightly lower-lying plains, in many of which a valley character is still dis- tinctly recognizable in spite of the fact that they especially have been favored with deposits of modified drift. The present drainage, in its main lines, largely follows these older valleys, although much confusion, which is especially noticeable in the details, has of course resulted from their: partial choking by the drift. 334 THE CRYSTAL FALLS IRON-BEARING DISTRICT. DRAINAGE. Nearly all the surface water of this district finds its way to Lake Michigan through the Michigamme and the Sturgeon rivers, which are inde- pendent branches of the Menominee—the largest river flowing into Lake Michigan from the west. A few square miles along the eastern boundary, however, are tributary to the Ford, which flows into Green Bay north of the Menominee. Of these the Michigamme drains by far the largest part of the district. This river heads in Lake Michigamme, which it leaves in sec. 9, T. 47 N., R. 30 W., near the northeast corner of the area shown on the general map (PI. II), at an elevation above the sea of 1,580 feet. Thence it flows for 8 miles southeast to Republic, in a synclinal valley cut - out of the soft schists of the Michigamme formation. This valley, which is nearly a mile wide at the northern end and less than half as wide at the - southern, is bordered on both sides by the harder Archean granites, which rise with rather steep slopes to the general level of the plain. Throughout the length of the valley the river flows over glacial drift, but at Republic, - where the soft rocks come to an end, it breaks across rocky barriers in a succession of rapids, and continues first nearly due south (leaving the dis- trict covered by our map), and then flows southwest over glacial deposits, which completely mask the bed rock for 10 miles. South of the Archean oval, which occupies the western part of T. 44 N., R. 31 W., and the east- ern part of T. 44 N., R. 32 W., the limestones and slates of the pre- Cambrian are again exposed, and over these the Michigamme flows in close conformity to the general strike as far as the range line. In the southern sections of 'T. 44 N., R.31 W., the Michigamme receives two tributaries from the north—the Fence River, which comes from the eastern side of the Archean mass just mentioned, and the Deer River, which comes from its western side. The headwaters of the Deer and of the west- ern branch of the Fence flow through the same section (21) in T. 46 N., R. 32 W., north of the Archean oval, but farther south they diverge to an extreme distance of 10 miles, and afterwards converge so that their points of junction with the Michigamme are but 4 miles apart. The area thus inclosed is broadly concentric with the Archean oval. In the case of the Fence, at least, the river is placed within a wide depression coincident with the softer stratified rocks of the Algonkian, and follows very faithfully their DRAINAGE. 33) general strike. Deposits of glacial sand and gravels are very abundant within this valley, and for these the river often swings aside across the strike for a mile or more. In secs. 21 and 29, T. 45 N., R. 31 W., and in sec. 10, T. 44 N., k. 31 W., excellent rock sections are afforded by such digressions. The old valley between the two Archean ovals west of the Republic tongue (see Pl. IIT) is on the south entirely filled with glacial gravels to the level of the old divides, and the large brook known as the east branch of the Fence is diverted to the till-covered western of the two Archean ovals. The valley is clearly indicated, however, by an interesting series of lakes, of which Squaw, Trout, and Sundog, each about 1 mile in length, are the most considerable. The area drained by the Sturgeon lies in the extreme southeastern part of the district, wholly within the marginal fringe of sandstone. The rela- tion of its course to the geology is known in detail only within portions of the Felech Mountain range. This it first enters in the northern portions of sees. 35 and 36, T. 42 N., R. 30 W., in a loop into the Algonkian, from the northern Archean margin, to which it again returns. Five miles farther east it crosses the trough from north to south, transverse to the strike of the Algonkian formations, to the contact with the southern Archean mass. It follows this contact eastward for 2 miles, and then strikes southward across the Archean to the Menominee River, not again returning to the Felch Mountain range. The river valley in the Felch Mountain range is very distinct, and where bordered by Potsdam outliers is rather deep, with pre- cipitous banks. It is but slightly affected by drift deposits. Its course shows an almost complete disregard of the structure of the Algonkian and Archean rocks, and so has the usual characters of a superimposed stream. The Michigamme River, as was early noted by Pumpelly, has practically no eastern branches within this district. The Escanaba and Ford rivers, which reach Lake Michigan directly, and the Sturgeon, which joins the Menominee below the mouth of the Michigamme, all head within 2 or 3 miles of the latter, the course of which is transverse to their general direc- tion. ‘The Michigamme thus flows along the eastern edge of its drainage basin. This fact—the most striking in the general distribution of the streams of the district—is the result of causes which, in part at least, go back to very remote geological periods. “% Ole ANNIE OTe IIL MAGNETIC OBSERVATIONS IN GEOLOGICAL MAPPING. SECTION I. INTRODUCTION. As has been said already, the area in which our work was done is largely drift covered, to somewhat varying but usually considerable depths; the mantle on the whole is so evenly spread that outcrops of any rocks except those belonging to the Archean are in many sections few and scat- tered and sometimes are almost entirely lacking over whole townships. Under these circumstances, and since also the pre-Cambrian rock structure is complex, even a general outlining of the old formations would be. impossible by the usual geological methods, and if we were restricted to these there would be no alternative but to map most of the territory as Pleistocene. It happens, however, that the Algonkian rocks of Michigan contain a large amount of magnetite, which is known from observation in the developed ranges to be characteristic of certain geological formations. It undoubtedly occurs in more or less amount in all the sedimentary rocks and is also present, sometimes in considerable quantities, in rocks that are not sedimentary, as, for example, around the margins of the old intrusive diorite bosses. But generally speaking, its occurrence in large quantities is confined so closely to definite geological formations, in which it is found in characteristic association with certain other minerals, or to horizons within those formations, that it can be guardedly used in identifying them, and in tracing them from localities where they outcrop through areas in which they are buried. This use is not only justified, from an empirical standpoint, by the presumption in favor of analogies to which no exceptions are known, but it has a rational ‘basis, in the view of the late Professor Irving,” which is This chapter is abridged from a paper of the same title presented at the Colorado meeting of the American Institute of Mining Engineers in September, 1896. 2 Classificatior of early Cambrian and pre-Cambrian formations, by R. D. Irving: Seventh Ann. Rept. U.S. Geol. Survey, 1888, pp. 451-452. 336 MAGNETIC OBSERVATIONS. 337 steadily gaining ground, that at least much of the iron of this magnetite was originally buried in the same formations in which it now occurs, through the agency of organic life. From this point of view the magnetite is in a cer- tain sense a fossil, but with the important practical advantage over other organic remains, that it need not be dug up in order to prove its existence. These magnetite-bearing rocks always produce disturbances in the compass-needles held in their neighborhood. By a systematic location and comparison of these disturbances the position of the rocks which produce them can be determined with a considerable degree of precision, even when they are deeply buried. Besides their position on the map, the magnetic observations may, and often do, indicate certain other geologically impor- tant facts, such as whether the rocks are flat lying or highly tilted, the direction of strike and dip, and, in some cases, the depth to which they are buried. The methods employed in the field work were based on those described by Maj. T. B. Brooks,’ who perfected the dial compass and pre- dicted the importance of magnetic methods in geological mapping; but the results reached in interpretation were gradually developed in the progress of this work, as we were daily brought face to face with phenomena which called for explanation. . It must be clearly understood at the outset that in the iron ranges of the south shore of Lake Superior magnetite is rarely concentrated in large bodies, and that, in fact, its known occurrence as such is restricted to a small part of the western Marquette district, where in one producing mine it now forms practically the whole product and in another a variable but usually important part of the whole. It is therefore well understood in the Upper Peninsula that disturbances of the magnetic needle, however great, do not mean workable deposits of magnetite. Whatever significance such disturbances possess is stratigraphical, and properly interpreted may lead to discoveries of rich ore, other than magnetite, in formations to the position and attitude of which the attractions may furnish a clew. But it may be asserted as a general proposition, the essential truth of which has been established by the experience of many years, that in the region referred to magnetic disturbances usually mean that magnetic iron ore in a workable deposit does not exist in the area of disturbance. 1 Geological Survey of Michigan, Vol. I, Part I, 1873, Chapter VII. MON XXXVI 22 338 THE CRYSTAL FALLS IRON-BEARING DISTRICT. SECTION II. DESCRIPTION~OF THE MAGNETIC ROCKS. The magnetic rock of the Lower Huronian series of the western por- tion of the Marquette area, which is of special importance to notice, since it forms one of the chief horizons of reference to which our work is tied, is the Negaunee iron formation. It is finely exposed at the south end of the Republic trough; but farther north has been greatly reduced in thick- ness, or locally cut out altogether,’ by the Upper Marquette denudation, and, where present at all, is usually drift covered. This rock often possesses a very distinct banding, caused by the alter- nation of layers, in which one of the constituent minerals predominates over the others, sometimes, indeed, to their total exclusion. In the lower part of the formation, quartz and griinerite constitute the bulk of the rock, with magnetite scattered soniewhat indiscriminately through them. Higher up, the magnetite and quartz relatively increase, until near the top, but below the jasper, the griinerite goes out almost entirely, and the rock consists of quartz bands, heavily charged with magnetite, in alternation with bands of nearly pure magnetite. In the Negaunee formation, as exposed at Republic, the magnetite therefore occurs concentrated in some of the parallel bands and disseminated through the others. In the same district there is another much less prominent locus of mag- ‘netite at and near the base of the Upper Marquette or ‘‘shanging-wall” quartzite. Along the strike of this zone, which is of small thickness, the distribution of magnetite is very irregular; and for this and the additional reason that when the magnetic portion of the Negaunee formation comes up to it the disturbances which it produces can not be discriminated from those produced by the latter, the position of the plane usually can not be inferred. In the Menominee district and its extensions there are two horizons in the lower series, characterized by the presence of magnetite. The lower of these is not known to outcrop, but it occurs somewhere near the junc- tion of the dolomite and the underlying quartzite. The magnetic disturb- ances due to this formation are feeble, but they are quite persistent in the Felch Mountain area, and have thrown some light on the geological structure. 1The Marquette iron-bearing district of Michigan, by C. R. Van Hise and W. 8. Bayley, with a chapter on the Republic trough, by H. L. Smyth: Mon. U.S. Geol. Survey, No. XXVIII, 1897, pp. 531, 537, —- MAGNETIC OBSERVATIONS. 339 The other formation which produces disturbances is that which I have correlated with the Negaunee formation and named in a former paper’ the Michigamme jasper, but which is here renamed the Groveland formation. This rock, while varying a great deal in character, is generally much like that magnetic phase of the Negaunee formation in which the griinerite is rare or absent. From the fact that it now survives for the most part only in shallow and shattered synclines, it often lacks the regular banding; and hematite is always present in greater or less amount. The relative propor- tions of the two iron minerals vary along the strike also. The rock as a whole, however, is very magnetic, but not so strongly so as the Negaunee formation in the Republic trough. In the Felch Mountain range there is still a third magnetic formation, which seems to overlie unconformably the lower series, and is therefore provisionally assigned to the Upper Huronian. This formation consists of ferruginous schists, interstratified with layers of ferruginous fragmental quartzite. It is generally much less highly inclined than the magnetic rocks of the lower series as well as less rich in iron, and the disturbances produced by it are correspondingly small. Besides these rocks of sedimentary origin, with which this paper prop- erly deals, it may be mentioned that along the Fence River there is a considerable area of metamorphic eruptives, which are often exceedingly magnetic. These are restricted to a definite geological horizon, within which the magnetic disturbances are remarkable for their complexity and irregularity, no doubt as the result of a very irregular distribution of magnetite and of the formations which chiefly contain it. The rocks in portions of this belt outcrop freely, and the disturbances can therefore easily be assigned to the proper causes. SECTION III. THE DISTRIBUTION OF MAGNETISM IN THE MAGNETIC ROCKS. Magnetite occurs, therefore, in these Algonkian rocks in different ways. In some instances it is mainly concentrated in nearly pure parallel layers; in others, it is more or less evenly disseminated through non- magnetic material; and still again it is present in both forms. Moreover, ‘Relations of the Lower Menominee an Lower Marquette series of Michigan (Preliminary), by H. L. Smyth: Am. Jour. Sci., Vol. XLVII, 1894, pp. 217, 218, 223. 340 THE CRYSTAL FALLS IRON-BEARING DISTRICT. these rocks have all been folded, more or less strongly, at more than one period; and wherever they are exposed, they are seen to be inclined to the horizon, often at high angles, and to be traversed by intersecting sets of joint-planes and cleavage-planes, some of which always cut the bedding, and often have been the seat of the development of secondary minerals. By the crossing of these various surfaces, the rocks are divided into small unit masses, at the boundaries of which there is either an actual physical parting or a break in the continuity of the magnetite. It is well known that when a bar magnet is broken and the severed ends are again joined, the two pieces do not unite to form one magnet, but -remainas two. It may be conceived, therefore, from the manner of distribu- tion of the magnetite, and the secondary partings existing in these rocks, that their magnetism is seated in an enormous number of small separate magnets, at least one for each of the physically distinct unit volumes. It is a fact of observation, as will appear hereafter, that the upper surfaces of these magnetic rocks invariably attract the north end of the compass needles and, of course, repel the south end. From this it must be inferred that the small magnets are generally similarly oriented, and have their north ends, which would repel the north end of the compass needle, pointing downward, and their south ends, which attract it, pointing upward. As this is the arrangement that would result from induction from the earth’s magnetism, it can be concluded further (as, of course, might be assumed) that these rocks are magnetic from the earth’s induction. It is also well known that when several bar magnets are joined in line at opposite poles, the effect upon a compass needle within the range of influ- ence is nearly the same as if the jomed magnets were replaced by a single magnet of the combined length. For each member of the pairs of inter- mediate poles, one attracting and the other repelling, is about the same distance away, and their effects so balance each other. he result, there- fore, is to leave one pole unchanged in position and to remove the other to the end of the last magnet added. If enough magnets are added, the final result is to carry the moving pole so far away that it has no appreciable influence upon the needle. This is a condition which, from the distribution of the magnetite and the parting surfaces which run through the magnetic rocks, must always be realized more or less completely. It is a necessary consequence of such an arrangement of the small magnets that, in the case MAGNETIC OBSERVATIONS. 341 of a thin sheet of magnetic rock lying at a low angle of dip, the buried north poles would not be much farther removed than the upper south poles, and consequently the compass needle should be relatively only slightly dis- turbed. This is precisely what is found to be the case. Thus there is firm ground for the conception of the magnetic rocks as made up of sheaves of small magnets, all similarly oriented in a general way and all having their south poles upward at or near the rock surface, while the effective north poles, by the continual addition of similarly ori- ented sheaves below, are carried down, when the rocks are vertical or nearly so, to such depths that their influence is greatly diminished or altogether imperceptible. In equal small areas the individual magnets are no doubt of very unequal number or strength. This can be proved by holding a swinging needle close to the surface of a magnetic rock, shifting its position without moving it out of the parallel plane and observing the changes in the pointing. These are almost always large and are undoubtedly due to the variations in strength of small areas of the upper poles. In consequence of the law of magnetism, by which the attraction (or repulsion) varies inversely as the square of the distance, the areas immediately surrounding the needle are very much more important factors in the resultant than those farther removed. When the needle is held higher up, or, what is the same thing, the magnetic rock is buried, the effects are much more regular, since a larger number of the unit areas enter into the resultant with equal weight due to equal distance, and the extremes of individual variation are lost in the general mean. Since successive magnetic cross sections over buried rocks show on the whole a great degree of regularity, we can finally con- clude that the magnetic force of these rocks is seated in an immense, prac- tically an infinite, number of small magnets, which furnish free magnetism at the upper and lower bounding surfaces of the magnetic formation, and that on the average there is about the same number, of about the same ageregate strength, or, in other words, equal intensity in equal areas of these surfaces, if the areas are taken large enough. SECTION IV. THE INSTRUMENTS AND METHODS OF WORK. The instruments used in this work are simple and well known. The dial compass is an ordinary compass, carrying a 23-inch needle swinging inside a circle graduated to degrees, which is further supplied with a grad- 342 THE CRYSTAL FALLS IRON-BEARING DISTRICT. uated hour circle. It is.therefore a portable sundial. The gnomon is a thread, which is attached at one end to the center of graduation of the hour circle near the rear sight and at the other to a point in the forward sight so taken that the angle made by the thread with the plane of the hour circle is equal to the latitude of the place. When this instrument is leveled and set up in the meridian on a sunny day, the thread will cast a shadow on the hour eircle at the correct apparent solar time, from which mean time may be determined by applying the equation of time. Conversely, if it is so set up that the shadow of the thread falls on the correct apparent time, the sights of the instrument are in the true meridian. In this position the declination of the* horizontal needle may be read off from the graduated circle. At work, this instrument is mounted on a light Jacob's staff, or it may be held in the hand. The Jacob’s staff, although often inconvenient to carry, is preferable, as with it the readings are all taken at the same height above the ground and the leveling is more exact and steady. In a correctly constructed instrument, with good time, the readings may be made to half a degree. Correctness in the time, however, is indispensable to good work, and this is best secured by keeping a standard watch in camp and referring the working watches to it daily. The dip needle needs no description. In geological work that form known as the Norwegian, in which the needle is pivoted on a universal joint, is not so useful as the type in which the needle is rigidly confined to one plane. In taking the readings, this plane in which the needle is free to swing is made to coincide with the vertical plane determined by the pointing of the horizontal needle. The circle is graduated to single degrees, and with skillful work the readings are reliable to one or two degrees. It may be added that the south end is weighted, in order, either partly or completely, to balance the vertical component of the earth’s force. It was found better not to balance it completely, but only to such an extent that the north end of the needle would dip about 10° (the eraduation ZerO being horizontal) in an area removed from local disturbances. It is no doubt desirable that all the dip needles used in the same work should be brought to approximately the same index error, in order that the readings may be more directly comparable. In practice, however, it was found quite impossible to keep our three needles in unison, on account of the rough usage to which they were unavoidably subjected. As, however, the form MAGNETIC OBSERVATIONS. 343 of the dip curves is real-y the subject sought, and since these, in the pres- ence of considerable disturbances, are sensibly independent of small differ- ences in the index error, it is not indispensable that the needles should be exactly together. These instruments are simple, and, of course, do not give precise results. But the observations are rapidly and cheaply made, and to a sufficient degree of accuracy for the end in view. It may be stated again that the object is to detect and compare relative magnetic disturbances, and to find out the bearing of these disturbances on the distribution and attitude of the rocks which produce them. For this purpose the instruments are exceed- ingly well adapted. The field work was carried out by parties of two men each, one of whom, a skilled woodsman, carried along the line and observed the hori- zontal needle, while the other read the dip needle, kept the notes, and attended to the geology. According to the general plan of the field work, -a series of parallel lines was run either north and south or east and west across each section. The direction of the lines of travel was chosen so as to cut the strike of the rocks at the largest angle. The probable direction of strike for each day’s work could be inferred in advance from what had gone before. If it were more nearly north and south than east and west, the traverse lines were run east and west, and vice versa. These directions were in many cases not the most desirable for the magnetic work alone, but the choice was practically limited by the lines of the United States Land Survey, which give for each square miie eight points of departure (at the four corners and four quarter posts of each section); which are generally identifiable on the ground. On these lines of travel the imstruments were read at various intervals, from 5 to 10 or 100 paces, depending upon the local complications. “The intervals between the lines varied from one- sixteenth to one-tourth of a mile, and were determined not only by the magnetic complications, but by the character of the surface, it being especially desirable that the ground should be so closely covered that no outcrop could escape detection. The distances along and off the lines of travel were measured by pacing. The general accuracy of the pacing is remarkable, and is essentially within the platting error of the scale of the maps. The average closing error for August, 1892, during which about 100 miles of traverse lines were run, was 20 paces per mile, or 1 per cent. 344 THE CRYSTAL FALLS IRON-BEARING DISTRICT. Two-thirds of the errors averaged 10 paces per mile, or 1 in 200, while the maximum was 1 in 30. But this was better than the average for the season. The observations at each station consisted in a reading of the horizon- tal and dip needles. When there was no local magnetic disturbance, the horizontal needle would come to rest in the magnetic meridian, which in this region is about N. 2° to 8° E., or almost coinciding with the true merid ian. The dip needle, when held in the same meridian, would indicate the index error. When, however, disturbing material was present, the horizon- tal needle would point-to the east or west of the magnetic meridian, at an angle determined by the direction of the resultant of the horizontal com- ponents of the earth's and the local forces. The dip needle would come to rest in the same vertical plane, at an angle with the horizon determined by the amount and direction of the three forces, the whole pull of the earth’s force, the whole pull of the local forces, and the balancing weight, and in general would show a downward deflection. After making and recording the set of observations at a station, the party proceeded to the next, and so on to the end of the day. At the end of each day, or as soon as possible afterwards, the day’s work was platted on a large-scale map, on which the readings of the horizontal needle were represented by short arrows drawn through the stations, turned east or west of the true meridian, as the case might be, and carrying the amount of declination written at the arrow point. The dip observations were laid off to scale immediately below the stations, measuring all from the same horizontal line, and the points thus established were connected by a free-hand curve. SECTION VY... FACTS OF OBSERVATION AND GENERAL PRINCIPLES. 1. OBSERVED DEFLECTIONS WHEN THE STRIKE IS NORTH AND SOUTH AND THE DIP VERTICAL. If a magnetic rock, striking north and south and dipping vertically, is crossed by an east-and-west traverse, it is found, as the disturbing belt is approached, say from the western side, that the horizontal needle points toward the east of north, and that this easterly pointing gradually increases to a maximum. Continuing east from the maximum point the eastward declination decreases, and soon a station is reached at which the needle points due north. Still farther east the declination changes to west- ward, and soon thereafter reaches a westward maximum, beyond which eS MAGNETIC OBSERVATIONS. 345 again the westward pointing in its turn gradually decreases, until finally the needle reaches its normal eastward declination, after passing through a second zero. The dip-needle readings at the same stations generally increase slowly at first, and then rapidly, and soon reach a maximum at the first zero point between the converging arrows; beyond this to the east they decrease correspondingly, so that the dip curve is symmetrical east and west of the maximum ‘These statements will be made clear by a reference to fig. 15, which represents an actual traverse in T. 45 N., R. 31 W. 2. DEFLECTIONS OF THE HORIZONTAL NEEDLE. It is evident that im crossing a rock belt which stretches away indefi- nitely in both directions, only a limited part of it will affect the readings on .) mM ee ih © 1 18 Tl 84 G 1 8B wf OMe I Tees me fe ios 1s WwW i= -—Dip Curve —_— TET ee eon sy fF ~, 7 ~. of ~. of SS Pa ~. -— _S So ™~, a SA ea ~ a —— Fig. 15.—Magnetic cross section in T. 45 N., R.31 W. a given cross section. Since the pull of the poles of a magnet on a com- pass needle diminishes with the square of the distance of separation, it follows that the limits to the material that would noticeably disturb com- paratively insensitive instruments would soon be reached. If we consider for the moment only the horizontal components, and call the distance a (fig. 16) at which the needle would respond to the attraction of material possessing the magnetic force of that with which we are dealing, then at any station, P, the material inside a circle drawn with P as a center and radius @ (shaded in the figure) would exert force on P, the material outside would not. If the circle drawn from a station, P’, does not reach to the magnetic belt, the needles at P’ will not be disturbed. For reasons of symmetry, it is seen that the attraction of the magnetic 'The actual distances at which disturbances of the needles can be detected are exceedingly variable, since they depend (as will be shown hereafter) not only upon the lithological character of the magnetic formation, but also upon its strike, dip, thickness, extent, and nearness to the surface. One formation in which all the conditions are exceptionally favorable distinctly deflects the dial- compass needle at a distance of 34 miles. 346 THE CRYSTAL FALLS IRON-BEARING DISTRICT. belt would act along the line P N, drawn through P perpendicular to the strike of the rock. Since there is as much material on one side of this line as on the other, the components perpendicular to it will balance each other, and the instruments at P will be affected exactly as if all the attract- ing material were concentrated along this line. The horizontal needle will take a position in the line of the resultant of the two forces which act upon it, namely, the horizontal component of the earth’s magnetism (which acts in the line of the magnetic meridian) and the horizontal component of the material within the circle of attraction (which acts along the line P N). The force which de- flects the needle from its general local direc- tion is the component along P N, and it is mk lz: A ce — TS x evident that the great- Se pees I Nar er this component the He ! Ne \ greater will be the de- gee we L ! ‘ flection of the needle, Alou sate a BEER ee eae ae sar Sees since the direction in ead a which it acts always 7 \ remains the same at N Nae all stations for any IN N given direction of > Sa Sao see we strike of the rock. R mine For if £ is the strike Fig. 16.—Circles of attraction. of the rock measured from the north, and H and H’ are the horizontal components of the earth’s and the rock force respectively, it is readily shown that 6, the angle of detlee- tion of the horizontal needle at any station, P, is given by the equation: ie matte diame eaten hotel (CL) H sin fo H’ From this equation it is easily seen that, no matter how great may be the horizontal component of the force of the magnetic rock, the horizontal needle can not be deflected past the normal. MAGNETIC OBSERVATIONS. 347 As P moves toward the magnetic belt the horizontal component at first increases, and with it the westward deflection of the needle. Finally, the maximum westward deflection is reached, beyond which the needle begins to return; it is evident, therefore, that at this point the horizontal component has reached a maximum value. 3. DEFLECTIONS OF THE DIP NEEDLE. The balanced, dip needle G. e., without index error), in an area of no local disturbance, is in equilibrium under the action of two couples, - namely, the vertical component of the earth’s magnetism and the added weight. When displaced from the position of equilibrium, the horizontal couple re- stores it. In fig. 17 let PP be a. balanced dip needle which has been displaced Fig. 17.—The forces acting on the dip needle. through the angle a. At the two poles the attraction and repulsion of the earth’s magnetism may be resolved into horizontal and vertical components, H and V. Taking moments about C, we have, if a=0, the needle in equilibrium under the couples, V . 2b—mg. a=0, where 2b=PP, mg=the added weight, and a its distance from the center. If this needle, so balanced, is carried to a station within the influence of a magnetic rock, its dip will be determined by the composition of the new forces with the old. The vertical plane will be that in which the hori- zontal needle points at the same station. The equations above give us a ready means of determining the angle of dip in terms of all the forces. Suppose the needle finally comes to rest at the angle a with the hori- zontal (the north pole being depressed). ‘Then V,. 2b. cos a—mga cos a—H,.2b.sma=0, . . . (2) where H, and V, signify the resultants of the horizontal and vertical com- ponents of the earth’s and the local force. 348 THE CRYSTAL FALLS [RON-BEARING DISTRICT. Equation (2) is easily reduced to 2b. V.— mga, POR MSE . . . . . . . (3) tan 4= If, however, the dip needle is not balanced, but has, where there is no local disturbance, a constant index error 6 (measured from the horizontal), it is readily seen that 2b. V—mga tan Gi In an area of local disturbance the angle of dip @ is given by equation (3). Since V and V’ always act in the same line, V,=V+V". Substituting this and the value of mga from equation (4), equation (3) becomes V' +H tan 6, =e Sa Paes.) tan a= If the index error is 6’, and the corresponding deflection a’, we have tana + V' +H tan9 fama 2 Ve2 Seta @/ Therefore at the same station, the greater the index error the greater is the angle of dip in the same or two similar instruments. It is also evi- dent that the greater the vertical component of the pull of the rock, the less will be the difference between the deflections in the two cases. From an inspection of equation (5) it is seen that tan a=oo, or a=90° only when H,=0. 4H, is, in general, given by the equation leh an) EPR IEEE 118 iin 5, where # is the strike of the rock measured from the north. H,, can there- fore equal zero only when £ ==, or the rock strikes east and west, and at the same time H’ is numerically equal to H, and acting in the opposite direction. Dips of 90° can not occur in other cases, no matter how strong. Ee MAGNETIC OBSERVATIONS. 349 the magnetic force of the rock may be. It is also evident that in general H, has its minimum value when H’'=—H sin ~. When the rock strikes north and south or 6=0, H, is a minimum when H’=0. 4. HORIZONTAL AND VERTICAL COMPONENTS WHEN THE MAGNETIC ROCK DIPS VERTICALLY. If we assume that the magnetic rock has a uniform strike in any direc- tion, a vertical dip and a surface width or thickness equal to 2a, it is easy to show that the horizontal and vertical components of the rock force are given by the following equations, where a is the horizontal distance of the station of observation from the middle plane of the formation, @ is the depth of surface covering, assumed to be uniform, and @ is a constant. aU aap Se CEO Sa ea Phe SRN iss PP pig acer (Gp) M30 Venn =e nt Se = } tan h tan h dime eee 5 alk cend CD) In equation (6) a = 0 when x = 0; therefore a point of no deflec- tion of the horizontal needle is found vertically over the middle point of the magnetic rock. It is also evident that at corresponding stations on opposite sides of the middle point, the horizontal components are equal, but act in opposite directions. To obtain the points of maximum or minimum values of the horizontal component, we differentiate the right-hand side of equation (6) with respect to w, and place the result equal to zero. This gives ON TPIS 0 2 on es 8 og oo (8) which determines two points, at equal distances from 0 on opposite sides of the rock, at which the horizontal component has maximum values. Writing for « the measurable distance d, and squaring, we have CVE G2 fol iene oh oe Behe ao) The thickness of the magnetic formation is therefore always less than the distance between the points of maximum horizontal deflection, except when =0, or the rock is uncovered, in which case the thickness and sepa- ration of the maxima are the same. 350 THE CRYSTAL FALLS IRON-BEARING DISTRICT, By differentiating the right-hand side of (7), with respect to 2, it is easy to show that V’ has a maximum value when v=0. When the rock strikes north and south, this also corresponds to a minimum value of H,, as has already been shown; and, therefore, by a reference to equation (5) it is readily seen that a point of maximum dip coinciding with a point of no horizontal deflection is in that case found over the middle plane of the - buried magnetic rock. Where the strike is inclined to the meridian, the points of maximum dip and zero deflection will not coincide, since the maximum value of V’ does not occur at the same station as the minimum value of H,. As has already been shown, H, is a minimum when H’=H sin £ ( being the angle of the strike), and this is in general on the side of the rock on which the angle made with an east and west traverse is obtuse. ‘The point of maximum dip will be situated on the same side of the rock between this station and the point of no horizontal deflection, and will approach the latter as the strike approached the meridian, and also as V’ increases relatively to Jel’, With strongly magnetic rocks the points of no deflection and maximum dip practically coincide on maps platted to the scale of 4 inches to the mile, except where the strike is nearly east and west. 5. HORIZONTAL AND VERTICAL COMPONENTS WHEN THE MAGNETIC ROCK DIPS AT AN ANGLE. , Under the last heading it was assumed that the magnetic rock dips vertically, and that it continues indefinitely downward at this angle. In consequence of this assumption, and also. of the conception of the manner in which magnetism is distributed through magnetic rocks, it has been con- cluded that the north poles of the rock, which repel the north end of the compass needle, are situated so far below the surface that their effect may be neglected. Therefore we have taken into account only the south poles, which are situated at the rock surface. In the case of rocks which do not dip at high angles this assumption can not safely be made, and the influence of the bottom poles must be taken into account. Since the force of these poles acts in opposite direc- tions from that of the upper poles, and since they are more deeply buried, it would seem that their influence in general must be to diminish the total force which acts upon the needles at any station, and therefore that the es MAGNETIC OBSERVATIONS. “135i deflections both of the horizontal and dip needles caused by the same rock should be less in amount, ceteris paribus, where that rock dips at a low angle than where it dips at a high angle. In the course of the field work certain peculiar deflections of the needles were encountered in traverses across rocks dipping at moderate or low angles. These were not thoroughly understood at the time, but the cause was believed to be connected with the angle of dip of the rock. For example, it was found along traverses crossing certain north-and-south- striking rocks, which were known to ‘have a westward dip that may have been either high or low, that the two points of maximum deflection of the horizontal needie were not situated at equal distances from the point of no deflection between them, but that the distance of the western maximum was much the shorter. It happens in this region that no east-dipping rocks occur which are so far removed from other magnetic formations as to be out of range of their possible influence, but, so far as they go, traverses across these showed that the nearer maximum was situated on the eastern side of the point of no deflection. It therefore seemed probable that the cause of the inequality in the distances from the zero point to the maxima was the dip of the rock, and that the dip was in the direction of the nearer maximum. If the magnetic formation has a surface width =), is uniformly buried to the depth h, and dips at the angle 4, then, if A = tan 4, it may be shown that the horizontal and vertical components at any point P, the hori- zontal distance of which from the lower edge of the formation is 2, are given by the following equations: Ho ye ee QA @~ 14d © h?+(a—b)? 142? _, «—b—iXh tan 2A An—Ab+th Axth tan . (10) NE — tan tant @ h h A Wo? 2 mee SP y 1427 hA+A)—Ad+a) _ ,. 1 D+Ah—% } tan (1-2) h—a aus ear) + Bia) THE CRYSTAL FALLS IRON-BBARING DISTRICT. If A=, and the coordinates are referred to axes in the middle of the rock, these equations reduce to equations (6) and (7). By differentiating the right-hand side of equation (10), placing the result equal to zero, and solving for , the positions of the stations at which H’ is a maximum may be determined. This gives: ADEA (MPLA HLA) C= a1 + om east Meera wena ali) Calling the difference of the roots, or the measurable distance between . ue c 2a é the maxima, 2d, and substituting for } its value ea oY being the true : y) thickness of the rock, we have: 2 2 Pie ER ie ied hg ete 85, (LSS) > sin?a For rocks of high dip, therefore, the distance between the maximum points is but little greater than it would be were the dip vertical, and it increases inversely as the angle of dip. A general algebraic determination of the points at which H’ is 0 and V’ is a maximum is impossible, since it involves the solution of equations of a degree higher than the fifth. However, by assuming numerical values : : H’ Ww! for A, h, and a (or b) curves expressing the relations between ae and = and xz can be plotted, from which the maximum and zero points can be deter- mined in any desired number of special cases. Let us first assume that A=3 (or that the rock dips at an angle of about 70° 34’), h=2, and a=6. The ordinates to the curves of fig. 1, Pl. XLVII, : a det! Wi ; give the values of — and — corresponding to different values of 2 ‘The @ (cy) / ordinates to sal do not represent the deflections 6 of the horizontal needle @ from the meridian, but quantities that are connected with those deflections by equation (1). The deflections, however, vary as H’ varies, and will have maximum and minimum values at the same points. From this figure it appears, first, that the nearer maximum is situated on the dip side of the rock; secondly, that the point of no deflection is not over the middle plane, but is nearer the upper edge; thirdly, that the hori- zontal force of the rock is numerically less at the nearer than at the more ES ee — ——EOoOo U. §. GEOLOGICAL SURVEY MONOGRAPH XXXVI PL. XLVIl SS (3) Hf RELATIONS OF MAGNETIC BEDS TO VARIATION AND DIP. " MAGNETIC OBSERVATIONS. 353 distant maximum, and, fourthly, that the distance between the maximum points is nearly the same for the inclined rock as for the vertical. In PI. XLVII, fig. 2, the constants have the same numerical values as before, except 4, which now =4 instead of 2. The rock is thus buried to twice the depth of the former case. The same conclusions are true for this case as for the first. The zero point is still nearer the upper edge of the rock, and the maxima are farther apart. : | Let it next be assumed that A=0.5 (or that the rock dips at an angle of about 26° 34°), h=2, and a=6. These data lead to the curves of Pl. XLVI, fig. 3, in which, as in the case of the rock ot higher dip, the maximum points are unsymmetrical to the point of no deflection, the nearer lying on the dip side. In Pl. XLVI, fig. 4, we have a rock of the same thickness and dip at a depth h=4; and the same conclusions hold true. From these four curves, which represent formations dipping at high and moderately low angles, and buried to depths which are in the one case small and in the other great, relative to the thickness, it is probably safe to draw the following general conclusions: (a) The direction of dip of a magnetic formation is toward the nearer and (for north-and-south-striking rocks) the numerically smaller maximum. (b) The point of no deflection between the converging maxima is not situated over the middle plane of the formation, but is nearer the upper edge. But with increasing depth and diminishing angles of dip, this point may pass beyond the upper edge. (c) With slightly inclined rocks, for moderate depths of surface cover- ing, the disturbances are spread out over a much wider zone on each side, and the maxima are less sharp, particularly the maximum on the dip side. Under these circumstances irregular and anomalous deflections would be expected in practice, as will be seen in the following sections. ‘(d) The curves of the vertical component show maximum values near the zero value of the horizontal component only in the case of the rock of high dip. In the case of the rock of lower dip, the vertical component has a negative value, or is directed upward over a wide zone on the side of the rock opposite to the dip side. Over this zone the readings of the dip needle will be less than normal, or even negative if V’>H tan @. This is in accordance with the facts of observation. MON XXXVI 23 304 THH CRYSTAL FALLS IRON-BEARING DISTRICT. It is also interesting to determine the relative values of the horizontal components at the two maximum points, for different angles of dip, and for different ratios of h to a—that is, for different depths of burial. Fig. 18 shows in graphical form these relations for all angles of dip between 90° . 5 P a 5 and 0° for seven different ratios between h = BO and h=4a. The ordinates to the dotted curves (concave upward) give the relative values of the hori- zontal component at the maximum point on the dip side (B maximum); those to the full curves (convex upward) the values at the maximum point fe =e Poe OVC | eee g0° 80° 70° 20° 10° Fic, 18.—Curves showing the relations between the horizontal ae at the points of maximum deflection, for rocks dipping at various angles and buried to various depths. on the other side of the rock (A maximum). These curves show that the horizontal component at the B maximum is always numerically much less than at the A maximum, and that for moderate depths of burial it diminishes very rapidly at both points with small angles of dip. — 6. DETERMINATION OF DEPTH. The relations between the dip and thickness of a magnetic rock, the distance between the horizontal maxima, and the depth of covering are given in equation (13). By assuming numerical values for 4, and either for a, the half thickness of the rock, or for d, the half distance between the maximum points, it is easy to plot the curve which expresses the relations MAGNETIC OBSERVATIONS. 