= = See ~ ~ — S = ~ tapenade ee ery paral ee ey Y ; Se ILE EET “. - Sesser arn ee — _ SN I S — ee peal enlace erciere eS Ea ep omen a SE Eee ey a pe EE REE EE re eat velcro cancers nema ete eo naar tate 9 Wy we tn Ye ean eae a eee ae ea Secon ae aome-o tan naa ~ x ~ Sn an te ras na enema ee 5 — ~ es = Ro so Hanannerwy Avan” seer ene Gripes thew ere eam Ween en nen ~ = = = SS se. 7 ‘= = = = —— em == 5 5 pesearrrtererienes any = Serer —- : Sse = ~ —— ae i | | th ‘e i | ne my ’ r ' I “ao 9 . . | ’ Lo Wy alas y ‘ ry, 1 a . « rf - hy * : “< 7h 4 y ¢ ’ iene t ha My ’ Wig Ae Ay DEPARTMENT OF THE INTERIOR MONOGRAPHS OF THE UNITED STATES GEOLOGICAL SURVEY VOL Ul Mone exe V TT. WASHINGTON GOVERNMENT PRINTING OFFICE 1896 557.4 U7G Vie] UNITED STATES GEOLOGICAL SURVEY CHARLES D. WALCOTT, DIRECTOR GHOLOGY OF THE aN VB eA lal IN COLORADO SAMUEL FRANKLIN EMMONS, WHITMAN CROSS AND GEORGE HOMANS ELDRIDGE WASHINGTON GOVERNMENT PRINTING OFFICE 1896 . GO MELE NTS. Page. Tether Of translit talents acre sateen iets eee eae a toaa.n ia os SEs See, ce peer enen eee XVil [EEO Shon BoeCes Se Soc TROCECOn ROS nee OEE COCO CORON ON EIS AAR NSA He aeRO oR eR aa ee aS 5 Sa xix Chapter I.—General geology, by 8. F. Emmons... - --.. ---2-- 225. 22-2 -- cnn oon = oe === 1 IB SRT NeW oes cece BaeCe Ooms On od. e see, S4Oons Boob cnet aas tar oor nAcdosonsechocbaan CSE Mente 1 WON Maye ORG Boa Se Soe cre One Sec Depa Se -aoecor Gans Ten oe Beso ene snsorn Sasecsec 2 FOOL NI UO RO RT Dye see antares ral = sa Aare tala ate ota rater hey alent aces aia lan, a 5 Plainsiboporrap hy. ase a2 alana an ee ata oot Sela aia china eee ane 7 DRIES AT CN Peay Ys oe os SR em i CODE CB ASR R GDN A5 aoe BESO BERR aAA CS Or SSCA BASAL ee 10 Ere- CSM DEAN OLOAUIODS see eles ote rete esas re Shah eee ae he Stereo eee eae 10 Mambrianplan diss erp ceio5 als oases ne aaa awh ota SSS aN SES Se Oe se et eat 13 TOES Ry, LEPPIG Oy Ce eteber Gah ees So A5 Saco coe 6 bce5 oo0S se cer Ree seeaencae caso cece 6 cnS= 15 Canboniferonasmovement@—.5- 2 cme. sor saree seen eee cee wa ae ce team Tae eas oe ne 17 AWiyOmMmuno: fonm atl oN ee ais = ats sfet ets oeietraye ecm arora nal tai intors eeiele oi eae eee ee ewe cme 18 LEGGE WA OIE: 5 aS ope Ee cer es oS arose Soe SenSSerc cCeds SAO Ease Baar Soe ose DROSS 19 LUPE EON OLE eee Nae pons DES np Seo nee Soo as HOSE SEO Seer ence coe oaohac Se 20 PUTAGSIC OON.OM ONG oe apes ncem'ns soja oe ewe ie mic See eee ewan s eae ees Seca sn eens Reeeie ma 21 IMOTMIBOUTONINA LION sepia eo = aos ee reece ee nod = Sete ats oe eee eee Eee oe 22 Early Cretaceous movement ....---. ~~. - - SRI ce Pence ak ee teas ie aE Se ees S 23 Dako baihormationecens cen <.c seen cen eh ax atiieaces cae alee cc eke be ec cnwseebaeiencits as 25 Colorado formation \-2-6 cnc cosine eee cs see wee ates eee ae Races s Coe eee ane oases 3 26 Mid-Cretaceouspmoyvement: o.o.cn- = = else wise cciss sis Row cs cone ws Dace ee eee oes oan ewes 26 Montana formations: 22. ccacee ema ece ses amt ceils cate ates nh seen ine cine. cv Ae oe ee eee mate 28 iaramie.formation’-ce2 ==. s:-cy-eesecmet chee ted Sehe ta naceesaelesics wa cae ae ateeeasriee = aes 28 POs UAramichmoveMment-=- ect ace sere ae wee ne Cea e eet ee ath ce kee emt one 29 Aran shoe Vor Atl ODNsa am oo te ae ary toms toe oe ato eect miarcinte Sink Sil loin spseere 31 PoOst-ATananop MOVeMeNn bsaeee eae. Saeco ens eee haar ec eh soos te seals 32 DenventOrmaouee: esc eeeiee poet ceer Sec ne See eae ais foe oe paehie cieeee oie esc ceere 33 iPost-Denvernmovementi: «ou cesccss bese eee emice = la ramineuie sess sie ninice Sania ns aoe aicle 36 Mon dmenit) Greek OLmAtl OMemer ss tise ee te on, rae aad oe nian cee ioe le Sen Scie oe ase 38 Later movements. ...-...----.-.-- BE ar a ea ee se se eed. tick wck Haeee hence scaoe we 39 PISISLOCENE AOLMARIONA sa) =-snone ase e eos ee eo ee natok sue cise os wa coca s ase 40 Sirgen Oa ok ae Aetna aoe Oao ue Bahasa gee Ber Ons ae EOS ene eee osoae 5 42 TREAD ORIE See RES Re Se lie MES IEE G5 SOS SIE A 43 MOOLHI Ser CHUTG wea eee ae steer = tenia clean a ale esters ee oo eyes oe bie ded ose ait wleisle 45 Vl CONTENTS. Chapter II.—Mesozoic geology, by George H. Eldridge--.---.....-..----------------+---------- 51 Section l.—Dbe LOTMA clon ss Soe Si bo cose Baa. «desacttas do sgccce 64 Colorado group’. -----~--- <- << -< <= == soo aa ww ew i 64 Benton formation: lower member of Colorado group ..--..-----...--..~----..----. 65 Shari iter ayy sooco sidan o¢oscoces sno ncecedas sasce 5s eonesons Scicis cacy Anse Seoccone 65 ite: Sota cbies we = se sten e Soeeoa oe e ee ee etene ee r 66 Niobrara formation: upper member of Colorado group .-.---.-.------------------- 66 Simp aD Pho SS oa ees Osage oSoS cos sa asoteLsassaasoscne mentor sechaaeossssess 66 (iife tess. cacac toca acne © cecte eet a cle ea pig Sete ale ea oele eat tee Tare ae re 68 ICS eb Ey fea) bh eee BB aSe so ochs poeoeenc cde merc Seo mmeroncm asa caceieso ea de oss oscseae 68 Pierre formation: lower member of Montana group------.-----------------.---.---- 69 Stratigraphy ~~. ----- <<. <<< 222. 22 ana a nw es = we = en nm =e 69 1 et peo eeond SSoGsoes cone Sues CUOOnoSoss aconee Scaun sjas cease csepcses sesaescoseo 70 VAG NENW oY Intope Talley AeSee SospoocarisesoaScen Hedcoo Ssocee coc oaS soeosy Hoss fal Fox Hills formation: upper member of Montana group ......-.-.---.-------------- ae SENNA ENA NN? ne 65m SoS em Seno seo eemonaassS os sso Saedse code SSSeess rece cSaneces 71 Tie tes < sotcn eo tee violet aise Se oe he ae ee to eeee ces Seat ae ete eee 72 Piaramiie form atone he sists re ata a ree eae eae rear ee 72 Stasi A Dye, LO wy OLY Oi yA OTN eat ee re 73 SEK tendsy Oebgs Whe NO RGU AO Oo a55 seco sopSao sass copmseeeaodosondasehoabsasoc 75 Stirarti ovr sy lt Cau re) eat OTN ate re 76 1 flit ees DE Sep O Saat Se oe Son Reena eet seSese cost pace co tons mee oren onde nseoses> 77 SOU LOYD HUM —— sy LUT ATs sil a 79 Tntrodetion ~~~ nye:— ein om mm lara mlm i a lm dl me 79 LU blitcba over d SoU Vee gee nego e aed Qeemme sen omc Sa Osh aS os eaten aG gle coc eesores cass 80 Genera) SEGU GTN ae ate oe eam mle te lel alt 2. =e a 80 Region about Golden ......------------------------------------------------------- 82 General features\of, the area) aitected 22 == 2. = saa ee meen eee 83 Theformations and their Telatlons=--— a2 4 —2— = ee ee 84 Strnctural features): 22 fone we sete los eie ole = ala avneiicie ay oleate tote nee as eae ee 89 Development of the area..-...---..--------------- ---- ---- === ---- ~~ + = =e 91 Discussion of movements producing the present structure....-.--..--.-..-.--- 97 Views of others on the structure of this region........-------.-------.---..--- 101 Special irregularities in the Golden region...--.-.----..----.------------------ 103 Region about Boulder ...-..---.-------------------------------------+---+-+------ 105 Topographical features ..---...----.--------------- ---++- +++ +++ +++ - 222-2 20e-- 105 Chapter III.—Post-Laramie and Tertiary geology CONTENTS. Chapter II.—Mesozoic geology, by George H. Eldridge—Continued. Section II.—Structure—Continued. The foothills—Continued. Region about Boulder—Continued. Genlonical tea tures meena asta case eee Seen Seeeen saloons eee ceca evelopment Oh ule Merion scenes ss eine seni toe eee ee eee ese cae Inferences from the series of unconformities at Boulder and Golden. -.-......-.. .... Rep tonvah Come Cree kel Obkives se == lose oe al sel DIE GOW OS) sasceose cosee cee Seg escoostoc chore nSces ced pase ose e er eoreameconocce Laramie and Denver...-. --..-. Se Sump e sn Shecntouk soc 250 6ste sebdieses Soca st ouEecsnoSnee Section II.—Vertebrate fossils, by O. C. Marsh..-...--..--.---. 2... -2.. --2- +--+ ---- ------0- Initino diietions soo eseaaeenese occ ccsccoscinn~ sossoeee eam able Seas Salone s ones oe coe aac ees Tin is (Game GA WaARS oeec SRS RBBE poets ae sco es Son eHe aT en eo apeC SONS aos RODS e UTR ET 5 3 3 Sed cR oa S56 Scone One tes BE OROEe a obos5 SS Ae ee eeInSs mit SSE aren Hallopoeibeds meer ease 4-= Scaae ea see ee ee ees) antes ace seme aro one IDET ete NE Corecc = ASE n SHO eB OSS ora im SES COB SE Be Cann OOS 7 CAPE OSAB AAS ALAN CORANTUE) OWS ia -tece ce. soo inos oe ee eeioa > Sal soe er cin cin Se Moaetee ne eee Cretaceous ....-..- eae a ays oe ont oo Ieee e eens Hata sete eens Same cece ese IRieranod on Dedspersr ns haacccn a tn eee Cee ae Nee oe oe See sete we Ne Snore Geratops Deeds) jaccc- en cone cecen = weiner ees ewninie es Shee coos Ses cae eee soon tetas PROLtIRE Yin so-so ais eee oe = spc = ce eee eesiew eel ae eee cleicinc vais ce eee eelen/soceeceusl IBLONLOCHOL UM DOGS Getcha sae eee eee tee ot cases sso ea cence ote onc Pliohippus beds ........--...- oe COS DO SIDE Bo ORE ORO ass oo Ree Seer Patt ie TAssOpvence DEAuO ud OSSTIST tars teats ie alchein cota amin Cee oie Sn ac pie abe Cio ciate sinew ai RGD Ol arses ee meee ae wa asa nee secon eae ease teehee amen seen Sern ce seen TRE QUEM CS seme comom en Got Coton GOODS SER SR BOS eB ECe Bae pats MS toao ASTOR RMN oer eens So a ee eae ee Samat e eto ee nse aces said cies am SAP LAILOC OU ase e askin <5 ee ce ete ete orn Selon ee cin oe orale inl oii ataiaurwies ew emte m= FATE ANTON SLU ieee et ese = ee teIse eee tee Mesias obicic Sols Sale ee eee Sahrce ec estas JIN VEG SP nc ee oon Bodeos ceoe abnaad “Se Mean eee CeeeOd Baames one Ber secea soar [ESO LOM SIL (18 een eer tes At ee IO le eta clare alain c Sere ww aiaaia crate nie sate winte tie 8 LOCO CUS reetertersta seein ere a se eects eterna este ete Cae eine aida maintain inc reise aoe xii CONTENTS. Chapter VIT.—Paleontology—Continued. : Page. Section I1.—Vertebrate fossils, by O. C. Marsh—Continued. Part II. Jurassic vertebrate fossils—Continued. Reptilia—Continued, MOLOSAULUSE Jos -jocbc ecco oc comin vic tom ioeie eee ete ra eee eae 496 PSE OS EU IUO e ame = le 498 (Camp COs Ren UB rasa mm tele a 502 CoeratosauruS) 25.2 5.cis see semen om oo ofa Sees sieeve eae ee 503 Other vertebrates .%.o: 5-525 2 saecs, sce ce weosle eee See e Een ee eee saa) 1006 Mammals). 22s <%r-seidoec/scccsentwice tat sseeaeae ae o> oes ee On ee eee eee 508 Part JIL. -Cretaceous*vertebrate fossils... - 2s. sae. ne sana see ieee eee seas eee eee OOS 509 509 509 Ceratopsidie 509 @laosaurus oe oe ee eb tigee wees Sesie tee ecw — ener os cine ire ete eee ae 516 Ornithomiinoig! ss oo. 2 seek see ates Rose en cee Cae eee te a ee 518 Mammals 3. .2.cccwns cc icstimcecde cans esc seetesibon sees tee se eee hee eee ae 520 PartiLV. (Genozoie.vertebrate LOssilsc.-oc soccee See se oa oe kee eee eee 520 Parti Vi. Conclusion 2 sscc2 2 a6 ose oxo ue tne 6 aw era See eee eck Ean geese -C eee 525 List of ‘vertebrate: fossils... Seis 25 és cndc Co ceec men eae eee hee eee See ee oe 526 100 ee AIOE SIAR ICOn SEO SOO n ena aden Se COU aE RE Ca OoUASCOosSnc6 5b1 EL UST RA DOWN Tw Page. PUATHR MLopoprap ly Ofethe s)OUVvel BASIN ...j- 2.5 << oan elene main en nem ae ewice Seen on ain In pocket. i Areal reolory of the: Denver Basin. -<.-. =< 6. soo ocr woes ones ccuecv--cs--ee- In pocket, ii: Heonomic'reology of the Denver Basin - ...- ---~ --.- --6 2 o-- 5 ese ewes oon = oe In pocket, LVe omucture sections: of the Denver Basin... =~. 22s cen cdemeces oceeee gee ae mete nace 374 XXI. Restoration of Brontosaurus excelaus Marsh ....-........-.---..----- -2---eee-- one 530 XXII. Restoration of Stegosaurus ungulatus Marsh..............-.-.----.-.-------------- 532 XXIII. Restoration of Camptosaurus dispar Marsh. ..........-.---------- eee eee eee eeee es =. bod MERI MREALOLADLON Of, UAOsalrUs Consors) Marsico ee. so-csq5 ecb perio ine eas cece arcs + cece es 536 XXV. Restoration of Ceratosaurus nasicornis Marsh ......---..---.----.------------------ 538 XXVI. Restoration of Ichthyornis victor Marsh and of Hesperornis regalis Marsh......-.--- 510 XVII Restorationor Triceratops! prorsus) Marsh os. 2. 252 cnn oe ance ie spt were se soe wn 542 XXVIII. Restoration of Claosaurus annectens Marsh ....-..-......--.--- ---202 e202 eeeeee eens Dd MERON: MEOSTOLALLON/ OL OLONLOPS.LODUStUS; MAIS fec0 ms a naieani = slaneaar- te be wanting. Still, the breaks in their continuity are in general of so lim- ited extent that the foothill belt can nowhere be followed for any great distance without meeting them, if not in typical development, at least in some modified form of fringing reef. In the Denver Basin area the hogback is found to extend in most pertect form from the southern boundary of the area nearly to Table Moun- tain, a continuous knife-edge ridge of Dakota sandstone or quartzite, broken only by the narrow gorges of the mountain streams; with a valley behind, separating it from the mountain slopes, as regular and continuous as any 6 GEOLOGY OF THE DENVER BASIN. . . coast lagoon or sound within a sand bar or frmging reef. At Golden the hogback is wanting for a few miles, by reason of structural causes to be explained later. It is resumed again opposite North Table Mountain and continues with a few unimportant breaks, due in every instance to some readily explainable geological cause, nearly to Boulder, where is again a gap due to structural causes similar to those operative at Golden. In all this area the hogback ridge, when typically developed, is formed by hard sandstones or quartzites of the Dakota formation upturned at an angle of 45° or more, which are underlain by clays and easily eroded argil- laceous sandstones. ‘The variations from this type and the causes therefor may be explained in detail as follows: The disappearance of the hogback at Golden results from the non- deposition of the more resisting sandstones and of the easily eroded sedi- ments beneath, caused by an arching or bowing up of the sea bottom at this point during the time of sedimentation. A similar cause, combined with the subsequent leveling off of the surface by Pleistocene detritus washed down from the mountains, accounts for the disappearance of the hog- back in the immediate vicinity of Boulder, near the northern edge of the area mapped. At Coal Creek, on the other hand, the hard Dakota sand- stone is present, but it rests directly on a projecting boss of granite, with no intervening softer beds by whose erosion the hogback valley might be carved out and thus separate it from the underlying Archean. Immediately north of this point the hogback valley reappears with the recurrence of the softer clays below the Dakota. The prominence of the Boulder peaks, which is a striking feature in the foothill topography, is due to a series of north-and-south thrust faults, combined with a tendency to the échelon structure before alluded to, or the formation of minor folds oblique to the direction of the range and pitehing southeastward under the plains. A more evident instance of the échelon fold and its influence on foothill topography is found immediately north of Ralston Creek, which has resulted in the offset to the eastward of a portion of the hogback ridge. Pl. VIT is the reproduction of a photographie view, looking southeast- ward from Ralston Peak, on a line with the axis of this échelon fold, which U. S. GEOLOGICAL 6URVEY VIEW SOUTHEASTWARD FROM MONOGRAPH XXViI PL. Vil PEAK, ALONG BASE OF FOOTHILLS. U. 8. GEOLOGICAL BURVEY MONOGRAPH Xxvil_ PL. vit ORR NTT ee : : non MON PERK 4), VIEW SOUTHEASTWARO F » ALONG BASE of FOOTHILLS, FOOTHILL TOPOGRAPHY. if well shows the typical features of foothill topography. In the immediate foreground are outcrops of Wyoming red sandstones resting upon the Archean, and just beyond them, over the tops of the trees, can be seen a rounded hill formed by the échelon fold in the Dakota sandstones. To the left of this is a long, narrow ridge of basalt, known as the Ralston dike. Ralston Creek, issuing from the Archean area in the lower right-hand corner of the view, flows first southeast, then bends sharply east, cutting through the Wyoming sandstone ridge, and, curving round the hill of Dakota sand- stone, takes a northeasterly course to the plains around the north end of the Ralston dike. Inthe middle distance are the Table Mountains, with Green Mountain beyond, seen over their western point. Between them and the foothills of the range can be distinguished the long line of the main Dakota hogback, whose continuity is broken at Golden, in the valley of Clear Creek, and which in the foreground of the view is offset to the eastward by the échelon fold. Instances of horizontal Tertiary beds overlapping the upturned edges of the Mesozoic strata do not occur within the Denver Basin area, although topographically the Table Mountains and Green Mountain somewhat approach this type of structure. They are, however, no longer in actual contact with the mountain slopes, but are separated from them by a valley of erosion. PLAINS TOPOGRAPHY. The topography of the plains area is that characteristic of a series of recent and easily degraded horizontal beds long exposed to subaerial erosion in a semiarid climate. It is a region of broad, shallow valleys with gently slopimg sides, the higher ridges being plateaus or mesas whose surface is formed by some harder or more resisting stratum. The lower ridges within the basin areas have also the mesa structure wherever they are capped by a harder stratum, but when composed of homogeneous and comparatively yielding material they have softly rounded outlines. The stream bottoms between are wide and shallow, and sometimes are bordered by indistinct terraces. The whole area in Colorado is divided by the erosion of the Arkansas and Platte rivers and their tributaries into two shallow basins, each nearly 8 GEOLOGY OF THE DENVER BASIN. 150 miles wide on a north-and-south line. The divides between these basins and those on the north and south, respectively, are flat-topped ridges, sloping gently eastward from the foothill region and having average eleva- tions 2,000 to 3,000 feet above the lowest points of the basins. The more enduring beds which form the divide south of the Arkansas Basin are the basal sandstones of the Laramie, capped to a considerable extent by still harder sheets of basalt. The Arkansas Basin itself is largely eroded out of the clays of the Middle Cretaceous. The Arkansas-Platte divide is formed of the conglomerates and coarse sandstones of the Monu- ment Creek series, in part capped by beds of rhyolitic tuff. The South Platte Basin is largely eroded out of the clays of the Upper Laramie and later formations, and the divide to the north, between it and the North Platte, consists of Miocene limestones and conglomerates of Pliocene age. Within these basins the details of the topography are dependent on more recent geological phenomena. Thus, in the Denver Basin area, which occupies the southern half of the South Platte Basin, while the broader outlines of its topography were roughed out in Tertiary time, these have been more or less effaced by Pleistocene deposits of river drift and loess, upon the subsequent erosion of which the present details of its topographical form are mostly dependent. Relies of the older topography of this region may be distinguished in the mesa-topped spurs of the Arkansas-Platte divide, only the extreme points of which appear within the area of the map, but which are charac- teristically developed in the vicinity of Castle Rock, just south of that area. Green Mountain and the twin mesas known as the Table Moun- tains, near Golden, are other features which have not been essentially changed since the Tertiary erosion. In the more modern features are to be recognized remains of a series of terraces, some of which are undoubtedly ancient river terraces of a period of earlier Pleistocene erosion, others being apparently lake terraces, formed in a sheet of water of lake-like dimensions, which occupied the area subsequent to this period. The most prominent river terrace is that along the east side of the Platte. River. Its surface is uniformly horizontal or with a slight slope PLAINS TOPOGRAPHY. 9 toward and down the river. The present stream has carved its channel within this terrace, its flood-plains being about 80 or 40 feet below the ancient terrace. The ancient terrace is with difficulty distinguishable along the west bank of the Platte. Similar but less prominent ancient river terraces are found along Clear and Bear creeks, but are wanting along the streams entering the Platte from the east, where the topograph- ical forms are those of simple modern erosion. The lake terraces are more prominent in the northwestern portion of the area mapped, owing probably to its having been less deeply cut away by modern erosion. They are particularly well marked in the area between Ralston and South Boulder creeks, where there is a blending of lake and river terraces. Here five distinct terraces are traceable, the lake terraces extending from 100 yards to 3 miles eastward from the foothills, while those more distinctly of stream origin are but from 200 to 700 feet in width. In the southern half of the area, the conditions for their preserva- tion being less favorable, the remains of the lake terraces are less contin- uous, being found here and there on the flanks of the hogback and on the granite slopes within the hogback valleys. Above the 6,000-foot contour there is a frequent recurrence of terrace remains along the foothills and on the flanks of Green Mountain, but they are not found against the Tertiary . bluff along and beyond the southern border of the area mapped. The influence of faulting upon the topography of the plains is less evident than might haye been expected. No doubt much of its effect has been obscured by more recent formations, deposited since the faulting took place. Nevertheless, it is readily observable that in the northern part of the area the trend of the leading topographical features has a northeasterly direction, which is also that of the greater faults that have been detected in the coal areas round Marshall and Erie. Many of the faults that have been traced in this region, however, have had no perceptible influence upon the present surface configuration, and were detected only as a result of artificial excavations, such as ditches or mine workings. In the region east of the Platte and north of Sand Creek within the area mapped, and on the same side of the Platte beyond this area, sand 10 GEOLOGY OF THE DENVER BASIN. dunes constitute another topographical feature, distinct from any hitherto mentioned. ‘These are formed, as in other parts of the Rocky Mountain region, on the west side of depressions broad enough to admit of a consid- erable accumulation of wind-borne sand on the leeward side of the basin, and on the slopes of divides too high for the wind to carry them over. Those within the area of the map, being not over 40 feet in height, come within the mterval of its contours, and are not outlined by them. The generally regular and gentle slope of the entire area is partie- wlarly favorable to its most important and permanent industry, namely, agriculture wader irrigation. Wherever there is sufficient supply of water, it admits of its distribution from irrigation ditches over the areas below their level with great uniformity and with a minimum expenditure of manual labor. In consequence, the upland prairie country, which twenty years ago was looked upon as practically valueless except as a grazing country, is now covered with rich fields of grain and alfalfa, abounds in gardens, and constitutes one of the most valuable farming areas in the West. HISTORICAL GEOLOGY. PRE-CAMBRIAN FORMATIONS. The present report has to do mainly with a comparatively recent phase in the geological history of the Rocky Mountains. That considerable por- tions of these mountains represent original land masses that have never been completely submerged, and that the sedimentary beds now resting on their flanks and in part covering the crests of certain mountains have been formed from the abrasion of these original land masses, has long ago been demonstrated by the writer.' These original land masses, which consti- tuted an archipelago of large islands in the Paleozoic seas, consist, so far as determined by such examinations as have been made of the portions now exposed, almost entirely of crystalline rocks, among which granites, gneisses, and micaceous or hornblendic schists are the prevailing types. A distinct unconformity and a pronounced change in lithological con- stitution almost invariably mark the contact between this older crystalline complex and the succeeding sedimentary beds of Paleozoic or later age, ‘Second Ann. Rept. U. 8. Geol. Survey, 1882, p. 211; Mon. U.S. Geol. Survey, Vol. XII, 1886, p. 20; Bull. Geol. Soc. America, Vol. I, 1890, p. 252. PRE-CAMBRIAN FORMATIONS. 11 while the latter contain among their constituents a considerable proportion of material which can be distinctly recognized as derived from the base- ment of crystalline rocks, and the amount and relative coarseness of such material vary with its distance from the ancient shore-line. According to the classification in vogue at the time the earlier geo- logical maps of the Rocky Mountain region were made, the whole of this older series of prevailingly crystalline rocks was mapped as Archean, although, even in the earliest reconnaissances, various observers had recog- nized that it included two or more distinct groups, in some of which a dis- tinctly sedimentary nature and stratified structure could be distinguished, In later years, since the discovery in the Lake Superior region of immense thicknesses of distinctly stratified clastic or fragmentary beds older than the earliest Cambrian, yet more recent than, and as a rule unconformable with, the underlying crystalline complex, the practice has been adopted by the Survey of confining the term Archean to the oldest crystalline rocks, and of grouping all fragmentary or clastic rocks which are older than the lowest fossiliferous Cambrian in a new system called Algonkian. Although no systematic study has yet been made of the great areas of more or less crystalline pre-Cambrian rocks in the Rocky Mountain region with the view of differentiating the Algonkian from the true Archean rocks that may occur in them, several small areas of Algonkian rock series have been recognized which possess such distinctly sedimentary characteristics, and are so situated with regard to Cambrian beds on the one hand and to large bodies of rock hitherto classed as Archean on the other, that there can be little doubt of the correctness of their assignment to this age. Such are those of the Quartzite peaks, south of Silverton, and of the Uncom- pahgre Canyon, south of Ouray, in the San Juan Mountains. In the Colorado or Front Range occurrences of distinctly fragmental or clastic rocks, more or less completely inclosed in the granite-gneiss complex, have been noted at various points, but their relations with adjoin- ing rocks have not yet been worked out over any large or connected areas. In the Pikes Peak region Mr. Cross' has described several bodies of >Geologic Atlas of the United States, Pikes Peak folio, Washington, 1894, Explanatory text, Algonkian period. 