355 between the other two quantities. If d is taken as the constant, the equa- tion represents a circle; if a, it represents a hyperbola. This equation may have a useful application in making it possible to judge, in advance of actual test pitting, of the probable depth of surface covering over a magnetic rock, for which the original assumptions are ful- filled, and the numerical values of 4, a, and d are determinable. It will be remembered that the assumptions upon which equation (13) rests are the fundamental ones of Section III, and also that the rock has a uniform strike. In practice, the uniformity of the strike can be established by other traverses on each side of the one in question, and 4, the angle of dip, may usually be determined by the outcrop of other formations in the same series. For any practical application it is also necessary that a (half the thickness of the rock) and d (half the distance between the stations at which the horizontal component is a maximum) should be known. From an inspection of the equation it is evident that any close determination of h, except for great depths of covering, depends upon very precise knowl- edge of the ratio between a and d. ‘The practical difficulties in the way of the measurement of a and the ever-present probability that the rock may vary from point to point, not only in actual but in effective magnetic thick- ness (which is what @ actually signifies), make it clear that for the most part can only be found approximately. Also, in the case of a rock striking due east and west the methods fail, from the fact that 2d can not be determined on the ground. The determination of h is therefore hedged in with important limita- tions; yet in many cases the information supplied by the equation may be very useful. The difficulties in the way of measuring a are disposed of in the event that along one traverse on the sirike of the rock h is known, as it | may be, by the sinking of a test pit. This value of / at once gives a value of a, which may be used on other traverses across the same rock with much “more accurate results, in the lack of disturbing factors, than if a were known only by measurement It should be added that when the traverse crosses the strike of the magnetic rock at the angle y, the distance d meas- ured on the line of the traverse must be multiplied by sin y in order to get the value of d to be used in the determination of /, and also that h must be corrected for the height of the instrument. 356 THE CRYSTAL FALLS IRON-BEARING DISTRICT. More general information as to relative depths of burial is also given by the dip curves. It is easily seen that where the superficial covering is small the vertical component of the rock force must remain small, except immediately over the rock. This condition is, therefore, indicated by steep slopes in the dip curves. On the other hand, where the depth of covering is considerable, the vertical component increases slowly and steadily, begin- ning at stations at a distance from the rock, and the resulting dip curve approaches the maximum with gentle slopes. 7. SUMMARY. (1) The strike of a magnetic rock is given by the line joining the points on successive traverses, at which the horizontal needle is not deflected from the local meridian between the converging arrows, or at which the dip angles are a maximum. When the rock is vertical, this line lies in the middle plane of the rock and fixes its position. It may be called a line of mag- netic attraction. (2) The dip of a magnetic rock is toward the nearer horizontal maximum. (3).The thickness of the magnetic formation must, if buried, always be less than the distance between the maximum points. (4) Where the superficial cover is not very great, a change in the dip of a magnetic rock from moderate or high angles to low angles is attended with a rapid decrease in the values of the horizontal component, with a corresponding decrease in the deflections of the horizontal needle. SECTION VI. APPLICATIONS TO SPECIAL CASES. In the preceding section certain general conclusions have been estab- lished with regard to the relative positions of the stations at which the horizontal and yertical components of the force of a magnetic rock have maximum and zero values. The deflections produced by these components from the positions which the magnetic needles assume under the action of the earth’s force: have maximum and zero values at the same stations at which the components have maximum and zero values, and therefore the conclusions as to the relative positions of these points are true for any angle of strike. .But certain numerical relations between the deflections depend upon the orientation or strike of the magnetic formation and upon the direction of dip, and these will now be considered. MAGNETIC OBSERVATIONS. BINT 1. THE MAGNETIC ROCK STRIKES EAST OR WEST OF NORTH AND DIPS VERTICALLY. Let. us first take the case of a rock striking east of north. At the stations within range of the local influence on the east side of such a rock belt the horizontal needle is pulled west of the meridian, reaches a west- ward maximum, then points north, then on the west side of the belt, east of the meridian, and reaches an eastward maximum. It is observed, how- ever, that the westward deflections on the east side of the belt are generally not so great as the corresponding eastward deflections on the west side of the belt. The reason for this is easily seen. At each station east of the belt the local pull acts along the normal to the belt drawn through the station. This normal makes with the local magnetic meridian an acute angle. The needle will come to rest within this acute angle along the line of the resultant of the horizontal components of the two forces, the earth’s and the local force, which determine its position. However strong the local pull may be, the horizontal needle can not be deflected past the normal. : At the corresponding stations on the west side of the disturbing belt the local pull also acts along the normal from the station to the belt, and has the same numerical value. But in this case the normal makes an obtuse angle with the magnetic meridian. For two points equally distant from the magnetic belt, one on the east and the other on the west, the resultant for the western point will, therefore, make a larger angle with the meridian than that for the eastern. On the other hand, when the rock strikes west of north, it is observed that the horizontal deflections are greater on the east side than on the west, and the explanation is entirely similar to that given above. The dip-needle observations at the same stations show general phe- nomena quite like those in the case in which the strike of the rock coincided with the meridian. They gradually increase to a maximum near the sta- tion, where the horizontal needle stands at zero between the converging arrows, and gradually decrease from this maximum on the other side. It is noted, however, that the readings are not equal at corresponding stations on opposite sides of the maximum. When the strike is east of north, the western station shows a higher dip than the eastern; when the strike is 358 THE CRYSTAL FALLS IRON-BEARING DISTRICT. west of north, the eastern station shows a higher dip than the corresponding station on the west. Generally stated, then, the stations on that side of the magnetic rock on which the angle between the strike of the rock and the line of traverse is obtuse show greater dip angles than the corresponding stations on the side on which this angle is acute. As the angles of dip are represented graphically by a continuous curve, this is the same thing as saying that the dip curve is steeper on the side of the acute than on that of the obtuse angle. These facts are easily explained by the following considerations. The vertical components tend to lower the needle, and would carry it to a ver- tical position except for the action of the horizontal forces, which tend to keep it horizontal. At any station on the acute-angle side of the magnetic belt the resultant of the two horizontal components is larger than at the corresponding station on the obtuse-angle side, the two being represented by the longer and shorter diagonals of a parallelogram. Since the vertical forces are the same at the two stations, it follows that on the obtuse-angle side the angle of dip must be larger than on the acute-angle side. Or, expressed algebraically, since H, is the only variable on the right-hand side of equation (5) it is evident that tan a, and therefore a, the angle of dip, increases with a decrease in H,,. : If the rock dips at an angle less than 90°, these results are either intensified or greatly modified, depending upon the direction of dip. It was shown in the last section that the horizontal component of the rock force is smaller on the dip side. If the strike and dip are both toward the same side of the meridian (e. g., if the strike is northwest and the dip south- west), it is evident that the numerical difference between the deflections of the horizontal needle on the two sides of the rock will be still greater than if the rock were vertical. On the other hand, if the strike and dip are toward opposite sides of the meridian (e. g., if the strike is northeast and the dip northwest), the difference between the deflections on the two sides is less than for a vertical dip, or may even be reversed. The deflections of the dip needle in the case of rocks dipping at angles less than 90° are also greatly influenced by the direction of dip. If strike and dip are toward the same side of the meridian, the difference noted above in angle of dip on the two sides of the rock is neutralized, and the dip curve tends to become symmetrical; while if they are toward opposite . MAGNETIC OBSERVATIONS. . 359 sides of the meridian, the difference is increased. It is to be noted that the influence on both instruments of the direction and angle of dip of the rock becomes weakened with an increase in surface covering. 2. THE MAGNETIC ROCK STRIKES EAST AND WEST. When a vertically dipping magnetic rock strikes east and west, or nearly so, the traverse lines must be run north and south so as to cross it as nearly as possible at right angles. In approaching such a belt from the south the instruments give little warning. The readings of the horizontal needle show either no deflections, or else very slight deflections, from the magnetic meridian. Past the middle of the formation the horizontal needle is strongly deflected, often through an angle of 180°, so that it may point due south. But as the magnetic rocks having this strike which were encountered in our work were not deeply buried, and had also quite irregular upper surfaces, generally the needle pointed either east or west of south on account of the weight which the nearness to the surface gave to the adja- cent material, either from the irregular distribution of magnetite or from the protrusion of small masses above the general level. Continuing north, the horizontal deflections gradually diminish and eventually disappear. The behavior of the horizontal needle is explained in the same way as in the preceding cases. The position of the needle at any station is deter- mined by the resultant of the horizontal components of the two forces—the earth’s force and the rock force—that act upon it. South of the magnetic rock these two components act in the same direction and essentially in the same line, since the magnetic meridian practically coincides with the true meridian. The resultant, therefore, is equal to their sum and coincides with them in direction, and consequently there is no deflection. North of the magnetic rock the two horizontal components act in opposite directions, and when they are in the same line the needle takes up its position in the direc- tion of the greater, which determines that of the resultant; when H’ is greater than H (which often happens near and north of the rock) this diree- tion is due south. When the two components do not act in exactly the same line, the needle will point east or west of south at an angle which -depends on the angle between the two forces and their ratio. Still farther north the horizontal component of the rock force dimin- ishes rapidly and we consequently first pass through a zone of large and 360 THE CRYSTAL FALLS IRON-BEARING DISTRICT. diminishing deflections to the east or west, depending on the side of the meridian on which this component falls; and finally, when it becomes insensible, the needle rests again in the meridian. In the case of a rock striking east and west, the points at which the horizontal component of the magnetism of the rock has maximum values become indeterminate by the methods hitherto described, from the fact that throughout the traverse the two components act in or nearly im the same line, and the deflections from the local magnetic meridian, therefore, do not indicate the relative strengths at different stations of the horizontal com- ponent of the rock force. The dip-needle readings for an east-and-west-striking rock are as fol- lows: At some distance south of the rock the angles are constant at the index error. As the rock is approached, the angles of dip depend upon the depth of burial. If the surface covering is considerable, an increase in the dip angles begins at a considerable distance away, and progresses continu- ously as the magnetic belt is approached. If the rock is near the surface, the dip needle shows either the constant index error or else angles of dip less than the index error for all stations south except those very near the southern margin of the rock. The maximum reading is attained north of the middle plane of the rock, at a distance from it which also depends upon the depth of covering. Farther north the dip angles decrease slowly and are in general greater than at the corresponding stations south. The form of the dip-curve, therefore, shows a steeper slope south of the magnetic rock than north of it. The reasons for these differences will be evident from the following considerations. Let it be supposed, for the sake of simplicity, that throughout the north-and-south traverse the two horizontal components act in the same line in the meridian. At any station south of the magnetic rock they act in the same direction, and their resultant will be their numerical sum. At the corresponding station north they act in opposite directions, and their result- ant will be their numerical difference. The angle of dip is given by equation (5): ; V’+ 4H tan 0 oo tan a = For the two corresponding stations, V’ will be the same. The other quantities are all constants except H,. For the south station H,= H’ + H; MAGNETIC OBSERVATIONS. 361 for the north station, H,=H— H’, where H and H’ are, respectively, the horizontal components of the magnetism of the earth and of the rock, as before. The numerator of the right-hand side of the equation will be the same for both stations, while the numerical value of the denominator will be less for the north station than for the south. Consequently tan a, and therefore a, will be greater for the north station. For great depths of superficial covering, however, these differences become almost imperceptible, owing to the fact that H’ is so small that H, is essentially the same at the two corresponding stations. The tendency, therefore, as / increases is for the dip curve to become symmetrical. In the special case in which H’'=—H, H,= 0, and the dip needle stands at 90°. his can only take place north of the rock, and may, depending on the strength of H’, be found at two stations, one on either side of the station at which H’ is a maximum. At the same stations the horizontal needle is not acted on by any unbalanced force, and rests indifferently in any position. The dips less than normal which are often observed at stations south of a magnetic rock which lies very near the surface are also easily under- stood by a reference to equation (5). At these stations the resultant pull of the rock is so nearly horizontal that the vertical component V’ is very small in comparison with the horizontal component H’. If V’ is a negligible quantity, equation (5) becomes tan a= ay . tan @ In such cases the angle of dip is therefore less than the index error. With north or south dipping rocks, where V’ is negative, tan a becomes negative when V’>H tan @. 3. TWO PARALLEL MAGNETIC FORMATIONS. The cases so far considered have involved only one belt of magnetic rock, which has been assumed to have a uniform dip in one direction, or, in other words, to be a monocline. In practice, however, owing to complexi- ties of structure and other causes, which will be considered hereafter, it frequently happens that two or more approximately parallel belts are found within the range of one another’s influence. Under these circumstances 362 THE CRYSTAL FALLS I[RON-BEARING DISTRICT. the effects produced upon the magnetic needles are correspondingly com- plicated. : For the purposes of illustration it is sufficient to consider a few extreme 7 / H Vv : eases, and to represent the values of ~~ and —— for these graphically. Let it first be assumed that the two parallel belts are vertical, that the distance between them is 8, and that a=3, and h=2. This represents the conditions when the distance of separation is large compared with h. The ordinates to / H the curve of Pl. XLVIII, fig. 1, give the values of — which correspond to the different stations of observation. Those parts of the curve which are above the horizontal axis of coordinates represent the portions of the trav- erse in which H’ is directed toward the west; the parts below, those in which it is directed toward the east. It is seen that besides the middle point there are two other points of no horizontal deflection, which do not exactly correspond with the points vertically over the middle of the magnetic forma- tions, but are somewhat nearer the adjoiing edges; and also four points of maximum deflection, one on each side of each rock. The maximum points inside of the two formations have smaller deflections than those outside, as shown by the relative lengths of the ordinates to the curve, and also between the inside maximum points the horizontal components are directed away from the middle point. / The curve of =a represented by the dotted line, has two maximum values, which fall nearly over the two rocks. If next we assume that the distance between the rocks is 8, and that a==3, and h=8, we obtain the curve of Pl. XLVII, fig. 2, which represents / the value of @ When his large compared with the distance of separation. This case shows but one point of no horizontal deflection between the two rocks and but two points of maximum deflection, one on the outside of each. / The curve of ae represented by the dotted line, shows the interesting feature of three maximum points, one at the center and one over each rock. If h were relatively a little greater, these would evidently coincide at the center. Si a i a tall U. S, GEOLOGICAL SURVEY MONOGRAPH XXXVI PL. XLVIII 23 25 29 30 23 26 33 39 40 ¥7/110 90 68 G2 56 50 45 49 3 24/727 26 Fo HO) ut QERNEEN EE NEHER 38 RELATIONS OF MAGNETIC BEDS TO VARIATION AND DIP. voy rh ¥ | ‘ 4 a : ; ’ 1K i Vcr inne « yay " Ns i ie > Ha ; ee ar y i ony ; ; : r ‘ ; i % : ; 5 he 1 7 j , i iis i ; ; ¥ iN k y ; : ik b 4 My ry. is ik me “ 4 : 4 1] \' LF, i } + vee u | ‘ oa ik } f 4 y 1 } i iy an 4 cy mn , v ¥ i . _ * y ms ' 4 4 a sh, 4 MAGNETIC OBSERVATIONS. 363 If the two parallel formations are not vertical, but dip in the same direction at the same angle, the resulting curves are somewhat different. Pl. LXVIII, fies. 3 and 4, show two cases in which the elements are the same, except the depth of covering and the thickness of the intervening non- magnetic material. Here the rocks dip at an angle of 71° 34’. and the width at the rock surface is 6.3 for each. In fig. 3, Pl XLVI, where h=2 and the width of the nonmagnetic bed is 10.7, and the covering is, therefore, relatively small, the presence of two rocks is distinctly shown by the curves of both components, and the chief result of their interaction is to introduce an additional point of no hori-. zontal deflection between them, on each side. of which the horizontal arrows diverge. The positions of the maximum and of the other zero points are hardly disturbed, and consequently the direction of dip is very clearly indicated. 4 In fig. 4, Pl. XLVI, where h=4 and the formations are separated by nonmagnetic material 4.7 wide, there is but one zero point, nearly over the middle of the upper formation, toward which the pointings of the hori- zontal needle converge. West of this are two points of maximum eastern deflection, between which a faint minimum represents the backward pull of the lower formation. Tf the two magnetic formations are parallel in strike, but dip toward each other at equal angles, the resulting curves of the two components are shown in Pl. XLVIII, figs. 5 and 6. Fig. 5 illustrates the effects on a syncline with steeply dipping sides, the superficial covering being relatively shallow. These conditions result in a point of no horizontal deflection over the mid- dle of the trough with diverging arrows on each side, and besides a point of no horizontal deflection over each rock, toward which the arrows con- verge. The positions of the two maximum points for each rock, and of the zero between them, is nearly the same as if the other rock were absent, and consequently the fact that the rocks dip toward each other is clearly indicated by the unsymmetrical distances. In fig. 6, Pl. XLVIII, the depth of the rock surface is much greater relatively to the inside distance between the legs of the syncline, and the dip is flatter: In this case there are but two points of maximum deflection, one on each side of the syncline, and but one point of no deflection, over the middle of the trough. 'The maximum points represent the outside maxima ’ 364 THE CRYSTAL FALLS IRON-BEARING DISTRICT. of the former case, and the result of the interaction of the two legs is to increase the numerical values of these, as well as to bring them nearer together. It is evident that the deflections of the horizontal needle in this case could hardly be distinguished from those that would be produced hy a single vertical formation buried to a considerable depth. | Let it next be supposed that the two rocks dip toward each other at different angles, the strikes remaining parallel, and also that the rock of lower dip is buried to the greater depth. This, then, is a case in which the magnetic effect of one limb of the syncline is much stronger than that of the other. In Pl. XLVIII, fig. 7, the curves of the two components are given for the special case in which the right-hand limb of the synclinal dips at an angle of 90°, and has a surface covering h=2, while the left-hand limb dips atan angle of 26°°34’, and has a surface covering h=4. It is interest- Fig. 19—Truncated-anticlinal told with gently dipping limbs. ing to compare the theoretical results of this figure with the curves of Pl. XLVIII, fig. 8, which represent deflections actually observed, and not components. : =i In the latter figure the strike of the two rocks is represented by the heavy lines. The two rocks are the same formation, brought up by folding on opposite sides of a synclinal trough. The synelinal is slightly pushed over, so that the eastern rock dips nearly vertical, while the western has a much lower dip toward the east, and is also more deeply buried. These facts rest on independent evidence, yet they might all be inferred from the observations recorded in this figure. The dip curve in this case shows two distinct maxima, a smaller under MAGNETIC OBSERVATIONS. 365 the zone of retardation and a larger over the point of no horizontal deflee- tion, which correspond respectively to the two magnetic rocks. If the two formations are parallel in strike, but dip away from each other, the curves of the horizontal and vertical components for different angles of dip and different relations of thickness and depth of covering are shown in figs. 19 and 20. In fig. 19 the formations are widely separated, h is relatively small, and the angles of dip are equal and low; the inter- action of the two rocks therefore extends over a narrow zone only, and the curves of the components clearly indicate the presence of two formations and the direction of dip of each. In fig. 20 the anticlinal is so truncated that magnetic material occupies the whole space on the rock surface between the outer boundaries of the two formations. The angles of dip are equal, and are higher than in the preceding case, while the depth of cover- ing is relatively much greater. The horizontal component is zero in the axial plane of the anticlinal, and has maximum values at two points, one on each side of the zero. The ver- tical component is a maximum at one point, also in the axial plane. The deflections pro- Fig, 20—rruncated anticlinal fold with steeply dipping limbs. duced by these conditions could not be distinguished in practice from those produced by a single vertically dipping formation. In general, therefore, when two magnetic formations lie within range of each other's influence, the deflections are determined by the relative magnetic strengths of the two rocks, by their distance apart, by their strike and dip, and by their depth of burial. It is evident that for certain given relations among these factors the special cases above described will occur, and it is found that they really do occur in practice. For other relations it is not possible to make a general statement either as to the number or the position of the maximum and minimum points. 366 THE CRYSTAL FALLS IRON-BEARING DISTRICT. SECTION VII. THE INTERPRETATION OF MORE COMPLEX STRUCTURES. The existence of two parallel belts of magnetic rocks may be accounted for geologically in more than one way. They may represent two distinct formations occurring at different horizons in the same series, or they may represent the same formation either duplicated by folding or faulting or separated into two parts by the intrusion of a sheet of igneous rock parallel to its bedding. Since, then, two magnetic lines, the existence of which has been established by observation, may have more: than one interpretation, the discrimination of these cases, when possible, is of special importance. The question whether any given case belongs to the first of these cate- gories can generally be settled only by following the lines of attraction into a district which affords a geological section across the formations involved, or by the occasional outcrop of the rocks which give rise to the disturbances, in which case lithological resemblances or differences, the relations to other formations, and the observed structure will decide the matter one way or the other. In the special case in which either or both lines can be followed completely round an anticlinal dome or a synclinal basin, which of course can only rarely happen, the question would be settled affirmatively, even if outcrops were entirely lacking. In the other instances the magnetic observations themselves often give means of discrimination, even when the outcrops are so few or so obscure as to be in themselves indecisive. It is characteristic of the folds in the pre- Cambrian rocks of this region that the axes are not usually parallel with the horizon for long distances, but are often inclined to it; in other words, when followed for greater or less distances they pitch. The outcropping edges of any formation involved in an anticlinal or synclinal fold which has | been cut by a plane of denudation will be parallel to each other wherever the axis of the fold is horizontal, but will approach each other where the axis is inclined. In an anticlinal fold they converge in the direction in which the axis sinks, while in a synclinal they converge in the direction ‘in which the axis rises. If the formation is a magnetic one, conformably placed between beds of nonmagnetic character, the magnetic lines to which the outcropping edges give rise will therefore run parallel to each other when the axis is horizontal and will converge or diverge when the axis pitches. The convergence or divergence takes place gradually, since the angles of pitch usually are not large. - MAGNETIC OBSERVATIONS. 367 In the case also of a single formation which stands on edge and has been split by the intrusion of a sheet of eruptive rock parallel to the bed- ding planes, the magnetic observations will often show two parallel lines, which, at the extremities of the eruptive rock, where it wedges out, merge into one. In general, therefore, two parallel magnetic lines which represert two distinct formations preserve their identity, and do not pass into each other; when, however, they represent the same formation, they will often come together if followed far enough. The principles which have already been applied to the analysis of simpler cases are useful in discriminating among the three cases of convergence. 1. PITCHING SYNCLINES. Let us first consider a pitching synclinal fold, which is represented in plan and by successive cross sections in fig. 21. It is evident that on the lines of traverse along Sections ‘I and II the deflections of the needle will observe the usual sequence for two parallel belts, the details depending upon separation and depth of cover- ing, tions III, IV, and V the phe- nomena will be those caused by a single belt of magnetic i CROSS SECTIONS rock. Also, on account of the Fie. 21.—Plan and cross sections of a pitching syncline. while on lines along Sec- rise in the axis, the south poles of the rock are brought continually nearer the surface on these successive cross sections, and therefore the two components of the rock force will be smaller for each traverse than for the one preceding. Since the magnetic material comes to an end at A, it is no longer true that there is as much magnetic material on one side of these sections as on the other. Conse- quently the horizontal component due to the pull of the rock does not become zero at any point along these sections, but for every station has a positive numerical value and acts in the general direction in which the syn- clinal pitches. At the station in the plane of symmetry of the fold this 368 THE CRYSTAL FALLS IRON-BEARING DISTRICT. component acts parallel to the axis. The direction and amount of the deflee- tion depend upon the direction of strike and pitch of the synclinal. Let us suppose, first, that the axis of the synclinal strikes north and pitches north. In this case Section I, in fig. 17, is the most northern, Section V the most southern. The traverses along Sections I and II display the usual phenomena for two parallel belts. Hast of the eastern limb and west of the western the horizontal needle will be deflected toward the syncline. Between the two limbs there will be at least one point of no deflection, and frequently, depending upon the relations between the depth of burial and the thickness of the intervening nonmagnetic material, either two other points of no deflection or two zones of retardation, one on each side of this middle zero. Along Sections III, IV, and V there will be but one point of no deflection of the horizontal needle, which will correspond with the axial line of the fold. Since this axisis north and south, and so coincides with the magnetic meridian, the horizontal component of the rock force coincides in direction with the horizontal component of the earth’s pull, and consequently there is no deflection of the horizontal needle. For other stations east and west of the central station the deflections are toward the west and east, with the usual maximum points. The deflections on successive sections south grow smaller, since the angle between the two horizontal components progressively diminishes. The relative value of the horizontal component of the rock force also progressively diminishes, since the thinning of the magnetic material due to the rise in the axis of the fold brings the buried north poles into promi- nence. Therefore the deflections of the horizontal needle after the mag- netic rock has been left behind very soon become imperceptible. The dip needle deflections for the northern sections, I and I, reach their maximum values at the usual points, over the central zero and near the outside zeros or points of retardation. For the southern sections the dips grow less, since the horizontal restormg couple due to the rock has always a positive numerical value, and also because the vertical component of the rock force diminishes,-owing to the nearness of the south poles. As’ the section approaches the limits of the magnetic material the points of maximum dip become less and less clearly defined, and the dip curve passes into an irregular line, slightly depressed below the line of no deflection. EE MAGNETIC OBSERVATIONS. 369 The reasons for this are, of course, obvious from what has been said above. In the case of a synclinal fold pitching south, Section I (fig. 17) becomes the most southern, Section V the most northern, line of traverse. Sections I and II present the same general phenomena as before for both needles. In Sections III, IV, and V the horizontal component due to the rock has a positive value for all stations as before, but in this case acts in a generally opposite direction to that of the horizontal component of the earth’s force. Therefore, on these sections we should expect at first greater deflections of the horizontal needle, which would diminish rapidly as the sections approached and passed beyond the northern limit of the magnetic material, but which, for corresponding sections, would be greater than for the northerly pitching fold. The deflections of the dip needle would also be greater for the same reasons. For a synciinal pitching west, Section I is the most western, Section V the most eastern, traverse. In this case, along I and II, the deflections of the horizontal and dip needles are dependent for their details upon the ratio of depth to distance of separation, but if far enough to the west will show clearly two belts of magnetic material, approximately parallel, and striking approximately east and west. For Sections I, IV, and V, in which the distance of separation is either nothing or relatively small, the phenomena will indicate but one belt. On these sections, owing to the faci that the horizontal component of the rock pull is nowhere zero and has everywhere a general westerly direction, the deflection of the horizontal needle will be westerly throughout, and will reach a maximum north of the east-and-west axial plane of the material, where the angle which it makes with the magnetic meridian is more than 90°. In accordance with the general principles stated in the discussion of a single belt with the same strike, the angles of dip are in general smaller south of the syncline than north, and the maximum dip is reached at a point north of the axial plane. On sections farther to the east, near the limits of the rock and beyond them, the dip-needle deflections, like those of the horizontal needle, rapidly diminish and soon become imperceptible. These facts are well shown in fig. 22, which represents a series of north- and-south traverses across the Groveland basin, the limits of which are defined by outcrops on the eastern side. MON XXXVI——24 °370 THE CRYSTAL FALLS IRON-BEARING DISTRICT. In this figure it is instructive to notice the small dip angles in the sections east of the end of the syncline. In the first of these the dip curve shows a hollow near the axis of the fold or angles of depression less than the normal. This is easily understood upon considering that, since the surface covering is here small, the vertical component of the rock force becomes very small at these stations compared with the horizontal component. For an eastward-pitching syncline it is obvious that the facts will be entirely similar to those stated above, except that the deflections of the horizontal needle will be toward the east instead of toward the west. This (} —— = = Lowe) —— eee. a Oo ~ —_—-—— = —— -——— -— SSS SS Soa as SSS ooo = =~. - Fig. 22.—Magnetic map of the Groveland Basin. is also well shown at the western end of the Groveland basin in fig. 22. This basin does not show the phenomena of two lines, however, from the fact that it is so narrow and shallow that it does not include in its interior any overlying nonmagnetic material, and there is accordingly no separation of its rims. 2. PITCHING ANTICLINES. In the cases of pitching anticlines (fig. 23) the sequence of observa- tions in the area of separation of the rims is very similar to that of pitching synclines. In the zone of coincidence the structural difference in the two cases is that the material does not come to an end, but continues as one band, which, as the axis sinks, is progressively buried to a greater depth. MAGNETIC OBSERVATIONS. 371 Therefore, in general, the buried north poles of the magnetic formation are not brought nearer the surface; and this, together with the fact that the material continues on in the line of the axis, produces characteristic phe- nomena in the magnetic sections. These phenomena, the details of which can be easily followed out for any given direction of pitch, and need not here be described, show in gen- eral two lines of attraction merging into one, which continues in the same direction as a strong line, showing, as it is followed, the peculiarities due to an increasing depth of burial. The points of maximum deflection of the horizontal needle continue to separate from each other on suc- cessive sections. ‘The dip curve shows a definite maximum closely corresponding, except for due east-and-west strikes, to the point of no horizontal deflection. I te Where the axis of the fold is so / j \ oriented that these points can j be established, they indicate the nature of the fold. If the strike is east and west, in which case oe /\ they become indeterminate, the CROSS SECTIONS 2 . 5 s Fig. 23.—Plan and cross sections of a pitching anticline. continuity of the line and its very gradual decrease in power may give an excellent basis for inference as to the nature of the fold. 38. FORMATIONS SPLIT BY INTRUSIVES. When a single formation has been split into two by the intrusion of a nonmagnetic igneous rock, there are in the area in which the igneous rock oceurs two parallel magnetic formations, which give rise on cross traverses to phenomena the precise features of which depend upon the strike and dip of the formation and upon the relation which the width of the intruded mass bears to the depth of burial. To describe these would involve a mere repetition of what has been said before. Such intruded masses always have a définite limit in length, which is usually not very great. When the limits are reached, the two parallel lines pass into a single line which continues on 372 THE CRYSTAL FALLS IRON-BEARING DISTRICT. in undiminished vigor. Also, such intruded masses are seldom confined to definite horizons for great distances and seldom split the formation into symmetrical halves. Two nearly parallel lines of unequal strength, which, as they are followed, become equal, and then again become unequal, with the stronger on the opposite side, are often, therefore, characteristic phenom- ena of this case. A good illustration of the unequal division of a magnetic rock at different points along its strike by an intruded sheet which wedges $8 84 95 86 40 40 40 50 57 1659 76 69 64 60 53 Fig. 24._Magnetic map of a single formation split by an intruded sheet. out at both ends is given in fig. 24. Between the second and third trav- erses from the north end the existence of this sheet has been proved by drilling. 4. SUMMARY. The means of discrimination among these cases of convergence are therefore founded on the deflections in the critical areas, where the separated bands of magnetic material merge into one. Strong deflections toward the point where they run together, with a rapid disappearance of all disturb- ances within a short distance of this point, indicate a pitching synclinal MAGNETIC OBSERVATIONS. 373 fold. A long continuance of the disturbances, with the characteristic phe- nomena attending deeper burial, beyond the point of coincidence, indicates an anticlinal fold. A coincidence in both directions and the continuation of the disturbances without diminution indicate an intrusive sheet. To these may be added the delicate criterion which the unsymmetrical distances of the horizontal maxima from the central zero may afford. If in the area of separation the two belts depart from each so far as to be out of range of each other’s influence, and it is found on successive cross sections that the nearest maxima are inside the lines of no deflection which directly indicates the position of the rock, it can be concluded that the rocks dip toward each other, and, on the other hand, if the nearer maxima are outside these lines, that they dip away from each other. In the one case a syncline and in the other an anticline would be indicated, and, of course, in either case it would be certain that the phenomena could not be due to an intruded mass. CHAPTER IIt. THE FELCH MOUNTAIN RANGE. SECTION I. POSITION, EXTENT, AND PREVIOUS WORK. Our map (Pl. XLIX) of the Felch Mountain range includes 12 sections in the southern tier of 'T. 42 N., Rs. 28, 29, and 80 W., beginning with sec. 33, T. 42 N., R. 28 W., on the east, and ending with sec. 34, T. 42 N., kh. 30 W., on the west. The range is known to extend beyond these limits both to the east and to the west. Rominger states’ that it has been traced 4 miles east of our eastern boundary, and also west of our western boundary to the Menominee River north of Badwater Village. From a hasty recon- naissance of the country to the east it seemed probable that but few addi- tional facts could be determined, because of the swamps and the extensive cover of the Paleozoic sandstone, and these sections were therefore not studied in detail. We were not able to continue the work to the west, on account of the lateness of the season, but it is desirable that this should be done at some future time. The sections surveyed include, however, that portion of the range in which outcrops are most abundant and which has been the principal seat of exploration for iron ore. The strong magnetic attractions in several of these sections and the prominent outcrops of ferruginous jaspers at Felch Mountain in see. 82, T. 42 N., R. 28 W., and in sec. 31, T. 42 N., R. 29 W., were early noticed by the United States land surveyors and indicated on the township plats. With the rapid development of the Marquette range after the close of the civil war the attention of miners was quickly drawn to these as to other outlying prospects, with the result that vigorous exploration was begun on this range even earlier than on the Menominee range proper. ' Geological report on the Upper Peninsula of Michigan, by C. Rominger: Geol. Survey, Mich.; Vol. VY, 1895, p. 35. 374 U.S. GEOLOGICAL SURVEY nl i | ~ i} ' | i r il Hu im) i) sina Ay, = = bi itt if sea lal we annul = it Sey a an Mi TRG Octet tego eee 6.70 on oe OAS | Mag... -22scc0 seeese cesses esate: 2.50 AQpR) |e OLS Wake Ofa tall OCeeeee eta cus eau 19) ||Loecoee ee [Reels sean ELD PUDONG INOS) cenaeeece eeeeeees Ad faye een ieetes creas Sng Sea le ee eh pea ae COG LES SUaE Ree ae | ' Ba, Sr, Li, Cl, S, SO; were not looked for. 2 Water not determined. *Includes POs. No. 1. Specimen 34826, Lake Superior Division, U.S. Geol. Surv., 240 N., 1,250 W., sec. 35, T. 42 N., R. 29 W., Upper Peninsula of Michigan. No. 2. Specimen 36058, Lake Superior Division, U.S. Geol. Surv., 325 N., 1,225 W., sec. 36, T. 42 N., R. 29 W., Upper Peninsula of Michigan. No. 3. Specimen 36080, Lake Superior Division, U.S. Geol. Sury., 15 N., 1,025 W., sec. 31, T. 42 N., R: 28 W., Upper Peninsula of Michigan. 392 THE CRYSTAL FALLS IRON-BEARING DISTRICT. (3) The mica-schists are not widely distributed in the portion of the Archean areas included in the Felech Mountain map. They are well rep- resented in the northern Archean area beyond the limit of the area mapped, but within this limit they are known only in sees. 34 and 35, T. 42 N., R. 29 W., where an overthrust fault brings them into successive contact with the Randville dolomite and Sturgeon quartzite for a distance of three- fourths of a mile. An excellent section, which includes the faulted con- tact with the dolomite, is exposed along the Sturgeon River below the dam in the northern portion of section 35. Though so feebly represented, they possess an unusual interest both in their field relations and in their microscopic characters. The mica-schists when fresh are dark gray, rather soft rocks, of fine to medium grain, with a generally well-developed schistose structure. The most noticeable constituent, in spite of the dark color, is muscovite, which occurs in pearly flakes of large size plentifully sprinkled along the cleav- age surfaces, and is especially characteristic of thin seams, which are much more fissile than the rest of the rock and part it into parallel bands of much regularity. Biotite, however, is the more abundant mica, although m smaller and less conspicuous plates, and to it the dark color of the rock is due. Quartz and sometimes feldspar may also be recognized. These rocks offer little resistance to the weather. The biotite gives up its iron with great ease, staining the outcrop a dull red. The final product is a slightly coherent ferruginous mixture in which the large muscovite plates alone are recognizable. At a less advanced stage of weathering the alterna- tion of layers more rich in biotite produces color banding in reds and grays. The mica-schists contain many intruded dikes and sheets of flesh-colored pegmatite and also of amphibolite, both of which are generally parallel to the foliation. The pegmatites are typical ‘‘schrift-granits,” the feldspar being microcline. Bothpegmatites and amphibolites show ragged and intrusive con- tacts with the schists when these are examined indetail. Both also are foliated. Under the microscope the mica-schists are thoroughly crystalline aggre- gates of quartz, biotite, and muscovite, always with more or less microcline. Magenetiteisalways presentasa primary mineral, and hematite orsome hydrous oxide of iron between hematite and limonite is very abundant in the zone of weathering. Besides these, tourmaline is an abundant accessory in some slides, and apatite, zircon, titanite, pyrite, and chlorite also commonly occur. ARCHEAN IN FELCH MOUNTAIN DISTRICT. 393 Quartz occurs in small and often partly rounded areas, some-of which have a very clastic appearance. Exceptas stated below, it is generally free from inclusions of the micas, which surround and terminate against it in such a way as to indicate that it crystallized the earlier. It is often crowded with fluid and gas inclusions, and an occasional grain bristles with radiating clusters of rutile needles. Minute crystals of magnetite are also frequently inclosed. The inclusions of all kinds are frequently grouped in roughly oval areas near the centers of the grains, while between the nuclei and the wandering perimeters the quartz is relatively free from inclusions. Biotite, varying in color from dark brown to light yellowish green, is the predominant mica. It occurs in irregular plates, generally much larger than the quartz; the great abundance and uniform alignment of these plates produce the schistose structure. As already stated, it cludes and is there- fore younger than the quartz generally, but it is also found, though rarely and always in very minute plates, included in the small quartz grains which are so abundant in the fresh microclines. The latter occurrences belong to an earlier generation than that of the larger biotites. The chief interest attaching to the biotite is in its alteration under the attack of the weather. The iron separates out along the cleavages in little spheroidal drops and flattened plates, which are red and translucent, but not quite of the deep color of hematite. Doubtless they contain some water, and are possibly close to géthite in composition. Between the red globules the biotite sub- stance becomes paler, its pleochroism diminishes, and double refraction increases, and finally, in a slide containing no basal sections, it can not be distinguished from muscovite. The separated ferric oxide remains in the mica, and while the rock remains firm does not travel and stain the other constituents. In these stages the slide contains a very faintly colored bleached biotite, which is sprinkled through and through with the little dots of bright red iron ore. Muscovite is not very abundant. It is sometimes intergrown with the large biotites, and occurs under similar conditions, but it chiefly comes in little ragged inclusions in the secondary microcline. In the form of aggre- gates of sericite it composes the macroscopically conspicuous pearly micas, and also is an abundant constituent, and sometimes the only representative, of the partly absorbed and older feldspars included in the microcline. Microcline is always a secondary mineral, and is present in variable 394 THE CRYSTAL FALLS TRON-BEARING DISTRICT. amounts in different sections. It incloses quartz, the micas, magnetite, and an older feldspar. These inclosures are usually small; they often he in par- allel alignment in the same and adjoming microclines, and the lmes in which they are disposed sometimes bend, apparently indicating that the original rock was minutely puckered. The imclosed quartz sometimes incloses smaller flakes of biotite and muscovite, as well as magnetite and rutile needles. The inclosures in the little grains of quartz are frequently con- centrated in the centers, as in the case of some of the quartzes outside the microclines, as described above. The microcline sometimes occurs in a few scattered grains; sometimes with its inclusions it makes up almost the whole rock. In its manner of occurrence, its inclusions, and the way in which these are disposed within it, it is strikingly like the secondary albite of the Hoosae schists of western Massachusetts, described by Prof. J. E. Wolff? The microclines are distinctly elongated in a direction parallel to the foliation, to which they thus contribute. In a few cases the elongation is parallel to a line, and does not appear in thin sections cut normal to this direction. But in most cases the crystals are flattened parallel to a plane. These forms are those of crystallization; except along the secondary fracture planes the microline is entirely free from breaking or granulation. The following is a complete analysis of a representative specimen of the mica-schist: Analysis of mica-schist. [By Dr. H. S. Stokes, U. S. Geol. Survey.] 1 1 SIO SS 55 eee eee see (oa i MOE) Ones nere sete eeeete see 0. 08 (iO esde sane eoaboseososces 2 oy OM ccarsers sera eee seer 297 COs Seer Ee ee oa eemesenicee None. |WKeOPeons. ceeeme ccs ee ace ee 5.63 Det Ot ae Hone By aos Sea OOor 2 |MINias © eet ees en emteeset sate 11 AI @35c45 ee eta ia GHA Sa ELS O rate 1hl OSES arenes all 1 or O Sa aE Ree ee ee 1.83 || H:O above 110°_.-__....__-- 2.79 | BOO seecec cess: seen coea os 3.34 Total wate eee eee 99. 44 Min Qs. eae ae ee ee Trace. No.1. Specimen 34822, Lake Superior Division, U.S. Geol. Surv., 1900 N., 1310 W.., sec. 35, T. 42.N., R. 29 W., Upper Peninsula of Michigan. 1 Mon. U.S. Geol. Survey, Vol. XXIII, pp. 59-63. ARCHEAN IN FELCH MOUNTAIN DISTRICT. 395 Tn its low silica and lime, and high iron and magnesia, this rock differs in important particulars from the granites, to which in its mineral com- position it is allied. In these respects, as well as in the great excess of potash over soda, it closely approximates the composition of certain clay slates.’ The original character of the mica-schists is indeterminate. They may be altered sediments, as the chemical analysis indicates, but if so they no longer contain any material which can be proved to be in its original form, and in view of the complete recrystallization, for which the evidence is clear and striking, this could not be expected. heir mineralogical relationship and close association with the granites and gneisses is perhaps a reason for regarding them as autoclastic rocks, derived from originally massive granites by dynamic metamorphism. If this be true, then the crust movements which crushed the parent granite belong to pre-Algonkian time, for the later stresses which folded and brought the schists into faulted contact with the Randville and Sturgeon formations found them with a parallel foliation which it bent and crumpled, and no period of great stress earlier than this is known in Algonkian time. - The complete recrystallization may be referred with probability to the period of quiescence following the faulting and folding, during which also occurred the recomposition of the older Algonkian formations. (4) The amphibolites or hornblende-gneisses are widely and abundantly represented in the Archean. Macroscopically they are black or dark- green rocks of medium to fairly coarse grain, the fresh fractures of which glisten with the cleavage surfaces of hornblende, which is much the most abundant and often the only recognizable constituent. They are universally foliated parallel to the foliation of the associated gneisses, and exhibit, but in a more marked degree, the same varieties of structure. The folia- tion is easily recognized by the eye as due to the parallel arrangement of the hornblende prisms. Depending mainly upon the position of the hornblendes relative to the other constituents, the structure is either of the plane-parallel or limear-parallel type, the latter often superbly developed. The essential constituents of these rocks are common green horn- blende, plagioclase, biotite, and quartz. The structure is thoroughly crys- ' See analyses quoted by Kemp, Handbook of Rocks, p. 107, nos. 4 and 5. 396 THE CRYSTAL FALLS IRON-BEARING DISTRICT. talline. The hornblende occurs in long prisms 3 to 10 mm. in length, which lie close together, and inclose, partially surround, and abut against smaller angular grains of plagioclase. The plagioclase is quite unstrained and is usually fresh and clear, and entirely without crystal boundaries. Brown biotite is universally present in small amount, in long plates parallel with the foliation. It does not seem to be an alteration product from the horn- blende. Quartz is the least abundant constituent. It is crowded with fluid cavities and needles of rutile, and often incloses minute crystals of horn- blende. The plagioclase, from its high extinction angles and alteration products, is evidently basic. A little magnetite is present, but titanite has not been observed. The structural features are well brought out in thin section In the linear-parallel type the hornblendes all lie with their crystallographic axes parallel to a line. A thin section parallel to the foliation cuts essentially all in the zone of the prism or near it; one across the foliation gives only sec- tions across the prism. The grains of plagioclase are generally elongated without strain. Their outlines are most irregular and quite independent of the twinning lamelle. Their general appearance is that which would be presented if numerous crushed contiguous grains had united by some proc- ess of annealing or absorption to form the new individuals. In the plane- parallel type the only difference is that the hornblende prisms have grown parallel to a plane, in which, however, they may have any orientation An indistinct banding is also often observable in this type, caused by a partial grouping of the light and dark constituents in parallel layers. The order of crystallization seems to have been plagioclase first, but nearly contempo- raneous with the hornblende and biotite, and the quartz last. The amphibolites occur in comparatively narrow bands of indefinite length in the granites and gneisses. The width usually does not exceed 8 to 10 feet, and their dip is always at high angles. The boundaries are invariably sharp, and frequently cut the foliation of the amphibolite within and of the gneisses without somewhat obliquely. There is a general uni- formity of grain throughout the width; the wider bands are not coarser than the narrower. ARCHEAN IN FELCH MOUNTAIN DISTRICT. 397 The following complete analysis shows the chemical character of a rep- resentative specimen of amphibolite: Analysis of amphibolite. [By Dr. H. N. Stokes, U. S. Geol. Survey.] 1.! 1.1 Osco ee er ee BONG eI Naki © eee eae eae ecaieetae STEKO) enette UNC UL NS lat CaO, aes ee wee 7. 85, (COs soopessecenossoeeeenee None. WO asecccencecosesosccor ' 5.55 POs | aki de Sa eens a 300) Ree eee eR ee levees SINTLEC stag ng ae ae 326." |i NasO™ somes men aceeen eae 19. su1 SiEOs suesbe Snes ee ee Beene eee (ESR ON MU at. Sec cc.5 . 16 JOSH O iste Sete ieee Se eS 6. 30 He Olaboventl 0S =e see 15 FeO... ------------------ 9.34 | PSEA Lear eco raee 99, 59 Vin Ofer se aaa ie SONS oy. Trace. | ‘Ba, Sr, Li, Cl, 8, SO; were not looked for. No. 1. Specimen 36407, Lake Superior Division, U. S. Geol. Survey, 1140 N., 1000 W., sec. 32, T. 42 N., R. 28 W., Upper Peninsula of Michigan. From this analysis it appears that the rock has essentially the compo- sition of diabase or basalt. The composition of the amphibolites, as shown by the above analysis, and their field relations leave little room for doubt that they are old dikes of basic rock. Their present crystallization is of course not that due to original cooling, since among other reasons it bears no relation either to their thickness or to distance from the walls. The evidence of complete recrys- tallization im place after consolidation which they thus afford, and the unquestionable community of origin between their foliation and that of the gneisses, are significant facts in the metamorphic history of the Archean of this district. It is for this reason that they are described with the Archean and not with the intrusives. Whether they are really Archean intrusions and. not of Algonkian age can not, perhaps, be known with certainty. Basic rocks haying approximately the same composition are known to have penetrated the Algonkian, but they have not undergone the same recrystallization. These last besides have their known analogues, equally unmetamorphic in the Archean itself. For these reasons it seems probable that the amphib- olites were intruded into the Archean before the Algonkian rocks of this district were deposited. 398 THE CRYSTAL FALLS IRON-BEARING DISTRICT. SECTION IV. THE STURGEON QUARTZITE. The lowest member of the Algonkian in the Felch Mountain range is a formation consisting mainly, but not exclusively, of coarse vitreous quartzite. Typical exposures of this formation, as well as one of the rare contacts between it and the underlymg Archean, occur along the Sturgeon River, and it is therefore named the ‘Sturgeon Quartzite.” DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY. The Sturgeon formation, next to the Randville dolomite, is the most widespread member of the Algonkian series in the Felch Mountain range. Its general distribution throughout the area mapped is in two parallel zones, of varying width, immediately adjoining the northern and southern Archean, except when displaced from this position for relatively short distances by faults. These zones extend east and west for the whole length of the range. Their surface width varies with the complexity of the structure and the depth of erosion. In part of sec. 35, T. 42 N., R. 29 W., the higher formations have been entirely removed, and the two zones come together, leaving the quartzite as the only Algonkian rock at the present surface. On the whole the Sturgeon formation is fairly well but very unevenly exposed. Beginning at the west the zone in contact with the southern Archean furnishes frequent outcrops from the south quarter post of see. 34, T. 42 N., R. 30 W., to the south quarter post of sec. 36, T. 42 N., R. 30 W., a distance of 2 miles. Then follows a gap of a mile in which no outcrop: have been found. Near the north and south quarter line of sec. 31, T. 42 N., R. 29 W., they begin once more, and are supplemented by test pits as far as the west sixteenth line of section 32, next east. Then follows another gap without exposures, 24 miles in length. Near the north-and-south quarter line of sec. 34, T. 42 N., R. 29 W., outcrops begin again and continue for a mile to the east, with frequent inter- ruption, as far as the north-and-south quarter line of section 35, where in the valley of the Sturgeon the southern zone broadens and joins the north- ern, in consequence of the general westward pitch which has carried the higher formations above the present surface of denudation. East of this poit the quartzite is known in only a few scattered localities. In the southern part of sec. 36, T. 42 N., R. 29 W., it is in contact with the Archean on the south bank of the Sturgeon River. South of Felch Moun- —— STURGEON QUARTZITE IN FELCH MOUNTAIN RANGE. 399 ‘tain, in sec. 32, 'T. 42 N., R. 28 W., it outcrops immediately south of the abandoned Northwestern mine, and has also been found in drilling on the west and in test pits on the east of the natural exposures through a distance of half a mile. In sec. 33, T. 42 W., R. 28 W., a small ledge, a few feet square, occurs between the overlying dolomite and the Archean, 200 feet east of the road to the Calumet and Hecla (iron) mines. East of this the contact between the Archean and the Algonkian is a faulted one, and the quartzite is buried beneath the overlying formations. The northern zone of the Sturgeon formation is not nearly so well exposed, nor for the most part does it fall within the artificial line that bounds our detailed work on the north. Sees. 34 and 35, T. 42 N., R. 30 W., on the west contain a few scattered outcrops, one of which is of exceptional petrographical interest and to be noticed later. The next exposures are 5 miles east, along and just north of the north line of see. 35, T. 42 N., Rk. 29 W. The main northern zone of the Sturgeon formation coming from the west lies south of these exposures and is entirely covered. Between the two the tongue of Archean schists already described is faulted up. Two miles farther east quartzite again appears in test pits, low-lying outerops and drill holes along the northern border of sec. 31, T. 42 N., R. 28 E., and in section 29, immediately north of section 32, is well exposed in a broad belt that reaches north almost to the east-and-west quarter line. The quartzite often forms distinct linear ridges, which in spite of the chemical stability and apparent homogeneity of the rock seldom rise to the mean altitude of the neighboring Archean areas. An exception to this rule is the succession of ridges formed by the southern zone in the 3-mile stretch west of sec. 31, T. 42 N., hk. 29 W.; these frequently overtop the adjacent Archean plateau. Very frequently, also, the quartzite zones occupy lower ground not only than the Archean but even than the immediately overly- ing dolomite. The southern zone, for some unknown reason, is a distinctly weak belt east of sec. 32; T. 42 N., R. 29 W., and for several miles forms the bed rock of the Sturgeon and the connecting valleys. FOLDING AND THICKNESS. It is extremely difficult in most cases to determine directly the attitude of the Sturgeon formation, owing to its generally massive and homogeneous character. This is due, as will be shown hereafter, to the completeness of 400 THE CRYSTAL FALLS IRON-BEARING DISTRICT. the recrystallization, in consequence of which the ordinary sedimentary . features that it originally possessed have been almost entirely obliterated. Faint color banding, itself of secondary development, but no doubt pre- serving a distinction in original composition, alone remains, and only here and there, as a guide to the former stratification. By scattered indications of this sort, and by the better evidence afforded by the overlying dolomite, often very distinctly banded, it is known that the southern zone of quartzite on the whole dips toward the north. Southward dips also occur in this belt, by which it is known that subordinate folds occur within the quartz- ite itself. From the considerable variations in the surface width of the forma- tion we are led to suspect the existence of more of these little folds than we are able to prove. However, the secondary syncline, which extends from the offset already referred to in sec. 35, T. 42 N., R. 30 W., for 6 miles to the east to sec. 35, T. 42 N., R. 29 W., and includes no formation higher than the quartzite, is very definitely determined. In the northern belt of the Sturgeon formation the indications of dip are generally northward at very high angles. These indications, not in themselves conclusive, are reenforced by a corresponding attitude in the overlying dolomite, and it is therefore probable that there is a general, or at least widespread, overturn in the dip of the northern belt. Since the contacts of the Sturgeon formation with the underlying Archean and with the overlying dolomite are (except in one case) covered, it is impossible to obtain the data for very accurate determination of its thickness. The uncertainty in most outcrops as to the dip of the quartzite introduces an additional difficulty. However, in sec. 35, T. 42 N., R. 30 W., on the west end of the range, and in sec. 33, T. 42 N., R. 28 W., 11 tniles farther east, the covered intervals to the limiting formations are not great, and if the contacts are not faulted (which is far from certain), the minimum thickness is determinable within a reasonable limit of error. In the western locality the surface width of the zone probably under- . lain by quartzite is about 500 feet. The quartzite itself is structureless, but the overlying dolomite dips northward at an average angle of about 70°. If the same dip holds in the quartzite, its true thickness is about 470 feet. In the eastern locality similar data lead to a thickness of nearly 430 feet. In these two sections the quartzite zone is much narrower than it is else- where, either because undetected faults have reduced it, or because it is STURGEON QUARTZITE IN FELCH MOUNTAIN RANGE. 401 uncomplicated by subordinate folds. It is probably safe to conclude, in view of the uncertainties, that the average thickness of the formation is not less than 450 feet, and may be considerably more. In a preliminary paper on the district,’ written before the field notes were fully analyzed, I have placed the thickness of the quartzite at about 700 feet; but this figure is probably too large. PETROGRAPHICAL CHARACTERS. The Sturgeon formation includes a few very closely related rock varieties, of which quartzite furnishes the great majority of the exposures. The quartzites are usually light gray in color, and break with a coarsely ‘granular or glassy fracture. To the eye quartz is often the only recogniza- ble constituent in the body of the rock, although the numerous joint and shearing planes shimmer with little silvery plates of muscovite. Occasion- ally a weathered surface is dotted with minute specks of an opaque pinkish substance, which leads one to suspect the presence of feldspar. Chlorite also is now and then visible in the darker varieties. The quartzites are almost uniformly massive, except for the secondary fractures above mentioned. At scattered localities, however, a faint color- banding, due. to the presence of layers of a pinkish hue, which are inde- pendent of the secondary fractures, seems to indicate the original stratifica- tion. The color bands are generally only vaguely defined; occasionally, however, they are numerous and sharp. Closely associated with the massive quartzites are sheared quartzites, or micaceous quartz-schists. These rocks are merely varieties of the quartzite in which secondary shearing planes, with their attendant growths of new muscovite, are more abundant than usual. The shearing surfaces almost invariably intersect, with the result that the new structure tends toward the linear-parallel type, and is often as similar in appearance as it is in origin to the structure already described in connection with the sheared granites. In a locality already referred to, on the south bank of the Sturgeon, in sec. 36, T. 42 N., R. 29 W., where the Sturgeon formation is in visible contact with the Archean, the quartzite is underlain by a considerable thickness of very fissile muscovite-biotite-gneiss, which incloses rather sparingly obscure pebbles of granite and quartz. This gneiss, which no ‘Relations of the Lower Menominee and Lower Marquette series in Michigan (Preliminary): Am. Jour. Sci., Vol. XLVII, 1894, p. 217. MON XXXVI——26 402 THE CRYSTAL FALLS IRON-BEARING DISTRICT. doubt was formerly an arkose rich in feldspar, has recrystallized and after- wards been sheared; the coarse micas to which the fissility is due, together with other new minerals, have grown between the fractured surfaces and recemented the broken mass. It affords beautiful examples of foliation parallel to a line. The thin sections of the Sturgeon quartzite are of exceptional interest, The principal constituent is, of course, always quartz. With the quartz are associated, in much smaller amounts, and not necessarily all in the same section, numerous accessories, including muscovite, biotite, chlorite, micro- cline, orthoclase, plagioclase, titanite, rutile, zircon, apatite, and the ores. The relations of the quartz to the other constituents present very unusual features, and indicate that the metamorphic changes by which the present completely crystalline rock has been made from an original granitic sand have proceeded along lines not hitherto distinctly recognized in the forma- tion of rocks of this character. Among the large number of slides examined, a broad distinction can at once be made between those which show the effects of stress in a pro- nounced degree and those in which such effects are subordinate or hardly noticeable. Connecting these two classes is a perfectly graded series; and it is therefore certain that those of the first are merely the more or less modified varieties of an earlier stage, represented more nearly by the second. In the slides in which the effects of pressure are least apparent the micro- scopic characters are as follows: The background is composed of large irregular grains of quartz, the edges of which interlock with the most minute and sharp interpenetrations. ‘The longest dimensions of these grains range from 1.5 to 6 mm., averaging perhaps 2.5 or 8. They often have a rather vague parallel elongation, which corresponds to the alignment of the minerals which they inclose. Scattered very abundantly through these large quartz grains are the accessory minerals, some predominating in one slide, others in another, but the micas and chlorite occurring in all. Through each slide the accessory minerals, with the exceptions noted below, lie with their long axes in a common direction, and frequently cross the serrated boundaries between adjacent quartzes. The inclusions in many cases have the form and other characters of clastic minerals, and thus preserve the only microscopic evidence of the original nature of the rock. The included micaceous minerals are usually in small plates, ranging STURGEON QUARTZITE IN FELCH MOUNTAIN RANGE. 403 from 0.05 to 0.75 mm. in longest dimensions, but few, however, exceeding 0.2. Many of these are bent and split, the clear unstrained quartz of the host pene- tratmg from the frayed edges into the interior between the partly separated leaves. Biotite and muscovite, and sometimes chlorite, occur in the same individual, indicating alteration before inclusion in the quartz host took place. Besides its common occurrence as an alteration product of the biotite, a few rounded areas of chlorite, made up of little radiating tufts, seem to be pseudomorphs of garnet. Inclusions of titanite and magnetite, or a related ore, are not uncommon in the larger micas, and the biotite and chlorite sometimes inclose beautiful sagenite webs. Many of the smaller micas, however, have clear sharp edges and depart from the general paral- lelism of the other inclusions. These are either contemporaneous crys- tallizations or else, perhaps, were primary inclusions in former grains of clastic quartz which has since disappeared. Some of the clastic plates of biotite are bleached and include spheroidal blebs of red iron ore, similar to those described in the case of the Archean mica-schists. The microcline inclusions are usually elongated in form, and frequently, particularly in the cases of the larger, have well-rounded clastic outlines. The long dimension, which usually coincides with one of the cleavages of the mineral, rarely exceeds 0.5 mm. or falls below 0.08 mm. The periphery is frequently partly surrounded by a thin film of biotite. Within the micro- clines are sometimes contained little blebs of quartz, which are not oriented optically with the host, and also, more rarely, small plates of biotite. The microcline individuals are sometimes broken into two or three differently oriented parts, which may be separated from each other, in which cases the quartz of the host has completely filled the interspaces. Fracture in the feldspar is often unattended with the shghtest appearance of strain in the inclosing and cementing quartz, which extinguishes as one individual, and is therefore unmistakably to be attributed to stresses previous to the erystalli- zation of the quartz. Besides microcline, both orthoclase and plagioclase are sometimes inclosed in the large quartzes, but much more sparingly. They are invari- ably more or less decomposed, and are sometimes surrounded partially or wholly by a film of ferruginous material. They show the same phenomena of fracture, and occasionally of separation with penetration of the host, as the microcline, and occur in grains having a similar range in size. 404. THE CRYSTAL FALLS IRON-BEARING DISTRICT. Titanite is of frequent, zircon of rather rare, occurrence. The titanite is found not only inclosed, as already stated, in biotite and chlorite, but also in well-rounded clastic grains which are often bordered with an opaque ore. Zircon occurs in broken grains, without doubt clastic, and also in small crystals which show no signs of wear. These last were probably entirely embedded in original clastic grains of quartz. Besides the above minerals of usual occurrence, small quartz grains of ~ different orientation from the matrix are very rarely found included in the large quartzes of the general background. Only two or three such cases have been observed, and in these the included grain is surrounded almost wholly with thin plates of mica. It is believed that these are original clastic grains which, perhaps because protected by a film of material now represented by the micas, have escaped the general fate of their neighbors. One or two composite inclusions, made up of microcline, the micas, and quartz, have also been noticed. These seem to represent original pebbles of granite or a crystalline schist. The pressure effects begin with the appearance of optical stram and decided elongation in the large quartzes of the groundmass. This is fol- lowed by fracture, either along or quite independent of the original sutures, the crack often halting in the interior of a grain. The fractures preserve very roughly the same general direction, but frequently mtersect at very acute angles, or come together in sweeping curves. The breaking is fol- lowed by movement, and this results in the production of a fine-grained quartz mosaic between the parted surfaces. In the final stages shown in the series of slides in my collection, the rock is made up of long, narrow lenses, each of which is an enormously strained quartz individual, separated by narrow anastomozing zones of very finely subdivided quartz. After the fracturing took’ place there seems to have been no further distortion of the lenses, for the edges of adjacent individuals follow similar curves, which are often reversed, and in many cases could be brought together with an accurate fit. If the Sturgeon quartzite represents an original sandstone, it is evident from the facts stated above that the old quartz grains have undergone com- plete reerystallization. The usual conception, since the time of Sorby, of the process by which quartzites are formed from original deposits of sands is that new quartz is deposited around each original fragmental quartz grain, STURGEON QUARTZITE IN FELCH MOUNTAIN RANGE. A405 in similar crystallographic orientation with it, and that neighboring grains thus enlarged finally interlock by mutual limitation of one another's growth. This explanation evidently can not account for the background of large interlocking quartz areas in these rocks, for if it were true it would be nec- essary to assume that the quartz grains were less numerous in the original deposit than those of almost any other mineral, in some slides even than the titanite or chlorite. There seems to be but one escape from the conclusion that the large quartz areas must each represent a number of original frag- mental quartz grains, which, as deposited, must have lain in the rock with their crystallographic axes disposed entirely at haphazard; and that is the hypothesis that this quartzite was not originally a sandstone, but consisted mainly of soluble and easily replaceable material, such as limestone, with the fragmental particles scattered through it, and that the large quartzes of the background have replaced this soluble substance. I have been able to find no positive evidence to support this hypothesis, and I am com- pelled to believe that the rock was a sandstone in which, in some way not easy to understand, considerable numbers of adjacent quartz grains have united to form or have been absorbed into a new individual, leaving absolutely no trace of their former separate existence. The introduction of new silica, or the separation of silica from decomposing silicates in the rock itself, may well have been essential factors in the recrystallization. I shall make no attempt to explain the process further than to point out its probable analogy with the process by which the new microclines were formed in the Archean mica-schists. The close alignment of the clastic minerals inclosed in the large quartz areas, their frequent fracture, and their occasional separation, indicate that the time of crystallization probably followed a period of stress; while the very vague parallel elongation of the individuals of the background in the unstrained sections would seem to show that they crystallized under static conditions. Unquestionable proof of a period of stress later than the erys- tallization is given by the numerous slides, in which these grains are seen to have suffered fracture and distortion. The microscopical study of the quartzites thus supplies important evidence, not afforded by the outcrops, as to the orogenic history of the district. 406 THE CRYSTAL FALLS IRON-BEARING DISTRICT. SECTION V. THE RANDVILLE DOLOMITE. The Sturgeon quartzite is succeeded by a formation consisting, so far as is known, almost wholly of crystalline dolomitic rocks. Excellent exposures belonging to this formation are situated within a short distance of Randville station, on the Milwaukee and Northern Railway, and it may therefore conveniently be named the Randville dolomite. DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY. Owing both to its great thickness and to its mtermediate position in the series, the Randyville dolomite in the Felch Mountain range covers a larger share of the surface than any other member of the Algonkian suc- cession. The overlying formations are frequently interrupted, because of the changes in direction of pitch of the secondary synclines in which they occur. In these gaps the dolomite covers the whole interior of the syncli- norium. Where the higher formations are present, they divide the dolomite into two or more parallel east and west belts, one of which lies south of the northern quartzite and the other north of the southern. Only in portions of secs. 85 and 36, T. 42 N., R. 29 W., where the rise in the axis of the main syncline has lifted it above the present surface of denudation, is the dolomite entirely absent from the main trough. Natural exposures of the dolomite are not so numerous as of the quartzite, but they are more evenly distributed. Moreover, owing to its proximity to the Groveland iron formation, the dolomite has been penetrated by many test pits and diamond-drill borings put down in search of ore, and these supply important information in the covered areas. From the western end of the map to sec. 34, T. 42 N., R. 29 W., the dolomite is for most of the way separated into two or more parallel belts. The southern belt is especially well exposed in secs. 35 and 36, T. 42 N., R. 30 W., and in sec. 31, T. 42 N., R. 29 W., and for 2 miles to the northeast, beyond which it has been found only in test pits and drill holes. In the middle of sec. 35, T. 42 N., R. 29 W., the base of the formation is brought to the surface by the westerly pitch of the main fold and is well exposed along Sturgeon River. North of the strike fault, which, as already described, has brought the —_— RANDVILLE DOLOMITE IN FELCH MOUNTAIN RANGER. 407 Archean mica-schists into contact with the dolomite and quartzite in the northern part of the same section, the Randville formation runs east in a single belt, which probably continuously widens as the throw of the fault diminishes. It has been found in several places in the north half of see. 31, T. 42 N., R. 28 W., and near the east line of this section the appearance of the overlying mica-schists again divides it into two belts, which pass to the north and south of the Felch Mountain syncline. The northern belt has been proved by test pits only, but the southern is well exposed naturally in the neighborhood of the Northwestern mine. Other exposures also occur south of the unconformable mica-schists and quartzite of the upper series, in the central portion of sec. 33, T. 42 N., R. 29 W. The dolomite is relatively a weak rock, and generally occupies lower ground than either the quartzite below or the iron formation above it. The belt in contact with the southern belt of quartzite especially is valley making throughout most of its extent. The outcrops usually form low, steep-sided knolls elongated with the strike and of slight relief above the basement; these occasionally unite into linear ridges, as in sec. 35, T. 42 N., R. 30 W. The northern belt is one of low general relief, from which, how- ever, similar isolated knobs often protrude. The largest and most prominent of these is the peak in the northeast quarter of NW. 4 sec. 36, T. 42 N., R. 30 W., which rises 80 feet above its base, covering 8 or 10 acres. No actual contacts between the Sturgeon and Randville formations have been found, but from their close association and continuity, as well as from the structural characters, when these are determinable, they seem everywhere to be strictly conformable. Near the quartzite the dolomite becomes distinctly more impure and contains a larger proportion of silicates and quartz. It is altogether probable that between them come transition beds, as indeed is shown by some of the drill records. In one of these ‘“taleky mica-schists, micaceous limestone, altered actinolite-schist, and quartzite” are described as being interbedded near the junction. The determination of the thickness of the Randville formation is beset with the same difficulties as are encountered in the case of the quartzite, namely, the uncertainty as to the exact position of the contacts and the possibility of faults and subordinate folds within the formation itself. The best sections give a wide range of values from a minimum of about 500 408 THE CRYSTAL FALLS IRON-BEARING DISTRICT. feet near Felch Mountain to a maximum of nearly 1,000 feet im the western part of the district. While the discrepancies may be partly due to lack of precision in the data, it is probable that the thickness of the formation is not uniform, but really increases from east to west. On the Fence River, 18 miles northwest of Randville, the thickness is probably about 1,500 feet. Accordingly, accepting each of these determinations as approximately correct, 700 feet may be taken as a fair estimate of the average thickness of the Randville dolomite within the Felch Mountain range. PETROGRAPHICAL CHARACTERS. The outcrops of the Randville formation consist exclusively of dolo- mite, more or less pure, and always thoroughly crystalline. A few comparatively thin layers of schists, probably both micaceous and amphibolitic, and also of quartzite, are mentioned in certain drill records to which I have had access as occurring interbedded with the dolomite; and while the lithological determinations are perhaps not entitled to much weight, they at least prove the existence of rocks which are not dolomite within the formation. In the field, however, such interbedded layers do not outcrop, and they must constitute an extremely small part of the total thickness. From the results of my work the Randville formation appears as a lithological unit. . Macroscopically the dolomites are rather coarse-grained marbles, of various shades of color, of which pinkish or bluish white are the most common. They always inclose, more or less abundantly, large flakes and aggregates of tremolite, which are particularly noticeable from their projec- tion above the weathered surface. Occasionally tremolite and other silicates are the most abundant, and sometimes, for small thicknesses, are essentially the only constituents. Quartz and chlorite are also often present, but in much smaller amounts. The weathered surface is usually dulled to a light brown or creamy yellow in a thin superficial skin, but is not deeply iron- stained, except when the silicates containing ferrous iron are present. The following partial analyses of three specimens from different parts of the range show that the carbonate is normal dolomite. The insoluble portion consists chiefly of tremolite. These analyses were made for me by Mr. G. B. Richardson, a graduate student in geology in Harvard University. RANDVILLE DOLOMITE IN FELCH MOUNTAIN RANGE. 409: Analyses of Randville dolomite. at, | II TIL | | ss Innsrolmliniey rem IBKON p35 5ks 5 ame cece 2.0 | 9.7 29.1 Hes OMeme eae jae Seas ee eee eneos Hoe | Shi 22 CHICO) 5 SAE a tek eas a ete 53.2 | 48.9 39.3 Me COs soya ne saetiosseemascee CPEB |) BELO || Bair RO tale eee Res Fh 98.7 98.7 | 98.3 eet” EI = | The outcrops, while often entirely massive, usually possess decided structural features. These are indicated by color banding, by differences in texture, and by the banded arrangement of the components. Slight variations in the body color of the rock, proceeding from no distinguishable variation in composition, often occur in alternate parallel layers, which are _ persistent within the limits of observation. With the color banding often go variations in texture, which, however, are neither so regular nor nearly so persistent. The characteristic form taken by these is in thin layers, which as they contmue open out into nodules. Such layers consist of closely packed crystalline grains, very much coarser than the body of the rock, which have grown normal to the boundaries. Adjacent layers are not strictly parallel and sometimes cross each other. They are believed to represent ancient fracture and slipping surfaces, which followed very closely the original bedding, in which the new carbonate individuals have had room for larger growth. The arrangement of the accessory minerals, especially the tremolite, also is usually a banded one. Layers rich in tremolite alternate with layers poor in tremolite, while within the layers the orientation of the tremolite individuals is usually at random. The structure brought out in these various ways is, on the whole, a parallel structure. It corresponds with the strike and dip in all the localities where these can be independently confirmed by the attitude of the adjacent for- mations, and it also has been thrown into minor folds. I therefore regard the structure as having originated partly in chemical differences in the material originally deposited and partly in secondary growths in the open spaces and rubbing zones determined by relative movements along the sur- faces of easiest fracture at the time of the earliest folding, and for both reasons preserving in the subsequent metamorphism the true stratification of the formation. 410 THE CRYSTAL FALLS [RON-BEARING DISTRICT. Under the microscope the dolomites show no features of special inter- est. They are thoroughly crystalline rocks, chiefly composed of coarse grains of dolomite with which is associated a considerable number of acces- sory minerals. Of these the most important are tremolite, diopside, chlorite, muscovite, phlogopite, quartz, and rutile, while apatite, tourmaline, pyrite, and magnetite are rare. The dolomite is by tar the most abundant constituent in most of the slides, and furnishes the general background for the accessories. The shape of the grains in many sections is decidedly oval, and the long axes lie in the same direction, thus producing a foliation. Tremolite is abundant in some of the sections, and is entirely absent from none. It occurs in long-bladed individuals and aggregates, usually bounded by the prism, but one or both pinacoids are also sometimes present. It includes portions of the carbonate background. Diopside is rather rare; it occurs usually in small single individuals, with sharp crystal outlines. It is sometimes surrounded by tremolite, from which it is distinguished by its high obliquity of extinction and its almost rectangular cleavage. Partings parallel to both pinacoids, as well as a transverse parting in prismatic sec- tions, are also observable. Quartz occurs in irregular grains completely interlocking with the dolomite, and in some cases with tremolite. In the slides examined it is in all cases a secondary as well as a rare constituent. In no case is there any indication that it is clastic. Chlorite is an abundant constituent of some of the slides, while from others it is entirely absent. Muscovite in little frayed plates is plentiful in some sections. Quite pos- sibly some of these may be original clastic particles. The most interesting mica, however, is phlogopite, which is very abundant in one locality near the base of the formation. It occurs in large, cleanly bounded plates, each of which is a multiple twin, and evidently a product of secondary crystallization. Some of these plates have been strongly bent, thus showing that the dolomite, like the quartzite, has been deformed since it crystallized. _ The thin sections therefore show that the rocks of this formation have experienced even more nearly complete reconstruction than is shown in the case of the quartzites, for here none of the constituents, except possibly some of the smaller micas, are present in their original form. Also the evi- dence for disturbance after crystallization is of similar character and equally MANSFIELD SCHISTS IN FELCH MOUNTAIN RANGE. 411 strong. Accordingly, a close agreement in the sequence and in the charac- ter of the principal events thus indicated in the history of the two rocks may be recognized. ‘These considerations make it quite certain that the recrystallization of the two formations was essentially contemporaneous. From the character of the accessory minerals in the dolomite it is probable that the crystallization was not accompanied by the introduction of foreign material from outside, in notable quantities, but consisted in a mineralog- ical rearrangement of the elements present in the rock from the beginning. SECTION VI. THE MANSFIELD SCHISTS. Above the Randville dolomite comes a formation composed chiefly of fine- to medium-grained mica-schists. Owing to their exceedingly soft character and small thickness, these rocks are exposed naturally in only a few localities in the Felch Mountain area. A series of phyllites less meta- morphic but otherwise similar, and occupying the same stratigraphical position, immediately above the dolomite, outcrop characteristically at the Mansfield mine, and especially north of it, near the Michigamme River, in T. 48 N., R. 31 W. For these reasons it is convenient to name the forma- tion for the Mansfield locality. DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY. The existence of the Mansfield formation in the Felch Mountain area is known mainly from test pits and the records of diamond-drill borings and early explorations. Fortunately, these are so widely distributed that the persistence of the formation is well proved. Many drill holes have passed through it into the dolomite. Immediately above it comes the mag- netic Groveland formation, which even when covered betrays its presence to the compass needle. With the upper and lower limits thus determined, and with the large body of data supplied by the test pits and records, there is no difficulty in indicating its approximate boundaries for the greater part of the map. On the west, mica-schists belonging to the Mansfield formation have been proved by diamond drilling to occur between the dolomite and Grove- land formations in the south half of sec. 34, T. 42 N., R. 30 W. Farther east there is a line of outcrops in the eastern portion of section 35, and the 412 THE CRYSTAL FALLS IRON-BEARING DISTRICT. schists have also been found in test pits on both sides of the western exten- sion of the Groveland syncline in sec. 36, T. 42 N., R. 30 W. In see. 31, T. 42 N., R. 29 W. (the Groveland section), they have been penetrated in 10 drill holes, besides numerous test pits, giving altogether a cross section more than half a mile in length from north to south. In the northern half of sections 32 and 33 numerous test pits have exposed the Mansfield formation, proving that it borders on both sides the narrow syncline, the interior of which for a mile and a half is occupied by the magnetic Groveland jasper. Through secs. 34, 35, and 36, T. 42 N., kh. 29 W., and sec. 31, T. 42 N,, R. 28 W., the mica-schists have not been discovered, probably both because they are but feebly represented and because but few test pits have been sunk through the Cambrian blanket. In sees. 32 and 33, T. 42 N., R. 28 W., the mica-schists have been found in scattered test pits and borings on both sides of the interior jasper of the Felech Mountaim syncline, and also on the south side of section 33. The thickness of the Mansfield formation is so small—not more than 200 feet—that it produces no very noticeable effects on the general topog- raphy, in spite of the ease with which it weathers. In the western portion of the district, through secs. 34 and 35, T. 42 N., R. 30 W., with the dolo- mite it underlies a broad low-lying plain, which is bounded on the south by a ridge of the Sturgeon quartzite backed by the Archean plateau. On the north, a broad ridge, through which diagonally pass the Archean gran- ites and gneisses, the quartzite, and the dolomite, defines this valley as far east as the middle of section 35; in the northern and central portions of this section it spreads out into a swampy lowland, diversified by glacial sand plains, expressive of the gradual widening of the trough and of the gen- erally horizontal attitude of the soft rocks of the interior. The most defi- nite topographical feature directly due to the Mansfield schists is the narrow steep-sided valley which runs east from this lowland for nearly 2 miles, on the south side of the Groveland syncline. The ancient stream valley filled with the Cambrian sandstone, already mentioned, follows along this narrow belt. PETROGRAPHICAL CHARACTERS. The hand specimens from the various test pits, the drill cores, and the few small outcrops indicate that the Mansfield formation is quite uniform in character throughout the Felech Mountain area. The great majority of MANSFIELD SCHISTS IN FELCH MOUNTAIN RANGE. 