12 GEOLOGY OF THE DENVER BASIN. quartzite that are not only older than Cambrian, but are almost entirely surrounded by granite, which also sends apophyses into them. These he regards as undoubted Algonkian, while much of the granite must be of even later age. Along the lower canyons of South Boulder and Coal creeks, within the area of the present map, and also near Big and Little Thompson creeks to the north of it, there are certain beds of highly altered quartzite and conglomerate, associated with schists, which occupy a position between the Triassic sandstones and the more massive gneisses of the interior of the range. These were first noted by Marvine, of the Hayden Survey, in 1873,! who regarded the quartzites as the first phase in the successive metamorphism of great series ot sedimentary rocks, of which the final expression, in his idea, is a structureless granite. The Coal Creek occurrences have since been more particularly exam- ined by A. Lakes for C. R. Van Hise, in 1890. In analyzing the various observations Van Hise? finds evidence of the existence of a general funda- mental crystalline complex of pre-Cambrian rocks in the Colorado Range, and regards these quartzites and conglomerates as undoubted pre-Cambrian elastics, but is unable from the evidence at hand to say definitely whether there is a sharply defined line between the two or only an insensible eradation. In the present work, inasmuch as but a very limited extent of pre- Cambrian rocks comes within the limits of the map and the internal structure and mutual relations of these rocks have no bearing upon the subject-matter of this investigation, it was not considered advisable to enter upon the necessarily lengthy and complicated study that would be required to determine these relations. Hence, beyond noting the oceur- rence of the quartzites and conglomerates in the area round Coal and South Boulder creeks and recognizing their distinctly sedimentary character as contrasted with gneisses, granite-gneisses, and massive granite, which appear successively as one approaches the center of the range, nothing has been done toward definitely delimiting the areas occupied by either ‘Seventh Ann. Rept. U. S. G. and G, Survey, Washington, 1874, p. 187. 2 Bull. U. S. Geol. Survey No. 86. Correlation Paper, Archean and Algonkian, Washington, 1892, pp. 312, 325. PRE-CAMBRIAN FORMATIONS. 13 class of rocks. The distinctly sedimentary rocks are, however, not over 1,000 feet in thickness, and hence they occupy necessarily but a very small proportion of the total pre-Cambrian area represented on the map. It has, therefore, been judged best to preserve the Archean color, in accordance with existing practice, for this area, and it will be referred to under this term throughout the text; at the same time it is freely admitted as possible that future study may show that a much larger portion of this area than at present appears probable should properly be considered as Algonkian. As the geological history of a region must be primarily determined by a study of the sedimentary beds exposed in it and of the organic remains found in those beds, and as thus far no representatives of the Lower and Middle Cambrian strata, so abundantly developed in Utah, Nevada, and British Columbia, have been found in the Rocky Mountain region, it is only for the period commencing with Upper Cambrian time that any attempt can be made at present to trace out the details of its geological development. Within this period the histories of earliest and latest phases are necessarily the most obscure—in the one case because the sediments deposited in the earliest times have been more generally covered up and buried beneath the accumulations of later ages; in the other because the latest sediments have had less time to consolidate into hard rock and have been the first to be removed by the destructive agencies that have acted on the surface during the long modern period in which the region has been exposed to subaerial erosion. CAMBRIAN LAND. The principal land areas of the Rocky Mountain region which pro- jected above the waters in which Upper Cambrian sediments were deposited were the Colorado and Sawatch islands. The latter included the present Sawatch Range, and was possibly connected with other land masses to the south, but was entirely distinct from the Colorado Island and was separated from it by a bay or strait, in which considerable extents of Upper Cambrian sediments were deposited. To the east of the Colorado Island, covering the present area of the Great Plains, was the ereat mediterranean sea, a shallow ocean which stretched with few known interruptions to the present summits of the Appalachian Mountains. 14 GEOLOGY OF THE DENVER BASIN. The Colorado Island corresponded in a general way with the present form of the Colorado and Wet Mountain ranges. Its northwestern exten- sion was, however, greater, probably taking in the present area of the North and Middle parks and reaching to the Park Range beyond them. On the southeast, on the other hand, it was divided up into a series of peninsulas or islands, by bays or straits extending into it in a northwesterly direction from the ocean. The most important of these bays was that between Canyon City and Pikes Peak, forming part of the South Park depression, which may have entirely cut off the connections between the Wet Mountains and the main island. A second important bay was that which stretched up the present depression of the Ute Pass and Manitou Park; and if, as is quite possible, the waters of the latter connected with the South Park depression, the present Pikes Peak massive would also have formed an island. There is some probability also that the present promontory known as the Rampart Range, lying between Manitou Park and the plains, was more or less submerged beneath the Cambro-Silurian seas and received sediments perhaps as late as the early Carboniferous, having been arched up into its present form during the later orographic movements to which the region has been subjected. Another prominent bay was that now occupied by Huerfano Park, at the southern end of the Wet Mountains, which it now separates from the Sangre de Cristo Range. Conditions have been so modified by later movements that in our present state of knowledge it is impossible to determine definitely whether the Wet Mountain Island was connected at that time with the Sangre de Cristo and Sawatch islands or not. A similar uncertainty exists with regard to the northern portion of the eastern shores of the Colorado Island, but judging from the present form of the Mesozoic deposits it is probable that there were smaller bays, extending to no great distance inland, which served to give an indented form to the shore-line. As shown by the character of the sediments deposited there, on the western shores of the island conditions differed somewhat from those on the eastern shore. The depression of South Park had certainly a connec- tion with the ocean westward between the northern end of the Sawatch CAMBRIAN LAND. ry and the Gore mountains, which may then have formed part of the Park Range, and consequently of the western shore of the Colorado Island. — It is probable, though less certain, that a water connection also existed through Canyon City Bay with the eastern sea; but if deposits were formed in this strait they have since been removed by erosion and can no longer be traced continuously. Only Mesozoic or later beds are now found in contact with the elder crystalline rocks on the west flanks of the present Colorado mountain range, which conceal the older deposits, so that the position of the ancient shore-line can be only approximately determined. This is also true of a considerable portion of the eastern shore-line. EARLY PALEOZOIC SEDIMENTS. The deposits formed in the early Cambrian seas around these islands were continued through Silurian into early Carboniferous time in a sort of cycle of deposition; that is, the successive beds formed during these periods show in their present outcrops no decided discrepancy of angle or other evidence of any pronounced orographic movement during the period which would have sensibly changed the form of the land masses around which they were deposited. That the region as a whole was subjected to changes of level with reference to that of the surrounding water, and that probably there were some local movements of elevation and subsidence producing differential changes of level on different portions of the shore- line, is evidenced by certain observed uncontformities by erosion, generally ‘of small amount, and by the thinning or even complete absence of some of the series of beds at certain parts of the observed outcrops. The thickness of beds deposited during this cycle of deposition was very slight, especially along the eastern shore, where in no ease, as far as observed, does it exceed an aggregate of 350 to 400 feet. They consist mainly of sandstones, more or less caleareous, and of siliceous limestones with some shales, generally in subordinate development. The series increases in thickness to the west- ward, being 500 to 750 feet around the Sawatch uplift, about 2,000 feet in the Grand Canyon region, and reaching an ageregate of 15,000 to 20,000 feet in the longitude of the Wasatch Mountains. In these outlying regions there is, as in the Rocky Mountain region, no evidence of any pronounced orographie disturbance during the deposition 16 GEOLOGY OF THE DENVER BASIN. of the successive beds, but, as more detailed studies of the respective exposures have been made, it has been found that certain members of the series thin out locally or are entirely wanting. Thus, in the Wasatch Mountains, where the ocean waters were apparently deeper than in the more eastern regions and the series is most complete, the Upper Cambrian deposits are apparently wanting. In the Grand Canyon region, on the other hand, the Devonian is represented by less that 100 feet of beds, while the Silurian is wanting. Evidences of erosion are there very distinct, but it is sti uncertain whether the Silurian was never deposited or whether it has been entirely removed by erosion. Finally, in the Rocky Mountain region, no Devonian has yet been discovered, but wherever the Lower Paleozoic section has been studied in detail the Cambrian, Silurian, and Lower Carboniferous beds have been found closely associated and quite conformable. Evidences of erosion are most frequent, however, between the Silurian and Lower Carboniferous, which would favor the hypothesis that during Devonian time ocean waters had retreated from this region and portions of it at least had been exposed to erosion. The studies that have been made of this series of rocks on the eastern flanks of the Colorado Island, in the Canyon City and Manitou bays, respectively, show besides a notable decrease in thickness a decided change in the character of the sediments from those observed on the west side of the island on the flanks of the Mosquito Range, as well as a considerable variation in either respect between those in the respective bays. The Cambrian is here very thin, and at some points does not appear, but the close association of its sediments and fauna with those of the succeeding Silurian renders it prob- able that there was no actual discontinuity in the deposition of the two series, the local absence being due to overlapping by the Silurian. The Silurian beds are relatively well developed and contain an abundant and characteristic fauna, which has rendered it possible to distinguish three distinct subdivisions or formations in this group. The local thinning out or absence of the uppermost of these formations proves that a considerable period of erosion must have intervened before the deposition of the suc- ceeding Lower Carboniferous beds, and in so far confirms the opinion, hitherto only tentatively put forth, that the absence of Devonian beds in EARLY PALEOZOIC SEDIMENTS. 1% the Rocky Mountain region is due to a depression of the ocean-level or an elevation of this region after Silurian time. Within the area mapped no outerops of Lower Paleozoic beds are found, but there is good reason to assume that they underlie all the later sediments and do not reach the surface simply because they are concealed along the Archean contact by the overlapping of Mesozoic and_ later deposits. A short distance south of this area, however, in Perry or Pleas- ant Park, at the base of the Archean foothills, is a small exposure of Paleozoic beds whose exact horizon has not been determined, but’ which, from descriptions and analogy with other exposures, are assumed to belong to the Lower Paleozoic series. The overlap of the Mesozoic over the edges of the older beds is there visibly demonstrated. CARBONIFEROUS MOVEMENT. At some time during the Carboniferous period not yet definitely determined, but probably during the latter half, an important orographic movement took place in the Rocky Mountain region which in certain localities was accompanied by some dynamic disturbance. Its most gen- eral effect was an elevation of the land, raising above water-level portions of the regions that were previously submerged and exposing them to subaerial erosion. In some cases it would appear that entirely new land masses were formed which in later movements were again submerged, and that in other cases land masses were depressed so as to be subject to sedimentation which had received no sediments during the early Paleozoic cycle. The movement was therefore of a differential and somewhat local character in this region, though it appears to have been nearly contempora- neous with important movements in far distant regions, notably that which preceded the coal-forming era in the Appalachians, and its effects were probably felt over a very large portion of the North American Continent. The most visible results of this movement in the Colorado Island are seen in the extensive erosion of Lower Paleozoic beds in Canyon City Bay, from whose present position it would seem probable that the water connec- tion with South Park was interrupted and a land connection between the main Colorado Island and its Wet Mountain extension was reestablished. MON XXVII 2 18 GEOLOGY OF THE DENVER BASIN. To what extent this elevation and erosion affected the eastern shore-line of the Colorado Island, immediately facing the mediterranean ocean, it is difficult to determine, on account of the uncertainty that exists with regard to the next succeeding deposits along that line. Following the Carboniferous elevation and erosion, there was a general subsidence throughout the Rocky Mountain region, which continued in some parts through Upper Carboniferous into Triassic time. On the west- ern slopes immense thicknesses of conglomerates accumulated during the Upper Carboniferous period, which passed upward into red sandstones, gradually growing lighter in color and finer in grain, the final development being the Red Beds, which have been generally considered Triassic, mainly on stratigraphical grounds. Along the eastern front no great conglomerate series comparable to the Upper Carboniferous of the western slope has yet been observed, and the beds occurring below the Jurassic have, on account of their red color, hitherto been assigned to the Trias, but the red color is not an infallible criterion, for red sandstones are known to oceur in beds carrying Upper Carboniferous fossils. WYOMING FORMATION. In the case of isolated’ and incomplete exposures of a series of beds deposited between two nonconformities, the lithological constitution of the beds is, in the absence of all paleontological evidence, the only, though necessarily very uncertain, means of determining their geolegical horizon, for the earlier formation of the series may be concealed by overlap if there was a continuous subsidence during the period, and the later formations may have been removed by erosion during a later elevation. The assignment of the lowest series of beds in the present field to the Trias is based mainly on lithological correspondence with the upper part of the Red Bed series as developed on the west flanks of the Rocky Moun- tain uplift. There is, however, a further reason for this assignment in the fact that the variation in thickness of the formation, which is very consider- able, is at the bottom rather than at the top of the series, and hence is mainly due to overlap rather than to erosion. Whether any considerable amount of beds not represented in the actual outcrops exists under the WYOMING FORMATION, 19 Denver Basin at some distance from the foothills can be only a matter of conjecture in the present state of knowledge. It is possible that the lower portions of the series, where the observed thickness is greatest, may corre- spond to what is elsewhere considered Upper Carboniferous on similar grounds of stratigraphical and lithological correspondence, and which, on the Pikes Peak sheet of the Geologic Atlas, has been designated by Mr. Whitman Cross the Fountain formation. That the upper part of the typical Red Beds are of Triassic age is rendered more than probable by the dis- covery in them at various points in the Cordilleran region of characteristic vertebrate and invertebrate remains, together with typical plants. It has seemed necessary, therefore, to recognize them as a distinct formation, and the name Wyoming has been assigned to them because of their widespread development in that State. In the present field Mr. Eldridge has judged best to divide the formation into an upper and a lower series on grounds of lithological composition and structure. LOWER WYOMING FORMATION. This formation varies in thickness in this field from about 500 to 2,500 feet. It consists essentially of coarse sandstones and conglomerates with subordinate red shales, and a few thin beds of limestone, generally compact, with conchoidal fracture, and of light-drab or white color. The base of the formation always consists of waterworn fragments of granite, gneiss, and schist, or their constituents, feldspar and quartz, the prevailing red color being largely due to the abundance of red feldspar. In the upper part is a series of white sandstones, made up almost entirely of quartz grains, called from their prevailing color the “creamy” sandstones. The variation in thickness of this formation is due mainly to the un- evenness of the floor or sea-bottom upon which it was deposited. Besides the regular slope away from the mountains, or eastward, it also deepened southward. Moreover, in the neighborhood of Golden a ridge or low prom- ontory extended for some distance from the foothills, upon which in the earlier part of the period there was no sedimentation. It is assumed, how- ever, that the ocean-bottom was continually sinking during this period and the sediments consequently advancing shoreward on the gentler slopes and 20 GEOLOGY OF THE DENVER BASIN. overlapping those previously deposited. Hence, at the present day, this formation is found to be thickest along the southern portion of the foothills and to reach its greatest attenuation at Golden. The neighborhood of Golden has been the scene of a peculiar series of deformations in this and sueceeding periods which merit especial mention. From the minute study made by Mr. Eldridge of the existing beds and the character and position of their cutcrops, it appears that already at the close of the Carboniferous movement there must have been a ridge or arch of Archean rocks about 4 miles wide extending out eastward at right angles to the general shore-line, which was above water during the early part of the Wyoming period and became subject to sedimentation by sinking below the ocean-leyel only toward the close of the period. It is not possible now to say what was the cause of the arching up of the sea-bottom at this point, whether it was due entirely to a movement within the rocks or in part to an unequal planing down of their surface by erosion. That there was a defor- mation of the crust, however, is rendered probable by the fact that in later movements there must have been a repetition of the arching which raised this portion of the surface successively above the general level of the sea- bottom and thus prevented sedimentation for a time at this locality. It is the proof of the repetition of this movement which forms the most convine- ing argument against the hypothesis of an overthrust fault at this point, which would be suggested by a first glance at the present disposition of the outcrops of the successive strata as represented on the geological map. The strain which produced this local arching may be assumed to be a compressive force acting in directions parallel with the shore-line, as con- trasted with that which produced the final general upturning of the beds, which must have acted at right angles to this direction. UPPER WYOMING FORMATION. The beds assigned to this division by Mr. Eldridge consist of about 185 feet of red sandstones and shales, with some thin limestone bands, followed by 300 to 400 feet of shales, variegated in color and gypsiferous in the upper part and ending in a persistent band of pink and brown sandstone. UPPER WYOMING FORMATION. 21 A portion of the beds included in this division have hitherto been classed in the succeeding Morrison group, or Jurassic, but as they contain, so far as observed, no fossil remains, the Hallopus fauna which occurs near Canyon City not having been detected in this region, the line of division has been drawn on grounds of lithological composition and structure. In lithological composition the variegated shales resemble similar beds be- longing to the Jurassic on the western slopes of the mountains, but the structural evidence was what finally determined Mr, Eldridge to draw the line where he did. ‘This point will be discussed later. JURASSIC MOVEMENT. A widespread orographic movement, resulting in elevation and erosion, and in some parts accompanied by folding and faulting, took place in the Rocky Mountain region previous to the deposition of the series of beds which, from their containing vertebrate remains, have been considered as of late Jurassic age. This has been designated the Jurassic movement, since its effects were most apparent during that period, though it may have been inaugurated toward the close of the Trias, when shallow water and in some places lacustrine conditions prevailed. As a result of this elevation, the marine deposits of Jurassic age, the Baptanodon beds of Marsh, which were formed in the regions to the north and west, were shut out from the Rocky Mountain region of Colorado, and during the depression which fol- lowed only fresh-water beds were laid down in this region; the character of the fossil remains found here indicates that oceanic waters did not enter the region until Dakota time, or at the commencement of the Upper Creta- ceous cycle of deposition. A notable feature in the Jurassic movement, deduced from the evidence which present conditions afford, is that the folds produced by it were in general at right angles to those formed by succeeding and more pronounced movements; therefore, after the crests of folds have been planed off by ero- sion and a later series of beds deposited over their upturned edges, when the whole complex is again folded at right angles to the earlier folds and again eroded, the unconformity between successive series is shown in the result- ing outcrops by discrepancies in strike rather than in dip of beds. This 22 GEOLOGY OF THE DENVER BASIN, effect is more pronounced on the western than on the eastern slopes of the region. On the eastern shore of the Colorado Range the movement and the succeeding erosion were so uniform that it is difficult to detect any discrepancy of angle either in strike or dip between Triassic and Jurassic beds. In the area mapped, however, the effect of the movement is shown in the Golden arch, whose axis runs just north of Clear Creek, between it and Gold Run. The crest of this arch and the Triassic beds already deposited over it were raised about 420 feet by this movement, and by the subsequent planing of the crest a great portion of the upper Wyoming formation, down to the limestone near its base, was removed. ‘That the absence of these beds from where the arch once existed is due to erosion rather than to nondeposition is proved by the fact that as one follows the strike of the present upturned beds toward Golden from either direction, the beds of the upper Wyoming formation disappear successively from the top downward, and the discrepancy in strike is between them and those of the next succeeding higher horizon. The movement is also assumed to have produced a similar though less pronounced arch in the vicinity of Boulder, from the crest of which the upper Wyoming formation was eroded off and over which the succeeding Morrison beds were not deposited. The same general phenomena obtain in these beds in their present outcrops as near the Golden arch. It was for this reason that the upper beds of this formation, which from their lithological constitution were formerly consid- ered Jurassic, have been classed by Mr. Eldridge as Triassic. MORRISON FORMATION. The beds of this formation, which were deposited during the general depression that followed the Jurassic movement, consist mainly of marls with varying proportions of sandstones and thin limestones, the whole having an average thickness of 200 feet. Lenticular bodies of drab lime- stone occur in the clays of the lower two-thirds of the formation, which also carry the remains of gigantic saurians characteristic of the formation, and from which the name ‘“Atlantosaurus beds” has been given to it. The upper third of the formation is more arenaceous, sandstones sometimes predominating over the clays and passing into a conglomerate at the base. MORRISON FORMATION. 23 Its thickness in the Denver field is somewhat variable from point to point, the variation being assumed to be mainly the result of erosion. The Morrison formation is chiefly remarkable for the abundant remains of gigantic saurians and other reptiles found in its beds, together with fishes, birds, and a few diminutive mammals. The abundance of the land animals and the discovery of fossil plants in the beds is further proof, if any were needed, that they were formed along the shores of a large land mass. Molluscan remains, of fresh-water habit, indicating that the sediments were deposited in an inclosed lake, are also found in these beds. As both mollusean and plant remains found at this horizon have too wide a range to be of value in the determination of the age of the inclosing beds, this determination has been based exclusively upon the vertebrates. Although the latter have some affinities with the European Wealden or Lower Cretaceous, to which Professor Marsh was at. first inclined to assign the horizon, he found the evidence in favor of late Jurassic age to be so much stronger that he assigned to this period not only these but other beds with a similar fauna, notably the Potomac beds of the East, which on other grounds had been considered early Cretaceous and which present many structural analogies with the Morrison beds. From the point of view of the stratigrapher, the assignment of the Morrison beds to the Lower Cretaceous rather than to the Upper Jurassic is much more desirable, not only because it accords better with the sequence of sedimentation thus far disclosed in the adjoining regions of Kansas and Texas, but because it places the physical break whose effects are recognized over the whole continent between these two great time divisions rather than in the midst of one of them. EARLY CRETACEOUS MOVEMENT. It has been hitherto assumed by the writer and others that a move- ment must have occurred in the Rocky Mountain region between the time of deposition of the Morrison or Atlantosaurus beds and those of the succeeding Dakota formation, now considered as the base of the Upper Cretaceous. In this interval a considerable thickness of earlier Cretaceous beds has been deposited in other regions, notably the Comanche series 24 GEOLOGY OF THE DENVER BASIN. of Texas and Mexico and the Kootanie series of British Columbia, whose representatives have for a long time been supposed to be wanting in the Rocky Mountain region. The area of this assumed nondeposition has been somewhat circumscribed of late years by the discovery of Kootanie beds in Montana, and more recently in the Black Hills region, and of Comanche beds in New Mexico on the east front of the Rocky Mountains, not far south of the Colorado boundary. The intermediate region, however, com- prising the higher portion of the Rocky Mountain region of Colorado and a considerable portion of the Great Plains opposite or to the east of this region, appears, as in Jurassic time, to have been cut off from the access of ocean waters, since no representative either of the marine Jura or of the earlier Cretaceous or Neocomian beds has yet been found in it. From their fauna the Morrison beds are assumed to have been deposited in fresh or lacustrine water, whereas the Dakota beds are distinctly marine, and the character of the conglomerate at their base indicates that they were deposited in an ocean that was slowly advancing over an area that bad for a long time been exposed to subaerial disintegration. The physical data from which the character of the movement may be inferred are as yet somewhat meager. It was assumed that the lake in which the Morrison beds were deposited was separated from the ocean by a barrier raised during the previous (Jurassic) movement along a line extending eastward in the latitude of the Raton Hills and probably con- nected with the old Paleozoic elevations of northern Texas, the Indian Territory, and western Arkansas. A similar barrier may have extended eastward in about the latitude of the northern boundary of Colorado. It was assumed, further, that during the early Cretaceous movement these barriers were broken and the region possibly slightly elevated, so that the Morrison Lake was drained and its bottom remained above the ocean during early Cretaceous time, exposed to subaerial disintegration and to some slight erosion. At the close of the movement a general depression is supposed to have set in, which continued to the middle of the Upper Cretaceous cycle and during which the ocean waters filled the area formerly occupied by the Morrison Lake. The evidence of the movement in this area is found in the varying thickness of the Morrison formation from point to point. This EARLY CRETACEOUS MOVEMENT. 