413 the specimens are of fine-grained mica-schists, the color of which varies from light to dark, according as muscovite or biotite is the predominant mica. Garnets, in some localities, are very abundant, especially near the contacts with intrusives. It appears from the records of explorations that thin seams of jaspery iron ore interlaminated with the schists have been encountered in occasional drill holes and test pits, but no specimens of such occurrences have been obtained. Their existence is of interest, as showing the likeness in an important character of these more altered rocks with the slates occupying the same relative position in the Iron Mountain and Norway areas. The outcrops and specimens are frequently well banded in lighter and darker layers, the color banding in some cases not coinciding with the schistosity. Just south of the Groveland mine, in a test pit which was sinking at the time of my visit, the color bands which mark the true strati- fication, as shown by the contact with the underlying dolomite, are closely crumpled and cut by the foliation of the rock, which is much the more dis- tinct of the two structures. Near the contact with the overlying Groveland formation the mica- schists become both more siliceous and more ferruginous, and there is accordingly a distinct passage between the two formations. This does not necessarily signify a transitional character in the original sediments, but may be altogether due to the downward transportation of silica and iron from the upper rock. The mica-schists are generally very tender rocks, and the material on the dumps of test pits sunk in them is usually far gone in decomposi- tion after a few years’ exposure to the weather. From even the freshest specimens the little flakes of mica often rub off on the fingers. Where penetrated by intrusions, however, as in sec. 35, T. 42 N., R. 30 W., and in sec. 31, T. 42 N., R. 28 W., they become very much harder. Under the microscope the rocks of this formation are seen to be in the main thoroughly crystalline, though very fine-grained, agereeates of biotite, muscovite, chlorite, quartz, and feldspar, with the iron ores, rutile, tourma- line, and apatite as the accessories. Garnets are abundant in some of the sections, and with these also occur actinolite, epidote, titanite, and an unde- termined colorless amphibole in stout single prisms. In the eight thin sec- tions which I have examined from this formation I have found no material 414 THE CRYSTAL FALLS IRON-BEARING DISTRICT. which is certainly original and fragmental, although almost every slide contains grains that may possibly be such. On the other hand, it is evident that the large majority of the individual grains have formed in place. The micas are in most cases the most abundant constituent; sometimes muscovite, though usually biotite, predominates. The two micas are often intergrown. ‘The biotite is usually very deeply colored, both brown and green, and, except in the thinnest slides, is almost opaque even in cleavage sections. The larger mica flakes do not exceed 0.5 mm. in length, and average not more than 0.25 mm. Quartz generally occurs in irregular grains, full of fluid inclusions, and inclosing the various accessories. It frequently appears in little triangles in the interspaces between adjacent flakes of mica. Rarely part of the peri- meter is rounded and embedded in a mica, thus suggesting a clastic origin. Feldspar is very abundant in some of the slides and entirely absent from others. Both microcline and plagioclase occur, and in forms similar to the quartz. Biotite sometimes penetrates in irregular shredded edges and filaments into the interior of the feldspars, and in such cases may be a metasomatic product, as described by Irving and Van Hise* in the mica- schists of the Gogebic district. But much of the feldspar, as shown by its form and freshness, has recrystallized. The alignment of these minerals is with the schistosity of the rock, which they thus determine. When the schistosity cuts the lines of stratification, as it frequently does, the latter are but faintly marked in the thin section by very shght mineralogical differences. Thus a dark band, which may be very striking macroscopically, may be due merely to the predominance of deeply colored biotite; a light band, to the predominance of muscovite. Sometimes, however, in these bands a grain of quartz, or a stout flake of muscovite, lies out of the general orientation and with the direction of the band. Such grains are very possibly original. The schistose structure, as has already been stated, is determined by the general parallelism of the long axes of the constituent grains. Since the greater part, if not demonstrably all, of these grains have formed in this position, and have not been forced mechanically into it, the cases in which the schistosity cuts the bedding support the inference as to the time of the general recrystallization of the series grounded on the facts observed 'The Penokee-Gogebic iron-bearing district of Michigan and Wisconsin, by R. D. Irving and C.R. Van Hise: Mon. U.S. Geol. Survey, No. XIX, 1892. GROVELAND FORMATION IN FELCH MOUNTAIN RANGE. 415 in the lower formation, namely, that this time followed a period of great strésses. Also a period of still later stress has affected the recrystallized con- stituents of the schists, just as it has those of the quartzite and dolomite. It is shown by lines of fracture crossing the slides along which ferric oxide has infiltrated, and by occasional straining and bending of the quartz and mica. Garnetiferous varieties of the schists are found in close proximity to basic igneous rocks, probably in every instance intrusives, and are evidently the result of contact metamorphism. With the garnets occur actinolite in felted mats and clusters, and abundant magnetite and pyrite. A colorless amphibole in large single crystals bounded by the prism and clinopinacoid, and giving low extinctions, is often associated with the actinolite. SECTION VII. THE GROVELAND FORMATION. The ferruginous rocks which compose this formation are well exposed in the central portion of sec. 31, T. 42 N., R. 29 W., in the vicinity of the Groveland mine, and thus may properly be termed the Groveland formation. DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY. The magnetite, which is always an abundant constituent of these rocks, has made it possible to trace them for long distances throughout the trough, by means of the disturbances effected in the compass needles. The same disturbances had led to the sinking of a great number of test pits on the part of former explorers for iron ore, and the material thrown out of these has served to check and substantiate the inferences from the magnetic attractions. Finally, in several localities excellent natural exposures of the iron-bearing rocks occur. So, altogether, the available data as to the surface distribution of the Groveland formation are fairly satisfactory. On the west the presence of the Groveland formation through sees. 34, 35, and 36, T. 42 N., R. 30 W., is shown by one principal and other minor lines of attraction, as well as by test pits and outcrops. The principal line of attraction begins in section 34, near the southwest corner, and runs to the northeast, in conformity with the strike of the northern belt of dolomite, finally ending in the northeastern portion of section 36. This line of attrac- tion is very vigorous and strongly marked. Two other lines, parallel with the principal line, but more feeble and much shorter, cross the boundary between sections 35 and 36, and on the northern of these ferruginous rocks 416 THE CRYSTAL FALLS [RON-BEARING DISTRICT. outcrop in the western part of section 36. Near the center of section 36 another line, marking the western end of the Groveland syncline, begins and continues for a mile and a half east to the eastern portion of sec. 31, T. 42 N., R. 29 W. Along the western portion of this line are many test pits, and in section 31 the fine exposures of the Groveland hill. Four hundred paces north of the center of sec. 32, T. 42 N., R. 29 W., another line of attraction begins, and may be followed toward the east without interruption nearly to the east line of section 33 of the same town- ship. Along this line, which is comparatively feeble and crosses wet ground, there are but few test pits. In the eastern part of section 33, beyond the point at which the attractions cease, many pits have been sunk to and into the Mansfield formation, which is there somewhat ferruginous. From this point east for 4 miles the Groveland formation has not been recognized. . In the northern part of secs. 32 and 33, T. 42 N., R. 28 W., the fer- ruginous rocks are again well exposed on Felch Mountain for nearly a mile along the strike, and may be identified for half a mile farther by the vigor- ous disturbances produced in the magnetic needles. In the southeastern quarter of section 33 the Groveland formation is again encountered in a small and much-disturbed area, in faulted contact with the Archean. The most conspicuous hills within the Algonkian belt owe their relief to the fact that they are underlain by the Groveland formation, but else- where this formation has left but little impress on the topography, perhaps because the local base-levels are cut nearly to the bottoms of the synclines in which it is preserved. The two hills referred to—Felch Mountain, in sees. 32 and 33, T. 42 N., R. 28 W., and the Groveland hill, in see. 31, T. 42 N., R. 29 W.—stand 100 feet or more above the average level of the surrounding Algonkian territory, and in both instances the infolded second- ary synclines are exceptionally deep and broad. 'The magnetic lines which indicate the other synclines pass through low ground, and the belts of dis- turbance are much narrower than in the cases of the two principal hills. ‘There seems to be, so far as the collected material warrants a judgment, no lithological difference between the rocks of the narrow and those of the broad and deep synclines, and accordingly the relief of the latter is believed to be caused by their depth below the adjacent base-levels and not by their more resistant character. GROVELAND FORMATION IN FELCH MOUNTAIN RANGE. 417 PETROGRAPHICAL CHARACTERS. The rocks of the Groveland formation have a general family likeness, which makes it very easy to distinguish them in the field from all the other members of the Algonkian series. Among them two main mineralogical kinds may be recognized, the usual one of which consists of quartz and the anhydrous oxides of iron, while the other, which is much rarer, is made up essentially of an iron amphibole, quite similar to the griinerite of the Marquette range, with quartz and the iron oxides as associates. As seen in the field, the rocks of the first kind are generally siliceous, heavy, and dark colored, the weight and color, which has a tinge of blue, being due to the presence of abundant crystalline iron oxides. A large part of the silica is easily recognized as crystalline quartz, in some instances, indeed, in the form of detrital grains. The visible iron oxides occur both as little spangles of specular hematite and also in irregular dark-blue masses and single grains, the latter often having the crystalline form of magnetite. Many, if not most, of these last, however, seem to be really martite, as they give a dark purple streak, and in fine powder are not attracted by a hand magnet. In the first kind there is much variety in external appearance, deter- mined by the variable proportions in which the chief constituents occur and by the different ways in which these constituents are arranged. Con- siderable areas, for example, consist mainly of granular quartz merely darkened by the intimately mixed iron oxides, and in these, so far as the eye can judge, the rock is a ferruginous quartzite. Closely connected with such occurrences, or included most irregularly in them, are others in which the ferruginous constituents are so abundant and the quartz so subordinate that they would pass for lean iron ores. Between such rare extremes we find all intermediate proportions of mixtures of the quartz and the iron oxides. One form of arrangement of the constituent minerals is in narrow parallel bands, in which the quartz and the iron oxides alternately predom- inate. Such alternations are sometimes so frequent and regular as _per- fectly to reproduce the lean “flag ores” of the Marquette range.” Regular banding, however, is not common. Usually the light or dark bands are 1Geol. Survey Michigan, Vol. I, Part I, by T. B. Brooks, pp. 93-94. MON XXXVI Pal| 418 THE CRYSTAL FALLS IRON-BEARING DISTRICT. suddenly cut off, as if by faulting, or taper to thin edges, or occur in sepa- rated pebble-like forms. Neighboring lenses and fragments of bands are most frequently roughly parallel with one another, but often they are jumbled together in the greatest confusion. They no doubt represent an original more continuous banding, which has suffered brecciation. Masses thus shattered are also traversed and cemented by numerous small veins filled chiefly with quartz, chalcedony, and specular hematite. The posi- tions in which the separated patches of the Groveland formation now survive, namely, in and near the bottoms of synclines, and therefore at the points where sharp turning and crowding together have taken place, suffi- ciently explain the extensive brecciation observed in these brittle beds. Very prevalent in all the varieties of the first kind of rock, i massive, banded, and brecciated alike, is the occurrence of some of the constituents in small roundish spots, which give to the whole formation a very detrital aspect. In the quartzitic phases, as well as in the most ferruginous bands, the eye recognizes, besides the little grains of clear quartz, which seem to be unquestionably detrital, numerous small dots of blue hematite and bright red dots of jasper. These are more abundant in some layers than in others, but seem never to be entirely absent, and are exceedingly characteristic of the formation wherever found. In a few localities the iron constituent is almost entirely in the form of little micaceous scales of specular hematite, which have a parallel arrange- ment. Hematite-schists, however, are not very common. The best exam- ples occur in the northern part of sec. 36, T. 42 N., R. 30 W., along the northern syncline. The second kind of rocks of the formation, the griinerite-schists, have been found in small thickness and in one locality only, namely, in the southern parts of sec. 33, T. 42 N., R. 28 W., where they underlie, in a series of small anticlines and synelines, banded siliceous beds composed of quartz and magnetite or martite. Under the microscope the essential constituents of the first or prevalent kind of rock of the Groveland formation are quartz, magnetite, martite, and hematite. With these, much smaller quantities of chlorite, epidote, and apatite are generally associated as accessories. Of rarer occurrence are calcite and probably siderite, sericite, tremolite, griinerite, pyrite, limonite, chalcedony, rutile, titanite, tourmaline, microcline, and plagioclase. ee GROVELAND FORMATION IN FELCH MOUNTAIN RANGE. 419 Quartz occurs in two ways—first as rounded detrital particles, and secondly as grains which have crystallized in place. The detrital grains, which are easily recognized by their form, size, and freedom from inclusions of the ores, consist of single individuals, often surrounded with rims of later growth. They are also usually larger than the neighboring indigenous grains. While detrital quartz is not abundant and, indeed, is often entirely absent from the thin sections, its occurrence is of interest as conclusively establishing the sedimentary origin of the iron-bearing formation. The secondary quartz grains are the most abundant constituents of the thin sections, and form the general background for the other minerals. They always inclose separate crystals of the iron oxides, usually in great _abundance, and often also chlorite and little prisms of apatite. These grains usually have the shape of irregular polygons bounded by straight lines, frequently with reentrant angles, and adjacent grains completely inter- lock. In size the secondary quartz grains range from about 0.03 to 0.4 mm. in diameter. Grains of approximately the same size occur together in bands or in the rounded areas to be mentioned later. The iron ores include both magnetite, or martite, and crystalline hem- atite, the former being much the more abundant. The magnetite and martite can not be distinguished in thin section, as their color in reflected light and crystalline form are the same. They occur in irregular bands composed of aggregates of crystals, the edges of which interlock with the adjoining and inclosed areas of quartz, and show the triangular, rhombic, and square sec- tions of magnetite individuals. Magnetite also occurs in isolated, irregular’ aggregates interlocking with the secondary quartz grains, and of similar dimensions to these, but is especially abundant as single minute crystals interposed in the grains of secondary quartz, ranging in size from such as are barely recognizable under a No. 9 objective to octahedra 0.03—0.05 mm. in diameter. A single quartz grain 4 mm. in diameter may inclose a hun- dred or more such minute individuals. Hematite is much rarer than mag- netite, and seems to be found only in the secondary quartz grains or in veins. In the former it occurs in separate crystalline plates, of deep red color in transmitted light, under the same conditions as to number and size as the magnetic crystals. Throughout some sections, and in certain bands and rounded areas in other sections, it is more abundant as inclosures than magnetite. Such rounded areas formed of several quartz individuals, each 430 THE CRYSTAL FALLS IRON-BEARING DISTRICT. of which thus holds a great number of hematite plates, appear macroscopic- ally as the little jasper dots already described. Chlorite and apatite are also often embedded in the secondary quartz grains, the former in thin plates and the latter in small hexagonal prisms. Epidote is quite common in small irregular areas intercalated between the quartz grains or in the magnetite bands. Many of the slides contain a small amount of rhombohedral carbonate, much if not all of which is calcite. It occurs chiefly in the quartz bands, in irregular grains which interlock with.the secondary quartz grains, and, like them, inclose little crystals of magnetite and hematite. Specimens the slides from which contain carbonates effervesce freely in scattered spots with cold dilute acid. Most of the carbonates are clear white under the microscope, and are evidently calcite. Sometimes, however, the carbonate areas have a very light-brown tint, and are partially surrounded with a limonite border and penetrated by brownish filaments along the cleavages. In such cases it is difficult to decide whether they are calcite stained with limonite, or siderite partially oxidized to limonite. However, if part of these areas are siderite it is nevertheless certain that the small magnetite and hematite crystals which they inclose have not been derived from them. These little crystals are inclosed in the carbonates just as they are in the adjoining grains of secondary quartz, while the alteration of the siderite, if it is siderite, is to limonite. Carbonates also occur with tremolite, quartz, chalcedony, epidote, and hematite in the numerous thread-like ves which traverse some of the thin sections. The feldspars have been found in only a few thin sections, as well- scattered but minute angular grains of microcline and plagioclase. Many slides, however, contain areas of matted sericite and quartz which probably represent original grains of feldspar. Rutile and tourmaline are also occasionally inclosed with the iron ores in the grains of secondary quartz. Small roundish areas of titanite, prob- ably detrital, occur very sparingly in a few of the thin sections. The most interesting features of the thin sections are certain very distinct structural arrangements of the quartz and iron ores. In almost every slide, in ordinary polarized light (with the analyzer out), the minute interpositions of the iron ores are seen not to be equally distributed through- out the background, but to be concentrated in round or oval areas, never =. = ss Sl GROVELAND FORMATION IN FELCH MOUNTAIN RANGE. 42] exceeding a millimeter in diameter. These oval forms are confined to the more siliceous bands, and are much more distinct in some of the slides than in others. Often the outlines are reenforced by rims of closely set mag- netite individuals, somewhat coarser than the dust-like crystals within. The long diameters of adjacent ovals are parallel to one another and to the band in which they lie, and are often closely packed like pebbles. Occasionally the little grains of iron ore within the ovals have a distinctly concentric arrangement. Between crossed nicols these areas are seen to have had in some instances a distinct influence on the crystallization of the secondary quartz. When they are large and closely packed, each oval includes a large number of interlocking quartz grains, and occasionally in such cases there is some difference in size between the quartz grains inside and those outside the ovals. In the triangular and quadrangular areas lying between the larger ovals, and bounded by curving segments of their perimeters, the secondary quartzes are frequently larger than those within, and are placed normal to the boundaries, precisely as if they had grown outward from the ovals into free spaces. Often, however, a single individual of secondary quartz lies partly within and partly without the oval. On the other hand, when the ovals are small, one or more may be completely or partially inclosed within a single quartz individual. The interlocking quartz grains within the large ovals show no indications of having formed in open spaces, even when the included iron ores have a tendency, as occasionally happens, to a concentric arrangement. ‘The faulting and brecciation so plainly seen in many of the thin sections have also displaced and separated the oval areas. It seems perfectly clear to me that these forms represent a structure originally possessed by the rock from which the various phases of the iron formation have been derived, and which has been preserved through the subsequent metamorphism. From the facts described above, it is evident that the Groveland for- mation is made up of highly metamorphic rocks, which still, however, retain some originat clastic material as well as certain original structural characters. With the exception of the rather rare clastic grains of quartz, titanite, feldspar, ete., the minerals which now chiefly compose these rocks—namely, quartz and the crystalline iron oxides—are not clastic, but have crystallized in place. It is a matter of great interest, therefore, to determine, if possible, 422 THE CRYSTAL FALLS IRON-BEARING DISTRICT. in what form these constituents were present in the original deposit. On this question the microscopic structure seems to me to have a distinct bearing. Forms similar to the ovals in these rocks occur in the iron-bearing formations of other districts in the Lake Superior region. In the Gogebic district of Michigan and Wisconsin, R. D. Irving and C. R. Van Hise’ have supposed that such forms have resulted from processes of solution and redeposition after the rock was formed, and are therefore concretionary. They regard that portion of the formation—which they have named fer- ruginous cherts—in which such forms occur, as an alteration product from an original deposit of cherty carbonate of iron. On the other hand, J. E. Spurr’ has shown that similar forms are exceedingly abundant throughout the iron-bearing formation of the Mesabi range of Minnesota, and are there original. In the least-altered stages Mr. Spurr has found that these oval and roundish areas are filled with a green substance, which chemically is a hydrous silicate of iron, in composition very close to glauconite, with which it is also optically identical. The oval and rounded forms, moreover, are those characteristic of glauconite in green sands of all geological ages. Starting with this original substance, which is very unstable when exposed to oxidizing and carbonated waters, Mr. Spurr has traced an interesting series of changes, the final result of which along one line is the complete oxidation of the iron to hematite or magnetite and the separation of the silica as chalcedony and quartz. Throughout these changes the original form of the glauconite grains is-preserved in the new minerals. Without going into the details of these changes, and without accepting Mr. Spurr’s conclusions in their entirety as to the steps involved, he has clearly shown, as I have satisfied myself from the study of the large number of Mesabi slides in my own collection, that the green glauconitic substance is the source of the iron and silica of the ferruginous cherts of the Mesabi range, and that the peculiar spotted structure of these cherts is mherited from the original forms of the glauconite grains. Between the ferruginous quartzites of the Groveland formation and the ferruginous cherts of the Mesabi range there is a very close resemblance, especially in structure. The essential difference is that the former contain ‘Loe. cit., pp. 254-257. 2 The iron-bearing rocks of the Mesabi range in Minnesota, by J. E. Spurr: Bull. Geol. and Nat. Hist. Surv. of Minn., No. X, 1894, 259 pp., 12 pls. UPPER HURONIAN OF FELCH MOUNTAIN RANGE. 493 little or no chalcedony, the silica being crystallized quartz, while the latter have a great deal of chalcedonie silica. Also the former contain small amounts of detrital material, which the latter generally lack, but the essen- tial difference between them is one of degree of crystallization only. If the silica of the. Mesabi cherts had originally crystallized entirely as quartz, or if after passing through the stage of mixed chalcedony and quartz it had subsequently crystallized as quartz, there would be no essential difference between the iron formations of the two districts. There are, then, at least two possible forms in which the iron and silica of the Groveland formation may have been deposited originally, as indicated by the conclusions of observers who have studied the similar iron-bearing formations in other districts of the Lake Superior region in which these formations are less altered than here.’ Hither of these forms—namely, a cherty iron carbonate, as on the Gogebic range, or a glauconitic greensand, as on the Mesabi range—could give rise, under the action of vigorously oxid- izing waters, to rocks of the mineralogical composition of those in question, and since no trace of either original form has been found in the Groveland formation the choice between them may perhaps be regarded as still open. My own opinion, based on the microscopic structure which, as I interpret it, shows that the Groveland formation was in the beginning largely made up of rounded particles having the same general form as the glauconite grains of the Mesabi range, is that the iron and silica were originally present largely in the form of glauconite. SECTION VIII. THE MICA-SCHISTS AND QUARTZITES OF THE UPPER HURONIAN SERIES. ‘Through the eastern part of sec. 32, T. 42 N., R. 28 W., and entirely across section 33, runs a belt of mica-schists and thin-bedded ferruginous quartzites which seem to have unconformable relations with the formations just described. These rocks are seen on the west in a cut in the North- western Railway in the SE. 4 of the NE. 4 of sec. 32. At the western end of this cut the strike is northwest and the dip northeast at an angle of about 35°. At the eastern end there is a decided bending in the strike to a more nearly east-and-west direction, and the bedding surfaces carry striations which dip east at an angle of 10°, all indicating that these outcrops prob- ably lie on the south limb of a gently eastward-pitching synclinal fold, and 494 THE CRYSTAL FALLS IRON-BEARING DISTRICT. near the axial plane. Last from this point similar schists and quartzites form a ridge, low and flat-topped, which extends immediately south of the railway almost to the east line of section 33, and sinks gradually beneath the great swamp of the eastern portion of that section. The formation notice- ably disturbs the compass needles, and this fact, together with the rusty appearance of the outcrops, has probably led to the sinking of the numer- ous test pits by which the continuity is chiefly established. But low-lying natural exposures are not lacking. North of the center of the NE. 4 of sec. 33 similar schists have been found in two test pits. Also, parallel with the outcropping southern belt and a quarter of a mile or more farther north, a faintly marked zone of magnetic disturbances runs east and west through the swampy ground south of Felch Mountain and probably connects the last-mentioned occur- rences with the exposures of the railway cut. It therefore seems likely that the low ground through the middle of sections 32 and 33 is wholly occupied by an open syncline of these soft and easily disintegrating rocks. Between the exposed southern limb of this syncline and the southern Archean the lower Algonkian formations are found in the southeastern portion of section 33. Actual contacts are not visible, but there are note- worthy discordances in strike and dip, and especially clear proof of great disturbances in the lower rocks in which the upper have not shared. In the SE. 4 of the NE. 4 of the SW. 4 of sec. 33, about 200 feet thickness of the Randville dolomite, striking east and west and dipping north at about 70°, is exposed between the Sturgeon quartzite below and the mica-schists to the north. Between the dolomite and the schists is a covered interval of some 40 feet. The latter also strike about east and west, but dip north at 30° or less. Between a quarter and three-eighths of a mile east of this locality (the interval being without outcrops) the Mansfield and Groveland formations lie against the Archean gneisses with a faulted contact. They have been thrown into a series of southeastward-pitching minor folds, and have been intruded by a mass of diabase and also by a pegmatite dike. The true strike of these formations at this locality is toward the northeast, and the dip, as shown both by the direction of pitch and the order of super- position, is toward the southeast. Five hundred feet north of this disturbed area and directly across the strike of the lower formations therein, the UPPER HURONIAN OF FELCH MOUNTAIN RANGE. 425 upper schists and quartzites continue their southeastward strike without deviation. These general relations indicate that the ferruginous mica-schists and quartzites are part of an upper series which overlies unconformably the Groveland and all the lower formations. This series has not been found elsewhere in the Felech Mountain area. PETROGRAPHICAL CHARACTERS. The rocks of this formation, as seen in the outcrops, are principally soft and deeply iron-stained mica-schists in which occur frequent thin beds of ferruginous and micaceous quartzite. Under the microscope the schists are composed mainly of biotite, quartz, muscovite, and magnetite. Chlorite, as an alteration product of the biotite, is frequently abundant, and garnets also occur in some sections. These schists are much coarser in grain than those of the Mansfield forma- tion, and are wholly crystalline. No clastic material has been recognized in the thin sections. The quartzites also are thoroughly recomposed rocks, without recog- nizable clastic particles. Quartz is the most abundant constituent, and with it muscovite, biotite, and magnetite are constantly associated. The micas and the magnetite are frequently inclosed in a background of large inter- locking quartz grains, which is very similar to the background of the Stur- geon quartzite. Such inclosures lie in general alignment throughout the thin sections, but, unlike many of the inclusions of the Sturgeon quartzite, they seem not to be clastic particles but to have crystallized in place. In one slide among the inclusions in the large quartzes of the background is a colorless isotropic substance, of low refraction, occurring in large polygonal areas, but without definite crystal form. It is usually stained with limonite, which has penetrated from the margins along straight lines, as if following cleavages. This imteresting mineral, which is certainly not garnet, and probably not opal, deserves further investigation The rocks of the upper series, like those of the lower series, are greatly altered. From their mineralogical composition and structure it is evident that as originally deposited they consisted of beds of mud separated by thinner beds of sand. But as they now stand they have been as greatly changed from their original condition as the bedded rocks below. Also, 426 THE CRYSTAL FALLS IRON-BEARING DISTRICT. since the time of metamorphism they have been subjected to stress, as is clearly shown by the optically strained condition of the secondary quartz grains and the bending and twisting of the micas. From these facts we may reasonably infer that the general metamor- phism of both series was accomplished after the deposition of the upper series and before the latter was folded. Reconstruction so complete as that shown by the upper series is not believed to take place except at con- siderable depths below the surface, and hence the part of the upper series now visible must then have been deeply covered by overlying rocks, which were afterwards entirely swept away before the deposition of the Cambrian. In the earth movements which folded this mass of material and brought it up within the reach of denuding agents, we may recognize the causes which have strained and broken the secondary minerals of both series alike. SECTION Ix. THE INTRUSIVES. The Algonkian formations of the Felch Mountain area have been cut by later intrusives, among which both acid and basic rocks are represented. The latter have also been recognized in the Archean, in which, indeed, the freshest and least-altered occurrences have been found. The acid rocks consist of fine- to medium-grained pink granites, occurring in narrow dikes. A number of these dikes have been found in the Sturgeon formation, both in the area of fine exposure on the south side of sec. 35, T. 42 N., R. 30 W., and also im sees. 34 and 35, T. 42 N., R. 29 W. Two granite dikes are also known in the highest member of the lower series, but none have been detected in the Randville or Mansfield forma- tions. One of these occurs on Felch Mountain, the other, a very coarse pegmatite, is found cutting the Groveland formation in the southern part of SECuoon ley 42 ON eee On Ve Basic dikes and intrusive sheets are found in many localities. Some are highly schistose and greatly altered, others are massive and but little changed. They probably belong to many eras of eruption. The least altered are diabases, in one occurrence of which, from the Archean, the augites are almost intact. CHAPTER IV. THE MICHIGAMME MOUNTAIN AND FENCE RIVER AREAS. By reference to the general map, PI. III, it will be seen that an oval- shaped Archean area, about 11 miles long from northwest to southeast and having an extreme breadth of nearly 4 miles, runs through portions of Ts 44, 45, and 46 N., Rs. 31 and 32 W. The country to be described in the present chapter includes that portion of this Archean mass (together with the younger rocks on its eastern border) which lies east of the line between Ranges 31 and. 32 W., as well as the territory to the south in the prolongation of the axial line, as far as the south line of T. 43 N., R. 31 W. A gap about 6 miles broad not covered by our work intervenes between this arbitrary southern boundary and the western termination of the Felch Mountain work at Randville. In the northern portion of the area now under consideration (which lies along and is twice crossed by the Fence River) the geological structure is exceedingly simple, while in the southern portion, especially in the neigh- borhood of Michigamme Mountain, it is rather complex. The boundary between these two divisions falls in the neighborhood of the mouth of the Fence River in sec. 22, T. 44 N., R. 31 W. It is therefore convenient in what follows to refer to the northern portion as the Fence River area, and to the southern as the Michigamme Mountain area. By referring to Pl. III, the broad geological structure of the whole territory of which the above-mentioned Archean oval is the center is evi- dent at a glance. It is an anticlinal dome, the core of which is Archean, around which the younger Algonkian formations run in a series of concen- tric rings, on all sides dipping outward from the inner nucleus. In the Fence River area, on the eastern long side of the dome, the Algonkian formations have a constant eastward dip, and are free from important secondary folds. In the Michigamme Mountain area, however, which les in the prolongation of the main axis of the dome, these encircling forma- 427 428 THE CRYSTAL FALLS [RON-BEARING DISTRICT. . tions fall away gently to the south in a series of waves, produced by several concentric minor folds transverse to the main axis. Of these minor folds but one is at all distinct to the east of the general anticlinal axis, while to the west of this axis at least three are well made out within the Michigamme Mountain area. The much greater breadth of the Algonkian formations on the west side of the dome than on the east is probably due to the persistence of these minor folds toward the northwest. The general character and aspect of the formations of the two areas and their succession is in so many respects identical with the formations of the Felch Mountain range that no doubt can be entertaimed that they are really the same formations. Nevertheless certaim differences mark these rocks with a distinet individuality. These differences will be considered in detail in the descriptions of the several formations. In general they may be summarized as involving a great reduction in thickness of the Sturgeon formation, with a corresponding increase in the Randville dolomite, the appearance of surface igneous rocks at the Mansfield horizon in the Fence River area, and a less uniform and complete metamorphism in the whole Algonkian series. SECTION I. THE ARCHEAN. The rocks of the Archean core are well exposed through the west- central sections of T. 44 N., R. 31 W., while farther north in T. 45 N., R. 31 W., outcrops are few and scattered. Much less attention was paid to this area than to the Felech Mountain Archean; our work, as a rule, stopped with the location of the boundary, and, therefore, the following brief state- ments as to its character embody observations along the southern and east- ern margins only. The prevalent rock in the Archean is eranite, varying from medium to coarse grain, and often carrying very large porphyritic Carlsbad twins of flesh-colored microcline. Banded gneisses and mica-gneisses and mica- schists, such as are so abundant in the Felech Mountain Archean, are rare but not entirely absent. While in many localities the granites are much crushed and even sheeted along adjacent parallel fractures, their originally massive character is sufficiently evident. They have the composition and structure of typical igneous granites. The primary minerals are entirely without definite arrangement. | ARCHEAN OF MICHIGAMME MOUNTAIN AREA. 429 In the Archean areas granites of two ages have been found, the younger in the form of narrow dikes. Basie igneous rocks, also in dike form, are rather abundant. One of these under the microscope proves to be a but little altered diabase, in which the augite is almost intact. These acid and basic intrusions are probably connected with the surface flows of like character which are so abundant at the Mansfield horizon along the Fence River. Of much interest is the occurrence of a small mass of quartz-porphyry m contact with the Archean, and below the lowest Algonkian sedimentary formation. ‘The locality is in sec. 21, T>44 N., R. 81 W., in the southeast quadrant of the Archean oval. The upper surface of contact of this sheet with the lowest sediments is covered, and hence it is not entirely certain whether it is intrusive or extrusive, and therefore whether it belongs to Archean or Algonkian time. The general relations, however, appear to indicate that it is a surface flow which suffered erosion before the deposition of the basal Algonkian member, and is therefore to be classed with the Archean. The exposure is 250 feet long by 100 broad. The rock consists of a very finely granular matrix of a warm gray color, through which are sprinkled quite uniformly little grains of blue quartz, and larger rounded grains of pink feldspar. Flakes of biotite are scattered through the ground- mass and coat the cleavage surfaces, which are developed in two distinct systems, intersecting at an angle of about 10°. . Immediately below the porphyry is coarse porphyritic granite, sheeted in waving surfaces parallel to the contact, which dips eastward about 40°. The lower portion of the porphyry contains a number of fragments of the underlying granite, one of which is over 4 feet in length. Under the microscope the groundmass is a fine-grained crystalline aggregate of quartz, greenish biotite, and a little feldspar. The quartz phenoerysts are beautifully corroded, and have the characteristic bipyrami- dal form, while the feldspars are extensively altered to biotite, sericite, and quartz. Biotite-gneisses related to this porphyry in external appearance occur among the Archean outcrops inclosed in the “B” line of magnetic attraction im sec. 7, T.45 N., R. 30 W., and may be described here for comparison. They are dark-colored, fine-grained rocks, which weather to light pink. They are eminently schistose, and the cleavage surfaces are coated with 430 THE GRYSTAL FALLS IRON-BEARING DISTRICT. biotite plates of medium size. Minute grains of blue quartz are occasionally distinguishable by the eye. Under the microscope these gneisses have a fine to medium grained groundmass composed of quartz, microcline, orthoclase, plagioclase, green biotite, and muscovite, and a little scattered epidote. Within it are large roundish areas of quartz and feldspar, sometimes single individuals, but more often consisting of several fragments. he gneissic foliation is pronounced and is caused by a general elongation of the constituent minerals In a common direction. The only essential differences between these gneisses and the porphyries described above are this strong foliation and the coarser groundmass. SECTION If. THE STURGEON FORMATION. The Sturgeon formation as a distinct member of the Algonkian series is hardly known in this area apart from the Randville formation. Neverthe- less, purely clastic sediments unmixed with the carbonates of calcium and magnesium were deposited and are now visible along one section between the Archean granites below and the dolomites above, and for these it is convenient to retain the name, although their total thickness is so small and their continuity so uncertain that they can not be shown on the geo- logical map. The general conditions of sedimentation here were such, perhaps in consequence of the low relief of the neighboring land, that lime- stones began to form a relatively short time after the submergence of the Archean surface, so that the two lower Algonkian formations probably by no means represent equal periods of time with the same formations in the Felch Mountain range. The time represented by both together is perhaps not greatly different in the two areas, but since in the entire absence of fossil evidence it is impossible to draw the line of equivalence, while at the same time the lithological break is a sharp one, it seems desirable to carry over the Felch Mountain names, extending the Randville dolomite downward to the lower limit of limestone deposition, and retaining the name Sturgeon formation for the basal sediments which are free from carbonates. These basal sediments are found only in sec. 15, T. 44 N., R. 31 W., where they are exposed in low-lying outcrops in the banks and bed of the Fence River. Elsewhere throughout the 10 or 12 miles through which the Archean extends in this area no outcrops have been found in the flat and generally swampy belt which intervenes between it and the dolomite above. RANDVILLE DOLOMITE IN FENCE RIVER AREA. A431 The exposures referred to consist of soft, light-weathering slates and eraywackes, with which are interbedded layers of coarser texture. They are very evenly banded in pale shades of yellow, red, and green, and the structure thus brought out dips eastward at an angle of 52°. Besides this a secondary cleavage is quite prominent, especially in the finer-grained beds, also dipping eastward, but at a considerably higher angle. At the eastern side the slates are overlain by the lowest marble beds, here extremely impure and highly charged with chlorite and quartz sand. The thickness of slates exposed is about 100 feet, and between the Archean and the most western outcrops there is room for about as much more. The total thickness, then, can not exceed 200 feet. A thin section of a specimen from one of the coarser layers shows it to be a graywacke, the most prominent constituent of which is quartz in small roundish and oval grains. These are embedded in a groundmass composed of chlorite in minute irregular plates, ferric oxide, and kaolin. The quartz grains while having generally clastic shapes are bounded by minutely rough edges which interlock with the fibrous minerals of the groundmass. Evidently much new quartz has been deposited round the original grains. SECTION Ill. THE RANDVILLE DOLOMITE. DISTRIBUTION AND EXPOSURES. In the Fence River area the dolomite, as already stated, lies on the east side of the Archean, and occupies a belt over half a mile in width, which extends from the mouth of the Fence River on the south for about 10 miles to the north and west, to our western boundary near the north- west corner of T. 45 N., R. 31 W. In this distance it is twice crossed by the river, and on these natural sections and in their neighborhood the only known outcrops of the dolomite have been found. The northern river sec- tion passes through secs. 22 and 28, T. 45 N., R. 31 W., and discloses an excellent series of closely connected exposures for a distance of about 2,900 feet, measured at right angles to the strike. The southern section is 5 miles farther south, and is much less continuous, laying bare the extreme upper and lower portions only of the formation. Elsewhere through the dolomite belt the rock surface is concealed by swamps or glacial drift, to which last it contributes but few scattered bowlders of noticeable size. 432 THE CRYSTAL FALLS IRON-BEARING DISTRICT. South of the Archean dome in the Michigamme Mountain area the dolomite tops the low arch in a broad crumpled sheet, in the minor syn- clines of which the higher formations are more and more implicated as we go south. This broad sheet, with its mcluded tongues of phyllite, extends to the south line of T. 44 N., R. 31 W., beyond which it disappears beneath the higher formations, except in a single narrow belt which continues along the main axis for about a mile farther south. Exposures sufficient in num- ber to indicate several minor folds are found along the Michigamme River and scattered through secs. 28, 32, and 33, T. 44 N., R. 31 W., and see. 4, Aad, Newieroleyyie FOLDING AND THICKNESS. In attitude the Randville formation in the Fence River division of the district is an eastward-dipping monocline, the imelination of which is gen- erally moderate. The rocks are usually heavily bedded and nearly always show distinct alternations in coarseness and color, so that structural obser- vations are made with much more certainty than in the Felch Mountain range. ‘he more conspicuous minerals secondarily developed here—coarse carbonates and tremolite—have formed chiefly in the old planes of bedding. Oblique structures are generally absent except in the close vicinity of the basic dikes which intersect the formation along the upper river section. The surfaces of contact with the dikes stand at high angles, and nearly parallel to these the neighboring dolomite has well-developed cleavages, along which new minerals have formed, intersecting the true bedding. It is evident that the stronger igneous rocks in these cases have furnished resistant surfaces against which the dolomite has been kneaded in the general tilting of the series. The eastward-dipping monocline is a simple one, yet the observed angles of inclination are by no means uniform. Thus, along the upper river section the dip ranges from 25° to 60°, with 40° as the mean of about a dozen observations. The variable dips are so scattered through the cross section as to indicate no widespread roll in the formation as a whole, but rather a great number of minor undulations probably distributed through- out its thickness. Such undulations are visible in favorable localities, as, for example, on the north bank of the river in the NW. 4 of NW. 4, see. 28, T. 45 N., R. 31 W., where fresh surfaces have been exposed in blasting FOLDING AND THICKNESS OF RANDVILLE DOLOMITE. 