25 - variation, as Mr. Eldridge shows, is in part from the bottom upward, hence raused by nondeposition on more elevated portions of the sea-bottom, but mainly from the top downward, hence to be ascribed to erosion previous to the deposition of the sueceeding Dakota beds. The Golden arch is assumed to have been further elevated by this movement about 1,000 feet, for the reason that this thickness of the Creta- ceous sediments (including the Dakota and the lower part of the Benton Cretaceous) apparently did not cross it. A further effect of the movement is seen in the apparent overlapping of Dakota sediments across the under- lying Morrison beds onto the Archean floor at Coal Creek. D DAKOTA FORMATION. The first beds deposited in this epoch were characteristic conglomerates, consisting mainly of small, very well rounded grains of chert, jasper, quartzite, and often of other of the more resisting rock varieties, together with some limestone pebbles. The whole formation, which averages 250 to 350 feet in thickness in this field, is essentially a sandstone, which changes to the quartzitic condition with remarkable facility, and, thus offering greater resistance to erosion than the other rocks, generally con- stitutes the hogback ridge wherever the Cretaceous strata are upturned against the flanks of the mountains. From an economic point of view, this formation is important for the remarkably pure beds of fire clay which occur in bodies of varying thickness within the sandstones. In organic remains it is remarkable for the variety and great number of fossil plants found within its beds. Dicotyledonous plants make their first appearance in the Colorado mountain region at the Dakota horizon. Marine mollusks, though not yet found in the Denver Basin, occur plenti- fully at this horizon in the plains region of Kansas and Texas. Vertebrate remains are not known to the writer to have been found in these beds in the vicinity of the Denver Basin. Significant from a structural point of view is the fact that among the pebbles of the basal conglomerate of the Dakota formation in the Denver Basin Mr. Eldridge has detected some Silurian fossils and ill-shaped coral- line forms. It is not conceivable under existing conditions that these should 26 GEOLOGY OF THE DENVER BASIN. have been derived from the degradation of Paleozoic limestones resting on the flanks of the range immediately above the Dakota beach, for these had already been overlapped and buried beneath the Triassic sediments. They must be assumed, therefore, to have been brought into this position by strong long-shore currents from some Paleozoic exposures to the southward, either at Perry Park or still farther south; possibly from remnants left upon the flanks of the Rampart Range. COLORADO FORMATION. The cycle of deposition inaugurated in Dakota time was continued in a progressively deepening sea through the Colorado Cretaceous. During the Benton division of Colorado time there were deposited up to 600 feet of beds, mainly argillaceous shales, characterized by their dark, almost black, leaden hue, and containing thin fossiliferous and often bituminous lime- stone beds of widely varying thicknesses and frequent concretionary clay- ironstones. The shales carry considerable disseminated pyrites, together with gypsum and sulphur. The succeeding Niobrara division of the Colo- rado is characterized by a persistent limestone at the base, reaching 50 feet in thickness, of light color, and here somewhat dolomitic im composition, though at other points of remarkable purity, succeeded by about 100 feet of gray and up to 250 feet of buff shales, with some thin limestone bands and iron concretions. All these clays contain considerable amounts of alkaline salts. Remains of a remarkable series of vertebrate animals have been found in chalky beds that apparently correspond to the Niobrara limestone of this horizon, along the Solomon, Saline, Smoky Hill, and other rivers in Kan- sas. These animals include marine swimming reptiles, birds with teeth, and pterodactyls, and from the latter they have been called by Marsh the ‘“‘Pteranodon beds.” MID-CRETACEOUS MOVEMENT. Until within a comparatively few years it has been assumed that the cycle of deposition of the Cretaceous deposits of the Rocky Mountains was entirely uninterrupted. The character of the sediments of the middle part of the series, which is prevailingly shales with unimportant and unpersistent MID-CRETACEOUS MOVEMENT. 27 harder calcareous and arenaceous layers, is such that, in the absence of any discrepancy of angle, unconformity by erosion would not be readily distinguishable. Since the discovery in this field of unmistakable evi- dences of considerable erosion following the deposition of the Niobrara Cretaceous, attention has been called to facts indicating that, throughout the area of the Great Plains at least, there was a general elevation produc- ing a temporary recession of the ocean waters, which was followed in places by considerable erosion before the area was again depressed below ocean-level. In the Denver Basin area the principal evidence of this movement is found in the Golden arch, which, according to Mr. Eldridge’s observations and inductions, must have been elevated by it some 9,500 feet, so that its cross-section widened to 21 miles from north to south and the involved strata up to the top of the Niobrara were crumpled into minor folds. In the erosion which followed, the whole thickness of the Niobrara, Fort Benton, Dakota, and whatever may have been left from previous denuda- tions of the earlier Morrison and Wyoming formations, was entirel vy removed from the crest of the arch. The thickness of material thus removed from the crest of the arch Mr. Eldridge estimates at something over 1,000 feet. Over the Boulder arch the present disposition of the strata indicates the probability of an earlier movement at the close of the Dakota, this formation apparently having been eroded, to a certain extent, before the deposition of the Benton clays. The evidence of this movement is of less conclusive character than that of the post-Niobrara movement, and _ its effects were at best very local in their character, so that it may be passed over without further consideration. General depression followed the erosion of the post-Niobrara elevation. It has been suggested that this erosion may have been submarine and due to the action of strong long-shore currents in not yet consolidated material recently deposited on the ocean bottom. This depression lasted during a large part of Montana time and was probably followed by a gradual elevation and shallowing of the waters during Fox Hills and Laramie time. 28 GEOLOGY OF THE DENVER BASIN. MONTANA FORMATION. The lower subdivision of the Montana formation, the Pierre, consists mainly of plastic clays, generally gray in color, with lenticular bodies of limestone, and an arenaceous zone from 100 to 300 feet thick about one- third way up in the formation. Its aggregate thickness is taken at about 7,700 feet, no part of which is supposed to have crossed the Golden arch. The clays contain a few clay-ironstone concretions, and are impregnated with gypsum and alkaline salts. The formation has a characteristic mol- lusean fauna, whose remains are usually found in the lenticular bodies of impure limestone. The upper subdivision of the Montana, the Fox Hills, has an average thickness in this field of 800 to 1,000 feet, only the upper 500 feet of which is assumed to have been deposited over the crest of the Golden arch. The formation is characteristically more arenaceous than the Pierre, and is generally of a yellowish or buff color. The arenaceous shales in the lower part carry some ferruginous concretions and gypsum, and lenticular limestones are also found in subordinate quantity, which contain a charac- teristic molluscan fauna and some plant remains. The most characteristic and persistent stratum in the formation is a sandstone at the very summit, which in this field is about 50 feet thick, of greenish-yellow color, and carries an abundant and typical marine fauna. It consists of quartz grains, with a little muscovite and biotite mica and disseminated iron. LARAMIE FORMATION. Lithologically in close connection with the Fox Hills is the Laramie formation, which consists of a series of basal sandstones up to 200 feet in thickness, above which are in this field 400 to 1,000 feet of clays, with small lenticular bodies of sandstone. The basal sandstones of the Laramie and the sandstone at the top of the Fox Hills often form a single sandstone bluff. The fossil horizon forms a sharp division between these horizons, and the Laramie sandstone is usually white in color, consisting mainly of quartz, with minute grains of black chert and but few other impurities. The Laramie sandstones LARAMIE FORMATION. yas) are generally divisible into three benches by clay bands or coal seams, and it is within these sandstones that the best workable coals are found. Coal seams have also been found in the upper clayey division, but the coals are lignites with higher percentages of water and of inferior economic value. One band of sandstone above the two lower benches is of impor- tance as being more generally fossiliferous, and hence a valuable indicator in searching for coal. It contains a considerable percentage of lime. The fossils found are mollusks of brackish- and fresh-water habit, t ether with remains of plants. No vertebrates have yet been found in beds that could with certainty be assigned to the Laramie horizon, as defined in this report. POST-LARAMIE MOVEMENT. At the close of the Laramie period the general shallowing of the ocean waters, which had been going on slowly during the latter part of Creta- ceous time and which was probably accompanied by some elevation of the sea-bottom, culminated in a widespread orographic movement whose effects have been traced from one end of the continent to the other, but are most marked in the Cordilleran region. It is to this movement that the rough- ing out and outlining of the mountain forms of the present Rocky Moun- tain system has been generally ascribed, and while it is not possible to decipher with certainty in a given region the amount of deformation which was due to each of the orographie movements to which it has been sub- jected, it is evident that the post-Laramie movement must have played relatively the most important part in these deformations, since its results were to shut out the ocean waters from the plain as well as from the mountain areas of the entire Western region. In this post-Laramie movement not only was there a general conti- nental elevation of the whole region, but the mountain areas suffered a differential uplift in relation to the surrounding plains or lowlands, so that the edges of the strata resting against these flanks were in many places upturned at considerable angles. With the dynamic movements which caused the differential elevation of the mountain masses and which pro- duced folding and dislocation of the strata was associated considerable eruptive activity, which inaugurated a succession of outbreaks of eruptive 30 GEOLOGY OF THE DENVER BASIN. rocks that continued from time to time during the Tertiary era, and which formed an important and characteristic feature of that era. The dynamic effects of this movement, as seen in the Denver Basin region, show that it was more intense than previous movements and was produced by forces acting at right angles to the foothill region and to the general strike of the strata, instead of nearly parallel, as were those which produced the various uplifts of the Golden and Boulder arches. It was apparently in the nature of a powerful compression along the base of the foothills, or the contact of the later sedimentary beds with the basement complex of crystalline rocks, accompanied here and there by a certain amount of thrust-faulting, which tended to push the higher horizons forward toward the mountains and over the lower ones. A most important and somewhat singular effect of this movement, which Mr. Eldridge’s explana- tion of the Golden and Boulder arches renders necessary, was the flatten- ing out of these arches, so that the present line of contact of the lowest exposed sedimentaries with the Archean, which, since the Laramie move- ment, is a line of strike, is a comparatively straight lime; whereas the line of strike of the higher beds, which were deposited over the arch, have now a decided curve inward toward the mountains, and must, before being upturned, have been compressed into something like a synclinal trough, as explained graphically in a later chapter. It seems possible to the writer that some of the curve in the strike line of the upper (Laramie and Fox Hill) beds, or, in other words, of the irregu- lar overlapping of the strata by the Montana and Laramie formations, may have been produced by overthrust faulting along a line making but a slight angle with the bedding, which would naturally have taken place in the great clay horizons of the Middle Cretaceous, where little traces of the shearing would be left. The general upturning of the Mesozoic strata along the foothills, which has produced the characteristic phenomena of the hogback ridges, must have been inaugurated by this movement, but it was not completed, as the succeeding Arapahoe and Denver formations have also been upturned at steep angles in the foothill region, and there is some reason to assume that the forees which produced this upturning have been acting in comparatively recent times. POST-LARAMIE MOVEMENT. 31 That the Mesozoic strata were uplifted along the foothills above the general level of the beds, and their upturned edges exposed to erosion, is proved by the fact that rolled fragments of the rocks of the different formations are found in the succeeding Arapahoe and Denver formations. The period of erosion that succeeded the movement of elevation must have been of long duration, since as much as half the total thickness of the Laramie was removed from portions of the field before the succeeding Arapahoe beds were laid down. ARAPAHOE FORMATION. After an erosion of the Laramie beds which removed from portions of the Denver Basin 600 feet or more of the previously deposited sediments, a considerable fresh-water lake was formed and sedimentation again set in. What the exact area of this lake was it is not possible now to determine; its extent was undoubtedly considerably larger than that covered by its beds at the present day, especially to the northward. To the southward vertebrate fossils characteristic of the post-Laramie formations have been observed by Professor Marsh in Monument Park, and remnants of beds resembling the Arapahoe and Denver series have been observed near Canyon City which may have been contemporaneously deposited, but whether the lake was continuous along the mountain front or there were several small isolated basins it is as yet impossible to determine. For the present discussion it will be assumed that the Arapahoe Lake was confined to the Denver Basin. In it were deposited more than 600 to 800 feet of sediments, the excess above these figures being the unknown amount that was eroded off before the Denver beds were deposited. Of these sediments the lower 50 to 200 feet were conglomerates, the upper 400 to 600 feet arenaceous clays. In the persistent band 40 feet in thickness at the base of the formation have been found among the pebbles coal, silicified wood, and white sandstone from the Laramie; limestone from the Niobrara; the characteristic cherty conglomerate from the Dakota; limestone and red sandstone from the Jura and Trias; and silicified limestone with casts of Beaumontia. The last named, which are most abundant in the southern portion of the field, must have come from Carboniferous limestones in Perry Park or beyond, and Sy GEOLOGY OF THE DENVER BASIN. are an indication that strong long-shore currents setting northward pre- vailed in the Arapahoe Lake, as they did in the Dakota Ocean. The material distinctly traceable to sedimentary beds is throughout of subordinate amount in the beds, the bulk of the sediments being derived from the abrasion of the crystalline rocks here classed as Archean. It is noticeable, however, that none of the andesitie débris, which form so important a part of the succeeding series, are found in these beds. Vertebrate remains are found in both the conglomerates and the clays, more abundant and better preserved, however, in the latter. They are classed among the Ceratops fauna, which is also characteristic of the Denver beds. The forms found are, however, in general more fragment- ary and less well preserved than those obtained from the latter. POST-ARAPAHOE MOVEMENT. between the deposition of the Arapahoe and Denver beds a consider- able time-interval occurred, during which, as the record of the rocks shows, the Arapahoe Lake was drained and the sediments deposited in its bottom were considerably eroded. The movement which caused the drainage of the lake was, as far as present indications go, rather local in its effects, and produced no important deformation of the lake beds already deposited. In the mountain region, however, it was accompanied by outbreaks of ande- sitic lava, which must have completely covered the crystalline rocks in the drainage area tributary to the lake basin. This movement was succeeded after a considerable lapse of time by a depression sufficient to allow of the formation of a second lake in the Denver Basin, and probably of others in the Middle Park region to the west of the mountains and in other parts of the Rocky Mountains. The nature of the depression which produced .such lakes without admitting marine waters to any extent within the areas affected is not readily conceivable, yet its effects are shown to have been widespread by the considerable thicknesses of fresh-water beds, consisting largely of eruptive débris, which are found overlying the Laramie im various portions of the Rocky Mountains, and which are manifestly more recent than the Laramie, yet older than any Eocene deposits hitherto recognized. From POST-ARAPAHOK MOVEMENT. 30 evidence already obtained it appears that such lakes existed along the east front of the Rocky Mountains at Canyon City and in Huerfano Park; on their southern slope in the valley of the Animas; on the west along the flanks of the Elk Mountains; and in the principal interior valleys, the North, South, and Middle parks. To the northward, along the east front of the mountains, the continuity of the post-Laramie exposures is broken in northern Colorado and southern Wyoming by a covering of Miocene and Pliocene beds, but they reappear in the basin of the Cheyenne River, and are abundantly exposed in the valleys of the tributaries to the upper Missouri. DENVER FORMATION. The beds deposited in the Denver Lake reached a thickness of over 1,400 feet along the flanks of the mountains, but were probably somewhat thinner toward the middle of the basin. The total thickness of the beds as originally deposited can no longer be determined, owing to the extensive erosion to which they have since been subjected. The most striking characteristic of these beds is the extent to which débris of great varieties of andesitic lavas enter into their composition. The lower 400 feet of the series are composed entirely of eruptive débris; above this point Archean and sedimentary débris are found in small but increasing proportion, and above 900 feet the material derived from the abrasion of Archean rocks is largely predominant, eruptive débris being still present in small amount, however, to the highest remaining part of the beds. The distribution of these varying constituents shows that the eruptive material must have come from the mountains to the west of the lake. The Archean material contains large bowlders, and the sand grains are angular. With the first appearance of Archean débris are found a lim- ited amount of pebbles, traceable to the Dakota conglomerate and Laramie sandstones. That the Denver beds were deposited in shallow waters is shown by the frequent cross bedding observable both in sandstone and conglomerate, and by the plant remains and standing tree stumps that abound at certain horizons. MON XXVII——3 34 GEOLOGY OF THE DENVER BASIN. About midway in the period, or after 500 to 600 feet of beds had been deposited, several successive flows of basaltic lava were poured out upon the sea-bottom and rapidly covered by deposits of sand and tuff. This eruptive action had no traceable connection with that which produced the andesites, but proceeded from fissures in the strata of the plains. Although most of the flows were poured out upon the surface of the sea-bottom, some small sheets were evidently intruded between the strata already deposited. The conclusion drawn by Mr. Cross from his most complete and thorough study is that in the long period during which the lower por- tion of these beds was being deposited the Archean and sedimentary rocks in the area from which they were derived were entirely covered by flows of andesitie lavas, but that toward the end of the period these lavas had been almost entirely worn away. Beds composed largely of coarse ande- sitic material, resting unconformably upon upturned Cretaceous rocks, are found in the Middle Park, on the western side of the Colorado Range, which, as well from their contained plant remains as from this similarity of constitution, are evidently of the same period with the Denver beds. Somewhat similar beds of andesitic débris are reported from the north- eastern portion of the South Park. It seems probable, therefore, that the andesitic flows must have covered the mountains lying between these two depressions, but singularly enough no remnants of these flows have yet been observed on them. It must be added, however, that as no system- atic survey has yet been made of this interior region it is by no means certain that some may not yet be found. The vertebrate remains found in both the Arapahoe and Denver beds are considered by the paleontologists to whom they have been referred to have Cretaceous rather than Tertiary affinities, and so high an authority as Prof. O. C. Marsh is decidedly of the opinion that, in spite of the evidence of the two physical breaks and the long time-interval that must have intervened, both Arapahoe and Denver formations are properly to be considered a part of the Laramie. It is to be remarked, however, that in the Denver Basin these vertebrate remains are not found in the coal-bearing rocks here classed as Laramie; neither is there as yet any certain evidence that this fauna existed prior to the Arapahoe and Denver periods. DENVER FORMATION. oD Abundant plant remains are found in the Laramie and Denver for- mations, which have hitherto been classed together as belonging to one continuous and uninterrupted series of beds. A careful revision of all the fossil plants collected from these and corresponding horizons has shown, however, that the floras of the Laramie and Denver periods were quite distinct. Of those collected in the Denver Basin (240 species in all) only 10 per cent are common to the two horizons. In the post-Laramie formations of Middle Park and Montana over 75 per cent of the plant remains are common with those of the Denver beds. These facts, taken together with the stratigraphical evidence in this and in other fields of a great time-interval and physical break intervening between the original Laramie and these later formations, while as yet there is no evidence of any important physical break or erosion period in the time intervening between the deposition of the Denver and of the sue- ceeding Eocene formations, seem to render it in the highest degree inad- visable to include these two later formations under the general head of Laramie, as has hitherto been done and as some paleontologists would still do. The post-Laramie formations, as they have been provisorily called, constitute a very important part of the geological column, which, up to the time these investigations were undertaken, had either been entirely over- looked by geologists or else confounded with underlying or overlying formations, as the case might be. Consisting, as they generally do, of soft, slightly compacted material, they have been readily eroded, and as their remnants are generally found in regions where the strata occupy a nearly horizontal position—that is, where the unconformities to be observed are those of erosion and not of angle of dip—they are not likely to be recog- nized as distinct from preceding or succeeding formations in ordinary reconnaissance work. Beds that occupy a corresponding position with these formations have been recognized stratigraphieally by the present observers at so many points on the periphery, as well as in the interior of the Rocky Mountain uplift in Colorado, as to indicate a general prevalence of similar conditions of sedimentation throughout the region in post- Laramie time. The fossil fauna of most of these exposures is, however, not yet known. On the other hand, at the several exposures from which 36 GEOLOGY OF THE DENVER BASIN. representatives of the Ceratops fauna have been described, the true posi- tion of the particular beds in which their remains occur has in no ease, outside of the present field, been so definitely determined stratigraphically as to permit of their exact correlation with other known horizons. _ From information furnished by Professor Marsh and others it appears that repre- sentatives of the Ceratops fauna have been recognized at other localities along the east front of the Rocky Mountain uplift from New Mexico to Canada, and in the great bay that once extended across the uplift westward to the base of the Wasatch Mountains. Wherever these remains have been systematically studied by the vertebrate paleontologist, his attention has been principally directed to the biological prob!ems involved, it having been assumed that the horizon occupied by them was sufficiently defined as Laramie, since it was higher than the Fox Hills Cretaceous, and the affini- ties of the fauna itself were regarded as Cretaceous rather than Tertiary. After a careful weighing of all the available evidence furnished by invertebrate and plant remains, as well as by vertebrates found in these localities, and of the somewhat meager data as to their relative stratigraphic position, Mr, Cross concludes that the Ceratops fauna has not as yet been described from any locality belonging beyond dispute to the true Laramie, as defined by King in the Fortieth Parallel Reports, while several of the known occurrences may be correlated more or less definitely with the Arapahoe or Denver formations. POST-DENVER MOVEMENT. After the deposition of the Denver beds the region was subjected to another orographic movement, whose dynamic effects are particularly noticeable in the steeper upturning of the Mesozoic strata along the foot- hills. At this time the Denver Lake was drained and the Denver beds were thereafter exposed to erosion. In the absence of any recognized represen- tatives of the beds that in other parts of the Rocky Mountain region, nota- bly along its western flanks in the Colorado and Green River basins, were most abundantly deposited during the latter half of the Eocene period, it is impossible to fix with any definiteness the time of this movement. It may have occurred at the close of the Eocene, and hence been contempo- POST-DENVER MOVEMENT. ill raneous with that recognized in the above regions as the post-Bridger move- ment,’ since the only beds definitely determined to be of Kocene age which have been found on the east flanks of the mountains, viz, those at Huerfano Park, were upturned by this movement; or it may, on the other hand, have been more nearly contemporaneous with those recognized in the Green River Basin, prior to or following the deposition of the Green River Eocene.’ It is evident that some dynamic movement took place during the Denver period in connection with the outflows of basalt which formed the Table Mountains, for the faulting of the Laramie and Fox Hills strata near Ral- ston Creek, opposite the main vent or fissure through which the basalt is supposed to have been extravasated, is evidently referable to the same movement which produced this fissure. The faults which fracture the coal measures, and in one ease the overlying Arapahoe beds, in the northern part of the Denver Basin area, may also have been determined by the shattering which accompanied. this voleanie eruption, especially if, as it is reasonable to assume, the eruption of the Valmont dike in this region was contemporaneous with that of the Table Mountain sheets. Whatever may have been the time in which the effects now recognized as caused by the post-Denver movement were produced, whether it was a single movement, or a succession of periodic movements, or an extremely slow and long-continued movement, its character was peculiar and typical of the foothill region in general, and will be specially considered under the head of “Structural geology.” The erosion which followed the Denver movement was most extensive, but here again it is difficult to differentiate that which properly belongs to the period intervening between the deposition of the Denver and of the next succeeding Monument Creek beds. In the center of the Denver Basin something over 1,000 feet of the Denver strata have been removed up to the present day. Under the edges of the Monument Creek beds, on the southeastern edge of the area mapped, about 600 to 700 feet of Denver beds probably remain, which, on the assumption that their original thick- 'R. C. Hills, Orographie and structural features of Rocky Mountain geology: Proc. Colorado Sci. Soc., Vol. III, p. 408. *Fortieth Parallel Reports, Vol. II, Descriptive Geology, pp. 203-204. 38 GEOLOGY OF THE DENVER BASIN. ness was 1,200 feet, would indicate a removal of about 500 feet in the intervening period. This amount is relatively small if the period is bounded by the Cretaceous on the one hand and the Miocene on the other, and thus affords a further, though confessedly not very strong, argument against assigning a Cretaceous age to the Denver and Arapahoe beds. MONUMENT CREEK FORMATION. This series of beds, of which only projecting tongues from the large area forming the divide between the Platte and Arkansas waters extend into the region mapped, consists in general of much coarser material than the Denver beds, but,is most readily distinguished by the absence of the andesitic débris which characterizes the latter. It has been less carefully studied, and no fossil remains have been found in the portions examined. It consists in general of conglomerates, sandstones, and arenaceous clays of variegated colors, made up mostly of Archean débris. Two divisions have been distinguished, marked by an apparent unconformity and period of ero- sion. The lower division is capped by flows of rhyolitie tuff, which forms the present protecting cap of many mesas, as do the basalts of the Denver beds of Table Mountain. The upper division contains, in addition to the Archean detritus, fragments of rhyolitic tuff and of other eruptive rocks. The assignment of a Miocene age to the beds of this series is based on the discovery, by earlier explorers, of vertebrate remains of this period at points which, while not so definitely located by them as to make it possible to trace the actual connection of the beds, appear to have been sufficiently near the area under consideration to leave little doubt that they must have come from the Monument Creek beds, and probably from the lower series. On the other hand, there are grounds of probability for assigning the upper division to the Pliocene period, though they were not considered sufficiently definite to justify the distinguishing of the upper series by a distinct color or name. These grounds are, first, the discovery by O. C. Marsh in 1871 of a Pliohippus fauna in the beds capping the Arkansas- Platte divide, south of the Smoky Hill River near the eastern boundary of the State; second, the fact that fossils which probably belong to the MONUMENT CREEK FORMATION, 3 | same horizon have been discovered at various points within the area of the Denver Basin, and, though not actually in place, in such positions as to indicate that they must have come from the disintegration of beds in the near vicinity. In addition to this, there is the analogy of the beds forming the divide between the North and South Platte rivers, along the Colorado-Wyoming boundary, where the Miocene! (Brontotherium) beds are overlain by Pliocene (Pliohippus). Although these beds differ somewhat from the Monument Creek beds in lithological composition, containing more argillaceous and calcareous material, this difference is readily explainable by the different character of the rocks composing the mountain masses to the westward, from the abrasion of which the sediments composing the respective series were formed. In Wyoming, Paleozoic and Mesozoic formations once arched entirely over the Archean nucleus of the mountains and protected it from erosion, whereas in Colorado this Archean nucleus was never entirely submerged, but has always been exposed to erosion. It is probable that when the plains region to the east of the Denver Basin shall have been systematically surveyed remnants of these beds will be discovered that will be sufficient to prove that both Miocene and Pliocene lakes were continuous across the Denver Basin northward, as was probably the case with those in which the Arapahoe and Denver beds were deposited. With regard to their southern extension, there is more uncertainty, as erosion in the Arkansas Basin seems to have been deeper and more extensive than in that of the South Platte. Professor Marsh is of opinion that the Miocene deposits show signs of thinning out to the southward. his idea is negatively confirmed by the fact that no Miocene beds have been found in Huerfano Park, where Eocene beds are directly overlain by what are supposed to be Pliocene strata. LATER MOVEMENTS. Of later orographic movements in this field, the record is too incom- plete and fragmentary to afford anything more than a general indication or suggestion. ' The assignment of a Miocene age to the Brontotherium beds is on the authority of Prof. O. C. Marsh. Prof. W. B. Scott and some other paleontologists class them as Oligocene. 40 GEOLOGY OF THE DENVER BASIN. That there has been a general differential uplift of the mountain or subsidence of the plains area—a continued action of the same forces which produced the upturning of the Mesozoic (including the Arapahoe and Denver) beds—is indicated by the observed upturning, at angles of from 15° to 20°, of the Monument Creek beds near the flanks of the mountains. Elsewhere, so far as observed, they do not depart from a practically hori- zontal position, and apparently have not been subjected to deformation resulting from a general orographic movement. Movements of elevation and subsidence, rather of an epeirogenic or continental nature, are indicated by both Tertiary and Pleistocene deposits that have a lacustrine origin, since the present inclination of the plains region, which shows an average descent in round numbers of 10 feet to the mile from the foothill region to the valleys of the Missouri and Mississippi, would not. admit of the holding of lake waters on its surface. It has already been suggested by earlier writers that the present con- ditions of the Tertiary deposits of the plains region indicate a differential tilting of this region which has produced a relative change of level of 5,000 feet or more between its eastern and western borders. The area of the present investigation has been too circumscribed to furnish much additional data on this subject. It can only be said that movements of this general nature have in all probability been several times repeated during Tertiary and Pleistocene times, but until the extent and character of these deposits shall have been carefully studied over the whole plains region and their relations to the underlying beds determined it will be impossible to trace with any approach to accuracy the nature and. history of these movements. PLEISTOCENE FORMATIONS. In the absence of any phenomena in the region that can definitely be assigned to Pliocene time, whatever has occurred since the deposition and elevation of the Monument Creek beds is provisorily assumed to be post- Pliocene or Pleistocene. In this period the present drainage areas of the plains took definite form. The Monument Creek beds were removed from a part, and possibly from nearly the whole, of the area mapped, and the present outlines of the Platte Basin were generally established. How much PLEISTOCENE FORMATIONS. 4] of this erosion may have been already accomplished prior to the Glacial period it is impossible to determine, but it is probable that the greater part of the erosion was due to the melting of the ice. The floods of that period carved out river channels more or less coincident with those of the present stream beds of the Platte, Clear Creek, ete, which were subsequently filled as the reduced slope of the streams diminished their corrading power, and when, in a later period of erosion, these streams cut their present beds, which are of relatively diminutive size as compared with those of the ancient streams, they varied somewhat in detail, and their courses, though generally conforming to ancient drainage lines, cut into the old river gravels to a depth of 30 to 50 feet. Between these two periods of erosion there was deposited over the whole area of the Denver Basin, below a level of 5,800 to 6,000 feet, a fine silt or loess, which apparently extended out over the plains of Kansas and Nebraska, and which gives to this region its remarkable fertility wherever it is susceptible of irrigation. In its physical structure and composition this loess is very similar to that of the Mississippi and Missouri valleys, the principal difference being its greater coarseness of grain. With the exception of the alluvium of the river valleys and of modern sand dunes, it is the most recent deposit of the region, and hence, being unconsolidated and readily disintegrable, it has been so very largely removed by modern erosion that its original extent is not easily determined. Its thickness in certain portions reaches 200 to 300 feet, but in the valleys is generally under 50 feet. The origin of this peculiar and economically important deposit in different parts of the world is one of the problems in geology that has been found most difficult of satisfactory determination. The great loess deposits of China, the most important in the geologically known world, are now conceded by all geologists who have examined them to be of subaerial origin. On the other hand, the geologists who have made the most recent and detailed studies of the loess of the Mississippi and Missouri valleys consider it to have been formed by the fine silt produced by the grinding of the great northern ice-sheet. The great difficulty involved in the latter theory is to account for the existence of a body of comparatively tranquil water in which such finely divided material could have been deposited. 42 GEOLOGY OF THE DENVER BASIN. In the loess of the Denver Basin the evidence shows that the lower part at least must have been of subaqueous origin. It seems necessary to suppose that, after the rapid erosion which carved out the ancient river beds, by some tilting of the plains region its general slope was so reduced o that the abundant water, resulting from the melting of the ice in the moun- tains, gradually backed up in a temporary lake which existed long enough for the settling to its bottom of this fine silt, which was readily carried a long distance from its shore-line, and that subsequent tilting in a reversed sense produced the present slope of the plains and the conditions of modern drainage. It is quite possible that after the waters had entirely drained away there would have been, under favoring climatic and meteorologic conditions, a certain rearrangement of and adding to this material, which would thus have been of subaerial origin, similar to that forming at the present day in China. Before this subject can be satisfactorily treated, however, it is neces- sary to learn more than is at present known of the extent of this loessial material. STRUCTURAL GEOLOGY. The tectonic or structural geology of an area like the present, though at first glance apparently very simple, involves some of the radical prob- lems of geotectonics or mountain building, and it is therefore important to note all the details of its structure and endeavor to draw some conclusions as to the manner in which the present structure was produced. It is not proposed, however, to enter into any discussion of first causes—that is, whether the forces which produced the observed deformations are the result of the contraction of a cooling crust on a molten globe, of overloading of areas near old shore-lines, or of any of the various hypothetical causes which geologists and physicists have proposed to account for the observed facts of the geological structure of our globe. Geologists are as yet far from being in accord on these theoretical points, but those who have had the most extensive field experience are agreed that the observed effects are most readily accounted for by the action of intense forces of compression of the outer crust of the globe acting generally in certain determined direc- tions, whether these forces are the result of contraction or of any other cause. STRUCTURAL GEOLOGY. 43 In the Denver Basin the beds underlying the plains area, except for some slight fracturing in the developed coal beds of the northwestern por- tion of the area, are practically in the position in which they were origin- ally deposited. They have been lifted above sea-level and exposed to erosion at various times, and there is possibly a slight upward curve of the Mesozoic beds on the eastern edge of the basin sufficient to produce a basin structure, but of deformation or pronounced flexing of the beds there is, so far as observed, a notable absence. In the foothill region, on the other hand, there is pronounced defor- mation, manifested in both folding and faulting, but it has a character of its own, distinct from the intense plication found in highly disturbed mountain regions, though partaking of some of its characteristics. It occupies struc- turally, as it does geographically, an intermediate position between the relatively undisturbed areas of the plains and the intensely disturbed and plicated rocks of the mountain region. The Archean areas of the present region, as has already been stated, have not been specially studied; hence nothing need be said of their struc- ture except that they were already so intensely deformed prior to the deposition of the Mesozoic beds that it is highly improbable that any study of their present structure would enable one to trace the effects of the later movements which have affected the more recent beds, though these movye- ments undoubtedly must have been felt within the crystalline complex. PLAINS AREA. The departure from a horizontal position of the strata on the plains is usually so slight as not to be susceptible of instrumental measurement, so that it must be measured by the relative position of outcrops of the succes- sive beds as platted on an accurate profile. Such profiles, given on the structure sheet (PI. IV, in pocket), which are limited in extent to the width of the map, also show no very perceptible rise of the strata to the eastward, and from these one would judge that the basin in which the Arapahoe and Denver beds was deposited was a basin of erosion. In crossing the plains still farther east, though the surface rises slightly, one crosses successively lower outcrops of the Laramie strata as one goes eastward; hence there is apparently a slight rise or arching up of the strata toward the present 44 GEOLOGY OF THE DENVER BASIN. surface, which, combined with the effects of the erosion of the Platte and its predecessors, has produced the eastern rim of the Denver Basin. The amount of arching is not readily determinable, since it does not come under direct observation. It would be greater or less according to whether, on the one hand, at this distance from the original shore-line there is already a considerable thinning of the strata or, on the other hand, the arching is within the limits of what students of orographical geology have called the syncline of deposition,’ which, as they assume, is produced by the extra weight brought upon the sea-bottom by the greater thickness of beds deposited along the shore-platform of a continent or continental island. In the present case, if there had been a syncline of deposition whose eastern limb had a perceptible inclination to the west the subsequent movements of compression would have produced an anticlinal fold of vis- ible amount of arching along the limb, which is evidently not the case. On the other hand, it is to be remarked that, though the present surface rises slightly from the Platte Valley eastward, if one takes into consideration a longer distance eastward—say from the point nearest the foothills where the strata assume a horizontal position to the eastern boundary of Colorado—there is a general slope of the surface eastward, which may be sufficient to account for the appearance of successively lower beds of the horizontal formations, without having recourse to any supposed arching of the strata or of the sea-bottom to produce a syncline of deposition. The actual proof of the one fact or the other could under these conditions be obtained only by the accurate determination of the relative position of the bottom line of some formation or of an easily recognizable bed within a formation at a number of different points. Such a determination can hardly be hoped for under existing conditions. It may be assumed, then, as most probable that the existence of the Denver Basin is due rather to erosion than to curving of the strata into a low arch within a hundred miles from the mountains. It is probable that the long-shore currents with a general northern direction, which, as has already been shown, are proved to have existed in the Dakota Ocean as well as in the Arapahoe and Denver lakes, may have produced a series of 1 Conditions of Appalachian faulting, B. Willis and C. W. Hayes: Am. Jour. Sci. (3), Vol. XLVI, Oct., 1893, p. 257. PLAINS STRUCTURE. 45 low ridges or sand bars on the ocean or lake bottoms, more or less parallel to and at some little distance from the shore-line. The waters pouring out from the mountains at various periods of erosion would have come in greatest volume from the present general upper drainage system of the Platte River; these ridges would have tended to give an initial northern direction to their course, which, once determined, would have influenced all drainage courses of subsequent erosion systems. Thus, as is shown in a general way on the profiles, the hollowing out of the bottoms of the Denver and Arapahoe lakes to form the Denver Basin would have had at first a northerly direction, and would bend more and more rapidly eastward with the general slope of the country as its distance from the ancient shore-line increased. FOOTHILL STRUCTURE. _ At first glance the geological structure of the foothill regions seems extremely simple, being merely the upturning of the ends of the sedi- mentary beds along the mountain flanks where they were most nearly in contact with the erystalline core of the range. The first explanation to suggest itself for such upturning is evidently a vertical upward movement of that core, which carried up with it the beds immediately resting upon it. It seems hardly necessary to mention the theory maintained by early explorers in the region, when the study of mountain structure was in its infancy, that this upturning was merely the remaining limb of a great anticline whose crest had been eroded off and planed down, and that the whole series of upturned strata once arched entirely over the mountain crest. This theory has long been disproved by a demonstration of the rarity in nature of such conditions, once held to be typical of mountain ranges, and of the impossibility of explaining by it the actual phenomena in this field.’ The idea that mountain elevation was produced by a vertical upthrust or force acting directly upward under the center of the range was one of the primitive theories held when the field of geological observation was very limited and theories were based more on meditation in the office than on actual exploration in the field. As field study advanced it was entirely 'Mon. U. 8. Geol. Survey, Vol. XII, p. 20. 46 GEOLOGY OF THE DENVER BASIN. abandoned and was replaced by the theory of tangential compression, under which the more or less yielding interior of the mountain was, so to speak, squeezed up, thus producing in effect something analogous to a vertical upthrust, but as the result of a horizontally rather than a vertically acting force. Recourse has again been had by a few writers’ to a vertically acting force for the explanation of mountain uplift, but they have found few followers among the actual working geologists. In the present case the objection to the theory that the upward curve of the sedimentary strata adjoming the mountain flanks was produced by the rising of the mountains, dragging up the ends of the strata with them, is readily apparent on an examination of the existing conditions shown by the accompanying geological map and sections. In the first place, the movement of displacement on the various longitudinal or strike faults along the foothills is just the reverse of what it would be had the movement been thus produced. ‘The beds on the side of the fault planes nearest the mountains would then have been dragged up, relatively to those on the other side, whereas in point of fact it is the beds on the opposite side of the fault plane, or farthest away from the mountain mass, that have suffered upthrow. : Again, were the upward curve of the beds produced by the upward movement of the crystalline rocks upon which they rest, those nearest these rocks would have been more steeply upturned than those farther away; or in other words, in a series of beds thus upturned whose edges were subsequently planed down, the resulting outcrops would show decreasing angles of dip as one proceeded eastward from the Archean exposure toward the plains, whereas in point of fact the reverse is the case. This is shown diagrammatically in the following sketch (fig. 1), in which diagram @ shows the varying dips that would be found in a series of sedimentary beds that had been upturned by a vertical upward movement of the Archean shore-line upon which they had rested. Diagram 0}, on the other hand, represents the actual conditions of dip in the beds of the foothill region, in which the angle of dip becomes steeper as distance from the shore-line increases, and which are more readily explainable as a result of tangential pressure ! Sixth Ann. Rept. U. S. Geol. Survey, 1886, p. 197. FOOTHILL STRUCTURE. 47 The beds were probably deposited on a shelving shore; that is, the slope of the contact of sedimentary beds with the underlying complex of crystalline rocks was not vertical, but decreased in angle with the distance from the mountains. Hence a vertical upthrust of this complex could not produce the abrupt transition from the vertical to the horizontal position in the overlying beds that is shown near D in diagram b. Tangential pressure, or a force of compression acting in a nearly horizontal direction, seems to afford a more reasonable explanation of the observed phenomena of deformation in the foothill region, and accounts readily for most of them, though a certain amount of differential vertical movement seems to be required for certain phases. Fic. 1.—Upturning of strata along base of range. The observed phenomena to be accounted for may be briefly enumer- ated as follows: r. Eastward dip of the beds away from the mountains— This dip increases in steepness as one ascends in geological horizon, or proceeds eastward across the strike from the foothills toward the plains, from 25° to 30°, as a rule, in the lowest or Wyoming beds to 45° in the intermediate series, increasing rapidly to a vertical or even beyond in the Laramie strata, with which the overlying Arapahoe and Denver beds, where not eroded away, are found to be involved; then changing in a few hundred yards of hori. zontal distance to a practically horizontal position. The dip of the upturned beds, if projected upward into the air, would produce a sort of partial fan structure. (See diagram b, fig. 1.) The width of outcrop, which nearly corresponds to the distance between the Archean contact and the vertical dip, varies with the thickness of the beds from 4 miles on Turkey Creek, near Morrison, to about 1 mile at Golden. At the latter point the vertical Laramie beds approach very close to the Archean 48 GEOLOGY OF THE DENVER BASIN, foothills, and here it is easy to conceive of a horizontal shove of the strata in a westward direction against the unyielding buttress of Archean rocks, which, a slight initial upward curve of the beds being presupposed, would bend the beds in immediate proximity to the buttress into a vertical posi- tion. The condition of things on the Turkey Creek or Morrison line, on the other hand, is less readily conceived as the result of a simple horizontal shove, for between the vertical Laramie sandstones and the Archean but- tress is the great thickness of soft clays of the Middle Cretaceous, under which come the hard, unyielding Dakota sandstones at an angle of 45°, with other sandstones and clays below them at a still lower angle, and it would seem that the horizontal shove would induce a sliding upward Via, 2.—Restoration of overthrust fault near Boulder Peak. of the upper beds over the more gently inclined lower series, instead of bending the former almost at a right angle, as they appear to have been bent, unless there were some force to hold them down. If one examines the curve of the beds at the bend, on the sheet of cross-sections (see Pl. TV), which have been most carefully constructed from measured dips at many points and represent the most probable form of the curve of the strata underground that can be deduced from facts that actually come under observation, it will be seen that the most probable direction of a foree which would make such a curve would be one acting at an angle of 45° with the horizon, or in the general direction of the arrow in diagram 0}, fig. 1. Such a force might be considered as the resultant OVERTHRUST FAULTS. 49 of a horizontal force pushing westward and a vertical force directed vertically downward. The vertical force may be conceived to be the load of superin- cumbent strata; and if, as seems probable, there is a tendency of the plains area to sink, under the load of accumulated sediments, with relation to the adjoining unloaded mountain mass of Archean rocks, the relative effect would be the same asif the latter had been subjected to a vertical upthrust. 2. Overthrust faults— The second series of phenomena to be considered are the longitudinal or strike faults in the Wyoming beds, by which fragments of the-latter are left among the Archean rocks or a piece of the Archean basement is pushed up among the red sandstones. The former phenomenon is seen south of South Boulder Creek and at several points along the foothills to the north of the area mapped; the latter, at South Boulder Peak and in the hogback valley near Deer Creek. One of the latter faults is represented in section on fig. 7, p. 116, where it is seen that the fault plane dips to the east and would, therefore, at first elance, appear to be a normal fault in which the downthrow should be to the east; but that it is in reality an overthrust fault may be seen in fig. 2, p. 48, in which the part of the section adjoining the fault is turned so as to bring the sedimentary beds to a horizontal position. In this dia- gram the line A B represents the present surface as carved by erosion. A F G IT represents the ancient shore-line or contact of sedimentary beds with underlying Archean, broken, however, by the fault G G’, whose movement has carried the point G to G’. In addition to the thrust movement, there has evidently been some upward movement in the beds adjoining the Archean which brought about their steeply upturned position, and this may be accounted for on the ground that as this locality was so much nearer the shore-line than the points where the sharp bend from the vertical to the horizontal position of the bed oceurs there would have been a lesser load of superineumbent strata, either through nondeposition or by reason of subsequent removal by erosion; hence the element of the compressive force that tended to act vertically downward would have been smaller and the resultant force more nearly horizontal, and thus have admitted of a certain amount of pushing up along the shelving shore. MON XXVII——4 5O GEOLOGY OF THE DENVER BASIN. 3. Transverse folds —Next to be considered are the arches of the sea-bottom at Golden and Boulder, whose axes, as has already been stated, must have been more or less at right angles with the prevailing line of strike. These arches were formed previous to the main or post-Laramie movement, and must be conceived to be the result of a longitudinal compression. The writer's study of mountain uplift has shown him that in most every great mountain uplift there are evidences of two forces of compression acting more or less at mght angles to each other, as here, and producing a major and a minor series of folds. In the Archean area immediately adjoining the present field, indeed, the structural lines show evidence of considerable longitudinal compression. Hence the force which produced these arches is not difficult to conceive of, but it is less easy to understand why the arch should have occurred exactly where it did and not at other points. Once initiated, it is quite comprehensible that it should ever afterward be a point of weakness, or of least resistance to the compressing forces, and thus the succeeding movements are readily conceivable. That it continued to be a line of weakness after longitudinal compression had ceased is also evidenced by the fact that it was at this locality in Denver time that the basaltic eruptions were forced up to the surface. It might be argued that the extravasation of such a considerable mass of eruptive material from beneath the surface would account, in part at least, for the sinking of the upper part of the sedimentary series at this point (which is predicated by Mr. Eldridge’s diagram, p. 99, in order to account for the present relative position of their outcrops), and that this would obviate the difficulty conceivable in the ironing out or flattening of the arch previous to the upturning of the beds along the flanks of the range. This would, however, necessitate the assumption that the greater part of this upturning occurred since the deposition ot the Denver beds. If it were also assumed, as has been already suggested as possible, that the upper part of the sedimentary series above the Middle Cretaceous clays had been pushed in toward the foothills, prior to this upturning, by an overthrust movement over the arch, there would have been less of an arch to be planed down and the void left by the extravasation of the basalt might have been quite sufficient to have produced the flattening of the arch. SGO5D 3SHL JO N3GuVS 'AYSN30S 038,03y WOldAL Co ALP en bE. By GrorGce H. ELDRIDGE. MESOZOIC GEOLOGY. SECTION 1.—THE FORMATIONS. TRIAS. WYOMING FORMATION. STRATIGRAPHY, Within the Denver Basin, lying at high angle of dip along the base of the Archean slopes and often extending to a considerable height above them, is a prominent series of brilliant-red conglomerates, sandstones, and shales, with thin limestones and gypsums in the upper part. These are the well-known Red Beds of the Rocky Mountain region. They are commonly referred to the Trias. The term “Wyoming” is here adopted as their formation name. In the Denver field they rest directly upon the ancient erystalline rocks, although in many other localities there intervene thou- sands of feet of Paleozoic measures, Permian (?), Carboniferous, Silurian, and Cambrian. The thickness of the series exposed in the Denver Basin varies between 500 and 3,000 feet, but is generally somewhat under 1,500. The varia- tion is chiefly due to the unevenness of the ancient floor upon which the formation was laid down, the Archean having been thrown into folds and having suffered extensive denudation before the deposition of the younger formations upon it. In the southern half of the area mapped the topography of the Red Beds is that of a longitudinal valley between an Archean mountain slope 51 5? GEOLOGY OF THE DENVER BASIN. on the west and a ridge of Dakota quartzite or sandstone on the east. Rising from the valley bottom are lofty, cathedral-like spires of vertical or highly inclined strata, brilliant in coloring, and producing by contrast with the green of vegetation effects most picturesque. In the northern portion of the field these effects still prevail, but the lower half of the Red Beds rises high upon the mountain slopes, forming prominent peaks along the range-tront. The formation is separable about midway into a lower division, of soft, friable conglomerates and coarse sandstones, with few shales, and an upper one of shales, with some prominent sandstone bands, narrow beds of lime- stone, and small local deposits of gypsum. The following section, taken at Morrison, is typical, reading from top downward: Section at Morrison, Colo. UPPER DIVISION. Sandstone, fine-grained, often massive, pink and brown; pay DETSISC@N tes eos a oe 15 to 25 Clays, bright-colored—gray, yellow, green, pink, and lilae; gypsiferous and calcareous, especially 40 feet belowitheit sumMiG eee ee eee 125 to 175 Clays, more arenaceous than above; transitional in color, from grays, ete., above, to prevailing brick- LOGS DelOW < 2-256 ees ene I ee eee 150 to 200 Sandstone and shale, alternating; brick-red to pink; white dots; sandstones prominent .........-------- 50 Sandstones andeshale@saseam 4-6 see eee eee 60 Shales, sandy and argillaceous, brick-red, carrying nar- row bands (3 to 6 feet) of white crystalline limestone. 75 LOWER DIVISION. “Oreamy” sandstone; quartzose; conglomeritic at base; two sandy limestone bands in lower part; round fer- ruginous concretions near top; forms prominent out- crop in valley between Archean and Dakota (average 250 feet) << a. sare eee aa ee oe eee eee 200 to 400 Red Beds; conglomerates, sandstones, and shales, the last of minimum development; color, red; outcrops, lofty spires and pinnacles and towering masses of irregular: Shape’: --s-c0- 2 sass ia ee 270 to 2, 000 WYOMING FORMATION. 53 The lower division—A]though the contact between the Archean and the Trias occurs at a constantly varying horizon in both formations, the lower 5 to 20 feet of the Red Beds is nearly everywhere composed of coarse, subangular fragments of the adjacent granite, gneiss, or schists, with a small admixture of their derived sand, which shades in places to a red arenaceous mud. The materials are usually but loosely agglomerated, yet instances are frequent where the finer and less-worn débris is com- pacted to a rock so hard that, with the same mineralogical constitution, it closely resembles the underlying granite, their dividing line being very difficult to determine. Cross-bedding is developed at the base of this layer, while in its upper portion evidences of eddying shore currents exist in heavy deposits of pebbles in deep-worn depressions in the beds first laid down. Succeeding this layer is a series of heavily bedded sandstones and grits, with small local bodies of arenaceous shale. The normal thickness of this series is about 1,200 feet. The color varies from prevailing red to eray, according as the chief constituent of the rock is red feldspar or quartz. A very fine-grained, deep-red, shaly sediment, approaching mud in consistency, contains also a considerable per cent of black and white mica. Iron oxide is generally present and assists in the coloring, particu- larly in the more shaly varieties. The lower 200 feet of sediments are generally coarser and less compact than those overlying. Cross-bedding is a marked feature from base to summit. The foregoing beds pass by a broad transitional zone of lighter-red and more quartzose sandstones to the upper member of the division, a heavy bed of cream-white sandstones from 200 to 400 feet thick. From its color this sandstone has become known as the ‘‘Creamy sandstone.” It usually forms a+ well-marked ridge from 50 to 100 feet high along the middle of the valley of Triassic rocks. In the lower half it is somewhat more friable, and consequently more frequently eroded, than in the upper. It is also occasionally tinged a faint red, in irregular patches. Two small bands of dark-brown, quartzose limestone, from 2 to 8 feet thick, are usually present—one near the base, the other 40 feet up. The intervening 4 GEOLOGY OF THE DENVER BASIN. 1] sandstone is heavily cross-bedded, and in some layers conglomeritic, peb- bles and matrix consisting chiefly of quartz, with occasional admixtures of other débris from the erystalline rocks, and some brown arenaceous or cherty limestone. The upper half of the sandstone is also conglomeritic at the base, but becomes fine-grained above. It consists of quartz of great purity, and affords nearly the entire amount of silica used in the pottery and fire-brick establishments at Golden. Twenty feet from the summit are numerous small, brown or reddish nodules of cemented sand one-eighth to one inch or more in diameter, many of which first weather in relief and then roll out in balls; some of the larger, on being broken, are found hollow. The upper 6 feet of the Creamy sandstone locally becomes conglomeritie and calcareous, easily disintegrating and leaving a surface strewn with pebbles, all more or less angular. In this layer small geodes, lined with calcite crystals, also occur. The Creamy sandstone as a whole is remarkably uniform in texture and appearance. In the region of the South Boulder Peaks, however, where disturbed by faults or folds, the bed becomes a tolerably hard quartzite, fractured and slickensided, but its leading features are still maintained and its identity is easily established. The upper division—The lower half of this division consists of bright, brick-red, arenaceous shales and sandstones, with important intercalations of limestone. The limestones occur within 75 feet of the base, usually three or four beds from 6 to 18 inches thick in the lower 15 feet, and 50 feet higher up a bed 5 feet thick, a red, sandy shale intervening. ‘The upper bed is overlain by a succession of thin, wafer-like layers of white limestone and red mud, in all 5 or 10 feet; these present in cross-section a wavy structure, with sharp contrast of color and texture, the surtace weathering in delicate corrugations. Close examination occasionally reveals this structure in the limestone itself, the clay bands, however, being absent. Chert concretions, of purple color, which weather in con- centric circles and ultimately develop circular holes in the backs of the layers, are present, as are also vugs filled with calcite crystals. Minute grains of an undetermined black mineral also occur quite generally at this horizon. WYOMING FORMATION. 55D Following is an analysis of the upper limestone: Analysis (by L. G. Bakins) of Upper Wyoming limestone from Morrison, Colo, TMOG Or aoa Mon, U.S. Geol. Survey No. XIV, pp. 8-15, 1888. * Am. Philos. Soc., Proc., Vol. XXIV, p. 227, 1887. MORRISON FORMATION, 61 The marls are green, drab, or gray, and carry in the lower two-thirds numerous lenticular bodies of limestone of a characteristic drab color and a texture compact and even throughout. A small but persistent band of sand- stone and limestone in thin alternating layers occurs about 20 feet above the base; in some places the arenaceous elements largely predominate, and near Mount Vernon, 3 miles north of Morrison, and in the vicinity of Van Bibber Creek, there are at about this horizon from 10 to 15 feet of dull-gray or yel« lowish sandstones carrying small pebbles of flint of variouscolors. The clays of the lower two-thirds of the Jura are remarkable for their reptilian remains, and from the predominating form have been designated “Atlantosaurus clays.” The upper third of the Jura is generally a succession of sandstones and marls, of which the former predominate; locally, however, either may prevail to almost the entire exclusion of the other. The most important sandstone occurs just above the Atlantosaurus clays, is very persistent, and from contained Saurian remains has been called the Saurian sandstone. It varies in thickness between 5 and 35 feet, and in its distance below the Dakota from 10 to 125 feet, although more generally from 50 to 80 feet. The chief constituent is quartz. The sandstone is everywhere marked with small rusty dots sharply defined and round, one-sixteenth to one-fourth inch in diameter, the result of spherical stains of brown oxide of iron; oceasion- ally the appearance is one of irregular mottling. The sandstone is locally divided into several layers by narrow intercalations of drab clay. In the vicinity of Turkey Creek these clays reach the unusual thickness of 20 to 30 feet, the sandstones aggregating about 20 feet. The bed may be locally calcareous, especially in the northern half of the field, the lime being uniformly distributed throughout the mass. At the base is generally a conglomerate, of a maximum thickness of 8 feet, in which the pebbles so closely resemble those of the Dakota that, but for a slight admixture of red jasper and the characteristic brown dots, the two layers could with difficulty be distinguished from each other. The shales overlying this sandstone are similar to those comprising the bulk of the Jura, but carry through them a number of minor sandstones and occasionally one or two strata of limestone. A yariation in the Saurian sandstone occurs in the vicinity of Van Bibber Creek, which, on account of its determinative value in connection with the structural features about Golden is of considerable importance. 62 GEOLOGY OF THE DENVER BASIN. The bed is essentially a sandstone, but is divided into minor layers by bands of hard, white clay from 14 to 2 feet thick. Occasionally the clay also is specked with rust spots, and upon becoming coarser-grained is directly identifiable with the mottled or specked sandstone described above. It is also sometimes conglomeritic. The cause of the variation in the thickness of the upper half of the Jurassic could not be determined from conditions existing in the Denver field, but an oft-suggested unconformity at the base of the Dakota may be the explanation. CRETACEOUS. Wherever along the foothills of the range the Mesozoic beds have been upturned at a comparatively high angle of inclination, the different Cretaceous formations may be recognized by their topographic features and surface relations. The Dakota sandstone, dipping east, forms a promi- nent line of ‘“‘hogbacks”—sharp, serrated, monoclinal ridges, that extend with occasional interruptions along almost the whole front of the range. The black, slaty shales of the Benton occupy in general a shallow longi- * t=) tudinal depression, from 400 to 1,000 feet wide, between the Dakota terrane, and the white basal limestone of the Niobrara, which forms a second ridge, much lower than the ‘“hogbacks,” but still conspicuous. Kast of the Nio- s ’ | o a broad flat brara are the clays of the Pierre and Fox Hills, underlying belt 1 to 2 miles wide, sueceeded by the basal sandstones of the Laramie, which outcrop in low, somewhat irregular combs. Between the basal sandstones of the Laramie and those of the Arapahoe, next overlying, is another flat, from 600 to 1,200 feet wide, occupied by the clays of the older formation. The basal sandstones and conglomerates of the Arapahoe form local crests 10 to 20 feet above the adjoining prairie, east of which the strata gradually assume a gentle easterly dip and the surface features are chiefly those of erosion in approximately horizontal strata. DAKOTA FORMATION. STRATIGRAPHY. This formation is from 225 to 850 feet thick, and usually consists of two or three nearly equal benches of massive sandstone separated by narrow bands of shale which locally become fire-clays. A characteristic conglomerate occurs at the base of the formation; at the summit, a zone DAKOTA FORMATION. 63 of hard, white, slaty shales, 10 to 30 feet thick, transitional to the Benton; and a fossil flora is found throughout. Sandstones— ‘hese are composed chiefly of quartz, but contain a trace of mica and a small amount of iron which stains their normal white a brown, yellow, or red. Bitumens are locally present, which also impart a brown color. Cross-bedding and ripple-marks are common features. The sandstone is harder and more compact than any other Mesozoic or Tertiary rock of the field, and now and then verges upon quartzite, espe- cially in the upper bench. It can hardly have been stiibmitted to greater pressure or heat than the more loosely agglomerated Triassic sandstones, and its hardness may therefore be attributed to a more than usually large amount of silica in solution in the sea waters in which it was laid down. The conglomerate at the base of the formation is especially compact through silicification, fracturing across pebbles and matrix alike. A pecul- iar feature of certain layers of the sandstones is the agglomeration of the grains into forms which resemble short pieces of spaghetti from a half inch to 2 inches in length, with rounded ends, and woven in and out in the most irregular manner. ‘To what these forms are due it is impossible to say, but they suggest the casts of worm burrows. Conglomerates—T'he conglomerate at the base of the Dakota is char- acteristic. It varies from a thin, almost imperceptible layer to one 30 feet in thickness, and is composed of well-rounded, smooth, in some cases almost glazed, pebbles from the size of a pea to a diameter of 1 inch. The pebbles are derived from most of the older formations down to the Archean, including some of which no trace has yet been discovered in this field. Among the latter are a few of Silurian age, identified by contained fossils, and others carrying small corals of undetermined affinities; in others silicification is so advanced that their original composition is too much obscured to permit determination of their geological source. The pebbles comprise abundant limestones (some closely resembling those of the Trias), quartzites, clays, flints, jaspers, and rocks of granitic composition, together with the separate mineral constituents of the last. Besides the basal conglomerate, a thin sheet containing very minute pebbles is sometimes found at the base of the upper bench of sandstone, and a third near the summit of the formation. 64 GEOLOGY OF THE DENVER BASIN. Fire-clay—This occurs in local development in the shales separating the heavy sandstone benches of the Dakota. There are generally two horizons of the shales, and consequently of the fire-clays, one about mid- way in the formation and another nearer the summit. The fire-clay may occupy the entire space between the sandstones, usually from 2 to 8 feet, or may be interrupted by intercalations of hard, white, or bluish-white, quartzose shale. The typical fire-clay is blue or blue-gray, of fine, even texture, hard and compact, jointed or coneretionary, and very pure. It becomes impure through the presence of oxide of iron, by a varying amount of sand in fine grains, disseminated or in thin layers, or by carbonaceous matter. The iron weathers out in the form of minute brown spots, distine- tive of the horizon. ‘ LIFE. Animal—No trace of animal lite has been discovered in the Dakota of the Denver Basin. Plant —Plant remains, chiefly leaves of deciduous trees and enormous fucoids, abound in certain localities from base to summit of the Dakota. Wood tissue in minute fragments, or the impressions of the same, or the resulting stains, are of general occurrence. In a comparison of the Dakota of the Denver field, which may be regarded as typical for the eastern base of the Rocky Mountains, with that in the far distant and widely separated regions of Dakota, Nebraska, and Kansas, the similarity of the beds in composition, manner of occurrence, and flora is remarkable, the thickness of the formation alone being the only point of material difference. COLORADO GROUP. The two members of this group, the Benton and Niobrara, are broadly distinguishable from each other both in their sedimentation and in their fossils, but from the existence between the two of a zone of gradual transition in sediment, and from the common occurrence of many of their more abundant fossil forms, especially within the transitional zone itself, a definite line of demarcation can not be drawn. This relationship is in no manner a local one, but prevails in a greater or less degree wherever the two formations occur; it obtains even in the Montana and Dakota sections, BENTON FORMATION, 65 where the earliest work in the Cretaceous rocks indicated a much more distinct line of demarcation than that now recognized. BENTON. STRATIGRAPHY. This is essentially a formation of black argillaceous shales passing by transitional beds into the formations above and below. Its base lies within a zone of 15 or 20 feet, in which the unstratified black and white indurated shales that form the upper limits of the Dakota are succeeded by the black clays constituting the great mass of the Benton. The summit is in the vicinity of the persistent band of light-colored limestone which occurs near the base of the Niobrara and constitutes a secondary ridge east of the Dakota. The area underlain by the Benton is a narrow strip along the eastern base of the Dakota hogbacks, in width varying conjointly with the thickness of the formation and the angle of dip, but nowhere over 1,000 feet. The thickness of the Benton at Platte Canyon, its point of greatest development, is a little over 600 feet; at Deer Creek, about 590; Turkey Creek, 500; Morrison, 580; one mile north of Morrison, 400; at Ralston Creek, 430; at Bear Canyon, 34 miles south of the town of Boulder, 348; and at Fourmile Canyon, at the northern edge of the field, about 500. At Golden, under abnormal structural conditions, it disappears entirely. The lithological characteristics of the Benton formation are: a shaly nature; a dark leaden, almost black, hue; concretionary clay-ironstones; and thin layers of fossiliferous limestone. shates.—The shales are of fine clay, with a small amount of disseminated arenaceous matter. They are either jointed, concretionary, or broadly homogeneous in structure. They are compact and hard, but under atmos- pheric influence readily crumble to angular particles or thin scales. Iron pyrites occur disseminated throughout them in minute crystals or concen- trated in thin and often finely reticulated seams. Gypsum and native sulphur are also found in limited amount, the latter usually crystallized in small cavities. Ironstones— The ironstones occur in a zone 40 to 50 feet thick, about two-fifths the distance from base to summit of the formation. They are in MON XXVII 5 66 GEOLOGY OF THE DENVER BASIN. the form of concretions, from 1 to 3 feet in diameter, are very hard, break in angular fragments, and on fresh surfaces resemble in color the shales, though their exterior weathers either bright-yellow or rusty-brown. They are usually somewhat calcareous. A partial analysis of an ironstone follows: * Analysis of tronstone. Per cent. SUC a eee ccc nttts sore oS 2 Siete ws olojeo aieia = Se a Uiepslawisromiote ae isiee Ob tee ore Saye sme ermertee 26.31 Tron: (metallic).z2... 3. 