433 for the dam. The light-blue and pearly-white layers of the beautiful mar- ble here seen are thrown into a series of unsymmetrical folds. The western sides of the little anticlinals are short and overturned, while the eastern sides are long and gently inclined. Evidently, if the same system of sec- ondary folding holds throughout the entire thickness of the formation, sur- face observations would show everywhere eastward dips at variable angles, dependent upon the portion of the fold which happened to constitute the particular outcrop, and gentle dips would be more abundant than steep dips. This would completely explain the observed variations. Similar variations and lack of regular sequence in the dips are found in the southern river section. Five good observations range between 20° and 58°, all eastward, but none of the exposures is sufficiently extensive to show minor folds. The mean of these observations is about 40°. The surface width of the dolomite zone on each section is a little less than 3,000 feet, assuming that a fair proportion of the covered zones on each side is underlain by the same formation. If the average observed dip is taken to represent the average dip of the rock, the thickness in each ease would be a little over 1,900 feet. This is probably too great, and is certainly too great if the same kind of internal crumpling visible in parts of the upper river section is characteristic of the formation throughout. The average dip evidently would more nearly be represented by the dips of the long eastern limbs of the little anticlines. Assuming that these are less than the mean, we find the average of the dips below 40° to be 30° for each section. This gives a thickness of about 1,500 feet, which still is per- haps beyond the truth, but is probably much nearer it than the first value. It is interesting to compare this result with the thickness of 500-1,000 feet obtained on the two Felch Mountain sections. A part of the increase is probably due, as already explained, to the earlier beginning of limestone deposition in the Michigamme area. But an important part of it is prob- ably not depositional at all, but is the result of plications. The whole series here is but gently tilted as compared with the walls of the Felch Mountain trough, aud hence the strong horizontal pressures have acted in a direction but slightly inclined to the bedding. The result has been the secondary crumpling within the formation which must contribute in an important degree to its present apparent thickness. In the scattered outcrops of the Michigamme Mountain area the MON XXXVI——28 434. THH CRYSTAL FALLS [RON-BEARING DISTRICT, dolomite strikes and dips toward all points of the compass. This irregularity is caused by the gentle arching over the general northwest-southeast axis, combined with much sharper local folding about a series of axes which run more nearly east and west. The best-defined east-and-west folds occur west of the main axis in sec. 32, T. 44 N., R. 81 W., in which three syn- clines and three anticlines are found along a north-and-south section 4,000 feet long. ‘The two southern synclines are sufficiently deep to include the overlying Mansfield phyllites. The secondary folds die out toward the main north-and-south axis and broaden toward the west. Hast of the main axis but one secondary fold has been recognized, namely, the syncline which forms Michigamme Mountain. ‘This is the deepest of the secondary folds, and the only one containing the Groveland formation. PETROGRAPHICAL CHARACTERS. The Randville formation in this area is richer in lithological varieties than in the Feleh Mountain range. As originally deposited, a much larger proportion of sand and mud was mingled with the carbonates, and the prog- ress of subsequent metamorphism also has been less uniform. Depending upon the interaction of these two factors, we find, as the extremes of variation, on the one hand coarse saccharoidal marbles, sometimes very pure, but most often filled with secondary silicates, and on the other hand fine-grained little-altered limestones, which occasionally are so impure as to be rather calcareous or dolomitic sandstones and shales. The more impure varieties occur, as might be expected, near the contacts with the adjacent formations. On the Fence River, in sec. 16, T. 44 N., R. 31 W., the base of the dolomite rests on the Sturgeon formation. The rock is filled with grains of quartz and feldspar and scales of chlorite, and is so soft that it may be crushed between the fingers. In sec. 32, T. 44 N., R. 31 W., the top of the formation is in contact with the Mansfield slates, and between them is a com- plete series of transition beds. Near the junction the limestone becomes dark colored and contains thin bands in which the clayey material greatly exceeds the carbonates. These are succeeded by alternating beds of slate and impure limestone in nearly equal volume, and it is only high up in the slate member that the calcareous bands completely disappear. Apart from these belts of extreme impurity at the base and top of the formation, the presence of scat- tered fraemental grains of quartz and feldspar is rather general throughout. The prevalent colors are white, various shades of pink, both light and PETROGRAPHICAL CHARACTERS OF RANDVILLE DOLOMITE. 435 deep blue, and pale green. Where weathered, the usual colors are light brown or buff. he lighter-colored rocks in general are characteristic of the Fence River area where metamorphism is more uniform and more intense, and the darker colors of the Michigamme Mountain area to which the less crystallme forms are wholly confined. Bands differently colored are nearly always present in the same outcrop. In the Michigamme Mountain area the torsional strains attendant upon the formation of folds in two directions have developed two systems of frac- ture in the dolomite. In these secondary quartz has formed, occasionally in large amount. Of much interest is the occurrence in close connection with such vein quartz of occasional thin bands of pegmatite, doubtless aris- ing from the action of deeply derived waters. In similar spaces coarse secondary carbonates, tremolite, and oxides of iron also have commonly formed. Over the small anticlinal axes and domes of this area the original bands of the rock have often been shattered, and are now recognizable only in displaced fragments cemented together by the new minerals. In the Fence River area the general secondary folding has been attended with differential movements along the bedding, which left narrow open spaces where the adjacent surfaces failed to fit in their final position of rest. These spaces are now indicated by coarsely crystalline carbonates and silicates arranged nor- mal to the original walls. Where the space was a wide one the outer walls are usually lined with coarse calcite, while the interior is filled with quartz. In the Michigamme Mountain area certain pink bands of the dolomite have a beautiful odlitic texture, which is most clearly brought out in weath- ering by the geometrical regularity of distribution of the harder shells or cores of the little rounded grains. The forms are not different from and are quite as distinct as those in the oélitic limestones of recent deposition. The chemical composition of the dolomites is illustrated by the follow- ing partial analyses by Mr. R. J. Forsythe, of Harvard University: Analyses of dolomites from Michigamme Mountain area. alts | ut. eee Residmenm sols inth Clases aes ese eee 14, 25 oot oa ee ae ec PANS (Web) Og: occa toh eee aecec cee ee eanies 11. 15 12.57 5. 38 CAC Om sassas loos Seu eeae eerie aeeese 47.18 45, 98 36. 60 Mig © O35 seer aae - ss istinleetis lostes aac 18. 48 19, 22 16. 38 436 THE CRYSTAL FALLS [IRON-BEARING DISTRIC?. The ratio of CaCO;: MgCO; is too great for normal dolomite, but approximates that for 2CaCO,+MgCO,. Under the microscope the chief differences in the various thin sections are in the degree of metamorphism and in the quantity and character of the foreign fragments. The least altered varieties are those highest in the series from the Michigamme Mountain area. These consist of a background of extremely fine-grained calcite, with a few rounded fragmental quartz grains, and scattered particles of chalcedony. Mixtures of small quartz particles, chalcedony, and calcite slightly coarser than the background occur in short vein-like gashes. The prevalent deep color of these rocks is due to the even sprinkling through the background of a black opaque pigment, which may be carbonaceous. Altogether the microscopic characters are those of a little-altered, slightly cherty limestone. The more crystalline varieties of the dolomite contain several secondary minerals, namely, tremolite, diopside, chlorite, muscovite, phlogopite, pyrite, and the oxides of iron. Of these, tremolite is very common and abundant, especially in the Fence River area, where the rarer pyroxene, diopside, also is found. Phlogopite comes in but two of the thin sections, while muscovite occurs in nearly all. The general habit of these silicates is precisely the same as in the dolomite of the Felech Mountain range. They are developed pari passu with the passage of the unaltered dolomite into marble. The fragmental inclusions within the dolomite are of interest. These are little pebbles of quartz, feldspar, mica, titanite, magnetite, and augite; and are evidently derived mainly from preexisting granites or gneisses. Titanite and augite are very rare; the others are represented in almost every slide. The quartz grains are seldom more than a millimeter in diam- eter and commonly are much smaller. While the general shape is oval or rounded in most cases, the perimeters are usually extremely irregular and interlock with the carbonate grains of the background, which indicates that they have been enlarged since deposition by the formation of new silica. This is very evident in the few mstances in which the original smooth out- line, or part of it, is preserved by a film of different material inside the present perimeter. The feldspar pebbles include orthoclase, microcline, and plagioclase, microcline. being the common species. They are usually much decomposed and iron stained. The feldspars are especially abundant in the slides from the Fence River area. PETROGRAPHICAL CHARACTERS OF RANDVILLE DOLOMITE. 437 The clastic pebbles give us striking proof of the general and severe internal strains suffered by the dolomite, the effects of which have healed over without a scar in the carbonate matrix. The pebbles are always optically strained. Very often they are fractured and the parts separated, and sometimes they have been reduced to small fragments. In these cases the breaks have been completely healed by the flow or redeposition of the groundmass in the interstices. These effects are found in greater or less degree in every thin section. The oélitic varieties are very interesting under the microscope. They consist of little oval or round areas, averaging 2 mm. in diameter, packed together as closely as possible. Each oval consists of a single or compound nucleus, surrounded by several thin and very even concentric layers. The nucleus in a few cases is a single roundish quartz individual, evidently a clastic grain. In most cases, however, it is composed of a great number of minute quartz grains, or of several coarse calcite grains, with films of iron oxide between. The arrangement of these separate quartz and calcite indi- viduals is such as to indicate that they have filled interior cavities. The surrounding thin layers are calcite in all cases. Sometimes two adjoining nuclei, each within its own rim of several layers, are together included within a common series of shells. In one such case the outside rim trav- ersed the edges of the rings surrounding one of the nuclei with a decided unconformity, as if the latter had been eroded before the deposition of the former. The odlitic structure, I believe, has not hitherto been noted in limestones of undoubted pre-Cambrian age. SECTION IV. THE MANSFIELD FORMATION. The typical locality of the Mansfield formation is the Michigamme River valley in the vicinity of the Mansfield mine, which lies a mile west of the border of my field of work, and is described by Mr. Clements. The same formation, however, is present in the Michigamme Mountain area, where its relations to the adjacent formations are clearly defined. In the Fence River area rocks of very different character and derivation occur at the Mansfield horizon. These occur in typical development to the west, on the Hemlock River, and are hence called the Hemlock formation. 438 THE CRYSTAL FALLS IRON-BEARING DISTRIOT. DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY. The Mansfield rocks of the Michigamme Mountain area consist of phyllites or mica-slates of various colors. They are found in the series of east-west synclines, which have already been described in connection with the Randville formation. The best exposures occur in sec. 32, T. 44 N., R. 31 W., between the center and the west quarter post, and still farther north along the south bank of the Michigamme in the northwest quarter of the section. They are also found round the western edge of the Michi- gamme Mountain syncline in sec. 33, T. 44 N., R. 31 W., and im see. 5, T. 43 N., R. 31 W., but here the exposures are mainly in test pits. Test pits have likewise penetrated them in sec. 10, T.43 N., R. 31, where they succeed the dolomites as the surface rock over the general arch. Their extent in the covered portions of this area is probably considerable, but the structure is so complex and the outcrops so few as to forbid any but the most approximate outlining of their general boundaries. The geological position of the Mansfield rocks is free from doubt. In the principal syneline of section 32 they are seen to overlie the dolomites and to pass downward into them by a relatively slow gradation, while on the borders of the Michigamme Mountain syncline they are proved to underlie the Groveland formation. The passage to the higher formation likewise is graded, though more rapidly, and is marked in certain bands by an increase in clastic quartz grains and by changes in the character of the matrix in which these are set. The portions of the surface underlain by the Mansfield formation are without special features, and are indistinguishable topographically in the gently rolling plain, the greater portion of which is formed in the dolomites. In section 32 the outcrops are miniature ridges elongated with the strike, the height of which, however, is less than the contour interval of the map. FOLDING AND THICKNESS. The folding of the Mansfield rocks, so far as it can be determined in this area, has already been described in the account of the preceding formation, which they overlie. The rocks are known only in the sec- ondary synclines which lie transverse to the general direction of the main axis south of the Michigamme River. In the southern of these syn- clines, in sec. 32, T. 44 N., R. 31 W., between the lmestone rims on the FOLDING AND THICKNESS Ob MANSFIELD FORMATION. 439 north and south, a superficial width of about 1,800 feet of phyllites is exposed. The most southern exposures dip northward at a low angle. On the northern rim the true bedding is nearly vertical. Elsewhere the ver- tical cleavage structure alone is distinguishable. The upper limit of the formation is not found in this syncline. Making the most liberal estimate for possible minor crumples, it is improbable that a less thickness than 300 to 400 feet occurs here. On the eastern side of the main axis the phyllites below the Groveland formation are very much thinner than this, the thick- ness at the Interrange exploration, for example, bemg only about 100 feet; but there, as well as along the whole western edge of the Michigamme Mountain syneline, the lower contact with the dolomite is probably faulted. It seems entirely safe, therefore, to place the average thickness of the Mansfield formation m the Michigamme Mountain area at not less than 400 feet. PETROGRAPHICAL CHARACTERS. The Mansfield formation consists almost entirely of very fine grained mica-slates or phyllites. The prevailing colors are dark green, black, and light olive-green. These are often mottled irregularly with red, due to the infiltration of iron oxides along the secondary cleavages. The cleavage surfaces have a dull luster, caused by the parallelism of the micaceous minerals, which are too minute, however, to be distinguished by the eye or lens. The phyllites are often finely banded in different colors and shades. Near the base of the formation bands of limestone and near the top thin bands of graywacke are interbedded, as has already been stated. Quartz and calcite lenses are not unusual in the minutely puckered portions of the formation. The secondary cleavage is the prominent structure, and, indeed, the only structure of the outcrops where the color and texture bandings do not appear. Its general direction is transverse to the main arch, or nearly east and west, and its dip is almost vertical. The north-south compression thus appears to have been the stronger, or to have been active somewhat later in point of time than the east-west compression. Under the microscope the phyllites are seen to be composed principally of fine leaves of muscovite and chlorite, often also with a little biotite, and with a variable and usually small amount of quartz, feldspar, and sometimes 440 THE CRYSTAL FALLS IRON-BEARING DISTRICT. calcite. Magnetite, ilmenite, and limonite are usually rather abundant. Pyrite also occurs in a few grains in nearly every slide. The differences in color depend mainly upon the relative proportions of the chlorite and mus- covite, the former being characteristic of the dark-colored, the latter of the light-colored, rocks. The very dark-green or black varieties contain also an opaque and probably organic pigment in very minute particles. The quartz and feldspar grains are usually very small and irregularly shaped. The larger, however, of which a few occur in the slides from the less com- pressed rocks, have well-rounded contours. In other cases extremely flattened and strung-out lenses composed of many small particles represent what were doubtless originally single clastic grains. Two varieties of cleavage are well illustrated in the thin sections, namely, that caused by the parallelism of the component minerals, and “ ausweichungs-clivage.” The former is characteristic of the coarser- grained varieties, and the latter of the finer grained, where the direction of pressure has made a large angle with the bedding. In some cases the little leaves of muscovite outline parallel and equal folds, less than 0.2 mm. from crest to crest, each of which is ruptured, sometimes with slight displace- ments, sometimes with none, entirely across the slide. The structure is most distinct in the red phyllites, in which the fractures and the arrangement of the muscovite plates are clearly outlined by the ferruginous stain. Hach kind of cleavage in a different way tells the story of extreme pressure. SECTION V. THE HEMLOCK FORMATION. The Mansfield formation of the Michigamme Mountain area changes along the strike into rocks of an entirely different character, which, as already said, have been named the Hemlock formation. DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY. The Hemlock formation in the Fence River area consists of several varieties of schists which occupy a belt between 2,000 and 3,000 feet in width between the dolomite on the west and the Groveland formation on - the east. The best exposures occur on the two river sections already referred to (p. 431), but outcrops are by no means lacking elsewhere. At the northwest corner of the area, in sec. 6, T. 45 N., R. 31 W., the schists are found striking N. 60°-70° W., and dipping northeast about 40°. East THE HEMLOCK FORMATION. 441 of the center of sec. 16, T. 45 N., R. 31, a tew exposures occur, the struc- ture of which strikes a few degrees west of north and dips eastward at angles of 45° to 50°. In sections 21 and 28, a mile and a half south, numer- ous outcrops in similar attitudes are found along the northern river section. In sees. 3 and 4, T. 44 N., R. 31 W., a few scattered outcrops only have been found, but throughout section 10 they are very abundant. For the next 2 miles south through secs. 15 and 22, T. 44 N., R. 31 W., only a few small exposures have been discovered which have the same northerly strike and eastward dip. Thus for a distance of 11 miles along the strike exposures occur at comparatively short intervals, the longest gap being 3 miles. In general this belt is one of slight elevation above both the dolomite country on the west and the iron formation country on the east. The areas of best exposure are characterized by very rough topographical details, which are entirely lost in the generalized curves of the map. Abrupt strike ridges, separated by narrow ravines, succeed one another at short intervals. In the covered areas the surface, while retaining its general elevation, has been leveled off by the deposition of till in the hollows, and has the smoothly undulating contours characteristic of till-covered areas. FOLDING AND THICKNESS. No secondary folds have been detected within the Fence River area of the Hemlock formation, and on account of the metamorphism and cleavage structural observations are not possible from which they might be inferred. The only clear evidence as to the attitude of the rocks was afforded by the contact at one locality between beds of amygdaloid and agglomerate. There the dip is eastward at an angle of 50°. The surface width of the formation varies between 2,000 and 3,000 feet. If 50° is taken as the dip, the thickness would be from 1,500 to 2,300 feet. If the average dip is assumed to be 40°, or the average of the observed dips of the underlying dolomite, the thickness would be from 1,300 to 1,900 feet. Or if 30° be taken, the average of the lower dips of the dolomite, the thickness would be 1,000 to 1,500 feet. We may say, therefore, that the thickness north of the southern river section is probably not less than 1,000 nor probably more than 2,300 feet. South of the southern river section the thickness diminishes rapidly. 449 THE CRYSTAL FALLS IRON-BEARING DISTRICT, PETROGRAPHICAL CHARACTERS. The exposures through section 10 and the northern part of sec. 15, T. 44 N., R. 31 W.—the southern river section—give us a nearly complete sequence across the Hemlock formation, the principal gaps being on the extreme east and west, thus leaving the details of the relations with the dolomites below and the iron formation above undisclosed. In this section of 3,000 feet in length, the rocks are chiefly chloritic and epidotic schists, with which are associated schists bearing biotite, ilmenite, ottrelite, and amphibole, greenstone conglomerates or agglomerates, and amygdaloids. These rocks are characterized by a generally fine and even grain, by a lack of sedimentary characters, and by a double structure. In most of the varieties minerals, which have formed quite independently of and later than these structures, are macroscopically conspicuous. The prevailing color is green, passing to dark purple and black in the varieties in which biotite, hornblende, and magnetite abound. The distinction made in the field between the several varieties of the schists is a rough one, indicating the predominating minerals rather than implying the absence of the others. In fact all the varieties are intimately related. The chlorite-schists are very fine-grained green rocks, usually from their color evidently very epidotic; they weather to greenish or pink- ish white. The cleavage surfaces are often plentifully sprinkled with little flakes of biotite. Frequently also black needles of ilmenite, brilliant plates of ottrelite, and large clusters of actinolite run irregularly through them, quite independent of the cleavages. The biotite-schists are much darker, and lack the green coloring Through them also the same metamorphic minerals are frequently interlaced. By an increase in these minerals the passage to the other varieties in limited exposures is a very easy one. Greenstone-conglomerates and amygdaloidal rocks occur in a few exposures. In the former, light green or gray aphanitic inclusions, of angular shapes, ranging from an inch to 2 or 3 feet in long diameter, are inclosed in a matrix of chlorite-schist or biotite-schist. The chlorite-schists often hold round or lens-shaped eyes of epidote, and epidote and quartz. That these are filled cavities can in most cases be shown only by the micro- scope, yet some of the larger amygdules have a banded structure evident to the naked eye. These rocks are of structural interest since they are the PETROGRAPHICAL CHARACTERS OF HEMLOCK FORMATION. 443 only members of the area which possess undoubted bedding. The plane of contact between an amygdaloid and a layer of greenstone-conglomerate in SE. 4 sec. 10, T. 44 N., R. 31 W., dips eastward at an angle of 50°. Two well-marked systems of cleavage traverse all the rocks of the southern river section. The angle between their strikes is always acute toward the north, varying from 5° to as high as 34° in different exposures, while the direction of the bisectrix is almost constant at N. 8°-10° W. The dip of both systems is toward the east at about the same angle, namely 50° to 60°. The two systems are usually both well developed, so that the outcrop edges break down by weathering along zigzag lines. The character of the cleavages varies from fine partings which divide the surface into rhombs, sometimes extremely regular in the more aphanitie rocks to a single perfect schistosity capable of minute subdivision, along which the component minerals are visibly aligned, in the more crystalline. Along the cleavages seams of quartz and calcite have frequently formed. Along the upper river section the rocks of the area are distinctly more crystalline, and are chiefly biotite-schists and biotite-hornblende-schists, the latter often very coarse. They are sometimes banded, but very irregularly, the lenticular character of the banding suggesting the rhombic cleavages of the southern section. Some of the finer-grained biotite-schists contain round or elongated areas of quartz and epidote, which resemble amygdules. With these are associated considerable thicknesses of sericite-schists, full of little eyes of blue quartz; these are evidently metamorphic acid eruptives. The width of the northern section is about 2,000 feet. Under the microscope the Hemlock schists of the Fence River area have a general porphyritic habit. Two main divisions only are clearly distinguished. One of these is the fine-grained mica (sericite) schists, which are characterized by the presence of muscovite as well as biotite in the microcrystalline groundmass, and true phenocrysts of feldspar and bipyramidal quartz, while the other embraces all the other varieties, which, diverse as they undoubtedly are, have yet certain important characters in common and are connected by gradations. ‘The sericite-schists are obviously metamorphosed acid lavas, and need not be described in detail here. The origin of the second division, however, is far more obscure. The least altered of these rocks possess an exceedingly fine grained microcrys- talline groundmass, made up of very pale chlorite and a colorless aggregate 444 THE CRYSTAL FALLS IRON-BEARING DISTRICT. with feeble double refraction, which seems to be quartz. Between crossed nicols the groundmass is almost isotropic, and it is by no means improbable that certain reddish patches here and there may really be glass. Little erys- tals of magnetite are abundantly scattered through the groundmass, and are often arranged in parallel curving lines, very suggestive of the flowage lines brought out on the surface of weathered rhyolites by the ferruginous stains. In many sections the groundmass includes minute lath-shaped plagioclase feldspars, much altered and with indistinct boundaries, which are often arranged in parallel lmes.. The groundmass also is generally sprinkled with little irregular grains of epidote and calcite. In this groundmass are included in variable combinations and propor- tions much larger crystals and grains of common hornblende, actinolite, biotite, ottrelite, calcite, ilmenite, epidote, and zoisite. Of these biotite, calcite, ilmenite, epidote, and zoisite are the most constant and abundant. Biotite is present in all or nearly all of the thin sections. It is always brown, and is characteristically developed in stubby individuals, very thick for their basal dimensions. These individuals are large and lie scattered through the slides. They frequently inclose portions of the groundmass. The mica cleavage most frequently stands across the cleavage of the rock. In many of the darker-colored schists, however, biotite plates intermediate in size between the large porphyritic individuals and the small chlorite plates of groundmass are present in large numbers, constituting a sort of secondary groundmass. These are generally aligned with the cleavage of the rock and are sometimes gathered in bands, but in color and stubby habit are similar to the phenocrysts. Ilmenite in brownish-black prismatic sections is a common constituent. It usually lies at random through the slide. It incloses the quartz and epidote grains of the groundmass. Epidote and zoisite are exceedingly abundant, often in well-formed crystals. Many of the epidote and zoisite individuals contain darker colored inner nuclei, the nature of which is uncertain. In some cases the nuclei are irregular in shape and have the characteristic pleochroism of epidote, but are more strongly colored than the surrounding zones. In other cases they have sharp crystal boundaries, isomorphous with epidote, are brown in color, and inclose grains of magnetite; these may be allanite. The nuclei are too small, however, for determination. Generally they do not extinguish exactly with PETROGRAPHICAL CHARACTERS OF HEMLOCK FORMATION. 445 the surrounding zones. It is probable that many of these nuclei represent an early generation of epidote, like the small irregular grains of the ground- mass, which were subsequently enlarged to porphyritic size. Inclosures of zoisite are not uncommon in the large epidote individuals. Large lenticular aggregates of epidote with calcite, chlorite, and biotite are found partially replacing feldspar individuals, which were no doubt original phenocrysts. Similar aggregates unmixed with the remains of feldspar are not infrequent, and may reasonably be attributed to the same source. Epidote with quartz is also the common filling of the amygdaloidal cavities. Common hornblende, actinolite, and ottrelite are very common as porphyritic constituents of the schists. Hornblende occurs in very large well-formed single crystals and clusters placed at random through the eroundmass. It is characteristically associated with ottrelite and biotite, and often has formed somewhat later than the latter. It is always crowded with inclusions, which in the laminated varieties carry the structure through without reference to the position of the host. Ottrelite is abundant in some of the sections, and is distinguished by its characteristic pleochroism. It occurs in large individuals and multiple twins, and like the large horn- blendes and biotites is full of inclusions. The general characteristics of these schists then are, first, a groundmass composed of chlorite, quartz, magnetite, epidote, and in some cases contain- ing plagioclase microlites, and secondly the presence in this groundmass of much larger porphyritic individuals of several secondary minerals. The varieties are determined by the varying ratio of the porphyritic constituents to the groundmass, by the nature of the predominant secondary minerals, and also by the differences in grain of the groundmass. This, while gen- erally extremely fine grained is much coarser, but without mineralogical change, on the northern river section where the schists are more distinctly erystalline. The cleavage of the schists is determined by the arrangement of the minute particles of the groundmass, and not by the parallelism of the large secondary minerals. These last, further, are never faulted or broken, and in general are unstrained optically. They must have formed then after the compression and tilting of the series. The origin of these schists, I think, is not doubtful. As important points of evidence we have, first, the absence of rocks possessing any sedi- mentary characters throughout the whole section. Next we have the 446 THE CRYSTAL FALLS [RON-BEARING DISTRICT. undoubted presence of lavas in the series, shown by the sericite-schists, amygdaloids, and greenstone conglomerates or agglomerates. Furthermore, the minerals which compose the schists are those which would result from the alteration, in connection with dynamic metamorphism, of igneous rocks of basic or intermediate chemical composition. Finally, the grain and character of the groundmass, and in some slides the presence therem of plagioclase microlites disposed in flow lines, poit directly to an igneous origin and to consolidation at the surface. Mr. Clements has reached similar conclusions for the formation above the Randville dolomite’ on the western side of the Archean dome. There metamorphism seems to have progressed less far than in the Fence River area, and among the more basic rocks he has recognized andesites and basalts. I conceive, then, that the Hemlock rocks of the Fence River area are a series of old lava flows, varying in composition from acid to basic, which first underwent dynamic disturbance, which developed the secondary cleay- ages, and afterwards, in a state of rest, the porphyritic minerals were formed. It is an interesting fact, for which I can suggest no explanation, that metamorphism is further advanced in the northern part of the area than in the southern, and the schists more distinctly crystalline. This is also true of the underlying dolomite. SECTION VI. THE GROVELAND FORMATION. DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY. The Groveland formation in this area, as in the Felech Mountain range, consists mainly of siliceous iron-bearing rocks, which hold much fragmental material, together with certain subordinate schists. While it is of wide extent throughout the area, its known outcrops are limited to three local- ities, namely: The vicinity of Michigamme Mountain, in sees. 33, T. 44 N., 1 Voleanics of the Michigamme district of Michigan, by J. Morgan Clements: Jour. Geol., Vol. III, 1895, No. 7, p. 801. 2This formation was originally named by me the Michigamme Jasper (Am. Jour. Sci., March, 1894). The name Michigamme was subsequently used for one of the Upper Marquette formations, in the Preliminary Report on the Marquette District, 15th Ann. Rept. U. S. Geol. Survey. I now aban- don the old name, although it is entitled to stand by the rules of priority, in order to avoid the con- fusion which would necessarily arise from its retention. THE GROVELAND FORMATION. 447 kh. 31 W., and 3, T. 43 N., R. 31 W.; the exposures and test pits at the Sholdeis exploration in sec. 21, T. 45 N., R. 31 W., and the test pits at the Doane exploration in sec. 16, T. 45 N., R. 31 W. The last two localities are 1 mile apart, and the more southern is 8 miles north of Michigamme Mountain. In spite of the poverty of the formation in outerops, its distribution throughout the area has been well determined through its magnetic proper- ties (following the methods described in Chapter I]). Adjacent to the Fence River area of the Hemlock formation it gives rise to a strong mag- netic line which passes through the outcrops and test pits of the Sholdeis and Doane explorations. To the north this line was followed to the south- ern side of sec. 32, T. 46 N., R. 31 W., where it is said to connect with a magnetic line followed by Mr. Clements from the western to the northern side of the Archean dome. ‘To the south it continues into the Michigamme Mountain area to within a mile of the outcrops of Michigamme Moun- tain. There the magnetic line gives way to a broad zone of disturbances, feeble and difficult to interpret, but consequent I believe mainly upon the flattening of the formation as it begins to pass over the general northwest- southeast anticlinal axis. This zone connects directly with the exposures of Michigamme Mountain which produce similar irregular disturbances of the needles and which visibly constitute a thin crumpled sheet, on the whole but gently inclined. For the stretch of 13 miles just described the Groveland formation occupies a continuous belt on the east side of the main anticlinal axis. In the Fence River area it lies east of and upon the Hemlock formation, while in the Michigamme Mountain area it holds the same relations to the Mans- field formation. The eastern belt was not traced farther than a mile southeast of Michi- gamme Mountain. In the central and southeastern portions of T. 43 N., R. 31 W., however, in the direct prolongation of the anticlinal axis, we found a broad belt of slight magnetic disturbance, along the western mar- gin of which lie volcanic rocks, dipping west. In sec. 26, T. 43 N., R. 31 W., this magnetic belt splits into two branches, one of which runs directly east for a mile and then southeast indefinitely, while the other maintains a general southerly course to the south line of the township. In section 26 large 448 THE CRYSTAL FALLS IRON-BEARING DISTRICT. angular bowlders evidently derived from the Groveland formation are found in the zone of magnetic disturbance, but no outcrops have been discovered. There can be little doubt that these disturbances roughly outline the position © of the Groveland formation in the axial region. Except in Michigamme Mountain, the most elevated point of the dis- trict, the Groveland formation is not topographically prominent. In the Fence River area it produces a more subdued and somewhat lower-lying surface than the underlying formation, but the difference is slight and is of little moment in comparison with the confusing effects of glaciation. FOLDING AND THICKNESS. In the Fence River area there is no reason to suppose that the Grove- land formation contains within itself minor folds of any importance. Our knowledge of its attitude is supplied almost wholly by the magnetic obser- vations, and these indicate that it has a general eastward dip like the under- lying members of the succession. Here and there it may be divided into two or more parts by sheets of intrusive material, and also may be slightly crumpled, but on the whole it must be regarded as a single persistent sheet, having a general eastward dip. At Michigamme Mountain the Groveland formation caps the hill ma well-marked syncline, the axis of which runs northwest and southeast. The structure is distinctly shown by the attitude both of the ferrugmous rocks and of the underlying phyllites. At the Interrange exploration half a mile south, a secondary embayment of the same syncline, but more open, is found. These are the only folds of the Michigamme Mountain area sufficiently deep to include the iron-bearing rocks. The thickness of the formation can only be guessed at, as no complete section is exposed, and the data for determining its upper limit are decidedly shadowy. 'The mag- netic observations indicate a breadth of from 400 to 600 feet, and as in the Fence River area it is certainly much thinner than the two lower forma- tions, its thickness may be approximately 500 feet. PETROGRAPHICAL CHARACTERS. In general aspect the iron-bearing formation in this area is strikingly like that of the Felech Mountain range, and all the varieties there found are represented here also. It is therefore unnecessary to repeat the detailed descriptions already given. By way of broad comparison, however, it may PETROGRAPHICAL CHARACTERS OF GROVELAND FORMATION. 449 be said that the formation contains less iron than in the Felch Mountain range, and consequently the lighter-colored varieties are more abundant, that it contains more detrital material, and that in the Michigamme Moun- tain area the texture is generally closer and less granular. Moreover, in passing north from the Michigamme Mountain area to the Fence River area we find at the Sholdeis and Doane explorations that the lower portion of the formation is composed of ferruginous quartzite, which is succeeded higher up by actinolite-schists and griinerite-schists similar in all respects to the characteristic rocks of the Negaunee iron formation in the western Marquette district. In this change in character as the Marquette district is approached is found the lithological support for the view, first sugeested by the distribution of the lines of magnetic attraction, that the Groveland formation is the southern equivalent of both the Ajibik quartzite and the Negaunee formation of the Marquette district. The passage to a more crystalline condition in going from south to north is also in accord with the like changes already noted in the lower formations. Under the microscope the close texture of the Groveland rocks of . Michigamme Mountain is seen to be due to the minuteness of the quartz grains of the groundmass, and to the abundance therein of chalcedony. The coarse quartz grains are all detrital and are often beautifully enlarged. In many slides feldspar pebbles occur, and in many also sericite and chlorite are prominent in the groundmass. The iron oxides, including both mag- netite and hematite, in single crystals and also in aggregates, are well dis- tributed, as in the Felch Mountain sections. A similar grouping of these in round pebble-hke areas as in the Felch Mountain range is also beauti- fully shown. In one slide the matrix is a rhombohedral carbonate, prob- ably calcite, in which are embedded quartz grains and the iron ores in single crystals and irregular aggregates. The most interesting features of the thin sections from Michigamme Mountain are the pressure effects. In many slides the detrital quartz grains are strained to an extraordinary degree. In one case the stage was rotated 45° before the black wave of extinction completely traversed a little pebble 0.3 mm. in diameter. Almost every section is crossed in sev- eral directions by fractures healed by the deposition of coarse quartz and the iron oxides. MON XXXVI——29 fe 450 THE CRYSTAL FALLS IRON-BEARING DISTRICT. In the Fence River area the lower portion of the formation consists of quartzite, more or less ferrugmous and micaceous. It contains beautifully rounded and enlarged grains of quartz, and also less abundantly rolled grains of microcline. Muscovite, biotite, and epidote occur, with the gen- eral background of interlocking later quartz. The more ferruginous layers have a groundmass almost exclusively of hematite, in which the clastic particles are set. The hematite is in parallel micaceous scales, which com- pletely cover the cleavage surfaces. Above these layers come crystalline actinolite-schists and griinerite-schists, the former with garnets and both carrying particles of the iron oxides. These rocks are not distinguishable in the field or in thin section from certain varieties of schists of common occurrence in the Negaunee formation of the Marquette district. a + x N ‘ : : OO a ae eee f 4 wey E “ ™* ae x fo = ‘ = S ’ 5 ‘ i w Piss A ~ . ae / : eA + } an iE : : + S H 3 - ‘ r : f - i tes : i . i f > b z % ‘ £ ; - t My y 4 * ; : ; t ? ‘ i ¢ ; 4 i4 ' ¢ - 1 j f i : . 