0.82 224 Joc e va we dee ocaen deere coco eae Soe ee ees 22.90 ATTMIN Gas Soc cok cists cicsnnsioe wine SERRE ORE RE CRS eee eae 2.31 WANG sw si starnkie ac Ses soe ce osc came oe ost cone eeieicue WRC ER Cars Oeee ce ee ieee 3, 22 Magnesia. <2. siciccseclic cues caciisie's bles enivcac’e Came meicictens see paeeee Once seer 8.19 Limestones—'These occur in more or less noncontinuous bands, 1 to 3 feet thick, throughout the formation, but they are more numerous, thicker, and of greater continuity in the upper third. One only is persistent over the entire field, this lying about 100 feet below the summit. The lime- stones are of coarse texture, dark color, and generally yield a bituminous odor. The uppermost beds resemble the basal member of the Niobrara and thus constitute a zone of transition between the two formations. LIFE. The life of the Benton seems to have been rather poorly represented in this field as compared with other localities, for while there are evidences of organisms in profusion, both of plants and animals, the only forms especially abundant are those of the two genera of Mollusca, Ostrea and Inoceramus. To these must be added some undetermined Selachians, represented by their teeth, which occur mainly in the more strongly bituminous limestones. NIOBRARA. STRATIGRAPHY. The surface area occupied by the Niobrara formation has an average width of about 600 feet, except for a distance of about 3 miles north and south of Golden, where, like the Benton, it has entirely disappeared. The normal thickness of the Niobrara varies slightly on either side of 400 feet. In sedimentation the formation is sufficiently differentiated to ‘Report of the Colorado State School of Mines, Golden, Colo., p. 20, 1885. NIOBRARA FORMATION. 67 readily permit its division into three members, of which the lowest is lime- stone, having an average thickness of 50 feet; the middle is a succession of gray marly clays of various shades; the upper, a series of yellow or buff shales, 240 feet in thickness, containing several impure limestone bands. Lower division—The limestone of the lower division, from its prominent characteristics, forms an excellent datum-level in the study of the strati- graphic and structural geology of the foothills along the Colorado Range. These characteristics are, the bluish-gray, light-gray to clouded white color; the even, fine-grained texture; the superior hardness by which it effectively resists atmospheric influences and becomes persistent in outerop; the freedom from arenaceous and clayey matter; the conchoidal fracture; the evenness of the bedding planes; and the fossil contents. The thickness of this member is about 50 feet, but the above characteristics are especially applicable to the lower 30 feet, which is also a portion of particular eco- nomic value. The upper layers are usually much thinner, and graduate through shales into the overlying group of marls constituting the middle member of the formation. Occasionally the limestone becomes shaly from base to summit, when the prominence of its outcrop is greatly diminished. The presence of bituminous matter, though recognizable, is not so marked in the limestones of the Niobrara as in those of the Benton. Following is an analysis of a type specimen of the limestone, by Mr. L. G. Eakins, which shows it to be dolomite: Analysis of limestone. Per cent. TAT OT Bee a tBESe Gee CERO EIA e Be ccs so SCRE EOE eC nEceaaee 12. 01 CaO ete ee ane oa te, oe Sree ee roe ae Sn cin cerenem ose. 27.49 Wife OS eS Salle SA eRe ohn oe BOER ERE Eo ce DOC CORO OC Ce Ree Se eer 18. 03 RVI (0) eae ope NEN ER ce al ara hls = RRO een ere Lice me dere cicietyaiarat istotele 20 JAIN O)<9).3 Sie SE = ers Sp So RICE AS SO Terry Sr ea RR TI Ae ea ine ae eee 5d GO) seetnet re oa ES cies fe ice st ee oN os stale cintols Soles eae cismine sett 11 sO ppetee eters smokes Cieeie ee ce > Sains Soom mae oaalot ccc atewae, See ae arociss 029 RISO es Sak aad6 OR AG Ae ODOR RBS ES eo a0 SB HES Or nna ose SSE mee Bens fe 61 CO Fee eee Ie cialis Kio te teeta eee, Saka Soteert Se tee ete cide 41.40 100, 419 Local variations from the above composition may easily be seen under the lens alone. These consist of differences in the amount of silica present in the form of sand; in yarying proportions of iron, as instanced by the 68 GEOLOGY OF THE DENVER BASIN. presence, in some of the lower beds, of small limonite concretions; and in a variable amount of alumina, especially noticeable in a comparison between the shaly and nonshaly varieties. Middle division—"T"his consists of light-gray marls, here and there streaked with darker-gray and buff bands. It is usually a little over 100 feet thick. Upper division This has an average thickness of 240 feet and is an accumulation of buff and yellow shales of a peculiar earthy and more or less calcareous composition. Several narrow layers of yellow, impure, saccharoidal limestone occur, together with beds of fine, sandy material. The whole series is fossiliferous. There is a remarkable distribution of alkaline salts throughout the series, weathering out in small white spots. Gypsum is frequently met with, and iron occurs in small concretions. The contrast in composition and color between the yellow shales and the sediments of the overlying Pierre affords a clear lme of demarcation between the two formations. LIFE. The fossils of the lower limestone member of the Niobrara include Tnoceramus problematicus, Inoceramus deformis, Ostrea, undetermined fish remains, and sharks’ teeth. The last are coated with a heavy, shiny-black, carbonaceous material, much resembling bitumen in appearance, and in the limestone containing them the odor of this substance is frequently detected. The fossils of the yellow series embrace the forms Ostrea congesta, a species of Inoceramus, one of a Baculites, several long, black spines of elliptical cross-section, probably derived from the shells of some molluscan, and innumerable remnants of fish inteeuments, occurring as small brown or blue membranous fragments. The vertebrate remains of animals which constitute the Pteranodon fauna of Professor Marsh occur further eastward, in Kansas, in chalky beds which are supposed to correspond in horizon with the Niobrara formation. MONTANA GROUP. The Montana group occupies a highly inclined position along the foothills, and, with the exception of an area in the northwest portion of PIERRE FORMATION, 69 the field, the width of its outcrop is but little in excess of its thickness. The latter, under more normal conditions, reaches a maximum of approxi- mately 8,700 feet, of which the Pierre constitutes the lower 7,700 to 7,900 feet and the Fox Hills the upper 1,000 or 800 feet. PIERRE. STRATIGRAPHY. This formation is, in the main, a great body of plastic clays, carrying small, lenticular bodies of impure limestone and, at a horizon about one- third the distance from base to summit, a zone of sandstone from 100 to 350 feet thick. A variable quantity of calcareous matter and alkaline salts is also generally distributed throughout the formation, while iron in the form of concretions or thin seams, fine carbonaceous matter, and gypsum are of frequent occurrence. ciays—The clays are usually leaden-gray, but may be blue or yellow. Contraction cracks which reticulate the surface are characteristic of areas underlain by the formation, particularly in regions of low dip. Some of these have a linear extent of 30 feet, a width of 6 inches, and a visible depth of 3 feet; usually, however, they are much smaller. he clays retain their normal characteristics over the greater part of the Denver field, but in the immediate vicinity of the Ralston dike they have been metamorphosed to hard, dark-blue or black shales. In the vicinity of the Valmont dike their metamorphism is less pronounced. Limestones—These occur as small, lenticular bodies whose horizontal axes are from 2 to 6 feet long, and whose vertical axes are from 6 inches to 2 feet. Their composition is between that of a clay, with little carbonate of lime, and a very pure limestone, generally inclining to the more calcareous variety. Their color is gray, their texture very fine-grained, and both color and texture are extremely even throughout the same body. Oxide of iron is occasionally present in small amount. The limestones are often reticulated with calcite seams, the ramifications extending through the entire mass. Under a hammer blow the mass flies into hundreds of small angular fragments. This also happens with rocks showing no calcite reticulations, but of homogeneous appearance, and indicates a predisposition of the rock 70 GEOLOGY OF THE DENVER BASIN. to this structure. The limestones are irregularly distributed throughout the whole body of shales, one horizon, about two-thirds the distance from base to summit, showing a specially large number. The limestones are the chief source of the fossils, but, except at the horizon just mentioned, no part of the group is specially marked by either abundance of forms or the development of particular species. Sandy zon.— This is an almost continuous band of soft, friable, yellow- ish-gray, fine-grained sandstone, composed chiefly of quartz, with local admixtures of clay either in small seams of shale or disseminated through the mass. It rarely forms an outcrop except in deeply eroded gulches or in ditches, yet its presence is easily detected in the soil, and it constitutes an excellent datum for stratigraphical reference. Besides the beds of thin clay, two or three bands of impure limestone are locally present. The most peculiar development of this sandstone occurs about three- fourths of a mile north of the northern boundary of the map. It is here nearly 350 feet thick, and at first sight resembles the basal sandstones of the Laramie, but on inspection is found to be calcareous in certain layers, to contain an abundance of the more common Pierre fossils, and in its coarser mineral constituents to differ materially from the purely quartzose beds of the Laramie. Ironstones— These are chiefly small, irregularly shaped concretions of hardened, caleareous clay, from 1 to 3 inches in diameter, containing a variable amount of iron. Their color is between an ashes-of-roses and deep rust. of the Denver field. FOX HILLS FORMATION, Ql ZONE TRANSITIONAL TO FOX HILLS. Between the Pierre and overlying Fox Hills formation there is a change from the pure clay of the one to the arenaceous shales of the other. Limestones and small ferruginous nodules, similar to those already described, are present throughout this transitional zone, extending well into the Fox Hills. Fossils also occur, but the life of the zone is marked by the sudden increase in the members of the genus Mactra, a genus which below has only been occasionally met with, but which from this up is frequently found. FOX HILLS FORMATION, STRATIGRAPHY. The Fox Hills formation has a normal thickness of between 800 and 1,000 feet, falling below this only at Golden, where its decrease to 500 feet is attributable to the nondeposition of its lower portion. The formation consists mainly of soft, friable, arenaceous shales, with occasional interstratified bands of clay. At the summit is a persistent and characteristic sandstone, usually about 50 feet thick. The entire formation has a yellowish cast, but while the shales are generally of a grayish-yellow the sandstone itself has a pronounced tint of green. The composition of both shales and sandstones is very uniform. shale—The shales carry a small amount of ferruginous matter in concre- tions and seams, and a minor quantity of gypsum. Limestone concretions resembling those of the Pierre are numerous, but less abundant than in the older formation; like the shales in which they occur, they usually contain more or less sand, those near the summit resembling a calcareous quartzite. All are fossiliferous. Fine carbonaceous matter is generally present. Cone-in-cone structure is frequent. Sandstone—The sandstone at the summit of the formation is noteworthy on account of its position as cap to the great mass of Cretaceous clays, from its wide occurrence over the West, from the fossil remains in its upper stratum, and from the marked difference displayed in its materials from those of the basal sandstones of the Laramie which overlie it. Its composition is chiefly quartz, but it carries an appreciable amount of biotite and muscovite, and T2 GEOLOGY OF THE DENVER BASIN. iron oxide is distributed throughout its entire mass. It is fine-grained, of close texture, and usually occurs as a single bed. Occasionally it becomes concretionary. It is in close union with the basal sandstone of the Laramie; no transition bed exists; the passage from the one to the other is direct; combined they frequently enter into the formation of a single bluff 150 feet high. Notwithstanding this, the formations are easily distinguished by their lithological contrasts and by the fossil horizon marking the summit of z the older. LIFE. While invertebrate fossil remains occur throughout the entire thickness of the Fox Hills, there is an especially conspicuous array of characteristic forms at the very summit of the formation, in the uppermost layer of the capping sandstone, none of which is ever found above, and but few of which are met with in numbers below. This is, moreover, a paleontological feature of the formation in all its western localities. Among the forms found in the Denver field those especially characteristic are— Mytilus subareuatus, Crenella elegantula, Nucula cancellata, Cardium (Ethmocardium) speciosum, Solemya subplicata, Spheeriola cordata, Veniella humilis, Callista dewey, Callista (Dosinopsis) owenana, Mactra alta, Tellina scitula, Tanecredia americana, Liopistha (Cymella) undata, Fasciolaria cheyennensis, Pyrula bairdi, Fusus sp. ? Pseudobuccinum nebrascense, Anchura americana, Turitella sp. ? Dentalium sp. ? Cylichna sp. ? In plant life Halymenites major is generally met with in the upper portion of the formation, in both the sandstones and the limestones. LARAMIE FORMATION. The entire Denver field, excepting the belt along the foothills occupied by the older formations, is underlain by Laramie strata, but their surface exposures are confined to the northern and northeastern portions of the field and to a narrow strip parallel with the foothills at a distance from them of between 1 and 2 miles. LARAMIE FORMATION. 73 The formation is from 600 to 1,200 feet thick and is divisible into two parts, a lower of sandstones, aud an upper, composed of clays. The former has a uniform thickness of about 200 feet; the latter varies. Both divisions sarry workable seams of coal. STRATIGRAPHY, LOWER DIVISION. The two sections given in figs. 12 and 15, in the first part of Chapter VI, show the general succession of strata in this division. Sand- stones largely predominate, outcropping in successive benches separated by bands of arenaceous and lignitic shale with their intercalated coal seams. The two heavy beds of sandstone at the base of the division and the bed at the top are the only ones persistent over the entire field. The intervening ones not only disappear, but vary in the horizon at which they recur. The coal beds also vary, one of the several seams being workable in one locality and another in another. Sandstones— The sandstones are white and are composed almost exclu- sively of quartz, clear and opaque white; minute grains of black chert and rare traces of feldspar and mica are the only associates. The material is somewhat loosely held together by a cement, usually white, but occasionally tinged brown by iron oxide. The chert is characteristic. The three important sandstones of the lower division of the Laramie may, for convenience, be designated from base upward, A, B, and C. A and B are each about 60 feet thick, are separated by from 2 to 4 feet of shale, and with the underlying Fox Hills ‘sandstone often form a single continuous outcrop. Although of nearly the same materials, the A and B sandstones are distinguishable from each other by their stratigraphical relations to the overlying and underlying beds; by the presence in the lower bench of a greater amount of ferruginous matter, which imparts to it a faint-yellow tinge, not nearly so deep, however, as that of the underlying Fox -Hills; by the occasional development in this bench of a concentric structure; and by the occurrence near the summit of the upper bench of enormous concretions of the same material as the surrounding sandstone, but of extreme hardness. These concretions upon weathering out are of z various sizes and shapes, but a common form is one somewhat irregular in 74 GEOLOGY OF THE DENVER BASIN. outline, of circular or elliptical cross-section, from 2 to 4 feet in diameter and 30 to 40 feet long. A further feature of the upper bench of sandstone (B) is the polygonal structure.developed upon its lamination planes in weathering. The shale separating the A and B sandstones is frequently lignitie and in a few localities, as in the bluffs north of North Boulder Creek and in the Erie coal field, is partially replaced by coal. Sandstone C occurs about 60 feet above the basal sandstones A and B. Its thickness averages 8 or 10 feet. The upper half is usually the whiter and more solid; the lower is somewhat ferruginous and also strongly ripple- marked. As there are several bands at about this horizon, it is not always possible to recognize this particular bed, but the presence of the one or the other is sufficient for the determination of the general horizon where a good section is exposed. This sandstone is of importance in coal exploration, all the workable beds in the lower Laramie of the Denver field occurring below it. Ostrea bed—This is a sandstone occurring from 12 to 15 feet above sandstone B. It is but slightly developed in many parts of the field, but wherever observed consists of the same material as the other sandstones, with the addition of a considerable amount of lime. It often contains in abundance one of the characteristic Laramie species, Ostrea glabra, besides a few indeterminable fragments of shells of other types. Its position is of special importance in working out the coal horizons. Shales and coal beds ——The remaining portion of the lower Laramie con- sists, besides the coal beds, of shales which are almost always more or less lignitic and generally somewhat arenaceous. They contain abundant partially carbonized plant remains—bark, wood, leaves, and their impres- sions. From the presence of the carbonaceous material the shales are frequently dark-gray or brown, and upon its prevalence to the exclusion of clay and sand a bed may become true coal of economic importance. Ironstones are sometimes present, but they are not so abundant as in the upper division of the Laramie. Local alteration of Laramie beds——'Ihe sandstones and shales of the lower Lara- mie have locally undergone considerable alteration from the burning out of associated coal beds, the result of spontaneous combustion or prairie fires. LARAMIE FORMATION, (5: This is particularly the case in the small hill known as the Burnt Knoll, north of the Marshall-Louisville mesa, about 3 miles west-northwest of Louisville. Here the strata have in some instances been fused, developing both flow and vesicular structure, hand specimens being hardly distinguish- able from lavas; in other instances the alteration has been to jasper. A crystalline structure appears in some, while elsewhere the granular texture of the original rock has been maintained. Many are coated with hematite from alteration of the iron originally present. Clinker, resembling that from furnaces, is abundant. Columnar structure has been developed in some of the clays, probably from evaporation of their water. The colors vary—reds, browns, blues, and blacks prevailing. In many of the rocks, even those most altered, leaf impressions are still apparent. STRATIGRAPHY, UPPER DIVISION, Che thickness of this division, owing to uneven denudation from the top, varies between 400 and 1,000 feet. The strata are chiefly clays, through which are distributed small, lenticular bodies of sandstone, innum- erable concretionary ironstones from 2 to 4 feet in diameter, and narrow local seams of impure lignitic material. One or two beds of lignite are also present in its upper portion, east of Denver. Especially fine exposures of these clays occur on the slopes south and east of Rock and Coal creeks, where the strata have a slight dip east or southeast, forming bluffs looking westward, in which colors, lines of stratification, sandstone concretions, and ironstones are all clearly defined. clays—These are dark bluish-gray, relieved by bright bands of red, yellow, brown, drab, and white. Sandstones—T he sandstones differ somewhat from those of the lower Laramie in containing more or less lime, in their superior hardness, and in their occurrence as lenticular masses from 6 to 30 feet in diameter and 1 to 6 feet in thickness. They contain numerous plant remains and a moderate amount of iron. In weathering they often scale in concentric layers. There are apparently three or four horizons at which they especially prevail, although they may be found anywhere in the series. Ironstones.— These form one of the distinguishing features of the upper 76 GEOLOGY OF THE DENVER BASIN. Laramie. The weathered concretions outwardly resemble limonite, but in their normal condition they are probably carbonate of iron with a larger or smaller admixture of clay. They frequently show concentric structure, the outer coats weathering off in succession, but more generally they break up into angular fragments. They differ from the ferruginous concretions of the Colorado and Montana groups in their superior size, in the abundance of their plant remains, in the far greater amount of iron contained, and in their primary composition. Somewhat similar concretions occur in the overlying Arapahoe; they are, however, less abundant than in the Laramie. An analysis of a purer variety from near Trinidad, Colo., gave— Analysis of tronstone.! Per cent. HEY sa an55 coS ann bedoon CoCo Sopa SROs OOcew osisaSS sooo oes CoOp eS sesE 9.19 JErdo) AO. SKIL RTI R a ee Oma ObO CoO US SOC. ce accor Soinaae coc Son csoccaasees 245, O4 PMMA, nS aS conosco cocoa mere sabes Gob soba sachs aacd cess shecssencus ses 6, 29 IDB eo cn Re Goncten boone 6 coc coubasese date note S55 Sob oss ochousssascee 4. 02 IBY EEY $8 2556s eo csosboSasu Ocenia sdoootess scéccs soasosossegese 1.37 Lely leo OOOO ae eas moe sb asso gsc ssbb snog 3 ceacbootSeseoe cess. Ssa¢ 31.055 Carbonieracid ands Ole anys cm. = (cements ele eaten le eee ee eee 33. 035 100, 000 Although smelted in early times, the ironstones of the Laramie are no longer of economie value. STRATIGRAPHICAL RELATIONS, The stratigraphical relations of the Laramie formation to those of younger age are varied. After the period during which the Laramie was laid down, there followed a time of great oscillation and erosion, during which were produced a series of unconformities that included all the post- Laramie and Tertiary formations of the West. To the erosion is due the impossibility of assigning to the Laramie a definite original thickness, much of the deposit having been removed and its surface rendered most undulating before the deposition of the younger beds upon it. Upon this uneven floor ! Preliminary notes on the iron resources of Colorado. Prof. Regis Chauvenet, Ann. Rept. State School of Mines, Golden, Colo., 1885, p. 19. 2 Tron 35.03. 8 Phosphorus 0.46 by difference. LARAMIE FORMATION, Te were laid down in succession and at most varied topographical horizons the overlying formations—at one point, the Arapahoe; at another, the Denver; at a third, the Monument Creek; and finally the Pleistocene. Each rests not only on the next younger formation but also in places on the Laramie itself. LIFE, Animal remains— These, so far as at present known, are limited to two species of mollusks and one of a vertebrate. The invertebrate remains include the very characteristic Laramie form, Ostrea glabra, and a Unio sp.?, which are considered decisive as to the age of the formation The occurrence of Ostrea glabra is general for the field, and always at the same horizon, a short distance above the basal sandstones of the formation. Two Unios only were found, these occurring in different localities and well up in the shaly portion of the formation. The vertebrate, according to Professor Marsh, belongs to the order Ornithopoda of the subclass Dinosauria The genus is undetermined. The specimen was found by a ranchman about 30 feet below the surface, in a well sunk through the upper Laramie strata, on the slopes of Dry Creek, about 8 miles west-southwest of the town of Brighton. Plant remains — The plant remains in the Laramie are abundant and show a clear differentiation from those of the Arapahoe and Denver beds. Prof. F. H. Knowlton, of the Survey, has made an exhaustive study of the com- bined floras, and his results are embodied in Chapter VII of this report. 78 GEOLOGY OF THE DENVER BASIN. Table of invertebrate fossils in the Denver field. [Determinations by Dr. C. A. White.] Cretaceous. 2 Fossils. Niobrara. Pierre. Fox Hills. | Laramie. | Ben- | Arap-) Den- ton. | row-| Mid- Up- | Low-, Mid-| Up- |Low-| Up- |Low-| Up- ahoe. | ver. er. | dle. | per. | er. | dle. | per. | er. | per. | er. | per. Caryophyllia Hh aSadigosactacepasc Beaumontia?! solitaria Serpula - aie eB OOO COoe Sseceaa| eecce Boeeeel Hensal sekeas Ss beeas4 SS WWasshes Be joe sal noose bse cl aaas6 Crustacean fragments.............--- Discina Jesse sosnoo soncotsesnss: Eingola nitida <-~~- ooo. e eens Ostrea ? fragments Pecten Chilamys! (Pecten) mebrascensis =< .<.-|s.csceleesms|e< ty YH Pil | ae I i | : TN Mie: Ge i aa U.$.GEOLOGICAL SURVEY. ’ = . - , Z, Z / : WY7.Yj : a) yf i | i Hit He is = ———— . SSS SSS ——S—— =~ aS Contour Interval loo Feet - —— Scale of Miles T Condition at close of Wyoming (Trias) ~ Morrison (.hira) he ee es 4 — » Benton oa ee Lowe oes GEOLOGIC M Be OLDEN, COLO: } SE JURATRIAS Sas PRE SOERIAN GaANIXEE! | | —ii = a ee 5 5, As GNEISSES AND METAMORPHIC see Ous __ Pr = LORADO CRO ; MORRISON WYOMING (UPPER) WYOMING (LOWER) ee Se - pals | MONTANA | pieRRE SE an f= = WY), J 2 Beiraie a 2 THE REGION ABOUT GOLDEN. 