3 < 7 e x : = i ' ¥ A , i> : i MONOGRAPH XXXVI PL. L. U.S. GEOLOGICAL SURVEY Pate te TON (SG ea Ran : y ae | | C{MCLALG NL NOMVMMIOA AINOVOAN NOWMVAUON MOOINaH = aIZiavad wIairy WLINOTOd YNOM SLINVHD SCLNTT ag JILUNOVIN eae CNV GNVISAOND TIALS OULNI NVINOUNH Wadd NVINOUOH AHMO'T ; NVIMNOD TV NVOUHOUV Asosnps 10 ssounrys ym sdoroyng 4- dip pure ois pourumayep yu sdoaoyng + dip 20 oyays poadesqo qnowtm sdoroyng % LAAT OF TIVAHHLNI HNOLNOOD ATW T-HONT T'STVOS OrTanAd au lO LS aM LOMALSIG STIVA IVLSAUMO HHL HO NOILUOd VY HO dVW TVOIDOTOUD AN H117'09 8 NZIS snnnar U.S. GEOLOGICAL SURVEY MONOGRAPH XXXVI PL.L. GEOLOGICAL MAP OF A PORTION OF THE CRYSTAL FALLS DISTRICT WEST OF REPUBLIC SCALE. 1 INCH=1L MILE CONTOUR INTERVAL, 20 FERT ¢ Outcrops without observed strike or dip ¢ Outerops with determined strike and dip + Outcrops with slatiness or sclristosity ARCHEAN ALG ONKIAN pee oa Y grass ae ___LOWER HURONIAN _ GRANITE: KONA DOLOMITE K QUARTZITK HEMLOCK FORMATION NEGAUNEE FORMATION an a cas "Tei CH ACE ah hyve THE NORTHEASTERN AREA AND THE RELATIONS BETWEEN THE LOWER MARQUETTE AND LOWER MENOMINEE SERIES. From the northernmost outcrops of the Fence River area to the north- ern end of the Republic trough the air-line distance is about 11 miles. This intervening territory, on one side of which we find the typical formations of the Menominee district and on the other the typical formations of the Marquette district, remains to be described in this chapter. It may con- veniently be referred to as the Northeastern area. As was shown in the report on the Marquette district, in the pro- ductive portion of the Marquette range between Negaunee on the east and Republic on the west, the lower Marquette series consists of two or three clearly marked formations, which, perhaps, may further .be subdivided according to individual taste." The lowest of these, the Ajibik? quartzite, which rests on the Archean complex, is fragmental in origin, and is prevail- ingly a white vitreous quartzite, which in one or two localities is conglom- eratic near the base. Often it is represented by a muscovite-schist as the result of the dynamic metamorphism of the original arkose. In the eastern part of the productive area of the Marquette district and along the northern side of the main fold, in the western part of the district, this formation is overlain by the Siamo slates.’ Elsewhere the slates are not present, or are not known. The next formation is the Negaunee iron formation,* which has already been referred to in Chapter II. This rock, which has many phases, as there noted, is clearly marked off from the lower quartzite by its great richness in iron and by the fact that over the whole Marquette district it nowhere appears to contain fragmental material, except in the transitional zone between it and the lower formations. ‘The Marquette iron-bearing series of Michigan, by C. R. Van Hise and W. S. Bayley, with a chapter on the Republic trough, by H. L. Smyth: Mon. U.S. Geol. Survey, Vol. XXVIII, 1897, p. 221. 2 Op. cit., pp. 528-529. 3 Op. cit., pp. 313-315. 1Op. cit., pp. 328-407. 451 452 THE CRYSTAL FALLS IRON-BEARING DISTRICT. Above these conformable formations comes the unconformably placed Upper Marquette series, the base of which rests now on one member, now on the other, or on the Archean. East and south of Negaunee, and extending thence to the shore of Lake Superior at Marquette, is a series of rocks which resemble lithologic- ally neither the Upper nor the Lower Marquette series in the productive area. It consists, in ascending order, of quartzite with basai conglomer- ates, dolomite, and slates, and thus bears a close resemblance lithologically and stratigraphically to the three lower members of the Menominee series. This series, named by Wadsworth the Mesnard series, has been regarded by him as belonging with the Upper Marquette series, or at least as over- lying the Lower Marquette formations just described. Maj. 'T. B. Brooks had earlier correlated the dolomite with the Lower Marquette quartzite, and had supposed that there was a gradual passage from one into the other along the strike. Mr.C. R. Van Hise has recently stated that its position is below the Ajibik quartzite. This series is found only in the eastern part of the Marquette area, between, Goose Lake and Lake Superior, a distance of about 6 miles. Elsewhere, over by far the greater part of the Marquette district, no member of it has been recognized. The geological structure of the Marquette range presents the general features of an east-west striking complex syncline or synclinorium. The pre-Cambrian sedimentary rocks, with their associated imtrusive and extru- sive igneous rocks, occupy the trough, in which there is much local com- plexity of structure. The trough is flanked on the north and south by the older Archean crystallines. At the western end of the district the peculiar Republic’ trough branches from the main synclinorium, and runs southeast into the Archean rocks for 6 or 7 miles, having a nearly constant width of about one-half to three-quarters of a mile. In this trough the Algonkian rocks have been so closely compressed that they stand essentially on edge. The interior is occupied by the younger Upper Marquette quartzites and schists, between which and the underlying Archean walls the older Lower Marquette iron formation and quartzite here and there occur. The northwestern end of the Republic trough is about the western 1 Op. cit., p. 525. THE NORTHEASTERN ARBA. 453 limit of mining development, though not of exploration, on the south side of the Marquette synclmorium. Up to this point outcrops, producing mines, and old explorations are sufficiently abundant to permit the separate formations to be traced and mapped with comparative ease, and to indicate at least the larger structural features. At this northwestern end of the Republic trough the Lower Marquette series makes an abrupt turn to the south, and may be followed for a mile or more by occasional outcrops and test pits. The Negaunee iron formation is persistently present beneath the Upper Marquette quartzite, and gives rise to a very strong and persistent line of magnetic ati action, which was followed in our work for about 12 miles to the south and southeast into the Northeastern area. For about 4 miles from the sharp turn at the mouth of the Republic trough it runs nearly due south; afterwards it turns somewhat to the east of south, and follows that course for about 6 miles, after which it turns more and more toward the east, and finally, where we left it, its course was only slightly south of east. That this magnetic line is caused by and marks the position of the Negaunee iron formation there can not be the slightest doubt, for that rock outcrops in a few scattered localities, occurs abundantly in the drift, and has been found in occasional test pits and drill holes throughout this distance. The underlying quartzite outcrops beneath the iron-bearing formation near the northern end of the line, but farther south it is entirely covered by the drift, so far as the territory has been examined. The overlying Upper Marquette rocks are also known to be present just west of the Negaunee formation as far south as sec. 19, HSA GENE, ka OWE The magnetic line which accompanies the Negaunee formation may be called the A Ine. ‘Taking into account the connected Republic trough and its exposures of the Lower Marquette rocks, it is seen that the A line par- tially surrounds a dome of the Archean crystallines, aid that in going from the interior of this dome outward across the A line we pass from older to younger rocks. The dip along the A line is, therefore, on the whole, toward the west, although the observed dips at the few localities where determinations have been made are either vertical or slightly inclined from the vertical toward the east. The southern part of the A line, as far as it has been traced, passes through sees. 5, 8, 9, 15, and 16 of T. 45 N., R. 30 W. In section 5 it is just 5 miles east of the Groveland formation, which, 454 THE CRYSTAL FALLS IRON-BEARING DISTRICT. as was shown in earlier chapters, is a magnetic rock occupying a definite place in the Menominee succession, and is underlain by other typical Menominee formations, and finally by the Archean. Between the A line and the magnetic line caused by the Groveland formation, which may be called the C line, is a third magnetic line, which may be called the B line. This was traced parallel to the A line and less than half a mile away, from near the south end of the latter to the north end, and finally entirely round an elliptical area, closing again upon itself at the starting point, the perimeter of the ellipse being 25 miles in length. Throughout this entire distance not a single outcrop could be discovered along the B line. Within the inclosed area, however, in secs. 6 and 7, T. 45 N., R. 30 W., and in sec. 19, T. 46 N., R. 30 W., several exposures of granites and crystalline schists were found, which left no doubt that the greater part of the area inclosed by the B line is occupied by Archean rocks of the same general character as those partially inclosed by the A line on the east and entirely by the C line on the west. The area between the A and B lines as far south as sec. 19, T. 46 N, R. 30 W., has been proved to contain the basal member of the Upper Marquette series. The southwestern quadrant of the B-line ellipse is nearly parallel to the C line and only 15 miles away. The known facts with reference to the B line, then, are these: (1) It represents a magnetic rock; (2) this magnetic rock completely encircles an Archean core. It may further be inferred with practical certainty that this formation, which carries such constant magnetic properties for 25 miles, must be sedimentary. With regard to its structure, the foregoing con- siderations would necessarily involve the conclusion that it dips away from the Archean core on all sides, and this conclusion is fortified by the unsymmetrical separation of the horizontal maxima on the magnetic cross sections. It follows, therefore, that on the eastern side of the oval, where the formation is parallel to the A line, it dips toward the east, and on the western side, where it is parallel to the C line, it dips toward the west. This conclusion is further supported by the dips within the ellipse in the _outcropping Archean rocks that show structure. These all happen to le east of the major axis, and all dip toward the east. East of the B line, and between it and the A line, is found the basal member of the Upper Marquette series. The rock which is manifest in the ee THE NORTHEASTERN AREA. 455 B line must, therefore, be older than any member of the Upper Marquette series. The Negaunee iron formation, represented in the A line, dips west, while the rock of the B line dips east. They are both older than the basal member of the Upper Marquette series, and are both younger than the Archean. They are both strongly and persistently magnetic. For 8 or 10 miles they run parallel to each other less than half a mile apart. Their broad structural relations to the Archean basement of the region are pre- cisely similar. Therefore, although the rock that gives rise to the B line has never yet been seen, it may be concluded with the utmost confidence that it is the Negaunee iron formation, and that the A and B lines represent this rock brought up in the two limbs of a narrow and probably deep synelinal fold. This conclusion carries the Negaunee iron formation 34 miles farther to the west, and in the northeast part of T.45 N., R.31 W., leaves a gap of but 14 miles between the Lower Marquette and the Menominee series. Here, between the B and C lines, is precisely the same situation as between the A and B. One magnetic rock, represented by the B line, dips west; the other, the Groveland formation, represented by the C line, dips east. Between them no magnetic disturbances can be found. The area between them must have a synclinal structure, and if they are not one and the same formation each must undergo an extremely rapid and precisely similar change in lithological character (mamely, the loss of magnetite) in a very short distance and be represented on the opposite side of the syn- clinal fold by a nonmagnetic formation. Hach of these rocks is persistently magnetic in the direction of the strike for great distances. That each should independently lose its magnetite in the direction of the dip in this particular locality is very improbable. And, therefore, the grounds for the conclusion that the B and C lies represent one and the same formation are quite as firm as those upon which rests the conclusion that the A and B lines repre- sent the same formation. The greater portion of the Northeastern area is without outcrops, yet through the structural and lithological results of the magnetic work we are able to bridge over the gap and to show with a high degree of probability that the Negaunee iron formation of the Marquette range is identical with the Groveland iron formation of the Felch Mountain range. Further, when we recall the differentiation of the Groveland formation in the 456 THE CRYSTAL FALLS TRON-BEARING DISTRICT. northern part of the Fence River area into ferruginous quartzite at the base and griinerite-schist im the upper portion, it would seem probable that the Groveland formation represents the underlying Ajibik quartzite as well as the Negaunee formation of the western part of the Marquette range. This conclusion has an important bearmg on the interpretation of the: early geological history of what is now the Upper Peninsula of Michigan. If the formations which constitute the whole of the Lower Marquette series over the 25 miles or more of the productive and best-known portion of the range are represented in the Menominee district and the mtervening area by a single formation, and that the lighest m the Felch Mountain succes- sion—namely, the Groveland formation—the formations below the Grove- land formation are all older than the Marquette rocks and do not occur at all within the productive portion of the Marquette range. Why are these lower formations absent? To this question there seem to be two answers which are a priori possible. It is conceivable that the quartzite, dolomite, and slates of the south, or some of them, may have been deposited in a succession of unbroken sheets over the whole Marquette area, in continuity with the simi- lar Mesnard formations on the east, and that afterwards the main Marquette area was elevated above the sea and entirely stripped of these formations by long-continued denudation. Finally, when the time of deposition of the Groveland formation came round, this elevated area had again been reduced to sea level, and subsided below it, so that the Ajibik quartzite and the Negaunee iron formation, and their southern equivalent, the Groveland formation, were deposited in an unbroken sheet over the whole. If this hypothesis is correct, two consequences should follow from it: First, we ought to find some discordance between the Groveland formation or the Lower Marquette quartzite and the lower formations in the marginal areas between the Menominee and Marquette, and the Mesnard and Marquette areas, respectively, or at least a gradual cutting out of these lower forma- tions by the iron-bearing members and the lower quartzite, and, secondly, we ought to find, in the lack of discordance, rocks present in the areas of continuous deposition which represent the time of denudation. With regard to the first of these consequences no verification is possible, at least in the territory between the Marquette and Fence River districts, from lack of outcrops. Throughout the Northeastern area, from the north- THE NORTHEASTERN AREA. 457 western end ot the Republic trough in T. 47 N., R. 30 W., to the C line in T. 45 N., R. 31 W., there are no exposures whatever of the Algonkian rocks which underlie the Groveland formation. Somewhere in this distance of about 11 miles the lower formations disappear, but whether by unconformity or overlap is an unanswerable question; nor (for the same reason) can it be definitely settled whether elsewhere farther to the south there is any discordance. That there is general parallelism between the Groveland formation and the lower rocks, and strict conformity in some places, is true. But this is not at all inconsistent with a period of erosion between them, if that erosion antedated the later and more severe orogenic disturbance. In the Mesnard area the observed relations have been interpreted by Van Hise to mean that the lower formations disappear by overlap. The facts at present known on the Felch Mountain side are capable of the same interpretation, but they are not sufficiently definite to exclude the possibility of a period of erosion below the iron-bearing formation. With regard to the second consequence—the deposition in the sub- merged areas of formations which would represent the erosion period in the elevated area—the evidence at hand is decidedly against the existence of such formations. The alternative hypothesis is that the lower quartzite, dolomite, and slate formations of the Menominee area were not deposited over the western Marquette area at all, but disappear toward the north and east by overlap, and this hypothesis is much more likely to be the true one. We can suppose, as I have already pointed out,’ that this part of the Upper Peninsula was a slowly subsiding area, the central portion of which, now occupied by the Marquette rocks, stood initially at a greater elevation above the encroaching sea than the rest. While the quartzite-dolomite-slate triad was going down in the Mesnard area on the east and the Menominee area on the south and west, the central Marquette area still remained above the sea. At last, when the Groveland formation began to be deposited, the Marquette high land was finally submerged and covered, as the sea marched over it, first, with a sheet of arkose made up of its own disinte- grated débris, and, finally, with the same nonclastic sediments as chiefly compose the Groveland and Negaunee formations. 1 Relations of the Lower Menominee and Lower Marquette series in Michigan, by H. L. Smyth:: Am. Jour. Sci., Vol. XLVII, 1894, p. 222. COHPAGR an hes Vee THE STURGEON RIVER TONGUE. By WILLIAM SHIRLEY BAYLEY. DESCRIPTION AND BOUNDARY OF AREA. The Sturgeon River area of Algonkian sediments, like the Felch Mountain area, is an east-west tongue of conglomerates, slates, and dolo- mites, very narrow at its. eastern extremity and widening out toward the west until it finally plunges under drift deposits that separate it from the large Huronian area of the Crystal Falls district. The tongue occupies the western portion of T. 42 N,, R. 27 W., the central and northern portions ie 1D, INTL, 1k, BAS) WW, WDE 2 a dete 2) Ni enol “IDL 2U2} IN, Js, BO VAY, eum the southern parts of T. 43 N., Rs. 28 W., 29 W., and 30 W. The best exposures of the rocks constituting the tongue are found in secs. 7, 8, 17, and 18, T. 42 N., R. 28 W., and in sees. 1 and 3, T. 42 N., R. 29 W., on or near the northwest branch of the east branch of the Sturgeon River; hence the name Sturgeon River tongue (Pl. LI). On the south the sedimentary rocks are bounded by an area of granites, gneisses, hornblende-schists, and mica-schists, that are cut by granite and quartz veins, by dikes of diabases, and by other greenstones. This area separates the Sturgeon River tongue of sediments from the Felch Mountain tongu ying from 2 to 3 miles farther south. The exact line of demarca- tion between the granite-schist complex and the sedimentary rocks is difficult to draw, because for the eastern 7 miles the latter are bordered by green- stones whose position in the granite-schist complex or in the sedimentary series can not be determined at present. The line as drawn on the map places the greenstones in the Archean. It begins near the south side of sec. 7, T. 42 N., R. 27 W., and runs a little south of west to the quarter post between sections 17 and 18, in the next town west, then northwest to near 458 U.S. GEOLOGICAL SURVEY _ Hf) NG iN aN ill GEOLOGICAL MAP SCALE 1INCH-1™M a Outerops without observed strike or ‘¥ Outerops with slatiness or schistosi VERTICAL SCALE ARCHHAN _LO Granite Sturg eon. quart r-Granite A-Arkose G.S:Greenstone-schist MONOGRAPH _ XXXVI PL.LI. c : i ; - JULIUS BIEN &CO.LITH.N.Y. MAY Wilf yy wh 2y Ry GE = RIVE R TONGUE [OUR INTERVAL 20 FEET =- Outerops with determined strike and dip « Test pits bottomed in rock. TION 1 INCH -1320 FEET, IAN PLEISTOCENE LONIAN Randyille dolomite i ee] M -Limestone Sl-Slate U.S. GEOLOGICAL SURVEY MONOGRAPH XXXVI PLL Gr yen gee? Feat GF % Gr TUUUS MIEN SCO LTH NY Rer x oh eacmangl [INS r NK S07 TON al Sau ae rT y oe 7 a Ame ay f Tees ee ‘ te eee Konan UP fie DS EY ~ cnet Wh ~ — _ ‘ Py . Py US NN SA t yy; SAS lis we Li fe Ns 4 \ pees ANA r% = —_ POEL om, GEOLOGICAL MAP OF STURGEON ee SCALE 1 INCH-1 NILE @¥rotR . R IVER TONGUE : puserees without observed strike or dip = Bey csvicn pa =r Outerops with slatiness oF 5 DS wil termined strike P VERTICAL SteT1oy i erent pits ‘bottomed in rock. sand dip ARCHEAN 2 oa 4 S$ ‘on ¢ artzi * = Wi antes aR Gr-Granite -C mere G5-Greenstone-schist od St-Slate THE STURGEON RIVER TONGUE. 459 the N. quarter post of sec. 13, T. 42 N., R. 29 W., and westward to near the southwest corner of sec. 12, T. 42 N., R. 30 W. From this point the line leaves the Sturgeon River tongue, curves southward, and returns east on ‘the north side of the Felch Mountain tongue. The eastern boundary of the Sturgeon River Algonkian area is even less definitely determinable than its southern boundary, because of the thick drift covering the rocks. This boundary is placed at about the east lines of sees. 6 and 7, T. 42 N., kh. 27 W., because just east of this line, in the NW. 4 sec. 5, ledges of Paleozoic limestone occur. The northern boundary is the most indefinite of all. The southern portions of T. 43 N., Rs. 28, 29, and 30 W., are so deeply drift covered that but few ledges can be found in them, and these are widely separated. In sec. 6, T. 42 N., R. 27 W., and im sees. 13 and 24, T. 43 N., R. 29 W., are exposures of granite. These, so far as is known, mark the southern limit of an Archean area which stretches some miles northward and separates the Sturgeon River fragmentals from those of the Marquette district. The line marking the northern boundary of the Sturgeon River tongue begins at the southeast corner of sec. 6, T. 42 N., R. 27 W., and is assumed to run a few degrees north of west from this point until it reaches the west line of R. 29 W., where it turns north. Between the northern and the southern boundaries of the sedimentary area as defined, and in the midst of the sediments, are two areas of granite, the rock of one of which is unquestionably, and that of the other presum- ably, older than the conglomerates within the tongue. The best defined of these two areas lies in the northern portions of secs. 7 and 8, T. 42 N., RB. 28 W., and sec. 12, T. 42 N., R. 29 W. It measures about 24 miles in length and less than one-half mile in width. The extent of the second area can not be so accurately outlined. It occupies about three-fourths of a square mile and is entirely within sec. 3, T. 42 N., R. 29 W. LITERATURE. But few references to the existence of fragmental rocks in this portion of the Upper Peninsula of Michigan can be found in the literature of the region. The early United States surveyors* reported the occurrence of talcose 1General observations upon the geology and topography of the district south of Lake Superior, by Bela Hubbard: Thirty-first Congress, first session, Executive Documents, 1849-50, Vol. III, No. 1, pp. 846, 847, 848, 855. 460 THE CRYSTAL FALLS IRON-BEARING DISTRICT. and argillaceous slates in T's. 42 and 43, Rs. 29, 30, ete., of mica-slates in Ts. 41 and 42, Rs. 29, 30, etc., and of ‘‘calciferous sandrock” near the south boundary of T. 42 N., Rs. 27 and 28 W. In a list of specimens gathered from these townships Burtt mentions sienitic greenstone, trap, granite, granulite, and talco-micaceous slate. On the land plats made by these surveyors conglomerate is noted on the west line of sec. 8, T. 42 N., R. 28 W., end marble at the south corner between sections 3 and 4 in the same township. In 1851 Messrs. Foster and Whitney reported” the existence of an arm of Azoic rocks about 18.miles in length and 10 in breadth, extending east- erly into T's. 42 and 43 N., R. 28 W., and located its position on their map of the Upper Peninsula. Brooks,’ in his description of the northern iron belt of the Menominee district, refers to the existence of outcrops of hornblendic rocks, mica- schists, and gneisses, cut by trap dikes, which he regarded as equivalents of the various greenstone-schists exposed along the Menominee River. ‘“‘Near the center of this hornblendic belt, in the north part of secs. 22, 23, and 24, T. 42 N., R. 29 W., a line of weak magnetic attraction was observed. This is regarded as an indication here of the existence of an iron-ore belt.” The gneiss, granite, etc., north of the north quarter post of sec. 31, T. 42 N., R. 29 W., he declares to have the appearance of typical Laurentian rocks. ‘If future investigations prove theni to be Laurentian, a very troublesome structural problem would be presented here, as we would have Laurentian rocks conformably overlying beds unmistakably Huronian.”* The only distinct reference made by Brooks to the sedimentary beds of the district is in the following paragraph:° A range of marble associated with quartzite, chloritic and taleose rock, and overlaid by a chloritic gneiss, with beds of chloritic schist and gneissoid conglom- erate, the whole dipping at a high angle to the south, passes about 5 miles north of 1 Geological report of the survey of a district of township lines in the State of Michigan, in the year 1846, by Wm. A. Burt: Thirty-first Congress, first session, 1849-50, Senate Documents, Vol. III, No. 1, p. 84. 2 Report on the geology and topography of the Lake Superior land district, by J. W. Foster and J. D. Whitney, Part II, The Iron Region: Thirty-second Congress, special session, 1851, Senate Documents Vol. III, No. 4, p. 14. 3 Tron-Bearing Rocks (economic), by T. B. Brooks: Geol. Survey of Michigan, Vol. 1, 1869-1873, N. Y., 1873, p. 161. 4Op. cit., p. 175. 5 Op. cit., p. 176. LITHBRATURE ON STURGEON RIVER TONGUE. 461 the North belt (i.e., the Felch Mountain tongue). These may represent the north side of the trough or basin, of which this iron belt is the south outcrop. No iron has, however, been found, so far as I know, on this range. In Rominger’s first report on the Menominee district only a single reference is made to this area. He declares that a series of test pits put down in the W. 4 sec. 26, T. 42 N., R. 29 W., and im the SW. 4 sec. 14, T. 42 N., Rh. 30 W., are in decomposed granite.’ A specimen of the conglomerate referred to by Brooks as overlying the marble in the belt 5 miles north of Felech Mountain is described and pictured by Van Hise’ in his paper on the Principles of North American pre-Cambrian Geology (see also Pl. LIID). It is stated to be from the Felch Mountain district. The more exact location of the ledge from which it was obtained is near the northwest corner of sec. 17, T. 42 N., R. 28 W., in the Sturgeon River tongue. Thus the only distinct reference to a tongue of sediments north of the Felch Mountain range is that of Brooks, although the existence of sedi- mentary rocks in this portion of the Menominee district was reported by Hubbard and Burt. Brooks believed that the Sturgeon River rocks repre- sented the northern rim of a syncline whose southern rim constitutes the Felch Mountain range, although both he and Rominger discovered a granite-schist complex underlying the country between the two areas of fragmental rocks. RELATIONS BETWEEN THE SEDIMENTARY ROCKS AND THE GRANITE- SCHIST COMPLEX. As has already been stated, the country between the Sturgeon River tongue of sediments and the Felch Mountain tongue is underlain by a com- plex of granites and various schists, traversed by fresh and altered diabases and by granite and quartz veins. Brooks recognized these rocks as pre- senting a Laurentian aspect, although he felt constrained to call them Huronian, because of the supposed structural difficulties involved in any other view of their age. No contacts of this granite-schist complex with the bedded rocks of the Sturgeon River tongue have been discovered. Nevertheless, there can be little question as to the relative ages of the two series. As has been 1Geol. Survey of Michigan, by C. Rominger, Vol. IV, 1881, pp. 198-199. 2 Sixteenth Ann. Rept. U. S. Geol. Survey, 1896, p. 801 and Pl. CXV. 462 THE CRYSTAL FALLS IRON-BEARING DISTRICT. stated, the granites and schists extend southward to the Felech Mountain fragmentals, and here they are unconformably beneath the latter. Moreover, since the Sturgeon River rocks and the lower members of the Felch Moun- tain series are identical in character, it is probable that they are of the same age, in which case the granites and schists that are older than the Felch Mountain rocks are older also than those of the Sturgeon River tongue. The relations of the sedimentary series to the granites on the north have not been determined, because no contacts are exposed. The granites, however, can be traced northward until they are found unconformably beneath the rocks of the Lower Marquette series at Republic, and these, so far as is known, are the oldest sediments in Upper Michigan. There can be little doubt, therefore, that the relations of the sediments to the northern granites are the same as those with the southern schist complex. The granites of the two areas surrounded by the sediments are prob- ably of the same age as the northern and southern granites. The rocks of the area in secs. 7 and 8, T. 42 N., R. 28 W., and sec. 12, T. 42 N., R. 29 W., are demonstrably beneath the conglomerates, though their relations with the dolomites have not been determined. A well-marked contact between the granites and the conglomerates is exposed at the south base of a small hill of granite in the NE. 4 sec. 7, T. 42 N., R. 28 W.. The conglomerate here is well bedded. Its strike is N. 60° W., and its dip almost vertical. It consists largely of pebbles and bowlders of granite identical with the granite composing the hill, and a matrix constituted entirely of granitic débris. The contact, though exposed for only a short distance, seems to be an erosive one. It is certainly not an igneous one. From a consideration of the facts as given above, there can be little doubt that the rocks of the granitic areas within the Sturgeon River tongue and of those bounding it on the northern and the southern sides are older than the sediments within the tongue, though this has not been proved for the granites with respect to the limestones. From the lithological similarity of the Sturgeon River fragmentals with those of the Felch Mountain district, and from the structural relations exisiting between the rocks of the two districts, it is practically certain that the Sturgeon River sediments are of the same age as the Felch Mountain ‘The exact location of the contact is 400 paces N., 280 W., of the southeast corner of section 7. BASEMENT COMPLEX OF STURGEON RIVER TONGUE. 463 ones—i. e., Menominee (Huronian)—while the granites and schists belong to the Basement Complex on which the Lower Algonkian beds throughout Michigan have been laid down. THE BASEMENT COMPLEX. The Basement Complex rocks in the area studied comprise gneissoid granites, biotite-schists, and hornblende-schists, cut by dikes of greenstone and by veins of quartz and granite. The granites are best exposed in the NE. 4 sec. 7 and the NW. 4 sec. 8 and the NE. 4 sec. 7, T. 42 N., R..28 W., where they occur as bare knolls of a fairly coarse pink rock, separated from one another by stretches of sand. The best exhibition of rocks with the typical aspect of the Basement Complex is along the west half of the east-west quarter line of sec. 19, T. 42 N., R. 28 W., and south of the center of this section. Here we find hornblende-schists and hornblende-gneisses cut by veins and dikes of red granite and by greenstones that are usually schistose. Near the west quarter post of the section is a high hill bare of vegetation. On this hill the rocks are especially well exposed. In addi- tion to the types already mentioned, there is present here a coarse white pegmatitic-looking granite that apparently cuts the hornblende-gneiss. All the members of the Basement Complex in this area are so similar to the corresponding members of this complex elsewhere in the Lake Supe- rior region that they demand but little description. They are described here only in sufficient detail to establish their character. | THE GNEISSOID GRANITES. The gneissoid granites north of the fragmental tongue, and those of the two areas surrounded by the sedimentary rocks, are mediumly coarse aggregates of a dark-red feldspar, white quartz, and a dirty green chloritic substance. ‘The red feldspar is in excess, sometimes to the exclusion of the other components, when the hand specimen resembles a dense red felsite. Almost all specimens are gneissoid. The constituents are usually lenticu- lar, but in a few specimens, particularly those taken from near the contacts with the sedimentary rocks, they are drawn out into long slender string-like masses, giving the specimens a streaked appearance. The microscopical features of all the granites are those common to these rocks elsewhere in the Basement Complex. They consist of clouded ortho- 464 THE CRYSTAL FALLS IRON-BEARING DISTRICT. clase, some plagioclase and a little microcline, quartz in varying quantity, and more or less green chlorite that seems to have been derived from biotite. All the constituents present abundant evidence of the effects of pressure. In the least-crushed rocks the quartz shows undulatory extine- tions, and the feldspar grains granulation around their edges. As the crushing action increased, the granulation increased, so that the most crushed granites now consist of large grains of feldspar and of quartz in an ageregate of broken fragments of orthoclase, quartz, plagioclase and micro- cline, and a few wisps of green chlorite. Movement in the crushed rock mass has drawn out the granulated aggregate between the large grains of feldspar into bands and lines, thus producing the schistose structure noted in the hand specimens and in the ledges. In the more highly schistose granites a considerable quantity of new microcline and a small quantity of new plagioclase have developed within the granulated ageregate, and in a few instances muscovite has been found in fairly large plates of -pale-yellow color. This muscovite occurs on the contact between the larger quartz and orthoclase grains, but more particularly in the granulated matrix. The granites in the area between the Sturgeon River and the Felch Mountain tongues are not so abundant as those in the northern area of Base- ment Complex rocks, or in the areas surrounded by the sediments, but in their essential features they are identical with these. Occasionally the sur- face of a fresh fracture through these southern granites shows the outlines of porphyritic orthoclase crystals, but these crystals are not sufficiently numerous to impart a porphyritic aspect to the rock. Some of the granite specimens examined from this district are so com- pletely granulated that they can with difficulty be distinguished in the hand specimens from the schistose arkoses near the base of the fragmental series. In thin section they differ from the latter in containing no rounded quartz grains and in the possession of very little mica. The feldspathic constitu- ents are nearly all decomposed, and very much of the quartz present in the granites is of secondary origin. Thus in all essential respects the gneissoid granites of this district are like those in the Marquette district elsewhere described.' 'The Marquette iron-bearing district of Michigan, by C. R. Van Hise and W. S. Bayley, with a -chapter on the Republic trough, by H. L. Smyth: Mon. U. 8. Geol. Survey, Vol. XXVIII, 1897, pp. 171-176. —— BASEMENT COMPLEX OF STURGEON RIVER TONGUE. 465 THE AMPHIBOLE-SCHISTS. In addition to gneissoid granites, the southern area of the Basement Complex contains a number of ledges of dark-colored schistose rocks. These, in some instances, are cut by dikes of granite similar to the granite already described. These dark schists may be classed as greenstone-schists and as horn- blende-schists. The former are heavy rocks, with dull greenish-gray luster and distinct schistose structure. They resemble closely in their micro- scopic as well as in their macroscopic features the schistose dike greenstones to be referred to later. They are doubtless altered or squeezed diabases or gabbros. The hornblende-schists are usually fine-grained, bluish-black rocks, with a very even schistosity, closely resembling slaty cleavage. On the surfaces of cross fractures may be seen long slender prisms of glistening black hornblende arranged in as distinct lines as the lines of particles in an evenly bedded sedimentary rock. Often the cleavage surfaces are coated with thin layers of golden-yellow mica scales. In most specimens there may also be noticed a fine banding parallel to the foliation. In thin section these hornblende-schists differ from the schistose dike diabases and from the greenstone-schists, referred to above, in the presence of large quantities of quartz, and of some biotite, and to some extent in structure. The greenstones owe their schistosity to the flattening of their components, while in the hornblende-schists this structure appears to be due largely to the crystallization of the hornblende in elongated prisms with their major axes parallel. The parallel arrangement of the amphibole in the latter rocks is thus much more pronounced than in the schistose greenstones. The hornblende-schists are composed of hornblende, quartz, biotite, plagioclase, magnetite, and sphene. The hornblende is in long prisms of the usual yellowish green color. The mineral is compact, but it is full of inclusions of quartz grains similar to those constituting a large part of the matrix lying between the amphiboles. It was evidently formed in situ as an original crystallization, and not, like much of the hornblende of the schistose greenstones, by the alteration of augite or of some other component of a basic crystalline rock. The biotite MON XXxVvI——30 466 THE CRYSTAL FALLS [RON-BEARING DISTRICT. is in small dark greenish-brown flakes interspersed between quartz and feldspar grains, which together constitute a matrix surrounding the other components. The quartz is unusually free from inclusions. It contains a few liquid inclosures and occasionally a few flakes of biotite and needles of hornblende. The plagioclase, where present, 1s in irregular grains with ragged outlines, as though a newly formed mineral. It appears to act the part of a cement surrounding the other minerals with which it is in contact. Small round grains of sphene and magnetite occur very abundantly scat- tered through the matrix. Often the magnetites are surrounded by borders of sphene; hence it is probable that this mineral is a titaniferous variety and that the round grains of sphene are pseudomorphs after magnetite grains that have been completely altered. In a few specimens large colorless areas with the outlines of porphy- ritic crystals are observed in the midst of the finer-grained groundmass of schist. Between crossed nicols these break up into a coarse-grained agere- gate composed of the same minerals that constitute the rest of the rock, except that in it altered plagioclase is common and amphibole is rare. These probably represent phenocrysts of plagioclase which have suffered alteration into quartz and new plagioclase that may differ somewhat from the feldspar of the original crystal. The banding of some of the hornblende-schists has already been referred to. Under the microscope the only differences noted in the bands are the quantity of hornblende present in them and a variation in the coarseness of grain. The coarsest of the bands have the composition and structure of the schistose greenstones. They contain large quantities of plagioclase, both fresh and altered, and large grains of hornblende that are not in the definite prismatic form characteristic of this mineral in the main mass of the rocks. ORIGIN OF THE AMPHIBOLE-SCHIS'S. From the gradations often observed between the hornblende-schists and the greenstone-schists, it is plain that the two rocks are genetically related. he latter, from their similarity to schistose dike greenstones in composition and structure, are believed to have been derived from massive diabases or gabbros. The hornblende-schists are in all probability derived -from similar basic rocks, though the presence in them.of what appears to BASEMENT COMPLEX OF STURGEON RIVER TONGUE. 467 have once been plagioclase phenocrysts may indicate that the original rocks were in the form of lavas. The principal difference between the hornblende-schists and the greenstone-schists seems to be in the nature of the amphibole in the two rocks and in the presence of quartz and newly formed plagioclase in the first named. ‘The materials of the greenstone-schist were derived from the alteration of those of the original rock, as were also those of the hornblende- schist, but the former now consist mainly of the direct products of this alteration, whereas in the latter the substances now existing have been worked over and entirely recrystallized. THE BIOTITE-SCHISTS. Mica-schists are not common in the Sturgeon River tongue. They constitute by no means so large a part of the Basement Complex in this dis- trict as they do in the other portions of the Lake Superior region that have been studied. Indeed, only a few ledges of this rock have been observed in the country between the Sturgeon River and the Felch Mountain sedi- mentary tongues, and most of these are along the southern edge of a green- stone knob 300 to 400 paces north of the southeast corner of sec. 17, TT, 22 IN, TR, BS WY. The mass of this knob is a dark hornblende-schist. On the south side of the top of the knob this rock is in contact with a very evenly banded or streaked rock of a general dark-gray color. In the hand specimen it resem- bles very closely a fine-grained banded augen-gneiss. Near its contact with the hornblende-schist the rock is apparently porphyritic, with pheno- erysts of feldspar from 1 to 15 mm. in length, and an occasional one of quartz scattered through a matrix composed of narrow alternating bands of almost black and light-gray material. On cross fractures of the rock the phenocrysts are seen to be drawn out in the direction of the bands. Cleavage takes place very readily along the planes of the banding, yielding surfaces covered with tiny scales of black biotite. A little farther from the contact the light-colored bands are thicker and more distinct. At first glance they appear to be uniformly thick for long distances, but a more careful inspection shows that they wedge out rapidly and are replaced by other bands of the same character. The dark bands are not thicker than 468 THE CRYSTAL FALLS IRON-BEARING DISTRICT. sheets of paper. They are the cross sections of the mica coatings on the cleavage planes. — The inspection of this rock in the hand specimen and in the ledge leads to the same conclusion—that it is an intermediate or an acid lava, a porphyrite, or a porphyry that was squeezed until it became schistose and sheared until it became fissile. . Under the microscope the feldspar phenocrysts, though much decom- posed and filled with inclusions of quartz, muscovite, and other decomposi- tion products, are weli enough preserved to exhibit in some places twinning striations. The greater portion of the phenocrysts are untwimned. The twinned material borders the grains, fills in cracks between Carlsbad twins, and is irregularly distributed through the untwimned material, occurring more particularly in those places where the decomposition of the original feldspar is most complete. The twinned feldspar is fresher than the untwinned variety. This fact and the manner of its distribution indicate a secondary origin for it. The quartz phenocrysts are rare. ‘They present their usual characteristics. The groundmass in which the phenocrysts lie is a fine-grained aggre- gate of biotite, quartz, and plagioclase. The biotite is a greenish-brown variety. It occurs in large plates arranged in parallel position and in small flakes occupying the same parallel position and lying between the quartz and the plagioclase grains. The banding noticed in the hand speci- men is due to the arrangement of the large biotite flakes in bands. These are separated from each other by bands of quartz and plagioclase that are free from the large biotites, though they contain innumerable small flakes of this mineral. Only when a porphyritic crystal lies in the way of the bands do these depart from their uniform directions. Here they bend around the phenocrysts, leaving on both sides of them little triangular areas in which the components are much finer grained than elsewhere in the rock. The light-colored components are quartz and plagioclase. ‘These min- erals are'in small grains that appear to be intererystallized in the manner of the secondary aggregate that constitutes the fine-grained matrix of many ereenstones, of the aporhyolites, and of other rocks that have suffered intense metamorphism. ‘The quartzes are nearly always crossed by strain shadows and the fresh clear plagioclase by interrupted and bent twinning bars. —— a BASEMENT COMPLEX OF STURGEON RIVER TONGUE. 469 Here and there in the midst of this fine-grained groundmass are noticed lenticular and long narrow aggregates composed of grains of plagioclase that are much larger than the grains of this mineral occurring in the sur- rounding matrix. They look as though they might be the crushed remains of what were originally plagioclase phenocrysts. Thus the microscopic study of these rocks tends to confirm the results of their field study. They were probably porphyritic lavas or mtercalated flows that have suffered alteration as the result of intense pressure and movement. Their present composition suggests that they were originally quartz-porphyries or perhaps andesitic porphyrites. Whatever their original nature, their origin is different from that of the biotite-schists of the Mar- quette district.’ THE INTRUSIVE ROCKS. The intrusives in the schists and gneissoid granites of the Basement Complex are granites, identical with the gneissoid granites above described, and greenstones. The former cut only the schists. They are probably apophyses from the larger granite masses. The greenstones cut the schists and the granites. They are similar in all respects to the greenstones in the sedimentary series, and thus are the youngest rocks in the district, with the exception of the horizontal Paleozoic sandstones and limestones that cap some of the higher hills. The greenstones are all more or less altered diabases. In some the ophitic structure may be detected, but in most of them no traces of their original constituents nor of their structure remain. Nearly all are more or less schistose. The only evidence that the most schistose phases were once massive igneous rocks is in their composition and their occurrence in dike- like fissures. As the schistosity of these greenstones increases, the amount of their alteration also increases; there is a greater abundance of horn- blende present in them and a greater quantity of quartz, until in the most schistose phases the rocks are now typical hornblende-schists. One of the best examples of these greenstones occurs in the series of ledges extending in nearly a straight line for 6 miles from the southern portion of sec. 13, T. 42 N., R. 29 W., to the northeast corner of sec. 14, T. 42 N., R. 28 W. Except in its eastern ledges the rock constitutes bold, 'Mon. U.S. Geol. Survey, Vol. XXVIII, pp. 200-203. 470 THE CRYSTAL FALLS IRON-BEARING DISTRICT. rounded, bare knobs with almost perpendicular sides, usually situated in the midst of swamps. ‘The main mass of the knobs is a rather fine-grained, slightly schistose, gray rock exhibiting the diabasic structure on weathered surfaces. On the south sides of the knobs the rock is much denser, and in most cases is much more highly schistose than the main rock mass. Under the microscope these rocks present the usual features of schistose dike greenstones. ‘They consist almost exclusively of hornblende, plagio- clase, and quartz. The hornblende, which is the common yellowish-green variety, occurs in long plates and in columnar crystals, some of which are idiomorphic in cross section, and also in slender needles penetrating the quartz and feldspar. These two minerals form an ageregate between the larger hornblendes. The feldspar is mainly a calcium-soda plagioclase, though a small quantity of albite may also be present. It occurs as irreg- ular grains embedded in a mosaic composed of rounded grains of the same | feldspar and of quartz, and appears to be a new crystallization subsequent to that of the greater portion of the plagioclase. At any rate, a single large grain often fills the interstices between numbers of the mosaic grains and extinguishes uniformly over large areas. The magnetite in the rock is titaniferous. It occurs in little crystals and im small irregular grains that are often surrounded by a granular zone of leucoxene. This rock may serve as a type of nearly all the other dike greenstones in the district under discussion. Some may be more schistose than this one, while a few may be more massive, but in general characteristics they are all similar. The more schistose rocks differ from the less schistose varieties simply in the possession of a greater amount of quartz and a greater quantity of what appears to be newly formed feldspar. Their greater schistosity is due to the more uniform elongation of their components. The fine-grained greenstones found on the edges of the coarser-grained ones, and occasionally as independent dikes, are weathered diabases of the normal type. COMPARISON OF THE STURGEON RIVER AND THE MARQUETTE CRYSTALLINE SERIES. The Basement Complex in this area is essentially like that in the Mar- quette district, except that the altered tuffs so abundant in the northern area are absent from that now under discussion. The biotite-schists of the two ee | pte i i eel ALGONKIAN ROCKS OF STURGEON RIVER TONGUE. A(T areas seem also to be different in origin, although this can not be stated with certainty, since the origin of the Marquette schists is not so clear as is that of the Sturgeon River schists. There is enough similarity between the crystalline series in the two areas to leave no doubt as to their practical identity. If the Marquette Basement Complex is Archean, the crystalline series underlying the conglomerates in the Sturgeon River tongue is also Archean. THE ALGONKIAN TROUGH. The sedimentary rocks comprised within the Sturgeon River Algonkian tongue may be separated into a conglomerate series and a dolomite series. The conglomerate series consists of schistose conglomerates, arkoses, quartz- ites, slates, and certain sericitic schists that are squeezed arkoses. The dolomite series embraces crystalline dolomites, a few thin beds of quartz- ite, a few breccias and conglomerates, and some slates. It is possible that a third series, composed essentially of slates, also exists in the district, but if so it is not advisable to separate it from the dolomite series, since its exposures are very few in number, and the slates which comprise its main mass are so nearly like the slates belonging in the dolomite series that they can with difficulty be distinguished from these. Associated with the sedimentary rocks are great masses of basic igneous ones. Some of these are unquestionably intrusive masses, as shown by their relations to the conglomerates, while others appear to be interleaved sheets. A very few, apparently bedded greenstones, on close examination seem to be composed of intermingled sedimentary and igneous material. These may be altered tufts. Nearly all the sedimentary as well as the igneous rocks embraced in the trough are schistose, and thus are sharply distinguished from the brown Potsdam sandstones and the Silurian limestones that here and there lie approximately horizontal on their upturned edges. The squeezing of the pre-Potsdam formations has been so intense that both conglomerates and dolomites have been forced mto closely appressed folds, which in the con- glomerates are for the most part apparently isoclinal. The strike of the latter rocks is nearly east and west, and their dip nearly perpendicular, except in one or two cases. The dolomites are less closely folded than the conglomerates. Their dips are much less steep, and their strike varies 472 THE CRYSTAL FALLS [RON-BEARING DISTRICT. considerably, except in the narrow eastern portion of the tongue, where it is approximately parallel to that of the conglomerate, i. e., a few degrees north of east. RELATIONS BETWEEN THE CONGLOMERATE AND THE DOLOMITE SERIES AND CORRELATION WITH THE FELCH MOUNTAIN FRAGMENTALS. The relations of the conglomerates to the dolomites are best shown by the distribution of their respective outcrops, as members of the two series are nowhere in contact. In the central portion-of the tongue the conglomerate outcrops are limited to the district between the central granites and the southern area of the Basement Complex. The dolomites, on the other hand, are limited to the country north of the central granite. Its outcrops are found scattered over the northern tier of sections in T. 42 N., Rs. 28 W. and 29 W., and the southern tier of sections in T. 43 N., Rs. 28 W. and 29 W. Between them and the granite to the north is a belt of country devoid of exposures. It is heavily drift covered, consisting of sand plains and sand hills, from beneath which no ledges of any kind protrude. This barren belt measures about a half mile in width, in sec. 2, T. 42 N., R. 28 W., gradually increasing in width till it reaches the center of sec. 1 in T. 3 N., R. 29 W., where it opens out into the large Pleistocene area whose southeast edge is shown on the map (PI. LI). In the eastern portion of the district the northern granites and the conglomerates approach each other, and the dolomite belt becomes very narrow, finally disappearing toward the east side of T. 42 N., R. 28 W. The relative distribution of the conglomerate and dolomite ledges, when considered with reference to the triangular outline of the area embraced between the northern and the southern granite-schist complexes, suggests that the two formations constitute a western-pitching syncline with the dolomite in the center and the conglomerates with their associated beds on the two flanks. The conglomerates comprising the southern flank are well exposed, but those of the northern flank are not seen. They are believed to underlie the glacial deposits in the barren strip of country bordermg the northern granites. The conglomerates, according to this view, are older than the dolomites. Toward the center of the dolomite area, in the north half of see. 6, T. 42 N., R. 28 W., and at a few places farther west, there are ferruginous gil itm se tier peter mee tS i ee ALGONKIAN ROCKS OF STURGEON RIVER TONGUE. 473 beds in the dolomite series. If these represent the upper portion of the dolomite formation, as is the case with similar rocks in the Felch Mountain range, it is clear that as we approach the center of the Sturgeon River tongue the rock beds met with are younger than those on its borders. This is in line with the supposition that the Sturgeon River tongue is a westward- pitching syncline. The belief that the conglomerates are beneath the dolomites in the Sturgeon River area is further strengthened by the fact that the principal conglomerate in the Felch Mountain range is beneath a dolomite which is identical in character with the Sturgeon River dolomite. This conglomerate is regarded as the base of the Lower Menominee series in this district, with the dolomite above it, known as the Randville dolomite, immediately suc- ceeding it. If the conglomerates and dolomites in the two districts are the same, the Sturgeon River rocks are Lower Menominee. RELATIONS BETWEEN THE DOLOMITES AND CONGLOMERATES AND THE OVERLYING SANDSTONES. At several places the conglomerates and dolomites are overlain by well-defined Lake Superior sandstone. The sandstone usually caps hills, on the lower slopes of which ledges of the underlying rocks appear. The contacts between the overlying sandstone and the underlying rocks are rarely seen, but the fact that the former are always horizontal, while the latter are always very steeply inclined, leaves no doubt that there is a strong unconformity between them. THE CONGLOMERATE FORMATION. The conglomerate formation comprises very much squeezed granitic conglomerates, arkoses, sericite-schists, quartzites, a few beds of banded rocks believed to consist largely of tuffaceous material (see pp. 485-487), and occasional beds of slates. Nearly all the members of the series are schistose, the arkoses in some cases passing into very well characterized sericite-schists. Occasionally the arkoses shew obscure traces of ripple marking, and more frequently very well defined cross bedding. All the rocks of this formation strike in a nearly uniform direction, N. 75°-84° E., and dip almost vertically. In one or two instances observed the dip is as low as 65°, but in most cases it varies between 85° N. and 85° 8. The strike of the schistosity is approximately parallel to the strike 474. THE CRYSTAL FALLS IRON-BEARING DISTRICT. of the bedding, as is also the direction of the elongation of the pebbles so abundant in the conglomeratic layers. From the slight changes in dip observed in the beds, as well as the great width of the formation in some places, it is evident that folding must exist. It is probable that m the wider portions of the area occupied by these rocks there are present two or more folds, so closely appressed that the beds on the opposite limbs can not be correlated. Hence they appear as members of a consecutive series of conformable members with a nearly uniform dip throughout. In the narrower portions of the area it can not be told whether more than one fold is »resent or not. In any event, the folding is practically isoclinal. The ledges of the conglomerates and their associated beds occur in the southern portion of the Sturgeon River tongue throughout its entire extent. No exposures have been found north of the granite areas in the central and western portions of the tongue. IMPORTANT EXPOSURES. The arkoses, the sericite-schists, and the conglomeratic phases of the series can be best studied at the dam of the Sturgeon River near the north- west corner of sec. 17, T. 42 N., R. 28 W. Here they form a continuous ledge of well-bedded layers striking N. 83° E., and dipping 85° S., which measures at least 250 yards in width and 400 yards in length. (See Pl. LIL) The conglomerates are pink in color. They contain immense numbers of white quartz pebbles and bowlders, fewer and smaller ones of pink granite, and many fragments of red feldspar in a matrix composed of moderately coarse granite débris. All the fragments and pebbles in these rocks, as well as their matrix, show plainly the effects of pressure (Pl. LIII.) The matrix of all specimens is more or less schistose, and the coarse sand grains embedded in it are in many cases elongated in the direction of the schistosity. Most of the pebbles and bowlders in the conglomerate are also flat and parallel to the schistose plane. How far these phenomena are due to mashing, to rota- tion into parallel positions during flattening, and to original sedimentation, respectively, can not be determined in most cases, since the schistosity of the rock and the elongation of the pebbles are both approximately parallel to the bedding—i. e., the pebbles are nearly in the positions assumed by unequidimensional pebbles in a well-bedded conglomerate. In a few U.S,GEOLOGICAL SURVEY. Whiting, WANA a7) y, jj NY I) U ui) MiNi . UY Ui Hy /, y i Hi Hi 7 “I y ARCHEAN — Gr.—Granite I ! (GG SGM 1h AN OREN MAP OF EXPOSURES IN SEC.7 AND IN PORTIONS OF SECS. 8,17,AND 18, T. 42 N,R.28 W.,MICHIGAN VICINITY OF DAM ON EAST BRANCH OF STURGEON RIVER ALGONKIAN A. Arkose SI. Slate Q. Quartzite C. Conglomerate GT. Greenstonetuffaceous IN TRUSIE| Ah AY G. Greenstone G.S.Greenstone Schist Ti cae NHE JULIUS BIEN &CO.N.Y, Od aL J ‘ . * ¥ ® a mt é tye ALGONKIAN ROCKS OF STURGEON RIVER TONGUE. 475 instances the schistosity may be seen to meet the bedding at a very acute angle. In this case the pebbles are usually arranged with their longer axes parallel to the schistosity, though there are always present a large number that lie parallel to the bedding planes. In the least schistose phases of the rocks the pebbles are nearly round and the matrix possesses a well-defined fragmental texture, but in those beds in which the schistosity is more pronounced the matrix is sericitic and the pebbles are lenticular. The most completely schistose phases resemble augen-gneisses. In these the matrix is an almost typical sericite-schist. The quartz pebbles have been crushed and flattened into long narrow stringers or plates of quartz, some of which are continuous for long distances (6 or 7 inches), while others are broken into separate parts, which when rounded on their edges yield quartz lerses like the “augen” of so many augen-gneisses.* The nonconglomeratic beds interstratified with the conglomerates are usually more completely schistose than the latter. The least schistose beds are arkoses. These often’ show ripple marking and current bedding. As the schistosity increases, the quantity of sericite present also increases, until in the most highly schistose phases sericite-schists result. Some of the arkoses, as well as some of the finer-grained conglomerates, in addition to being schistose, are also foliated—i. e., they are built up of plates or leaves, along the planes between which they split very easily. When this is the case, the cleavage surfaces are covered by small scales of silvery mica. The foliation is so pronounced in many cases that the rocks are almost fissile. Besides these rocks there are present near the dam great ledges of coarse and fine grained greenstone (see Pl. LII), whose relations to the sedimentary beds at first glance appear to be those of interleaved flows. Upon close inspection some of these masses disclose intrusive features. Although they almost invariably follow the bedding of the fragmental rocks, some of the greenstones can be seen to cut across the layers in such a manner as to leave no doubt of their intrusive character. 1 The best examples of these extremely schistose conglomerates are not found in the exposures referred to above, but they are well developed along the line between secs. 11 and 12, T. 42 N., R.29 W. Here the width of the series is but one-fourth mile, whereas the total width of these rocks and their associated greenstones near the dam measures a full mile. 476 THE CRYSTAL FALLS IRON-BEARING DISTRICT. On the old road leading to the dam the conglomerates and arkoses are intruded by an altered diabase in a most complex way. To the north of the road is the great mass of the greenstone, within which are considerable areas of the conglomerate. Within the belt of conglomerate, on the other hand, are several bands of the eruptive rock which roughly follow the bed- ding of the sedimentary one, but which cut across it in a minor way. At the contact of the main mass of greenstone and the conglomerate are numerous interlaminations of the two rocks, the greenstone having intruded the conglomerate along its bedding planes. At one place a dozen alternations of the two were noted within a foot. Moreover, for some distance from the greenstone the conglomerate appears to be impregnated with material from the intrusive, so that it has taken on a greenish tinge. This impregnation in one instance has gone on so far as to produce what is apparently a greenstone matrix containing separate pebbles from the con- glomerate, the groundmass of the conglomerate having apparently been absorbed. The greenstone adjacent to the conglomerate is traversed by narrow pegmatite veins in various directions, some of the largest being not more than 2 inches in width. There is no evidence of a granitic intrusion, the pegmatites appearing clearly to be the result of an interaction between the basic igneous rock and the more acid fragmental one. At one place along the contact there is a belt of very coarse hornblendic material that is cut through and through by the pegmatite veins. East and west of the dam for some distance are other ledges of con- glomerate. They, however, as a rule, present no features different from those exhibited by the great ledge described above. In all, especially in those occurring in secs. 9 and 10, T. 42 N., R. 28 W., the interbanding of conglomeratic and nonconglomeratic layers is beautifully shown. Near the north quarter post of sec. 11, T. 42 N., R. 28 W., the arkoses have a purple rather than a pink tinge. On cross fractures they are seen to be spangled with glistening black needles and plates of hornblende, which lie with their long axes in all azimuths. The little crystals appear to be more abundant in some layers than in others. The best exposures of quartzite are found near the north quarter post of sec. 11, T. 42 N., R. 28 W., and at 1,300 paces W., 150 N., of the south- east corner of sec. 7, T. 42 N., R.29 W. The rocks are black. They occur in beds varying in thickness from a few inches to several feet. ee Se eee ee ee ‘| ped ‘Aaaing [0285 ‘Ss (p ydey “Uuy YyyUeexiS Uy ‘KB0\0a5H uruquied-sig Uday YON 40 sajdjoulig S,aSiH UeA Woy paonpusdey “M 82 ‘YN Zp L ‘LI ‘OAS JO YANYOO LSSMHLYON AHL YVAN ‘YSAIN NOZODYNLS JO WYG WOYS ZLVYSWOTDNOD LSIHOS L— lI] “Id IAXXX Hd¥HSONOW. ASYNETS WEIMER): a) ALGONKIAN ROCKS OF STURGEON RIVER TONGUE. 477 PETROGRAPHICAL DESCRIPTIONS. As might naturally be expected, the least schistose of the arkoses and conglomerates exhibit the fewest evidences of alteration in the thin section. In addition to the pebbles in the conglomerates, these rocks consist of rounded and angular grains of quartz, microcline, orthoclase, and of various plagioclases, and a few of microperthite, embedded in a fimer-grained ageregate of the same minerals, tiny flakes of green biotite and of color- less muscovite or sericite, a few plates of chlorite, particles and crystals of magnetite, and little nests and isolated grains of epidote, with occasionally some calcite. Many of the feldspar grains are altered into sericitic products, colored red by small particles of various iron oxides and by red earthy substances. The composition and microstructure of the schistose arkoses and of the schistose matrices of the conglomerates vary greatly in different speci- mens, being determined largely by the original composition of the different beds and the amount of squeezing to which they have been subjected. No attempt will be made here to describe in detail all the changes suffered by these rocks; a simple statement of the tendency of these changes will be given. The quartz pebbles in the moderately schistose conglomerates show plainly that they have been under great stresses. The smaller ones all exhibit undulatory extinction. The larger ones are sometimes peripherally granulated, and sometimes etched or corroded on their edges, as though they had suffered partial solution. By this process small portions of the original particles have been separated from them, and the dissolved silica has been redeposited among the grains of the surrounding matrix as sec- ondary quartz. In their interiors many of the larger pebbles have been changed to a mosaic of differently oriented parts, which interlock so per- fectly that they appear to have crystallized together. The groundmass in which the pebbles le is, in a few cases, a frag- mental aggregate of quartz and several feldspars, with the addition of seri- cite and other crystallized components. In most cases, and in all in which schistosity is marked, no fragmental structure is noticeable. The ground- mass is an interlocking mosaic of fairly large quartz grains that appear to have crystallized in situ, between which are smaller grains of the same 478 THE CRYSTAL FALLS ITRON-BEARING DISTRICT. character, large and small spicules and plates of sericite, crystals of magne- tite, and a few needles of chlorite and other secondary substances. Between these, again, is often a cement of what seems to be secondary quartz. The schistosity of the specimens is due to the arrangement of the sericite in approximately parallel positions, and to the elongation of the quartz grains in the same direction. The pink color of the rocks is produced by red earthy substances in the feldspars and in their decomposition products. In the most schistose phases of the conglomerates the quartz pebbles have been mashed into plates, several of which joi, end to end, forming sheets, which in the thin section appear as long narrow lines of variously oriented quartz grains, each of which is crossed by strain shadows. The larger quartz grains in their matrices are broken into parts, and these parts are differently oriented with respect to one another. Other grains seem to have entirely recrystallized, for they are now made up exclu- sively of the same kind of interlocking quartzes as are present in the fine portions of the groundmass in which the coarse quartz grains are embedded. In the groundmass of these rocks sericite is very abundant, and feldspar is rare. From the proportions of these minerals present it would appear that the former has been derived largely from the latter. Biotite is also present in many specimens as small green flakes, but this mineral is not widely spread. The conclusion from the study of the thin sections of the schistose conglomerates is that there has been a crystallization of new substances, principally quartz, sericite, biotite, and magnetite, from the materials of the original granitic sediments. Perhaps a portion of the crystallization was the result of alteration of the original components before squeezing took place. The larger portion, however, was accomplished under the influence of pressure. The result of the mashing and recrystallization is a schist, which between crossed nicols has the aspect of a typical crystalline schist, but which in natural light exhibits its conglomeratic nature in the presence of the large quartz lenses, with the outlines of flattened pebbles, in a fine- grained groundmass. The pink arkoses differ from the conglomerates simply in the absence from them of the pebbles. The schistose varieties are similar in every respect to the schistose groundmass of the squeezed conglomerates. Both in the hand specimen and in the thin section the schistose arkoses exhibit striking resemblances to muscovitic gneisses. | ALGONKIAN ROCKS OF STURGEON RIVER TONGUB. 479 The purple arkoses differ from the pink ones just described in contain- ing chlorite and hornblende, and in addition some apparently newly formed feldspar, notably a feldspar with the microcline twinning. As a rule, these rocks are more feldspathic than the matrices of the conglomerates, and they contain much less quartz. The larger grains of both quartz and feldspar are corroded as if partially dissolved. They have lost their smooth, rounded contours of sand grains, and now possess irregular jagged ones, which, however, are not due to secondary enlargements. The characteristic components of these rocks are the chlorite and the hornblende. The former mineral is present in plates intermingled with grains of epidote, while the hornblende is in dark-green or light-green plates, and in acicular or columnar crystals that are idiomorphic in cross section. The crystals are distributed indiscriminately through the rocks, with their longer axes lying m all azimuths. They were evidently formed after the squeezing that made the rock schistose. The plates, moreover, include within themselves such great numbers of the other components of the rock that their parts often appear to be independent. Under crossed nicols, however, many of these apparently independent plates are discoy- ered to polarize together. No evidence is present in any of the sections as to the source of the material that gave rise to the hornblende. The fact, however, that all of the hornblendic rocks are banded, that some layers are rich in amphibole while others are completely devoid of this mineral, sug- gests the notion that the hornblendic schistose arkoses consist partly of sedimentary and partly of tuffaceous materials. As we shall see later, this origin is ascribed with more confidence to some very peculiar rocks to be discussed later. Crushing effects are noticed in some of the hornblendic arkoses, but their present condition appears to be due more to chemical changes pro- duced in them than by mechanical action. The chemical changes were no doubt supermduced by the mashing, but this can only be inferred from the fact that they are more pronounced in the schistose phases of the rocks than in those phases im which the schistosity is poorly developed. THE DOLOMITE FORMATION. The dolomite formation comprises, as has been stated, both dolomitic limestones and calcareous slates, and occasionally quartzites, sandstones, , 480 THE CRYSTAL FALLS IRON-BEARING DISTRICT. and conglomeratic and brecciated beds. As a rule, exposures are small and scattered. Their distribution has already been described. All ledges observed may be seen by reference to the map (PI. LI). IMPORTANT EXPOSURES. Good exposures of the dolomites occur in the NW. 4 sec. 6, T. 42 N., kh. 28 W. The ledge nearest the northwest corner of the section is a hard flesh-colored dolomitic marble, containing here and there little quartz grains. This is cut by joints, and is traversed by small chert bands. The bedding is more or less contorted, but its general strike is N. 45° E., and its dip is 45° NW. About one-fourth mile east of this ledge is a small, bare knoll, composed of interlaminated pink marbles, conglomerates, red sand- stones, and red slates, varying in thickness from a few inches to a foot or more. The conglomerate consists of marble pebbles and slate and chert fragments in a calcareous quartzitic matrix. The strikes and dips are uni- form throughout the ledge, the former being nearly east and west and the latter 45° S. The difference in dip of the beds of these two exposures indicates plainly the presence in this place of a little westward-pitching anticline. Other prominent exposures of the dolomite series are in the NW. 4 sec. 1, T. 42 N., R. 29 W., and in the SE. 4 sec. 35, T. 43 N., R.29W. In the first-named locality is a high, bare knob, and a cluster of small ledges, in which dolomites, conglomerates, and slates are all well exposed. The dolomites, for the greater part, are massive pink marbles crossed by joint planes. In places the rocks take on a greenish-yellow tinge, and become schistose. At 1,500 paces N., 1,930 W., of the southeast corner of sec. 1, T. 42 N., Rk. 29 W., the dolomite forms a well-defined bed, striking N. 45° E. and dipping 70° SE. Above this, to the southeast, is a bed of coarse-grained granitic sandstone or quartzite, which in turn is over-lain by beds of gray quartzite alternating with thin slates and fine-grained conglomerates. Farther south is a ridge of well-bedded, fine-grained quartzite and bluish-gray slate, the individual layers being usually less than one-half inch in thickness. This rock grades into a gray schistose dolomite, and the whole quartzite-slate series strikes N. 75° E. and dips 63° S. ‘The exposures in section 35 are almost pure marbles, in which no traces of bedding have been detected. ALGONKIAN ROCKS OF STURGEON RIVER TONGUE. 481 PETROGRAPHICAL DESCRIPTION. In thin section the marbles appear as very close-grained aggregates of calcite and dolomite, usually untwinned, but occasionally twinned in the ordinary manner of these minerals. Here and there among the car- bonates are rounded quartz grains, but the greater portion of this min- eral appears to have crystallized in situ between the calcite and dolomite individuals. All the marbles are of the same general character. They differ only in the quantity of silica present and in the presence or absence of the tiny dust grains producing the color. The schistose varieties owe their schistos- ity to the elongation of their components. The quartzites and slates interbedded with the marbles possess no unusual characters. They are similar to the corresponding rocks inter- stratified with the Marquette dolomites. The conglomerates interstratified with the dolomites, slates, and quartzites are of two kinds. One is com- posed of marble and slate fragments cemented by quartzite, and the other of small granite pebbles embedded in granite sand. The latter are evi- dently composed of the detritus of the granites underlying the dolomite series, while the marble-bearing conglomerates, or perhaps more properly breccias, are interformational beds conformable with the beds below them, and also with those above. They are similar in every respect to the inter- bedded breccias in the Kona dolomites on the Marquette range. SLATES AND SANDSTONES ON THE STURGEON RIVER. The rocks in the SW. 4 sec. 34, where the road to Sagola crosses the Sturgeon River, are placed in the dolomite formation, although they differ somewhat from that portion of the series described. These rocks are white caleareous sandstones, that look very much like the Potsdam sandstone where it overlies limestones, and a light-green slate, which near joint planes and other cracks has a light purple color. According to Dr. J. M. Clements, who visited the spot, the slate overlies the sandstone. ‘The river,” he writes in his notebook, ‘gives a section through these rocks, and makes the strike seem to be N. 35° W., dip 50° N._ It appears to me, how- ever, that the true strike is about N. 85° E., and dip 40° 8.” If these rocks belong to the marble series, they constitute its upper part. The slate closely resembles some of the slates in the Kona dolomite formation of the MON xxxvi—— 31 : 482 THE CRYSTAL FALLS IRON-BEARING DISTRICT. Marquette range. It is a very fine grained rock composed of very small splinters of quartz, flakes of sericite, and a few of chlorite. THE IGNEOUS ROCKS. The igneous rocks associated with the sedimentary beds in the Sturgeon River tongue are all greenstones in composition. Many of them are unques- tionably intrusive; a few may be tuffaceous. The intrusive greenstones do not differ essentially from those cutting the Basement Complex. Some of them are in the form of small bosses. Others are clearly dikes, though for the most part these dikes follow the bedding of the sedimentary rocks. Still others may be intrusive sheets. The rocks regarded as possibly tuffaceous are distinctly banded. Some are made up of alternate bands of dark and light shades. The darker bands consist principally of a schistose greenstone, and the lighter ones principally of arkose or granitic sandstone. These rocks are well bedded, apparently constituting a definite portion of the conglomerate series near its lower horizon." THE INTRUSIVE GREENSONES. The intrusive greenstones are usually fairly massive rocks, with a dark bluish-green color and a moderately fine grained texture. On their edges they often pass into schistose phases, presenting the structure and appear- ance of chlorite-schists. A very typical schist of this character occurs on the southern edge of the great greenstone mass 1,525 to 1,600 paces north and 300 to 400 west of the southeast corner of sec. 18, T. 42 N., R. 28 W. In the hand specimen the rock appears to be a well-characterized chlorite- schist, spangled with plates of a light-colored muscovite measuring 1.5 to 2 mm. in diameter. : The intrusive character of some of the greenstones is clearly shown by the fact that they occur immediately on the strike of the conglomerate bands, and often cutting across them, as is the case at 300 paces east of the north- west corner of sec. 17, T. 42 N., R. 28 W. (see PI. LIT), and at 400 paces south, 100 west, of this same corner. PETROGRAPHICAL DESCRIPTION. The greenstones intrusive in the Algonkian sediments are not essen- tially different from those cutting the members of the Basement Complex. 1See Van Hise’s Notebook 184, pp. 21-23. IGNEOUS ROCKS OF STURGEON RIVER TONGUE. 483 They differ from the latter in containing, as a rule, less quartz and a very much greater abundance of epidote. The epidote is all secondary, as is also the quartz, so that the only noticeable difference between the two sets of greenstones is dependent upon differences in the nature of their alteration, which in turn are probably the results of differences in environment. Both sets of greenstones have been squeezed, but those in the Basement Complex are associated with crystalline schists, while those in the Algonkian series are associated with fragmental beds. In addition to hornblende, plagioclase, epidote, and a little quartz, almost all the later greenstones contain biotite, small crystals of magnetite, and irregular grains of ilmenite or of a titaniferous magnetite. Their structure is schistose through the arrangement of the larger hornblendes and biotites and the elongation of the feldspar grains in approximately parallel directions. Asa rule, their thin sections present no unusual features. They all show dirty green hornblende plates, greenish-brown biotite flakes, magnetite crystals, etc., embedded in a mass of irregular grains of decom- posed plagioclase, the principal decomposition product of the feldspar being in almost all cases epidote. Often the proportion of epidote present is very great. It occurs as colorless crystals and grains scattered through the hornblende, and as light- yellow plates and grains embedded in the mass of altered-plagioclase. In the rock at 500 paces east, 125 north, of the southwest corner of sec. 8, T. 42 N., R. 28 W. (PL LID), the replacement of the plagioclase by epidote has pro- ceeded so far that no trace of the feldspar can be discovered. In the hand specimen the rock is seen to be a massive mixture of black glistening horn- blende crystals in a yellowish-green groundmass possessing a sugary texture. In the thin section the hornblende is present as bluish-green plates that are often idiomorphice in cross section. The groundmass in which they lie is composed of epidote and quartz. The epidote is in large yellowish-green irregularly-outlined plates, including particles of magnetite and small rounded quartz grains. Most of the quartz is in isolated grains between the epidote plates and in little nests of interlocking grains. Small magnetite granules are scattered everywhere throughout the section, through all of the components indiscriminately The coarser greenstones show plainly in the hand specimen ‘ie ophitic structure, even where the rocks are schistose. In the section this structure 484 THE CRYSTAL FALLS IRON-BEARING DISTRICT. is often obscured by the abundance of decomposition products. Under low powers of the microscope, however, it can nearly always be detected. In a few of the finer-grained varieties, phenocrysts of plagioclase are occasion- ally met with. They are clouded by inclusions of biotite flakes and shreds of hornblende and by tiny particles of a kaolinitic or sericitic mineral. From their composition and structure, it is clearly evident that the intrusive greenstones, whether massive or schistose, are altered phases of diabase or of diabase-porphyrite. The dark-green chlorite-schist referred to as occurring in the edge of one of the greenstone masses is a chloritic biotite-schist spangled with large flakes of a light-colored mica. The rock consists of biotite, chlorite, musco- vite, quartz, and rutile. The biotite is in broad thin plates, arranged approximately parallel, and embedded in a mass of chlorite, the greater portion of which is a greenish-brown variety that looks as though it may have been derived from hornblende. A smaller portion of the chlorite is in light-green plates, like the chlorite so frequently found in chlorite-schist. The quartz is in small rounded grains exhibiting strain shadows, scattered here and there through the chlorite and between the biotite plates. It is much more abundant in some portions of the rock than in others, forming bands rich in quartz, between others in which very little of this mineral is present. The rutile is in large quantity. It constitutes large greenish- yellow grains. Some of these are rounded forms, others are prismatic crystals measuring 0.08 mm. to 0.12 mm. in length, while still others are clearly defined elbow twins. They occur everywhere throughout the slide, but are rare in the quartz. They are most abundant in the chlorite and in the large plates of light-colored mica that have been mentioned as character- istic features of the hand specimens. These have all the properties of mus- covite. They lie indiscriminately among the other components, irrespective of the schistosity of the rock, and contain very few inclusions, with the exception of the rutile grains. The lines of biotite, to the arrangement of which the rock owes its schistosity, do not bend around the muscovite as they do around the eyes in an augen-gneiss, but they continue their courses up to the edge of the muscovite grain, and there abruptly stop. From these facts it is clear that the muscovites have originated since the rock containing them was rendered schistose. As m the case of many other secondary minerals, it appears that these were produced from the components of the IGNEOUS ROCKS OF STURGEON RIVER TONGUE. 485 schist by a process which resulted in the absorption of all of them except rutile. The process may have been connected with contact action, but no evidence in favor of this supposition has been obtained. There are a few other types of greenstone occasionally met with among the dike and other intrusive forms of the district, but they do not differ in any marked degree from those described, except that some are quite schistose. One or two of these contain oval aggregates of epidote, plagioclase, and quartz, that may represent inclusions of foreign rocks. They are now, however, so much altered that it is difficult to determine their character with any degree of certainty. The rock of one or two other exposures in the area underlain mainly by the conglomerates deserves mention before the banded greenstones are discussed. The rock referred to is a heavy, lustrous, black schist that resembles in many respects a hornblende-schist. On fresh fractures across the schistosity parallel lines, darker than the main mass of the rock, may be easily detected. These are the edges of cleavage planes, whose surfaces are coated with brassy yellow mica plates. In thin section these rocks differ very little from the schistose greenstones referred to above. They consist of a heterogeneous schistose mass of green hornblende, cloudy plagioclase, quartz, epidote, chlorite, and magnetite. Biotite flakes are met with occasionally, but they are by no means common, except on the cleay- age surfaces. Rocks of this class have not only been made schistose by squeezing, but they have also suffered shearing along what are now the cleavage planes. They are almost identical in microscopic and macroscopic features with the hornblende-schists in the Basement Complex. THE BANDED GREENSTONES. Distinctly banded rocks, composed partly of basic material with the composition of greenstone, form a well-defined hillock in sec. 17, T. 42 N., R. 28 W., about 250 paces north of the west quarter post of this section, and a group of outcrops on the east bank of the Sturgeon River, imme- diately west of this point. The rocks in question are banded in mediumly coarse-grained dark bands, containing large quantities of green hornblende, and in fine-grained lighter ones, that resemble in the hand specimen bluish-black quartzites or cherts. In some bands there are large lenticules of white quartz, that show 486 THE CRYSTAL FALLS IRON-BEARING DISTRICT. plainly on weathered surfaces, like the flattened pebbles in a squeezed con- glomerate or the drawn-out parts of quartzose layers in a mashed bedded rock. These bands, though not very well defined, run continuously for long distances, and strike and dip conformably with the conglomerate beds exposed 200 paces to the north. PETROGRAPHICAL DESCRIPTION. In the thin section. the lghter-colored layers of these rocks are seen to be composed of very irregularly outlined and rounded quartz grains, cemented by a mass of finer quartzes and small grains of zoisite, little clumps of chlorite, some decomposed feldspar, and particles of magnetite. Occasionally a plate of yellowish epidote occurs in the midst of this agere- gate, and scattered here and there through it are large plates of green horn- blende with the cellular structure so common to secondary minerals. These hornblendes lie irregularly in the slide, and include grains of all the other components. The quartz grains are small and are independently oriented, but frequently little groups of them, with the outlines of sand grains, are met with. There is little evidence of schistosity in these layers, but they exhibit a banding produced by the alternation of coarser and finer constit- uents. In the darker layers the proportion of hornblende is much greater than it is in the lighter ones. Indeed, some bands consist almost exclu- sively of large cellular plates and radial aggregates of plates of this mineral, only the small interstitial spaces between the large amphiboles being filled with an aggregate of quartz-zoisite, small hornblende needles, and magnet- ite. In some sections biotite is also present. It occurs most abundantly in the quartz-zoisite aggregate, fillmg the interstitial spaces between the amphiboles, but is present also as inclusions in this latter mineral. Some of the biotite in the hornblende appears to grade into its host, and certain portions of the amphibole possesses the brown color of the mica, with the optical properties of the hornblende. The large amphiboles are evidently the youngest components in the rocks, though they were plainly produced before the schistosity. In those layers in which the schistosity is strongly marked this structure is produced mainly by the parallel arrangement of the biotite and the small amphibole needles and plates in the quartzose aggregate. The larger cellular hornblendes lie across the schistose planes, and when they do so, the lines of biotite and of small amphiboles pass IGNEOUS ROCKS OF STURGEON RIVER TONGUE. 487 around them exactly as they would do were the large hornblendes present before the rock was squeezed. Sometimes the amphibole masses that form so large a proportion of the schistose bands are single crystals, sometimes they are fragments of crystals, and at other times they are groups of radiating crystals. The magnetite is very much more abundant in the hornblendes than in the surrounding quartz aggregate, sometimes being confined exclusively to this mineral, as though it were one of the products (the hornblende being the other) resulting from the decomposition of some original constituent, probably augite. Little particles of hematite, on the other hand, are abundantly disseminated through the quartzose aggregate, and are practically absent from the hornblende. Much of it appears to have been derived from magnetite. The evidence derived from the microscopic study of sections of these banded rocks, so far as it relates to their crigin, is disappointing. The quartzose layers are, in all probability, sedimentary. The hornblendic layers, however, differ from these so much im composition that their material must have had a different source. It is possible that the quartzose layers represent sediments derived from the granitic portions of the Basement Complex, while the hornblendic layers represent sediments derived from the basic portions of the Basement Complex; or, it may be that the acid layers have the origin aseribed to them, while the basic ones are mixed sediments and basic tuffs. The sections of the dark layers of these rocks resemble so strongly the sections of the basic layers m the Clarksburg series of mixed tuffs and sediments in the Marquette district that the writer is inclined to regard the rocks as composed partly of tuffaceous material. On the other hand, the banded rocks occur so close to the boundary between the sedimentary area and the Basement Complex, which near this boundary is composed mainly of basic schists, that it would seem but natural that they should contain large quantities of basic material derived from these schists. The original structure of the layers has been so completely destroyed by mashing that it can not give any evidence as to the nature of the beds. We are therefore compelled to rely entirely upon their com- position to aid us in discovering their origin. This indicates simply that much of their material was derived either from volcanic ashes or from the débris washed from the basic portions of the Basement Complex. ENE eS Page. 120, 121, 122, 123, 124 Aa structure, sketch of (See Ellipsoidal structure.) Aci Castello, basalt from ------ Aci Trezza, basalt from -- Acid intrusives, described . age of .