83 GENERAL FEATURES OF THE AREA AFFECTED, The area affected by the phenomena now to be discussed extends along the base of the foothills of the Colorado Range west of Denver, from a point about a mile south of Bear Creek northward to Coal Creek, a distance of 21 miles, with a breadth varying from 24 to 4 miles, the greater ocewring along its northern and southern edges. It involves the hogbacks of the Dakota and the region within to the Archean, and includes the prairies as far to the east as Mount Carbon, the western slopes of Green Mountain, the Table Mountains, and the vicinity of the Ralston dike. Topography — The topography shows a marked variation from that normal for the foothills region in general, and its relations to the geology of the tract as displayed from point to point are so close as to warrant the asser- tion that for every topographical lmeament there is to be discovered an equivalent geological incident that has led to its development. For mile after mile along the mountains the normal topographical features may be traced with unswerving regularity, but within the area to be described they undergo rapid change, and midway the length of the tract, in the vicinity of the town of Golden, they are lost to recognition. For a dis- tance of over a mile north of the town, and an equal distance south of it, the Dakota hogbacks have completely disappeared; the low Niobrara ridges cease to exist at a point about a mile north of Bear Creek, not to appear again until the region of Van Bibber Creek, 10 miles to the north, is reached; the Laramie sandstones with their coal have gradually approached to within 500 feet of the Archean at Clear Creek, the vari- ation in their strike from that of the Triassic and Dakota outcrops below being apparent to the most casual observer; finally, opposite the center of this great topographical gap, appear the two great basalt-capped sedi- mentary masses ef North and South Table Mountain, originally continuous but afterwards cut by the waters of Clear Creek, which debouches from the main range midway their length. Surface delineation. Seen from any of the more elevated points within this remarkable area, another set of features, second in prominence only to the ones already referred to, at once strike the eye. These are 84 GEOLOGY OF THE DENVER BASIN. the clearness with which the lines of stratification are delineated upon the su face and the distinct tendency which they display to group themselves, with respect to direction, into two well-marked assemblages—the one embracing the formations of the Colorado and all below, and maintaining for the greater part of their extent the same parallelism to the general trend of the foothills which they have held beyond the affected area; the other embracing the Montana and younger formations, and though maintaining a parallelism of strike within themselves, nevertheless abutting against the older formations at an angle in places as high even as 20°. The latter formations, in fact, approach the range proper in a broad, well-marked, and regular inward-sweeping curve, the center of its arch lying a short distance north of Clear Creek. The features just noticed again occur in a minor degree and in a manner not at first liable to attract attention, in the relations between the Dakota and underlying beds nearer the middle of the area, where the beds of the younger formation lie across the edges of those of the older. North of the central portion of the area of unconformity and south of Ralston Creek for the distance of about 2 miles the topographical and geological features are somewhat complicated by the presence of intrusive masses; they are, however, still sufficiently clear to permit interpretation, and with the others in the south and center of the area and in the remainder of the tract to the north form one complete whole. THE FORMATIONS AND THEIR RELATIONS. The Archean.— This is but slightly involved in the special geological history of the region. It formed an uneven floor for the deposition of the Trias, and across its truncated edges the latter formation was deposited. The Trias—In their strike and dip the beds of both members of the Trias are conformable inter se. Their strike follows approximately the line of the Archean foothills, and their dip is to the east and varies between 35° and 90°, being shallower next the foothills, and increasing as distance from them is gained. The lower member of this formation, the Red Beds, maintains its usual appearance and, except in two places, a nearly constant thickness over the entire area under consideration. The two variations in thickness are found, THE REGION ABOUT GOLDEN. 85 the one near the southern extent of the tract, the other for a mile and a half on either side of Clear Creek. The former is of no particular interest in the present discussion. The latter is attributable to two causes—one, nondeposition at the base, due to a rise in the Archean floor and a conse- quent shallowing of the sea at this point, the beds of the deeper water abutting against this rise; the other, the disappearance from the top of the series of the beds last laid down, including the Creamy sandstone and at least 100 or 200 feet of the beds beneath. The linear extent of the disap- pearance of the Creamy sandstone is probably somewhat under 1 mile, and is confined chiefly to the region immediately north of Clear Creek, reaching to the south of it but slightly, if at all. In this interval the clays of the Fox Hills are found in close proximity to the Red Beds, the former conformable in strike with the Laramie sandstones above, the latter pur- suing their usual trend, approximately parallel with the base of the range. The upper member of the Trias presents nothing anomalous in its occurrence until within a distance of about 2 miles north and south of Clear Creek, when a rapid disappearance of its beds successively from top downward is found to occur as the center of the region is approached, the limestones and associated beds at its base apparently reaching within a short distance of the limits already assigned for the Creamy sandstones below. An extremely important point in this connection is the fact that this disappearance occurs where the overlying Jura is not only still present, but where it maintains even the greater part of its thickness; it occurs, in fact, between the Jura above and the lower member of the Trias, the Red Beds, beneath. The disappearance of this series of strata is most marked, because more sudden, to the north of Clear Creek and Gold Run, where, within a distance of between one-half and three-fourths of a mile, it has decreased in thickness from 650 to 270 feet. The diminution in thickness to the south of Clear Creek is also rapid, but over this portion of the region the Upper Triassic member is not limited altogether by the Jura above, but in part by the Dakota, with a discrepancy of at least 10° in their strike. Farther to the south, where the Jura is present in nearly its full thickness, the variation in thickness of the Upper Trias is more gradual, but still to be associated with the local phenomena of the region. 86 GEOLOGY OF THE DENVER BASIN. The Jura—No extraordinary diserepancy in strike or in general relations between this formation and either the underlying or overlying one is apparent until upon near approach to the confines of the region presenting the anomalies just described for the Trias. Any decrease in the thick- ness of the Jura beyond is little more than is usually met with from point to point along the range, From about a mile south and a mile and a half north of Clear Creek, however, the beds of the formation disappear in rapid succession as the center of the region is gained. ‘Their strike is, moreover, at variance with the formations both above and below; in the southern part it is in noticeable contrast with that of the Dakota, being some 10° or 15° to the east of the latter; in the northern portion not only is the same discrepaney probable between these two formations, but an equal one also appears between the beds of the Jura and those of the Trias below. ‘The thinning of the Jura is in part probably due to the absence of some of its lower beds, while the cause of its sudden and final thinning is found in the rapid and successive disappearance of, first, its upper beds, followed in turn by those lying beneath. The Dakota——As ascent is gained in the series of formations, the region of anomalies becomes more and more extended in north and south directions. The Dakota begins to display irregularities as far south as the northern end of the high hogback just south of Coon Gulch, and in the north at the southern end of the chain of hogbacks north of Golden. The noticeable points in the behavior of the southern half of the formation are, first, the disappearance of the characteristic hogback; second, the eradual decrease in thickness, which the outcrops of the remaining portions show to be both from above and from below, the fire-clays in the middle of the formation being the last to disappear, as evidenced at the bluffs of both Clear Creek and Gold Run; third, the discrepancy in strike between this formation and those below and above, its beds in the region of more pronounced irregularity Lying across the edges of the former, and abutted from above by the ends of the successive strata of the Montana group throughout much of their line of contact; finally, frequent changes in the strike of its beds over the central portion of the affeeted area, which changes are not paralleled by corresponding ones in the prominent sand- stones at the base of the Laramie, lying but a short distance to the east. THE REGION ABOUT GOLDEN. 87 In the northern half these same peculiarities are again met with, but in some particulars they are more strongly accented than in the southern, These are, the more sudden disappearance of the hogback; the rapidity with which the formation thins; and the marked crumpling, as shown in their strikes, to which its beds have been subjected without the overlying strata being in the least affected. In dip the Dakota varies from 45° in its more normal occurrence to vertical over the more disturbed, middle portion of the field. The Benton—This formation completely disappears a short distance north of Coon Gulch, and also at a point about opposite the middle of the first hogback north of Golden. The southern portion thins very gradually throughout a distance of 34 miles, while the disappearance of the northern member is completed in a little less than a mile. The Benton conforms in strike and dip to the Dakota, but is overlain, after the disappearance of the Niobrara, by successively higher strata of the Pierre and Fox Mills forma- tions as the center of the disturbed region is approached. The Niobrara —This, like the Benton, disappears only from above down- ward, but its limits are found considerably to the north and south of those of the corresponding members of the older formations. In strike and dip it conforms with the Benton and Dakota, and, like them, in passing from without inward, is overlain by successive strata of the Pierre, though: it is nowhere brought in contact with the higher member of the Montana group, the Fox Hills. The disappearance of this formation is especially well shown both in the north and south by the physical character of its sediments: the upper, bright-yellow or buff, sandy measures, which often form a well-marked outcrop, are first lost; then follows the destruction of the argillaceous middle part of the series; and finally the prominent basal limestones are themselves cut out. The disappearance occurs in the south only a short distance north of Bear Creek, and in the north a quarter mile north of Van Bibber Creek. The Montana—This group in strike conforms strictly with the overlying Laramie, the basal sandstones of which afford a prominent and reliable key to the relations of these upper formations with the ones already considered. The dip of the component beds of the group, where all are present and in normal occurrence, shows a gradual increase from 45° to 90° as the distance 88 xHOLOGY OF THE DENVER BASIN. increases from the base toward the summit. Over the middle of the anomalous tract, however, a vertical dip prevails, as in the case of nearly all the formations in this portion of the area. The fact of chief interest regarding the Montana group is its remark- able and rapid disappearance between Bear and Coal creeks as distance is gained from either of these streams toward the center of the region at Golden. Immediately north of Bear Creek its strike relations with the underlying formations are rather more exaggerated than at most other points, and consequently more clearly brought out in the surface exposures there occurring. In this vicinity successive beds of the Pierre may be traced over their general line of strike by means of their lithological characteristics and the general prevalence of certain fossils at particular horizons, from points 1,000 feet or more to the east of the Niobrara at the bluffs of Bear Creek to others within only 200 or 800 feet of the older - formation, 1 or 2 miles to the north. The angle thus made by the differ- ence in strike between the Niobrara and Pierre is on an average about 15°, but decreases to the north, opposite the middle of the Dakota hogback, beyond which the strikes are, for a considerable distance, more nearly parallel. In the northern part of the region, opposite the first hogback north of Golden, the exposures of the Montana group are rare, but a half mile north of Van Bibber Creek, and from this point to Ralston, the discrepancy in strike observable to the south has for a time almost wholly disappeared, beyond Ralston Creek, however, a divergence of 20° is still noticed in the general trend of the Laramie and Dakota sandstones, which extends to a line due east from the entrance to the canyon of Coal Creek. This area is regarded as corresponding to that in the vicinity of Bear Creek in the south, over which a thickness of Montana beds equivalent to that on the southern border of the field is regained, by which the geological symmetry of the region is rendered complete. Over the middle portion of the region the Montana beds follow closely the behavior of the Laramie, but show frequent variations in thickness and corresponding changes in their strike relations with the beds below. Regarding the individual members of the Montana group, if the general THE REGION ABOUT GOLDEN. 8Y thickness of the Fox Hills is taken at between 800 and 1,000 feet, the Pierre has not been deposited for a long distance in the middle portion of the region, having gradually thinned from the confines of the area toward the center of Golden by successive losses of its lower beds. The Fox Hills alone is of general occurrence, although its thickness also over the central portion varies greatly and often. The Laramie—'The prominent feature of this formation is its remarkable bend from a course approximately parallel to the foothills and to the formations below to a broad, sweeping curve, by which it is gradually carried to the westward until at Golden, its point of greatest deviation, it lies between 2 and 3 miles to the west of its former course. The general trend is slightly wavy, but with reference to the early Cretaceous and older formations is of notable steadiness, passing all their individual deviations without the least disturbance of its own. Its dip is vertical or slightly overthrown for the entire length of the area under consideration, and its basal sandstones form along their trend a characteristic series of combs. Arapahoe and Denver——The formations above the Laramie, although in reality markedly unconformable with it and with each other throughout the broad area over which they have been deposited, nevertheless in the present tract so closely follow the former in strike and dip that they display no peculiarities worthy of note in the present discussion, and, in fact, are only incidentally connected with the special geological history here discussed. STRUCTURAL FEATURES. Dies— A geological cross-section along Bear Creek would present a gradual increase in the dip of the several formations from the Archean outward at a rate about as follows: 35° E. for the Trias; 38° to 40° for the Dakota, Benton, and Niobrara; 45° for the lower part of the Pierre, increasing to 55° to 65° in the upper part; 65° to 80° from base to summit for the Fox Hills; and 80° to 90° and overthrown for the Laramie, Arapahoe, and lower members of the Denver formation. Three or four miles north of Bear Creek, 10° to 15° may be added to the lesser of the foregoing dips, while from Coon Gulch to the vicinity of the hogback first north of Golden the formations of higher dip, having now become vertical 90 GEOLOGY OF THE DENVER BASIN. or slightly overthrown, remain so, and the Triassic beds alone have an inclination under 80° or 90°. North of this, where regularity in the forma- tions once more prevails, the dips settle back approximately to their normal amounts as given at Bear Creek. The general fold parallel with the base of the Colorado Range —T he surface exposures of the prominent and sharply defmed fold occurring generally along the base of the Colorado Range and resulting from its uplift are, for the greater part of the area under consideration, to be found within a short distance of the line of union of the Denver and Arapahoe formations. North of Van Bibber Creek, however—where the Denver formation ceases to exist, and where, 2 or 3 miles farther, the Arapahoe also disappears—the bend is almost entirely transferred to the Laramie, the Arapahoe for that part of the distance over which it is present entering into it only in the slightest degree. Faults—There are along the line of the older formations in this region four easily recognized fault localities: One near the termination of the Niobrara just north of Bear Creek; a second in the isolated Dakota hill 2 miles south of Clear Creek; a third near the southern end of the Dakota hogback first north of Golden; and a fourth one-half mile to the south of the latter, near the line of union of the lower and upper divisions of the Trias. The faults of each region have the present appearance of approxi- mately east-and-west cross fractures, along which the ends of the upturned strata are thrown to one side or the other. In the southern half of the field the northern ends of the interfault blocks are carried westward, while in the northern half it is the southern ends that are carried westward. The fractures in the isolated Dakota hill south of Clear Creek are irregular and apparently local in their character. . As a rule, the extent of throw of the faults mentioned is slight and is confined to a single formation, one frac- ture only—of the group in the southern portion of the area—extending beyond 100 or 200 feet, this including the Niobrara, Benton, and Dakota, but being much less pronounced in the older formations than in the Niobrara. The faults in the vicinity of the Ralston dike and the foothills of Coal Creek will be discussed in a separate section. THE REGION ABOUT GOLDEN, 91 STRUCTURAL DEVELOPMENT OF THE AREA, Introduetory— The abnormal conditions which have been noted in the relations of the several formations to one another are directly traceable to a series of unconformities that exist at the particular horizons at which these conditions oceur. Excluding the higher ones of general oceurrence along the base of the mountains in this portion of Colorado—that is, those between the Laramie and Arapahoe, and the latter formation and the Denver beds— there are still to be found four which are in some respects peculiar to this locality: One between the Archean and ‘Trias, of special development in this area; a second at or near the close of the Trias; a third at the top of the Jura; and a fourth in the Cretaceous at the close of the Colorado. Entering most prominently into the history of these unconformities are as many folds, all of which occurred prior to the general uplift of the Rocky Mountains, and hence, with the erosion going on at the time, repre- sented a topography for the region completely different from that of the present day. When the great uplift of the Rocky Mountains brought the beds into the position they now have, all hills resulting from previous folding were changed in their individual positions from one in which the plane of their bases was horizontal to one in which it became vertical, or at least inclined at a high angle, and parallel to the direction of the mountains. In the subsequent erosion of the region, therefore, what would originally have been a profile section of the strata constituting these folds now appears in plan on the present surface of the ground, all originally north and south dips becoming present north-and-south strikes—in some cases slightly altered in character by incidental variations in the amount of folding in the general uplift of later times. The detailed character and the contours of these ancient elevations ‘an not be determined, the two dimensions given in the profiles being nat- urally the only ones admitting of observation. The profiles, however, afford data quite sufficient to furnish a clear insight into the general character of the unconformities and the moyements in the earth’s crust which led up to them. First period —The several events in the geological history of this region 92 GEOLOGY OF THE DENVER BASIN. by which it has reached its present state of evolution were as follows: First, that which brought about the unconformity between the Archean and Trias. That there everywhere exists a general unconformity between the rocks of these two ages is well known, but within the region in ques- tion there is direct evidence of a special development of the unconformity, which is, furthermore, borne out by the subsequent events which form the successive steps in the geological history of the area. This evidence consists in the observed termination of certain of the lower beds of the Trias against a slightly projecting portion of the Archean; in the impos- sibility on structural grounds of the whole amount of thinning which the Triassic beds have undergone being: attributable to disappearance from the top and the consequent necessity for its having taken place from below; and in the graphical development of an Archean eminence, as represented in Profile I, Pl. X, by tracing backward from their present positions through the series of figures given the relative movements of the rocks of the several ages by which they have been brought into these positions. The evidence is found to lead directly to the following conclusions regarding the first of the periods in the special history of this region. Prior to the deposition of the Trias there had been developed in the Archean, partly by erosion and partly, perhaps, by compression, the eleva- tion shown in section in Profile I, Pl. X. Its height was probably 800 feet, and it had a linear extent in a north-and-south direction of nearly 4 miles. Against the sides of this Archean elevation were laid down the coarse sediments of the lower division of the Trias—the Red Beds—which in time completely capped the hill along the line of profile given, and finally buried its summit deep beneath the accumulated material. General subsidence and sedimentation continued uninterruptedly to, or nearly to, the close of Triassic times, completing the first stage in the history of events here considered. Second period At the close of the Trias the region which embraced the above events yielded a second time in a marked degree to the forces of elevation and developed the gentle arch of Triassic and Archean strata shown in Profile I], Pl. X. The north-and-south extent of this arch was but slightly greater than that of the one already described in Archean times, its crown—coincident with that of the earlier one—lying about a half mile THE REGION ABOUT GOLDEN. 93 to the north of the present position of Clear Creek. The rise of the arch, as indicated by its upper beds, was apparently about 420 feet, but subse- quent erosion must have planed it down from its original height and shape to approximately the level line drawn across it in the figure as the base of the Jurassic formation. The evidence for the occurrence of the unconformity at this horizon and the fold which preceded it is found in the disappearance of most of the upper members of the Trias within the region of its influence, and in the divergence between the present strikes of the formations on either side of the line of unconformity—a divergence in strikes, it being remembered, corresponding to an equivalent discrepancy in the ancient dips, as shown in the profiles. This line of unconformity is naturally somewhat wavy, and it is possible, indeed, that at some points along the middle portion of the exist- ing arch, through insufficient erosion, the deposition of a part or even of the whole of the Jura may not have taken place. The weight of the evidence, however, is in favor of nearly complete deposition over the entire section, from the fact that wherever the formation now exists it displays no tendency whatever to a protracted, gradual thinning, as is the case in the disappearance of certain of the other formations, but, on the contrary, disappears by the sudden truncation of its strata in almost their full normal thickness, clearly the effect of subsequent erosion. The movement which brought about the elevation of the Triassic strata must be regarded as synchronous with at least a portion of that more prolonged or extensive movement by which the sea was sooner or later shut out from certain areas in the Rocky Mountain region of Colorado, causing either a partial or an entire absence of marine beds, according to circumstances, with a succeeding deposition of fresh-water strata in which a lacustrine life appeared. In the area under discussion the fresh-water Jurassic alone was laid down. General subsidence of the entire region continued during the deposi- tion of the Jura upon and against the sides of the Triassic eminence, and at its close the second period in the geological development of the area was completed. Third period —This opened with still another uplift of all the preexisting 94 GEOLOGY OF THE DENVER BASIN. sediments into the fold traced in Profile III, Pl. X, the rise of the arch in this case being approximately 1,000 feet. The figure shows the character of the fold on the line of profile given to have been that of a long, gentle slope from the confines well toward the center, where, on further yielding to the compressive forces, a clearly defined median ridge was produced. Erosion naturally went on in a more or less irregular manner, but the general position of the hill and its component strata relative to erosive forces was apparently such as to cause the disappearance from the top of the Jura, over those parts of the slopes of gentle inclination, of only the most insignificant amounts of material, while over the central or sharper portion of the fold the probable effeet was the complete removal of the beds of the Jura and Upper Trias, together with a partial removal of those of the Lower Trias, from the crown of the arch, and of the material from the adjoining flanks down to the gently sloping line of union shown between these formations and the Dakota lying across their edges. Whether erosion reached an extent sufficient to permit the deposition of the Dakota and the lower part of the Benton entirely across this rise is doubtful, but from the rate of disappearance of the Dakota from below, it is probable that neither this formation nor the lower half of the Benton was here laid down. The evidence for the conclusions given in the preceding statement is o clearly brought out in the strikes (ancient dips) and surface relations of the formations to one another, notably, in the divergence in strike and the truncation of the edges of the Jura by the Dakota on the southern side of the gap (Profile III, Pl. X); in the thinning of the Dakota in such a man- ner as to eventually leave the fire-clay in its upper half in contact with the older sediments at the two points where the formation appears to end, in the south bank of Clear Creek and the north one of Gold Run; and in the ready reproduction by graphic methods of the structural conditions observed in the field and the natural sequence of events based thereon. Sedimentation of the Dakota, Benton, and Niobrara continued unin- terruptedly to the close of the latter time, subsidence probably keeping pace. With this the third period of development ended. Fourth period—The fourth period embraces the time during which the ‘reat elevation shown in Profile IV, Pl. X, was created, and in which oO g1 THE REGION ABOUT GOLDEN. 