-.---.----------- distribution of . - Thal KONG Noo 56 seme sop econ a= pe ceoResobeSssooso5 in Felch Mountain Range---..--..--------------- _ 426 in Hemlock formation ---.-.-------------------- q7 jin Upper Huronian -..-.---..--.----------------- 164 | Acid lavas, of Hemlock formation, described -.----- 80-94 banding of .....--.-.-------------------=---- 91, 92, 93, 94 micropegmatitic texture in.--..----------------- 89 pressure effects in.----.-.----------- 87, 88 schistose, described ---.------------- 87-94 Acid pyroclastics, described --- 94, 95 Acid voleanics of Hemlock formation, described.... 80-95 Actinolite from hornblende 215 of adinole.--..-- 209 of chlorite-schist- - 442 of metabasalt. --- 133 of picrite-porphyry - 215 of pyroclastics ...-.--------------------+----+--- 147 OF BAU eesogasssoseas ooo se seebEeseee sbonsodcosS 205, 209 of spilosite...........--------------------------- 206 APE MANNON OP oacose coococenssosceososescmaasssec 105 (See Amphibole.) Actinolite-schist from graywacke...-.-..----------- 57 of pyroclastics ...----------------- 147 Adamello tonalite .....-..------------------------ 230 | Adams, F. D., on analysis of slates and granites. 58 | on pyroxene zone about olivine. - 256 Adinole, analyses of-.-...------.-------------------- 208 | compared with analyses of clay slate and spilo- site 210 in Mansfield formation. .-----..----------- BaeeaeS 64 AGORA coeszececsssocsossSocumesscomess00e 208-209 Ajibik quartzite -.---------------------------------- 451 conrelationlo Geese hee eee en eeree Ae eS ay XXY, XXVI relations to Groveland formation..----.....-- XXI, 449, 456 Albite from feldspar --.---.------------------- BEEeoaS 151, 201 of metabasalt ....----------------------.-------- 99 of spilosite, plate of ..--...-..------------------- 302 Algonkian, contact with Archean, effect on topog- raphy 22 386 | deposition of - - - 456, 457 distribution of - 331, 427 folding of - 427, 428 of Felch Mountain Range, distribution of ..---- 384 Succession in ----.-------------------------- 384-385 SYM (HDS) OH Samo ocoemonooeeeeSosooscaeacsese5 384-385 intrusives in, described --.----.------------- 426 Page. Algonkian of Marquette district, distribution of--.- epee of Sturgeon River tongue, described.--......... 458-487 comparison with Algonkian of Felch Moun- LEMON UOWVEING) oAnoomapasosseenasoosseSacecs 462 fOldin giofsstrsnee tee eee ee -- 471-472 igneous rocks of .............------ -- 482-487 LEssuLeletectshpe es eee eee eee te Reece 471 relations to Archean --.....-.-- BONSHC SOO Lee 461-463 relations to Lower Marquette-.---.-..-..--. 462 relations to Archean......---.----- XVII, 331, 399, 427, 458 relations to drainage .--....----..---..-.--.-.--- 334, 335 relations to Paleozoic formations.........--.---. 331, 383 , relations to quartz porphyry-.------------------- 439 (See Huronian, Upper Huronian, Lower Hu- ronian.) Allanite in acid lavas .--.....- 89 included in epidote-zoisite 444 Allen, Andrews, referred to .---- 22 Alteration of andesine.--......- 22, Ofmbasichvolcanicss=- sere seeeeeeeeeeeeeeee eae. 152 Oftbiotiteren-\.- sess ae sse eae eeae ae aie nee eat 43, 393 Ofsbronzitesplaterofeeese eens e ese ee ee eee 306 Ohi CANO pecopoSooeaccodoaqnaosncmecseosoacaanes 146 Ofidiabaseseaccecesneisere ae eoe eee eects 469 ofjellipsoidall basalitees=-eee=see eee nese 292 Ofptel dS paiteeenee eee eee 42,170, 171, 192, 201, 224, 228, 478 plate ofte2cs aac oes wee cosas 288 of glauconite. 6 422 Ofsenitie nse ---- 168-169 of hornblende 234, 235, 237 of metabasalt -. 117 described ....-. 126-135 of picrite-porphyry --.---.-..----.--.---.- ° 213 of slate 14 of tutts 141 INTO (WOVAN eqoascenccenosasdossnncoccd=sacsasase 12, 143, 162 Amasa area, ore deposits of .-.--..-..-----------.---- 177 STICCESS1 ODT amie a= eee eet ae ee 177-178 (See Hemlock mine.) American Black Slate Co., analysis of slate from. --. 61 Amphibole of greenstone -.---.----.-..--.-.----.--- 486, 487 Of metabasallt)s memes selene = ee ancl) 127, of mica-schist---- 415 of picrite-porphyry -.--------------- =. 214 OMEN AON Ohi Hose ssossconaccesecseeneTS 127, 214, 465, 486 (See Actinolite, Hornblende, Tremolite.) Amphibole-peridetite, described-.......----.-.------ 253-254 crystallization of.------- ~~~. 297 gradation to olivine-gabbro...-...-----..----.--. 254-260 gradation to wehrlite Amphibole-schist of Sturgeon River Archean, de- ROMA sodada ddatooasenodecesismoncaenosds 465-467 Amphibolite, analysis of ...........-...--.---------- 397 490 INDEX. | Page. Amphibolite of Archean, described -...-.----..----- 395-397 from) basic volcanics. ---- ~~. == 22 eee ee 152 | Dotrndin oymica-sChis hess esses ee seers 392 | Amygdaloidal texture in metabasalt- 102, 113, 122, 128, 442 GlaNeM wl .os5ssss-sssosoncase cose zssSpessonotes 124-126 plate of 280, 282, 284, 290 cause of distribution of, in basic lavas 95 in eruptive breccia..----.--.-.---.-.-.--.- 2 136 Tune Evenn) OGkss Chis teem=eer aera sane ae 445 TH pWPMOWARMOS 225s2sescce2cedsoesaess2552c0se209 138, 146 Amygdules of calcite, plate of -..--.--..-..-----.--. 282 @? Ohilomniney Waits Oio5-- cassasoneses cemsesasesoose 284 of feldspar, platejot se ose. oe- eee 22ers = 284 Ofequantziip altel Otaeenee meet eee eee eae 284 orderiofdepositionlone esse ee eee eee eeereeee 125) | pressuneretectswnn. merase eee ee eee 126, 128 Analyses of adinoles 208 of amphibolite - 397 of clay slate acon BRL GIL of clay slate, spilosite, and adinole, comparison of 210 of lolomites asta cence ee secs ees ce ee eee ee 409, 435 of gneiss - -- 391 of granite ..-- E 389 offhornblende mene steer eeesetre anes eeen ee eee 242 of hornblende-gabbro......-...-.---.---.-------- 263-264 of iron ore, by Brooks, referred to...--...-------- 19 OPO Casas eackeaesdeacocessoceoson peatanabens 181 of Mansfield mine.-.----..-..----.-.-.------ 69 OL Mansheldislatelcsssaccin ce. cae seen eones cecener 59, 61 comparison with contact products .-...----- 209-211 ofimetabasalteess.-ce soso nensesee ee scence 103, 106, 107 Ofmica-dioniterrss-seree reer eetee eres 231, 263-264 of mica-schist 394 of norite ..--- 263, 264 of peridotite---- 259, 263, 264 of picrite-porphyry Se 219 on Spilositeje see sssee ee ee 5 207 Anatase included in hornblende . 4 236 of gabbro and norite.-.--.-.--.- 5 236 OOM ASN ossosscosserseseosenceosossososscs 129 (See Octahedrite.) Andesine; alteration of -- 2.) .cence- esos eie eo oe 2! altering to muscovite. - 224 altering to epidote-zoisite-------..--.-----.------ 224 | Ofidi Orie yescee se eecle mae sane ee eet ae eine eee 224 | of gabbro and norite ...-...-.---.--..-....-..... 233 OMNES tio 505 2 coc dcoesesedeacerecasence 170 in Hemlock formation. - 2 77 of greenstone 486 described. ----=----- 204 of Hemlock schist - oes 444 in Mansfield slate described. - 203-204 of metadolerite .......------..- ay 202 in other intrusives described ..-........-.-..--- 204 of metamorphosed Mansfield slate..........---- 205 in Upper Huronian 164, 211 of mica-diorite, plate of. --.-<..------.---.------ 308 described tees asec ceo ee eee cee eee ae 204 Ofpmica-SChistiessat ase see eee ce eae ee selene 392, 393 metamorphism of Mansfield slate described. ---. 204-211 | of muscovite-biotite-gneiss, plate of .-..-....--- 298 492 INDEX. | Page Biotite of muscovite-biotite-granite -.....--.--.----- 193 Of perid OUte ee == le lee eee 252, 257, 261 of picrite-porphyry------.------------------.--. 215 of pyroclastics 147 ot sedimentaries developed by intrusion. : 195 of sedimentary inclusions in granite. --- os 197 of spilosite...--..---..---------------- ape 206 orientation of...--..------.-- 393, 425, 468, 486 paralle] growth with muscovite-.-----.--.------ 170 penetrating feldspar .--.-.--..---.-------------- 414 pressure effects im ----------------------------_- 43, 248 relations of orientation to hornblende ---------- 486 | relations of orientation to muscovite. ----------- 484 replacing feldspar. -..-.-...--.------.------------ 193 Biotite-gneiss of Archean....-.-.-----------.------- 429 Biotite-granite described.-...-..-.------------ 40-43, 191-193 WE Gb ye OF soso nessendsoscosasascose sescose=se9 43, 44 WOOO case se ssootddscongocuctecssosesseos 25 308 micropegmatitic structure in, described. -- 192-193 SC HISTOS Li yAO Dae eee atest eee ee ee eee 44 Biotite-schist of Algonkian of Sturgeon River tongue 484 of Archean of Sturgeon River tongue---..---.- 467-469 of Hemlock formation-....------------ 442 Birkinbine, John, on ore shipments 186 Blaney mine. (See Hope mine.) Blocklava. (SeeEllipsoidalstructure, Aastructure ) Bog iron ore of Upper Huronian.---.--.------------- 182 Bone Lake described....-.....-..----..------------- 35) referred to --.---.------.. FS BBEOCBHOEOSO AED EAOSO 156 Bone Lake crystalline schists described.----------- 148-152 Bonney, I. G., on alteration of olivine-.------------- 218 on ellipsoidal structure....--------- - 118-119 Botryoidall ore) -- === - =. eee eee 4 180 Brackett, R. N., on ultrabasie intrusives- - 220 Branner, J.C ,on ultrabasic intrusives------------- 220 Breccia, eruptive, of Hemlock formation, described. 135-136 volcanic, use of term-...--.--.---.---.--.-------- 137 Brecciation of Groveland formation... 418 of metabasal bieess= eee a ae ee ea eee 117 Brittany granite compared with Crystal Falls granite 44 Brégger, W.C., on diorite and gabbro families. .-.--- 242 on monzonite group-----.---- ------------------ 232 | On) rock analySeS- =e nae == ae eee eae 105 on use of term diorite-.--...-.------------------ 223) Bronzite altering to serpentine..-----...--.---------- 238 | WENO Ola ss. cssccsossbee croscoswpszeesencanesS 306 altering to tale_---------.--------- 238 NG Olson cokssosssossscaccsc] 306 included in hornblende. 250 plate of.----- 306 includivg ilmenite - 238 Ame lMd Im WW tee a ee 238 in zonal intergrowth with hornblende, plate of-- 318 of bronzite-norite ----_.----.----------.--------- 244 WEIO Ol esses -ocsesocsoresassb sonbasocosceons 318 alteration of, plate of-.-........------------- 306 of bronzite-norite-porphyry .-------------------- 246 of gabbro and norite-.-...-..-------.----------- 238 OP WOMEN onc osspnososct osnoceos sopeesasesasc 250 Bronzite-norite described ----.----.--- -- 244-247 WIENO OF ocosssssseaessseaoonsa] Bg 318 anally sigs} her ence eee 245 crystallization of minerals. --. 262 intruding hornblende-gabbro - 243, 249, 265 Bronzite-norite-porphyry 246 WENG OF oa ccc cansoccescopncasesosseroeseDe3e 320 Page Bronzite-norite-porphyry altering to serpentine ---. 246 intruding hornblende-gabbro ............------- 249 Brooks, A. H., referred to .-..--.-.----------.------- 22, Brooks, T. B., on composition of iron ore- 181 on correlation of Menominee rocks. 19 on correlation of Upper Huronian ..-........... 164 on Felch Mountain range . 376, 377, 378, 379 on iron-bearing rocks of Michigan--........-.-.- 16-17 on magnetic observations.-.-..-.-..-..--.------- 24, 337 on Menominee district -......--..-.-.-------.--- 19 on Mesnard series .-.--.---------------. 2. . 8. 452 on) Paint River district. --.--- 2222 -- 2 --- ee ee i on Sturgeon River tongue .-.----------..------- 460-461 on Upper Huronian.-...-....-..-..------------- 172, 173 BN BAWEGL KO -~ =o 2szo2e co sacseecess222s0css2520522 XV, 21 Brooks, Rominger, and Pumpelly, map of Upper Pen- insula of Michigan......----.--.-------- 18 Brown, E F., on analysis of iron ore - Brulé River described: .-.---..------- 31 referred to ..-.----- has -- 13,15, 161 Building stones of Hemlock formation described... 153, 154 Burt, William A., map of part of Upper Peninsula - 15 on) Crystal Malls rockstesssesssse-sss-e eee eee 13 on Felch Mountain range -----.----------------- 375 on Sturgeon River tongue .--.- pooseoegscorances 460-461 TeLeErned | tOn. +s 422 se cota see eee 16, 21 C. Calciferous limestone, relations to Potsdam ----..--- 383 Calcification of chlorite of feldspar --- of metabasalt - - CalcitesalterationlOfeeeeer ease ee eee eee ee oe pene eee altering to limonite -...- 153 developed by dynamic action......-.----.--..--- 432 from biotite 192, 225 MOON NOG — een scnonsonsneccacsososccse 82, 90, 131, 1382 frompliornblendeeeeeeecs--eeeceriee ae eee eee 203, 215 Ofaciddlavas. = soe sss anea snes s seater 89, 93 Ot Py ANNES oe e sce snssesoeas—eesseocor=ssn550 124 DAWIOOR osonc ve sac eotecqocedsanseseonsosseoce 282 Oh, Dasic Mikes 2 eee oan no 47 of biotite-granite -.-..--.---....-----.-. 192 of ellipsoidal metabasalt, plate of -- 292 of Groveland formation. .--.-...- 2 420 OP WPMD sossosabesossosace cs soccsoc eeesssescos9 481 of metabasalt 100, 101, 117, 127, 128, 129, 132 MTD OT asec csseosogsodnsoosencsacsesecesose 290 of metadolerite - 203 OP PUM G) 2-6 sccsnos sostezeaoesseresoesecese 252 Of) PY LOCIAS GCS ee ee a ee el 146, 147 OP (Witton sasconcas copsasesessec asa gse7Sezuscosane 142 orientation of ------------- ~~~ 132 pseudomorphs after feldspar.-....--.----------- 132 replaced by iron carbonate..---.-..---.--------- 133 replacing chlorite 132 replacing feldspar 131 Caledonia mine. (See Mansfield mine.) Calumet and Heckla mine.-..--..---....------------ 399 Cambrian sandstone -.-- 29 deposition of ------------- = ee XXIV GROSIN OF s-cosasesesaccasosseecgeosesestaeeasacs XXIV relations to Algonkian...---..-.------------.-.- 331, 473 relations to Archean ......----.------------ XXIV, 26, 331 relations to Keweenawan -.-..------------.----- 162 relations to intrusives -----..-----.------------- 188 INDEX. 493 : Page. Chlorite-schist intrusive in Algonkian of Sturgeon IPPs ciscacssoscseasbeseococsoecs acess 482 of Hemlock formation....-..-.--.--------------- 442 Of Upper Enronian)- easton niente 166, 174 Chloritization of metabasalt-.----.------------------- 117 Chrustschoff, C. von, on relations of pyroxene and PPM) NV). se sdaconsssces soeasso Tess =a50 258 Claire mine, location of ----- - 178, op. 186 table of shipments from op. 186 Claire Mining Company. (See Claire mine.) Clarke, H.W7., analysis\ Dy/----=-- --=----2-== 218 including sagenite 403 IMCclNd ins tibanite= == ea al 404 OP PAWIONG 2. 55sec gcse orossoseosszaceossocareses 208 Ofte altened) Slate emt ae aa ae oa 209 OW ArH pass - -—oo9s2s5e5 ences cossosasssosenese 124, 125 MENG OE soccescoccspescesocos= ses ctessadseocs 280, 284 OF MAKORE)| 2525 cose ssasess aes ossSsaseoS soseoeEess 479 OF SEGHO GWAGS) = ceosctoausssgenocencr aur cceonosess 47 of; biotite-granite------------.---.-.----. 192, 193 Of biotite-schist------------- ee 484 of Bone Lake schist- 151 of dolomite. ..- 410 | of graywacke -. 170 of metabasalt -- do 98, 99, 101, 117, 118, 127, 128, 129, 181, 182, 133, 134 OF WMCP ANONIS o-sosesaonesescacocosessoseasssse0 196 HE WOW) ooo ane aseoesoecetecorer sstoscseedscos 440 of picrite-porphyry OfspYMOC] AS TCS eae ieee 145 OH MOM IO-scs-ocoscotcescosecescessesneossesoc~ 206 WMO OR ossoce so sensossse sssosesecesassannces 302, 304 of spilosite-desmosite, plate of ----...-.---.----- 306 ofislatepeemes sence ster aden ornate eee. 205, 209 OP itis -petcheceaacetoosoeaccos -SEeeanosSendcossc 141, 142 of volcanic conglomerate. - ---- 143, 144, 145 orientation of ....---.----- 118, 127, 133, 146 pseudomorphs after biotite....-..-.-..---------- 217, 228 after garnet .-.--..-.-- = 403 Teplaced by; Calcite == een amen ce ee 132 Chlorite-schist from basic volcanics..----.-----.--.- 152 FERIA EN AEE ee cece ono ucbacanseobeSocuessscs 57 Compass, dial, use of.------.---- 24, 841, 342, a44 Concentration of ore in syncelinal iaayaelie (see Iron ore deposits, origin of) .--...-.---...---- 183, 184 Conglomerate altering to sericite-schist-..-----..--- 475 basalt of Upper Huronian.--.-....-----.--.------ 163 ‘Ofebemlockwtormatiolereessere erase see eee 76, 152, 153 intruded by diabase.-----...-.------------.----- 476 of Manstield ore deposit...--.-..--------------- 63-64, 68 intruded by greenstone -.----------.------------ 475-476 of Sturgeon River -.--...--.---- XVIL, XXIV, 461, 462, 481 Gero mhril oeapasssccoscctee ossogatensesasooes 472-479 -- XXII, 166 pressure effects in-- . 474, XVIII voleanic, described. --- 148-145 plate of. ...-.-- 4 284 WEO OE TOT Scone seedtcseasbosessmecsmacocos 136 Copper, absence of, in Huronian voleanics----------- 125 Corrigan, McKinney & Co. (See Crystal Falls mine.) Cortlandt serics, comparison with Crystal Falls in- URI Eiag seochccossmescneasagsacemacoos 222 494 INDEX. Page. Coutchiching.-........-.--.-------- 380 Credner, H., on Felch Mountain range 376 on Menominee district 5 377 on origin of iron ore... 6 fal Cretaceous subsidence. .-.----.----.------.--.-------- XXIV Cross, Whitman, on metadolerite.-.....-.-.----..--- 97 Crystal Falls area, ore deposits of..-....-.--.--.---- 178 comparison with output of Menominee mines-. . 186 comparison with output of region-.--. S CURDS RISUHE 186 discovery of ore deposits.-----.-.-.--..---.----- 175 Crystal Falls district, drainage of. 31-36 elevations in -.- welthesie sare ee ae ee eee 30, 31, 332 folding of. s208 =o 26° geographical limits of- =A 25 physiography .-..---------.---- “13, 29-37, 329-335 relations to Marquette district..........-..--- 11, 25, 329 relations to Menominee district..-..----....-- 11, 25, 329 structure and stratigraphy of....-.....--------- 25-29 Crystal Falls mine, analysis of ore from..-..-...---- 181 locationiofeemeee ae eseee ees nese r 178, op. 186 table of shipments of.-----.--------......--..--. op. 186 WE TMEMU)sooeoosc-soeses 161 Crystal Falls series, correlation with Marquette SOLICS ese eens eae ee eke acts XXV, XXVI | correlation with Menominee series--.-..-.-..- XXV, XXVI metamorphism of....-..--.-.----- XXIV, XXV, XXVI | Crystal Falls syncline. --- XXIM, 26,178 | descnibedees=sseE eae seae see eeeeriane -- 158-161 | Crystalline schist of Bone Lake deseribed.---....... 148-152 | Of UP PCL UTON anne eae sees 166, 167, 171, 172 Crystallization of minerals of basic rock described .. 257-259 | of minerals of intrusive series-..-.-..----------- 262 | Culver, G. B., referred! to.-----.-5------..----------- 22) | Current bedding in quartzite...-...--..--.-----..--- 5B) if D. | Dakyns, J. R., on plutonic rocks ------.....----.---- 222 | Dalmer, K., on ellipsoidal structure ....-.....-.----- 118, 119 | Dana, J. D., on ellipsoidal structure -- -- 120, 121, 124 | sketch by -- 120 on metadolerite- -- 96 on origin of voleani 78 REVEAL Hos = 5S 4h ceso soscoreHaSee 2 95 Darton, N. H., on ultrabasic intrusives.-----..--.---. 219-220 metenredstO psec ease ese ate ane eee eters 95 Dathe, E., on ellipsoidal structure..-.-.-.-.---.----- 118, 119 WYO KO Poko coecosbeocdabeooousebeacamaaoa9 38, 79, 88, 333 described’ soe oie te eee cee oe Been see 31, 32-35, 334 GOVEO} MINE O55 soos sos sop zessoosnsoosooeaseseS 32-35 | topography of valley.-----.--..--.---------- 29 Delphic mine, location of.-----..----.--..-----..---- op. 186 table‘of shipments from) 2222-2 —---- == =n a= op. 186 Desmosite gradation to spilosite, plate of-..-.------- 306 of Mansfield formation. -.....---.-.- 64 described. -- 207 De Soto Mining Co. (see Mansfield mine) - 65 | Devitrification of aporhyolite .-.--...-- 3 87 | of metabasalt-.-.--------- 102, 103, 126 OU NMI Ss secon Soo ca conaooseE conocecRooaOgscoss 138 Dewitt, N. Y., picrite-porphyry at-..--.--.-.---.---- 219 Diabasewalterationoferess- seee eee eee eee ee eee ee eee 469 altering to greenstone-......-..--.--...-----.... 466-484 | altering to hornblende-schist.....-.-----.------- 466 | intruding conglomerate ..........----..--------- 476 NOH ATTN CH DANE) = co comcodeSsasnasescesosoeds 429 | intruding Felch Mountain series......-.--.----- 426 | Page. Diabase intruding Sturgeon River series...------.-. 469 (See Metadiabase.) Dial compass, use of .---.- 24, 344 described Wssccesaserat eee aeaeeeoe -- 341-342 Diamond-drill work at Hemlock mine, figure of -.--. 177 in sec. 20, T. 45 N., R.33 W ..-- 176 Differentiation of magma ---...--- 269 Dike, associated with ore deposits at Paint igner MIN Oke sess esse esac ase nee e 183 183 183 acid, in Archean, described.-----.--------..----- 46-49 basic, in Archean, described -..-.-.....-.---.--- 46-49 effect on topography, sketch of.-...---.-.--. 46 (See Basic dikes, Acid dikes.) Diller, J. S., on ultrabasic intrusives - 220 neferrediito: |) oo nese oe eaee eee ee 95 Diopside of gabbro and norite.. - Diorite, comparison with granodiorite 231 crystallization of minerals of ......-...--------- 262 intruded by diorite-porphyry -..---------.------ 265 antinudedsbysonsni tease eee eee eee ner 194 intruded by hornblende-gabbro ........-.-.----- 265 of intrusive series described-..-- Sddovtieesoo0s05 222-232 WED OW WWM ocsegosoun sa sesesanounoonscessaasasLe 22),228 Diorite-porphyry intruding diovite --.....-..--..-.. 265 intruding hornblende-gabbro.......-.--..------- * 265 Diorite-schist associated with ore deposits. .-..----- 183 Diorite. See Metadiorite. Dip necdleyuselof sass Sate ee Sats hace 24, 344 described -.-. ie -- 342-343 Doane exploration....-.--.- -- 447,449 Dolerite, contact with granite ‘192 dikes associated with ore deposits .....---.-..-- 183 grading into basalt. ---7- <2 2- 4. . -eee ee ee nee 200 including sedimentary rocks..-.-..-....--.----. 2033 intruded ibysoranite ees ee aeeae 194, 204 Thane Des JO LO NEI So 555 eno osesnNsdecossoora ee 48 intruding Hemlock formation ...........-------- q7 intruding pyroclastics ---2--.-.--- esse see eee 147 intrusive, endomorphice effects of. Se 211 metamorphism of Mansfield slate, also. --- 204, 211 relations to intrusives of other districts... - 189 relations to picrite-porphyry ---- 212 URE OV ISOM oo eooasSSooseSsso nesses tsseos-enéeces 96 (See Metadolerite, Basic intrusives.) Dolomite described..-....-.--------- 408-411, 431-437, 479-482 analy SeSiObeee- tele ces Ree ee eee ee eee ee 409, 435 containing foreign minerals-...-...--.-.---..--. 436 MEeLAMOLPhISiO OLsseema eee sees eee eee 432 of Michigamme Mountain and Fence River areas, GENOME sococosesbecsaSsneseosdseasons: 431-437 of Randyille formation, described -.-.-...--..--- 408-411 of Sturgeon River tongue, described ....-.------ 479-482 k 472 es 471 relations to Felch Mountain fragmentals ... 472-473 relations to Lake Superior sandstone -- 473 orientation Of. = 2. eee oe ee = 410 (See Randville dolomite.) - Drainage ot Crystal Falls district.-.-.....--.-- 31-36, 334-335 Drift. (See Pleistocene.) Dunn Iron Mining Company. (See Dunn Mine.) Dunn mine, analysis of ore from -....-.-...-----.--- 181 depthiofecmtene pe asteneere eee eaeee bens aer sets 185 description of ore bodies ..............--..------ 182 INDEX. Page. | Dunn mine wocation\ Of. — 2. eee ene 179, op. 186 | itableyorsdipments trom ese. . cem ane eee op. 186 | Dynamic action. (See Pressure effects.) | | | Economic’ products of Hemlock formation de- SOME. ssooossos nous oSasuseSosoRCRSDAOS 153-154 | Elevations of Crystal Falls district - 30, 31, 332 | Elfdalen, Sweden, halleflinta of.-..-.......-.. eel 92 | Ellipsoidal structure in metabasalt described - figure of. - 112-124 | 112, 113, 114 | THEW Oi sroccensocosbecescosesas 116, 292, 258 Ells, R. W., on ellipsoidal structure..-..-...--...--- 118, 119 | Enlargement of feldspar. 144 | Olng Man tZpse see ee ee eee eee ae 57, 85, 404-405 Enstatite of gabbro and norite.--....-..-.-......--. 238 DOIVASMG, WIG? WON soscesccceaseeooosussesooseece 98 | J OMANOMAIG), MEO OF WHAM. ssaacenseoosossnssossesonase 97, 222 pidolenitey uSeloh termeeenem acme ee eee eee 97 LO OEOHS) WAN WMO) ssss5sssosasssocSsecns5 25205059 225) | from chlorite 132 drombpteldspamee sees 82, 92, 111, 169, 170, 201, 248, 483 1 Ween TOME -csaceesaccomseconeeconecsos eas 100, 203 | WACIMCA Hi WUC soosesocusesaozescsessceseo= 225, 226) | includediamichlorite ss -- =. eos eee eee c 248 included in feldspar....-....--.-.--.--.-.. 151 | included in hornblende. ----- Benes : 47 included in ilmenite-----.-....... 444 | included in quartz- © 47 | Taye he AbNeS PATTIE) ooo so camoe se mcodzpenooseecescue 445 | Oi WENGE CINROS —--scbasoscacsedossencmcohasoscesce 47 OF OUI DATO 2 cos scoese sabe sosoadecoecsecsca 443 of Bone Lake schist------2---.---.----------=--- 151 OF GNOME MGs cococnasoghocosobanéesarsnvesc 442 OF (HOM) .-sesaecssssescoses sogesesonsselssoenss 226 Of EGUORAHIOMO = doonacocnossseesocsansasccae 483, 485, 486 of metabasalt...--.-.-.-...-...- 101, 102, 117, 118, 127, 134 Oh! TYVOCESIIGS soc coco sodsoaooemoscHosscoscss605 147 Ol SPO ssascesered doasosnesosdbade ddassecese 206 DEMOOK, cosngsossooage secs coseessscasosésese 302 OW} Ui. - oo scoresonomecacesssosesemecomsonsnacane 141 Of varlolites=-—-----2=- <== 111 of volcanic conglomerate... .- 5 143 | Epidote-schist from basic volcanic 152 of Hemlock formation ----.--.--.-...--......_.. 449 Epidote-zoisite— IGRI WI GIMP s 261 Garnet of actinolite-schist. -. 450 Ot FAP EIO RD) cocsogusdesaccascqnosesoneasy, asces 167 ‘oteMiam sire! dischi siueepeeeeseeee reer ener sees 413 | ofemica-schistee-een wee ee eee ee eee 415 pseudomorphs after chlorite .--..............-.. 403 | Geikie, A., on ellipsoidal structure -.-.--.........-. 114, 118 | ont Dertianygbasaltsaeee ie eee ee eee ae 75 | Geographical limits of Crystal Falls district........ 25 of Feleh Mountain mange) ------ see eee eee 274 Glass in metabasalt 99 TWH onoososadessooasososepsdcodoshoeaaseane secs 138 (See Devitrification.) Glauconite, alteration of.--.. : 422 of Groveland formation .......--.- XxX of Mesabi iron-bearing formation. 2 422 Gliddenexploration). <<< - = 22-52 e-sne see eee eae 161, 163 Globular basalt. (See Ellipsoidal structure in meta- basalt.) Gneiss, analyses of..-.....--2-.--2--20--+---c eee eee 391 developed by intrusion.--.---.-----.-........... 198 OW INTOMNEAIN SANE Sosgescnosasoqeaccocemccasssoes 43 described eet at seenceeae sree eee ee eemeceee 390, 391 | Techy stallizaiionOfe-eeesee-sees =e eee ee 390, 391 | Gneissoid biotite-granite described .-..-............ 43,44 Gneissoid granite, included in granite-porphyry, EVRGUON Oi esccpodcasesesenseesa 45 of Sturgeon River tongue described - 463, 464 pressure effects in-....-.- 44 Granite altering to clay slate 58 altering toyphyllites 222. e-- seen e eee : 58 ERMAN WSES Ol ocosce see soc ores asosose soncsssa secs. 389 gradation into quartzite..-.----.----.------.---- 51, 52 (INGLIS OY s22s953s3ce 30s s9a7essensossosoosNTEs¢ 45 composition of -.-. 58 contact with dolerite........----..---.---------. 192 INDEX. Page. , --- 194-198, 300 | Granite, contact with sedimentaries .. AID OF roonssasss0s 00 cHcoodonoceosecoesonses 298 foliation of .. 287 including sedimentary fragments - 195 intruded by diabase..........--.....-..--------- 429 intruding iron-bearing formation ........... 376, 381, 426 intruding dolerite ..---...-.-.-..---..---..-.--.. 204 | intruding Felch Mountain range...-...-..--..-. 426 THAYER GHEY) sone soobs9s0eso=eseeSoSscogosS06 381 THT bys ERE WN) Sooeaeooces—e ose SssosssS8ss905e 429 intruding Sturgeon formation 426 intruding Sturgeon River series 459 Moribihan (Brittany) compared with Crystal ALG RONG soe conbccoceeemecsoeceanoace 44 of Archean...-....-.. - XVIII, 38, 45, 49, 428, 429 described - 40-43, 387-389 of Michigamme River 16 Oh? SHANA Coal ING coo coandescenSto ec oesscoeooreE 459, 464 Osi Shw@ulova..- = --asescosasseognacompsososedtansesas 44 TOWDIEN AERO s-ooe-oseapssosencoosesacsessesosenss 40, 45 pressure effects im ~ 2-2. < oo eee e cen ae ener nnn 44,194, 464 relations to quartzite... 376 relations to other intrusives..-.....--------..--- 194 (See Biotite-granite, Muscoyite-biotite-granite, Granitite, Gneissoid granite.) Granite-porphyry, sketch of - 45 Granitite.....-- 30 < 226 deseribed ses0 - 40-43 Granitic texture in diorite .........-.-....---+------ 223 Granodiorite, comparison with diorite of Crystal ONS assemsesscoscocmsncagoasoDoonconsesc 231 Granophyric texture--------- ~~. 85 Granular texture in hornblende-gabbro ....-.------- 240, 244 UGH Otssccosssese Coss se sasocsesascesnosesccesee 316 Ta WENA C OM) socnecoospoaccosoonepasecoshecaIso8 250 Granulation of feldspar....---.----....------------- 42, 169 plate of----------.- 276, 316 OH? QAM IZ cocoasacanseososoocécoascot - 51, 90, 169 Graywacke altering to actinolite-schist - 57 altering to chlorite-schist .-.-....- 57, 166 altering to mica-schist --- 57, 166 brecciated by intrusion. 195 intruded by gramite--------- 22.) - enn eee 194 EW OL scoccoccsseds cenccocssosoessonsosHosn 298 magnetitic of Upper Huronian...-.-..-........- 176 | ofeNTanefieldeslateseere ee emette eee aeeae see 56, 57 | of Sturgeon formation .........-...-..------.--- 431 of Upper Huronian -.----..--.-- 166, 167, 168, 169-174, 176 OBE SCH Ie) AE, AG W)og IN BP) WW! conescedonossnocssosas 27 pressure effects in..----.-.-.--.-.- 170 recrystallization Of.---.......-...0-.------------ 195, 198 THANG OR. 179 Page. Mines, Lincoln, analysis of ore, from.....-..------- 181 location Of..---<..---- <= ---..2---sesee- 2 e- eee eeceee 474 | Town. 41 N., R.29 W ..--- - 14, 375, 460 Sturgeon River tongue described..-.........-.-.---- 458-487 EDU Iie 185 ELD) Wcincosnosoans - 375,377, 460 Algonkian of, described .-...-..--....--....-.--. 471-482 ALTON EV 3U Why SCCULOI Ame aetna 385 Basement Complex of, described........--..----- 463-471 XENON 8 sooscd aso oritsecaecssSsocrsotascnese 385 COE ONICHA) OF acrasoccnopaennoceeeconson soe XVIII, XXIV CAV NG HE OH AW, Sascaccosceadsoscesseeessnsoecas ss 14 described --- 473-479 ZONE hind ei SOC HON O- Sees Eee eee eee eaeee 31 literature on EERO) 2), - coscesconobssmnadeaacodecoccoosare = 377 relations to Felch Mountain rocks ANG Ri Bob Wi sosent sec aeeee ne cel cece eee 14 relations to Lower Marquette -.- AINE; Riio2) Wins SCCULON LQ ene cle aces =e aoee eee eee 31 Sun Dog Lake.-...--..... MSM BI} «cece secsenossassoossosseeocssee0- 190 Sweden, granite of..-.... ¢ 44 MQ; Rs STOW) acess e - 458, 460 Syenite, distribution of. - 2 13 42 N., R. 27 W., section 6. Z 459 Syuclinal troughs, concentration of ore in. - 183, 184 section 7 ...---.-... -- 458, 459 Syncline, Crystal Falls, described .-....--..-..--.--- 158-161 SOCHONG eee ress ee eee 458 determined by magnetic observations... 366-370, 372, 373 ECMO IS) sco ssocomsemso pecs sseconeHSeesoese 458 of Groveland formation --<--. -2-52--<----- 22-5 = 416 POV IN lye) Wise abosoponcacoseSs 374, 376, 377, 458, 460, 472 Syracuse, N. Y., picrite-porphyry at ...-.-.--...----- 219 472 460 at £60 Table of succession in Marquette, Crystal Falls, and RCO Secnaaesosopasacassccocenossbodasss 480 Menominee districts....-...----......- XXV, XXVI SCCbiony(y- 5s -iaee eee ne eee eee ee 459, 462, 463 Table of iron ore shipments of Crystal Falls area... Op. 186 GEOMON ES) ocecsccosocnsonserconcesonss 459, 460, 462, 463 alektromybLonzilesaseeeease eae een eee 22 238 BGO MUNG) =o Sooesososecsonssonce soSasesosesass 476 WEUE Ole -soosssoscsceaseagooocsese oss 306 RHO MY) sanesanbocascosessonsoccsosessessoe 476 Teall, J.J.H., on ellipsoidal structure----.-.... 118, 123, 124 Section Lil ss22 3 2e2 eee secioes cece e ee eee 476 ON OME OME oooenass eonmedondeecoqeacase 222, SOO ee seo o550, spocoseseaneoaeconsssDe as 469 Test pits in Mansfield ore deposits ..-...-...-...---- 67 section 17 .---.... -- 458, 461, 467, 474, 482, 485 Timber of Crystal Falls district............--....-. 13, 36, 37 SOMOS scosoonsopacononsHososedoossonsescs 458, 482 TENT PEINS OF TATE coobeacsccedoscoscoccesosess ete 185 section 19 463 ‘Titanic iron altering to sphene.-..-...-... 144, 146, 170, 193, 239 section 21 377 al tenn pycomeutll e bee ese ap eal eae 170, 193, 239 section 29 399 OP IO CHE) < Se -5ac coosssoseasoseecosssecadsoas 47 Section(3 eam. esse -cieeeee cessee eee 399, 407, 412, 413 Ofpbloviie- eran Leeman sine sees elie ee 192, 193 section 32... 374, 377, 378, 381, 385, 399, 412, 416, 423, 424 included in biotite. - - 404 Repo 6B). cscs ssorscomssuecacossecossanesos 374, included in chlorite. - --- 146, 404 377, 381, 386, 387, 399, 412, 416, 418, 423, 424, 426 included in quartz. 146 42)N., R.29 W..--.--- 14, 329, 374, 375, 376, 377, 458, 460, 472 of quartzite ......- 404 BQ ONE COM icy SOC ULONE leet ae eee eee 458, 480 Thonschiefernadeln......--..--.. 93, 205 BECHION Seon esse eee eee eee eee nate 458, 459 Thiirach, on alteration of staurolite. - O 196 section 7 - 5 476 obinvlakomeeecseeriaa se ceeee sores eee eee eee oe 164 section 12 -- 459, 462 WYO N cosedossosoc5 so osszos seacberedsocastogsece 229 section 13 -- 459, 469 comparison with Adamello tonalite.....-.....-.. 230 section 460 Topography by Lake Superior Survey.......-.-..--- 22 section 460 by U.S. Geological Survey ----..-----.------.... 22 | section 460 influenced by intrusive rocks ..........--...---. 54 EEOGON AD ccosrecopgeeoosorcacacmoosonaeaneaG 461 510 INDEX. Page. Page Town. 42 N., R.29 W., section 31-.-...-..--..--.--.- 377, | Town. 43 N., R.31 W., section 26 ........---..------- 447 374, 378, 385, 398, 399, 406, 412, 415, 416, 460 section 27 .... sosoons 247 AN Be ceossspposesooseencopcaes 387, 399, 412, 416 section 29 ... - 54, 64, 190, 205 section 33... 386, 407, 412, 416 SCHON) OI anamnestic oe Sete we Soe ieee eee 164 section 34-- 392, 398, 406, 412, 426 REO MOM BE) sos-cosnecseses 392, 398, 399, 400, 406, 412, 426 RGGI Bieessonsssecuecsssenooce 385, 398, 401, 406, 412 E13) Shop 18%, BY) Wicocosecoeonsess 329 374, 375, 376, 377, 458, 460 42 N., R. 30 W.., section 12.--.-.---..---- =.=. 459 Secchi on 4 see eetee eel eee eee 461 EGON, PB cacncoossosdbb Sco teesacesescecoants 411 section 385 section 34---....---...-. 374, 385, 398, 399, 411, 412, 415 section 35--..-... 835, 399, 400, 406, 407, 412, 413, 415, 426 section 36..-..-- -- 335, 398, 407, 412, 415, 416, 418 LD Wo RoBi WV se coussoese te .. 14, 15, 16, 73, 199, 204, 329 42 N., R.31 W., section 1 .....-----.--.---.------ 158 ECAHON Msccesnocsedacooass cdossscedaconeogosa 158 ROAM 8) scosesoos Sos ocososos Dene scoasceand 163, 166 section 164, 190, 191, 211, 226, 240, 241 SACOM 10 - sccasoscosassoscegsosssstasascsse9 167, 211 RACHOM 18) osonasbeccoococHeEdecpesosaneS 164, 190, 194 REOMOM Weescecontaccoacosonbéoccoornsce 164, 190, 194 section 22)-----..-.------ 190, 191, 241, 249, 250, 253, 260 RECOM YI =o csonncancareccescsosonsasensssse0 164, 241 section 29......-.--- 164, 190, 194, 241, 248, 245, 249, 253 section -- 190, 194 section So 167 CO) IS(5, JE, BPA MWY So cossoccoons . 14, 15, 18, 156 42 N., R. 32 W., section 1 .- Baers 199 section 4--...----.-- = 164 SOW, MH sonsagosssoosegazasvaaosescossena0 158 section 14.....--.....----------------------- 167 AO Wh roocmacceqessoes udosasscosesqsaN2 74 AGO, JE} cesososeHeses cosnososesescooo9seen 74 section 19....-.-.---.-.---------.------.---- 74 section 20)------.---------- 2. - 74 OCI Ol ce sooedcnocuetonoSaecoeasoosacacs 164 ROCHON OY oo tnneccndeosccascndoelassoasecase 167 section 28 ..--....--.------------------------ 164, 229 EC ULONN Dee este atatatal arate fale alee 167 RGGHON BG co nces scones cee ccosseacecedesoceon= 167 A WL, IR5BB) Wi caconacesacbooncssbesecesescissoas2= 18, 156 42 N., R.33 W., section 13...-. 33) 18 section 24 -......--.--.-.- bs 74 POV Teg 185 PAB WY cocaosocougsasuscenucHoeoRssqccsea5 329, 460 43 N., R.28 W.---- . 375, 459, 460, 472, 458 ECMO BB} sasondosossoncenecdassesesesesacooa 400 OB} Wheg Ug) VW oe ssoces ..-- 458, 459, 460, 472 43 N., R. 29 W.., Section 1-.-----. 2222-2. 472 SOCHION 8) = 22 - em == sem meee oe en 459 SON BU = cemnciosmcenscocdaeoscasosscmagrece 459 ROCIO BD coco cesocoomeanuasoseEHeosseeese 480 ERVIN IRE) \iossasdesoosaoueHoosoyaeS070800 458, 459, 460 2B T8log Lea BIL Woe onacocwee noo enscae 76, 203, 204, 329, 427, 447 43 N., R. 31 W., section 2. ©. .--.--------- 4 432 ECORI B cocade socdecossasacusoossescansaq00 446 ROCHON cooc 55 soc onsecdsocemsascessasog0sa09 438 ROOTOM B seoses arecosccsnnbotsoeSssasesossca5 199 section 7 ....-.--.------ - 54, 64, 76, 203, 210 RECUIO S cocecoacedae cnodopdadsoecseecdlssesies 205, 210 section 10... seadaoste 438 section 17 - 54, 61, 65, 190, 210 section 18 - 54, 55 Ree I) ~ 53-5 consdosonssdésessssozesse2ees2 223 EGLO PU) coodiiconcos posnoaeoatecoosbussesén. 65, 223 section 32 section 33 section 34 5 SN5y 882) W/cpsecanscensc conser LENGE ad SED 18, 74 ASIN PR) Wie SECHIOND ese eiseniea( sence eae 54 SEAHIOIN sansagcseogacondeacoabaucsenoncacoee SOCIO) pooGbnconocosencarcnessgacobos sedans section 8 .. section 9 .. section 11 - section 13 - section 16 ... section 20 ... section 28 ........-- 43 N., R.33 W -.---.-- 430Ni-y ES DN Wi-y SCCULOD! ean cimsienicisecieieseiestee cls 158 44ONT pRUtO La Waciseinne cee cieteeicie cia esta 38, 334, 427, 428, 432 A4SNE OR lw SeChiONa peesee-caseeee eee encore 441 REWIOIIM SEaesodnca pee acodcochscoseucdos ascece 441 Sei WI) aeseeconsensanced sseconesseoososee 335 Sectionyl 5: ess ees sy sen sake eters 430, 441, 442 RECOM 1D 5 sbesonssoabososocctoosscensesscus 434 section 21 - os 429 section 22 - -- 427,441 section 28 . 432 section 32 . - 432, 434, 438 SECU GB gagsoosaeseosesss5so40-sbseeacS 432, 438, 446 AAU RS 2) Wistar isisisie = seleeeseisiseieceenisis: 38, 199, 334, 427 MAING, WRES2 8Wiss SOC HON some ae cee see ee ee ese 46, 53 section 4........--.- SOAS E AAA ean Specs be 91, 92 Section 9\=. 22. t cece sen aecen Set ety eee 212 section 10 ; 55 section 12 53 En iinale} eee atest daoccdsasecsoasaconsqceene 199, 204 RAO WS) 55 ae socso somone nessa sessebscoss 199 SGOUIOM 2 cacsastdosgscspnce saoanacdssnq90045 212 section 27 -- cn 212 section 28 .. -- 199,203 section 30 204 section 32 .. 35 95 section 36 .....-...--.- 20 88 44 Ni R83 Wie SOCtLON 4. -e- see ne eee ces ae eine 108 section 26 44 N., R. 42 W ADEN PRS Wie SCCULOD Dies aaa eee ete eee 453 SCHON If Sooo sesdscsanosecnseaseooosadonseess 429 RECOM denn coccdo dase sqsessoasesSusacees 453 Benn G) -SSasssascchsestoc sd5cose shoo osascS 453 section 15 453 section 16 453 AS WING wR ste LenW Vp ieerestaeleteraeerieietiaras 30, 38, 345, 427, 428, 431, 457 AABY Shop Jets BL NAY SEQUION (B saodscaseatoasaonbsssees 440 BEchionwlGH estes. see estes ee ao eee 441, 447 section 21 .- 335, 441, 447 ERC 2 ssceceaceass nasosadonosaessenenoscs 431 BECLON Binet see ea atoms etait 431, 432, 441 ROCHON PL) coseaseascee ce cocoscosuesososc5s005 335 BANS IR BP WW cacancocboscindbosdoacassc6 29, 30, 38, 73, 427 ADIN WA GOM Wis SC CULO Gl Gemteeiseist= state siete eae 39 SOMO 7) soscins shoscocsosccons sos sedabctoesss 51 COBY IN Tes IRBBRY MWY secacoadomaodnosaponcocsooasesag0es 73 . INDEX. Page. Town. 45 N., R.33 Wi., section 9-...................-- 157 AON MG) soso oossao cooeandosecasaeasosesseS 149 SOCIO IN). sdascoastnosaneenceoooseoadeaLaEs 149, 157 REQUTO UP). oannooegoongopassasosoocceccoe 158, 175, 176 SOCHONI 22 Seeman cecis- can siccies seisteeaeceeaeeee 149 SEGA 2 coos coc cco oso nenooooncaneescoses 149 section 34.............. : 153 46 N., R.30 W., section 19. 453 section 46....-....---.. sDeo 406 46 N.,R.31 W ..--..- -- 13, 38, 427 46)N/., Fol Wi, Section 32)------.--.-------------- 447 46 N., R.32 W....-- .. 38, 148, 155, 156, 427 AGEN eno om Vie SOCLLONG Ganeremee cris cieeiece eee a 149 SEUHOM Boos sco ose ocossssssnocmeooatcoDoecS 128, 334 SECMIOM Bil. ce seosceccocasepensoeaoosesoaseanesS 149 EGAN ON eseacocnssccoseoneomoncosaouaEdessosS 73, 149 AG ENIORYSS suet eet note sees 156 46 N., R.33 W., section 24.................-.. 147, 149, 199 REODON RT csogsoasesoacsood dootonessaceseanes 204 SCHON oo cesacceoorsocconceoas 149, 156, 163, 175, 176 LUE Ning 15a) MWY BoaasonodaSbeconneasorcaoseceEesae 329, 457 47 N., R. 30 W., section 9. S 334 section 19... 333 section 30... 333 47 N., Rol Wi - =.= -=. 13,329, 332 47 N., R.31 W., section 24. - Risers 333 BECHION ZO te eae elec cle ise ieee ee eee 333 RECON i oso ocenoonosopoonecacasaacosmesosec 333 ATANE RR ASOMW SOC HONG Dem seeisenrse seeeni- secs 199, 204 CD Wale sil WY ca sesdnoeaodaonbsddsdsoscaonesoqens 13 Trachytic texture in pyroclastics -.-......----..-.-- 147 Tremolite developed by dynamic action.....-......- 432 from olivine..... Didodocdeadosososeasnoccsacdsadae 217 includedsmyehloniuerseeeeseee-eeeeee tees 218 of dolomite..-.----- 408, 410, 436 of picrite-porphyry ------.---..--------00-cu=--- 214, 218 (See Amphibole.) HINDI WAVES) soncssonoascosssboonebasodocon 335 Tuff, basalt, perlitic parting in, plate of-. z 294 breccia, use of term..--.--.--........ 137 conglomerate, use of term .- 137 off Hemlock) formation seem seis tele eee 64, 76 Ges NAG. -occoooseenssss senna eserooessn soca 137-148 WINAVI Olasooneosnnes eccodo agsea4sbocsqade 141 OMIGIN OF soso csocossasacotasososnssdasSocoanose 141-142 palagonite, use of term....-.-.-..--..-..-------- 138 RelapiOusito ashy Ded Sleenase ree = cert sees 148 resemblance to Tertiary and Recent tufis....... 142 PEO OE WN ooeceasossscscsceasonesecseconesbees= 136 Tuffogene sediments described. - ---- 148-145 Turner, H. W., on granodiorite.......-.-.....------ 231 Twinning of feldspar.... 42, 104, 191, 201, 224, 228, 233, 261, 468 OfsMUSCOMALCH ese ee mice easter F 197 of pyroxene. . ocon 250 of rutile 56, 205, 236 U. Ultrabasic intrusives in Crystal Falls district CORCHINEG cscccsnccesdcdocsooosnoscasooSn 212-221 in \Ofay dere IS une soon osnboneesenconsoesconecea 164 Unconformity. (See Relations under particular formations and series.) Undulatory extinction in quartz (see Pressure effects)...-.. 41, 81, 82, 90, 133, 169, 225, 388, 464, 477 Union mine. (See Columbia mine.) United States Land Survey ....-.......-...-.-.----- 343 511 Page, United States surveyors on Sturgeon River tongue. 459-460 United States Geological Survey, topography by-.--- 22 Upper Cambrian. (See Potsdam, Lake Superior sandstone.) Upper Huronian described. ... correlation of XVIII, XXI, 27, 155-186, 423-426 -- 155, 164, 165 ChE TAA ONO Oe os 3c oocsneosensonodoocoossecdsoae 155, 156 HOGS OF socsacoscoocsne ss aasonpecoscceo feos 158-162, 188 SRO, Oigssscqssassassecseneses50 nonce soenee 179 relations to intrusives........------.-.------ 189-190 UA HAMDHYES) MN Soocas seascsess - 174,175, 187 magnetic observations in.-- - 156, 157, 339 metamorphism of Care - 28, 425, 426 of Felch Mountain range described - 423-426 of Penokee series ....-...---..--.--- ° XXI OOCEWSUTO Be oadsaanoaccodmncansenasqaaasosees 28 descnibedtcesersseereeereteeceeeer seen acer 175-186 table of shipments of.-.-.-.....-----.------- Op. 186 relationstboeAmch cantessa=e teem =eeeseeeeeeree XXI relations to basic intrusives-.--..-..- ococecd 204, 211, 223 relations to Cambrian sandstone ....-- setae 155, 161, 162 Pelatbions Ojo TiG sae eee eee eRe eee eee 155 relations to Groveland formation ..-.....--.---- XXI, 425 relations to Hemlock formation......-.......--. wi relations to intrusives ..-.... - 164, 190, 204, 211, 223 relations to Mansfield formation.-..........----. XXIL relations to Michigamme formation - 28, 165 relations to Lower Huronian........---.-------- XVII XXII, 158, 160, 161, 162, 163, 176, 424 relations to Randville dolomite ....-..---..-..-- 424 relations to Sturgeon quartzite........-......--- XXII relations to Upper Marquette..........---....-- 135 SUCOREETOM i cs55504bodnos-coeseadcedod s2c0s2 XXV, XXVI thickmessl0twecsenemaseeeeeeeeraseee se ree eee eee 157 UOMO? Of - coosnoseasnoscaaso oso zo caSeeoAess 155, 156 Upper Marquette, correlation of.................- XXV, XXVI iron-bearing series referred to..-.-.-.-..--...--- 20 iron ore deposits, comparison with Crystal Falls ITON| OLe|CepositS)-----eseeeeeeeeee eee ee 180, 181 relations to Upper Huronian...........--------- 155 Uralite from augite ..........-. - 104, 201, 212 from pyroxene .--... 48 including feldspar... 202 lofmetadolenite eae =e = eee eee ee eeeeee 200 of volcanic conglomerate ......--.......--------- 144 Wa Van Hise, C. R., connection with Lake Superior Sur- WEY) socsos gssansnescosensoscosesotoosasos 21, 22 12 168 on concretionary structure in ferruginous chert- 422 on correlation of Crystal Falls ore deposits..-.. 20 on Felch Mountain range’ on formation of crystalline schists... on Lake Superior stratigraphy.--... ‘one Mesnandt arcane ai ee mae ee eee on metamorphism of basic rocks ONIN Case his tase rer eee meee eee ele eee on Michigamme formation ............-.-..----- ORO COMA MMAINON soosococcssoncsoconsoussogcs on ore deposits of Armenia mine.---....-.....-. 183 on ore deposits of Hemlock mine ............-.- ON OMPIN OW SAGEM) concen omessbedopdacscesascs 168 OU OMAN GY! WON ONE) essen acessesco te 39, 70, 71, 180, 168 on sericite-schist from recomposed granite. ..-.. 52 512 INDEX. Page. Page. Van Hise, C. R., on silicification of ore formation on TYG Ayaan SHU) 635 Secoooododace daosesoceescoaD ees cogcassas 211 Sturgeon River conglomerate ..---.-.-----.----- 461 | Whitney, J.D. (See Foster, J. W.) Wiha) 18 (oye), JO, I, Onl OIG Saeco eonoeacooeseoonanes 235,247 | Wichmann, Arthur, on iron-bearing rocks south of Van Werveke, on spilosites...-.-.-..--.--.....----- 206 Lake Superior. .- = 21 Variolite in metabasalt described -....--....-.-..--- 108-111 on serpentine.....-----.... = 253 DEO O? caacoapesosssenpocononSSsseeneshopodaasoa 110 on Upper Huronian rocks.......-.....------.--- 173, 174 OH Letina A BOM Sooo noDessoSoncHSSEoasosmnocncans 108 | Williams, G. H., on alteration of ilmenite ..........- 202 Veins of iron oxide in Groveland formation on Cortlandt series....-.....-..--.----seeee----- 222 of pegmatite in greenstone. ... Onl Caven@lii®) -oscsossssodondbossonsnsccocosonss 254 Gh? (MEMS ossaccosse2sc0220sce5 on ellipsoidal structure . - 118, 119 in Groveland formation .--......-...--..---- On EROS) saaSoriconn coCoEosOO Cost ood eooSooboborS 247 in Randville formation.--...........-...--..- 435 on micropoikilitic texture......-.---...-...---- 84, 85, 87 Voleanic ashes altering to greenstone..-......------ 487 on production of schists from igneous clastics-. 152 Volcanic breccias, use of term .--...-..-.-.--------- 137 on pyroclastics of Marquette district........... 148 Volcanic elastics, plate of ..-...-.------.------------ 284 on pyroxene zone about olivine.--.....- 256 Voleanic cones in Hemlock formation.............-- 78 ov ultrabasic intrusives ---...-......----.------ 220 Volcanic conglomerate described ..............--.-- 143-145 MELSON ECs LO me eet elerel- elaine oe ieee ees ee 95 186 Of ermtehescstescn mass cee ee Oe eee eae 136 | Winchell, N. H., on ellipsoidal structure ............ 118-119 Volcanic rocks of Crystal Falls district ---- XX, 95-148 | Wolff, J. E., on Hoosac schists ......-......-..------ 394 OfME awaits te se ase iene oa ee emecee nema 16 on development of mica ....-.- : 130 Of Lcelandee. A. seo saee woes oa ee eee eee 75 | Wright, C. E., on Crystal Falls district.............- 20 of benokeeldistrict-peea-eeeeene ce esserennceeeee XXI on garnetiferous mica-schist .....-.-........-.-- 196 (See Hemlock formation.) fi on granite dikes in iron-bearing formation...... 381 Wolcanic sand) platelof <0. -2- 2s. eercs eeee ne eeneee 296 on Menominee iron region 21 Volcanic sedimentary rocks of Hemlock formation on staurelitiferous mica-schists. soossonns 196 describediemmceses nee ec aeeeeseee renee 136-145 on Upper Huronian .............--...-. cesasnece 173-174 Vulcan iron formation, correlated................ XXY, XXVI referred to ......... odtwaoosocuscsosecoos oNDOODSS 21 Byes Ww. Youngstown mine ..... oosaasctSsososcoestesoaocasos 182, 186 Wadsworth, M.E., on the iron, gold, and copper dis- OGRE OF