95 12 formations were laid down. the sediments of the Montana and overlyir The uplift of this time was of much greater vertical and areal extent than any of those which preceded it, the rise of the arch on the line of section given reaching at least 9,500 feet, while its lateral extent was not far from 21 miles, _ It is broadly symmetrical, though there are several sub-flexures of more or less pronounced curvature. The two of greatest prominence occur midway either flank. The others, of minor development, are confined chiefly to the higher part of the arch, and represent a crumpling of a secondary nature along this portion of the fold. This crumpling is well shown upon the present surface of the region in the changes in strike of the affected beds, which are in strong contrast with the unbroken direction to which the strata of younger age hold. The possibility of the presence of an occasional fault in the place of an unbroken flexure as drawn in the profile is to be remarked, notably, in the vicinity of Gold Run and again at points north of Coon Gulch. It so happens that here and there a space intervening between two outcrops of the same bed, lying in an indirect line from each other, is so covered that it is quite impossible to observe the position of the underlying strata; but since in no case a sharp break in the beds of the Archean and Trias lying below the more affected ones has been discovered, it is preferable to sketch the irreeularities as flexures rather than as faults. Concerning the recognized faults in the northern and southern halves of the arch, described on page 90, their true character now readily appears in Profile IV, where, upon the restoration of the beds to their position in pre-Montana times, the fractures are, with the local exception at the south end of the Dakota hogback north of Coon Gulch, all found to be a series of slip faults of the normal type, either vertical or hading to the down- thrown side and away from the center of the uplift, and similarly developed on either flank of the elevation. The explanation of the normal type of fault under the attendant-conditions may possibly be found in the readjust- ment of the strata, brought about by subsidence during a later period. The profile of this ancient hill, at least on the line given in the figure, is one of structure rather than erosion, the unevenness in its outline being clearly traceable to the flexures underlying, the comparatively little erosion that has taken place over the higher portion of the arch having been regular 96 GEOLOGY OF THE DENVER BASIN. in distribution, and thus having but slightly altered the original outline of the upheaval. The height of the elevation, however, has been reduced over 1,000 feet—to 8,481—by the removal of the Niobrara, Benton, and Dakota. The succession of events in the erosion, the transportation of the derived material, the sedimentation in the adjacent Montana seas, and the conditions which led up to each are in a degree speculative; but the inferences are, first, that soon after the completion of the Niobrara period elevation began, and so much of the hill as is above the altitude indicated in the section (Profile TV, Pl. X) by the line marking the upper layers of the Niobrara was then, sooner or later, brought within the erosive power of waves or currents, and the sediments last laid down, being now brought into a favorable position, and still in a condition sufficiently soft to permit their being easily broken down and comminuted, were removed by the transporting powers of the waters washing them; secondly, that the condi- tions of sedimentation in the immediate seas were the same as those in all mediterranean or large inland seas or along the margins of the continents at the present day—that is, there was comparatively deep and quiet water at a distance somewhat remote from the nearest coast line, which permitted the quiet settling of the sediments forming the clays of the Montana group, and corresponded to those under which the blue mud of subcontinental areas is now being deposited. The apparently complete removal over the space origmally covered by them of the materials resulting from the breaking down of the early Cretaceous strata is somewhat striking, but it may readily be accounted for in the nature of the formations removed and in the action of waves and currents throughout the long time the higher parts of the elevation were probably subjected to them. Furthermore, it can not be positively asserted that the line of unconformity is as clear of débris as represented, since on the steeper flanks of the arch it is rare that the beds above this line can be traced to actual contact with those below; still further, it is to be remem- bered that nothing whatever is known of the conditions on other profiles of this ancient hill. During the deposition of the beds of the Montana group gradual subsidence of the area at a generally uniform rate must have taken place, the sedimentation, with two exceptions, being that of quiet and deep THE REGION ABOUT GOLDEN. 97 water. The exceptions noted—the sandy zone midway the Pierre and are, however, not confined to the more arenaceous beds of the Fox Hills the area under consideration, and therefore bear no relation to the phe- nomena here discussed. With the general movement at the close of the Laramie and that which produced the unconformity between the Arapahoe and Denver formations along the range, the peculiar structural features here described have nothing to do. Fifth period —U pon comparing Profile IV, Pl. X, with the present surface section of the same beds upon the general map of the region (Pl. X), the early relations between the arched and horizontal strata, as shown in the profile, are observed in later times to have completely interchanged. The once highly arched strata below the line of unconformity have now assumed a practically direct trend, while the strata above the line of unconformity, originally horizontal, have at the present time a well- defined inward sweep toward the mountains, reaching their limit of devia- tion in the vicinity of Golden, or, the latter feature being considered with oO f=) reference to the profile itself, the Laramie and overlying strata have acquired a downward bend at the center of the area, directly over the crown of the arch of post-Niobrara times. Compare also figs. 4 and 5, following. The final movement which produced the present structural conditions, and the outpouring of the lavas of Table Mountain midway the period of the Denver formation, is regarded as constituting the fifth and closing stage in the geological development of the area under discussion. DISCUSSION OF MOVEMENTS PRODUCING THE PRESENT STRUCTURE. Statement of the hypothesis upon which the argument rests —H' yom the not infrequent occurrence, either within the present area or in other parts of the Rocky Mountains, of compound folds of the S type and of otherwise contorted strata, from the presence of reversed faults, and from the occurrence of the well-known folds en échelon, it is believed that the theory of lateral compression as the means by which the forces uplifting the range were generated, although not accepted by all scientists, does, nevertheless, more completely and satisfactorily fall in with the observed facts than any other MON XXVII——7 98 GEOLOGY OF THE DENVER BASIN. which can be suggested. It is not intended, however, that this shall pre- clude the acceptance in the future of any other grounds upon which it may be possible to establish a still more satisfactory explanation of the phenomena forming the subject of this section. Manner in which the forces of elevation have locally manifested themselves —A t VATiOUS points along the base of the Colorado Range occur strongly pronounced local peculiarities of structure, either faults, or folds of varying shape and char- acter, both secondary as to the general uplift of the range. It is highly probable that these structural peculiarities are attributable to the general forces of elevation that are acknowledged to have been in action throughout the several geological periods here represented. The unequal distribution of, or resistance to, the general force of elevation —Still further, it may unhesitatingly be granted that the general force of elevation or the resist- ance opposed to it has been more or less unevenly distributed from point Fic. 4.—Ilustration of unconformity near Golden. to point, and has acted, not always at an absolutely right angle to the axis of the range, but diagonally to it, in one or more directions at the same time. Its direction has in fact varied according to circumstances. The development of the post-Niobrara fold—In. the present area the distribution and directions of this force up to immediate post-Niobrara time had been such as to eventually bring into existence the fold of the general character represented in the profiles, and in fig. 4. An analysis of this distribution and its effects shows that, of the various components of this force, the major, which exceeded all the others combined, was that acting in the general elevation of the range and directly against its axis—that is, for the eastern base, with the atrows A (fig. 4), THE REGION ABOUT GOLDEN. 99 westward. This had undoubtedly been in action with probably but little interruption from earliest time. The other components, secondary to that just noted; and acting in directions more or less normal to it, B (fig. 4), were evidently periodical in character. They reasserted themselves with special intensity at the close of the Niobrara, effecting almost entirely at this time the pronounced elevation under discussion, ¢ (fig. 4), the cross-section of which is that in Profile IV, Pl. X. The Montana, Laramie, Arapahoe, and early Denver beds were then deposited upon this fold, closing the first four periods of history discussed above. The Profiles I-IV, Pl. X, inclusive, may be regarded as transverse (north-and-south) sections of this secondary fold in the several stages of its development according to the geological time represented by each. Fig. 4 is more particularly a diagrammatic representation of the condition of affairs at the close of the Laramie or at a point in time somewhere between this and a stage early in the deposition of the Denver formation. Fic. 5.—Illustration of unconformity near Golden. By the post-Laramie movement the strata were bent up against the range nearly at right angles and afterwards truncated by erosion. This effect is produced in fig. 5 by supposing a slice of the block represented to have been turned down through an angle of 90°, as if hinged along the line C D. The hinged portion is thus a diagrammatic representation of the superficial outlines, as shown in detail by the map. The readjustment of forces by which the structure of post-Niobrara and Laramie times was changed to that of the present day— At the close of the events constituting the fourth period there began a readjustment of the major forces acting against the range, by which the fold of pre-Montana age and its cap of horizontal strata gradually gave way to the structure of later times. The results of this readjustment 100 GEOLOGY OF THE DENVER BASIN. may have been developed prior to the time of the inclusion of the affected area within the general uplift of the range, but were more probably syn- chronous with it. The complex movement which brought about these results way prop- erly be resolved into two chief components. The first of these includes the movement by which the strata composing the pre-Montana fold were brought from their position, as represented in Profile IV, Pl.X, to that which they hold in the natural section given by the outlines on the map. The effect of this movement can be seen in diagrammatic representation, shorn of all complicated details, by comparing fig. 4, which shows the conditions previous to the movement, with fig. 5, which shows those subsequent to it. In this movement the strata resting horizontally upon the pre-Montana fold of necessity followed the recession of the beds beneath, assuming the position of the synclinal depression d in fig. 5, or the highly curved position—the result of the synclinal position—which they hold in the section on the map. The second component is the movement specially involved in the elevation ‘of the range, by which the strata were brought into the highly inclined position they hold along its base at the present time. Readjustment of the forces accounted for—'T‘he readjustment of the forces effecting such important structural changes can be accounted for by relief from the compression to which the strata had been subjected, brought about beyond the immediate region here considered. The exciting cause may have been elevations in other areas, or even an increase in the force of the general lateral thrust to the north and south of the field, accompanied by a variation from its normal western direction to directions diagonal to the range and divergent as this is approached, by which the original north-and-south compressive forces would have been compensated by the components of the later one acting in the reverse direction respectively (B, fig. 5). Equilibrium having been restored over the area in question, and a portion of the affected region having become involved in the general uplift of the range, which still continued, subsequent erosion and the formation of the plane surface of the present day exposed the underlying strata in the superficial section now existing. Relation of the basalt eruption to the above events— The eruption of the Table Moun- tain basalt took place early in the period of the Denver formation, THE REGION ABOUT GOLDEN. 101 approximately after the deposition of about one-third of the series had been completed and some time before the strata had assumed the extremely high angles they now have. With regard to its relations to the phenomena forming the subject of this section, it is possible that the subsidence of the Niobrara fold and the horizontal beds capping it may have been, in its later stages, synchronous with the eruption of the basalt masses in Denver times and perhaps, in a measure, due to it. The fissures through which the pent-up lavas found egress may have been the result of the almost constant bending to which the rocks were subjected, and their ejection may have thus constituted the final event in the history of a region remarkable for its dynamic movements. VIEWS OF OTHERS ON THE STRUCTURE OF THIS REGION, The views of Marvine—These are given in Vol. VII (1873) of the Hayden Reports, where he has expressed, in the briefest possible manner, the ide: of nonappearance of strata due to an actual “thinning of the original deposits * * * from conditions naturally attending the laying down of new formations upon the newly prepared and hence uneven surfaces of older rocks.” He also mentions, as an alternative, the possibility of a fault accounting for the structural peculiarities, but remarks the limited knowledge of the locality which he then possessed. The unpublished results of his work during the season of 1874 unfortunately can not be traced, and therefore his final views must remain unknown; but the brief statement given above leads one to believe that he would in the end have reached a solution not far different from the one presented in the foregoing pages. The views of Ward— These are to be found in the Sixth Annual Report of the present Geological Survey of the United States, pp. 537-538, where, referring to the strata in the vicinity of Golden, between Table Mountain and the Cretaceous (Montana group)—which embrace the Denver, Arapa- hoe, and Laramie formations, but which are all included by him in the Laramie, irrespective of stratigraphical evidence—he remarks: The strata are conformable, and both the Cretaceous and the Laramie are tilted so as to be approximately vertical. At the base of South Table Mountain the strata are horizontal, and the line dividing the vertical from the horizontal strata could be 102 GEOLOGY OF THE DENVER BASIN. detected at certain points. A measurement from this line to the base of the coal seam was made at one place and showed 1,700 feet of the upturned edges of Laramie strata. It is probable that we here have the very base of the formation. The geology of Golden is very complicated, but my observations led me to con- clude that during the upheaval of the Front Range a break must have occurred along a line near the western base of Table Mountain, forming a crevice through which issued the matter that forms the basaltic cap of these hills. The eastern edge of a broad strip of land lying to the west of this break dropped down wntil the entire strip of land assumed a vertical position or was tilted somewhat beyond the perpendicular. This brought the Laramie on the east side of the Cretaceous, with its upper strata at the extreme eastern, while the coal seam at its base occupied the extreme western side of the displaced rock. The degree of inversion varies slightly at different points and may have been much greater in some places. This will probably account for the discovery at one time of a certain Cretaceous shell (Mactra) above a vein of coal ina shaft about 4 miles north of Golden, and about which considerable has been said in discussing the age of the Laramie group. I visited the spot, but found the strata so covered by wash that I was unable to determine their nature. In the above views there are four points requiring notice, although one—that regarding a certain Cretaceous shell—is somewhat irrelevant. The first point is the remark as to the conformability of all the strata from the Denver beds to the Montana group. Although no discrepancy in dip or strike is noticed between them in the vicinity of the Table Mountains, : study of the whole region has abundantly proved the existence of several unconformities, by evidences of erosion, by the areal distribution of the outerops, and by the character of the component materials of the various formations. ‘The second point is the crevice near the western base of Table Mountain, through which issued the basalt of the region. As a matter of fact, no evidence of such a crevice, nor of the dike which would still remain as its filling, exists along the well-exposed base of the hills. Furthermore, the outpouring of the basaltic sheets is entirely accounted for by the great Ralston dike and the irregular eruptive body near its southern end, and hence there is no necessity for assuming a further fracturing of the strata to give it a vent at some other point in the field. The third point, the fault, into which Professor Ward has developed the break, beyond a doubt coincides in locality with the great fold which oceurs all along the eastern base of the Colorado Range, by which the beds to the west of it are sharply upturned, often to a vertical or reversed THE REGION ABOUT GOLDEN. 103 dip, while their continuance to the east of the axis is at a dip of but the slightest amount. This curve may at times be complete within a distance of 50 feet. A fault is therefore unnecessary to explain the abrupt change from the vertical to the horizontal position. Moreover, observations show that the Denver and Arapahoe beds actually take part in this fold at a point directly opposite South Table Mountain. The complicated geology of the region would very naturally lead one to mistaken conclusions unless a thorough knowledge were possessed not only of the area of disturbance here considered, but also of the general structure of the region far beyond. The fourth point in the quoted remarks of Professor Ward relates to the manner in which he accounts for the fossil Mactra found, according to prior statements, ‘‘over” the coal. As a matter of fact, the fossil does not oceur over the coal, but beneath it, in its usual position in the Pierre bed, its apparent position being due to its lying within a locally faulted area, the beds of which have been thrown to the eastward of the general trend of the coal in the unaffected area to the south and north. The views of others—In addition to the views of the above writers, others have from time to time been expressed, implying belief in a fault in the vicinity of Golden to account for the peculiarities of structure there displayed. In reply to this it need only be stated that no fault can be conceived which will at once account for the several features in the geology of this region as exposed over the present surface of the area and set forth in the preceding pages. SPECIAL IRREGULARITIES IN THE GOLDEN REGION, The Ralston faults—'These occur in the vicinity of the Ralston dike, mid- way between North Table Mountain and Ralston Creek. They entered only into the later development of the region in a manner incidental to rather than as a primary factor in the events that transpired. There are three fractures, and their peculiar features are due chiefly to exceptional local conditions connected with the neighboring eruptive phenomena. The fractures bound the western, northern, and southern sides of a rectangular block of strata of indefinite but not great thickness, beneath which there is probably a large mass of eruptive rock, a part of that which 104 GEOLOGY OF THE DENVER BASIN. oceupies the western of the three rents and forms the prominent north-and- south dike of this locality. The block of strata inclosed by the fractures extends north and south a little over a mile, east and west about 2 miles. Its component strata include rocks of the Pierre, Fox Hills, Laramie, and Arapahoe formations, completely shattered and in a most chaotic state, except in the extreme eastern part of the area, where their dip becomes more regular, and gradually shallows to the horizontal beyond the disturbed region. The amount of displacement which the interfault block has suf- fered is variable for the east-and-west fractures, while an actual estimate along the north-and-south fracture can not be made, on account of the shattered condition of the beds and the impossibility of recognizing definite horizons. The north-and-south fracture approximately coincides with a stratifica- o@ this. The z tion plane, the planes of the east-and-west faults intersecting north-and-south plane is now nearly vertical, though at the time of erup- tion it was probably more or less inclined, the strata having suffered some folding since. The east-and-west fractures are apparently vertical. The development of the three fractures is probably the combined result of the general folding that took place along the Colorado Range and of the presence of a mass of eruptive material seeking an outlet at the surface through the channel of least resistance. The north-and-south fracture, occupied by the main dike, probably antedates the others and was primarily of slight displacement, the more extensive throw of its southern half being due to a local enlargement of the eruptive body. The east-and- west displacements were concomitant features. The presence of the locally enlarged mass of eruptive material, the increasing pressure as the folding advanced, the yielding nature of the overlying clays, which also dipped toward the east, all united to at last compel a rupture and dislocation of the overlying beds in the location and directions platted. The resistance to the strain developed by this condition of affairs is well shown at several points in the disturbed area, but especially along the northern fault line, where there is abundant evidence in the normally flexed ends of the opposing beds, of distortion and bending before the strata finally yielded to the force of compression brought to bear upon them. TOPOGRAPHY AND GEOLOGY NEAR BOULDER. 10d THE REGION ABOUT BOULDER. The structural features of the region about Boulder are of the same general type, except for the absence of eruptive phenomena, as at Golden— that is, there is here another series of unconformities occurring at various horizons from Archean to Montana. The area affected by this series of unconformities extends along the foothills from a point a little over 2 miles south of North Boulder Creek to one about 1§ miles north; its breadth is nowhere much over 1 mile, and the phenomena are confined to the open slopes of the foothills and the bench lands and prairies adjoining. TOPOGRAPHICAL FEATURES, As about Golden, the topographical features, through influence of the geological phenomena involved in the region’s development, again diverge from normal. Instead of the fringing reefs of Dakota and Niobrara, the Triassic and Archean slopes directly overlook the prairie. The disappear- ance of the hogbacks, the wavy trend of the strata in actual outcrop or in soil delineations, the approach of formations in converging strikes, and the occurrence of short east-and-west faults all contribute to variation from the usual foothill character. GEOLOGICAL FEATURES The formations involved in the geology of the area include all that occur in the Denver field from Archean to Pierre. The Archean—T"his presents the usual wavy line of union with the Trias, significant of the unevenness of the floor receiving the deposits of the latter, but no feature of structure or sedimentation peculiar to this region alone appears. The Trias —The lower member, so far as the phenomena under discussion are concerned, has been unaffected, unless, perhaps, in the thinning which is observed in the vicinity of Boulder Creek, due to a rise in the Archean floor, which may have been the incipient movement of the series to follow. The local fold developed on the face of the Triassic slopes between Boulder Creek and Gregory Canyon is of an origin later than the phenomena here discussed, and unrelated to them. 106 GEOLOGY OF THE DENVER BASIN. The upper member of the Trias, within the limits of the area under consideration, rapidly diminishes from its normal thickness of about 600 feet at either end to complete disappearance at Gregory Canyon, near the center of the affected region. Measurements at two points along the line of gradual disappearance—one just north of Boulder Creek, between it and the wagon road leading to Sunshine, the other a few hundred yards south of Gregory Canyon, near the northern termination of its southern outerop—gave a thickness, respectively, of 350 and 50 feet. It is the lower ben} beds of the member that are the last to disappear, the limestones near its base appearing at several points after the formation has begun to thin out. Destruction of its beds has apparently been carried to its very base, though not beyond. The Jura——This formation, for the greater portion of its outcrop within the affected area, shows a marked decrease from the normal thickness, but nowhere disappears entirely. The decrease first becomes apparent, at the north, in the vicinity of the Sunshine road; at the south, in a gulch about three-fourths of a mile south of Gregory Canyon. The diminution almost wholly occurs near the limits of the area of unconformity, a considerable distance along the center showing a constant thickness of about 50 feet. The evidence of the strata in explanation of the thinning is in favor of non- deposition of the lower beds, the characteristic conglomerate and mottled sandstone near the summit of the formation appearing at several points along the trend of the strata. The Dakota—T'his thins very perceptibly between Polecat Canyon and the gulch next north, having within the intervening half mile decreased in thickness from nearly normal to about 100 feet, with a complete disappear- ance of the hogback features. At Gregory Canyon the decrease is still ereater, the actual exposure of Dakota here being only about 4 feet, but with a covered thickness of probably 30 feet more. Immediately north of Boulder Creek it is still at its minimum, but at Sunshine Gulch it is rapidly while a regaining its width—at this point being approximately 150 feet half mile farther north it has again reached its normal thickness. Over the area of minimum thickness it is quite impossible to determine whether it is the upper, middle, or lower portion of the formation which remains, but Site i] i] MONOGRAPH XXVII. PL. XI. N NS S No 25 : LS t \\ f i \ INS EE SSS INO SKS ‘) ~ H SSI TIS ete ) } iS SS ” < i | . wos, \7 4 4, i a 4 ¢ ¢ s\vQo se NANG £4 é | t fh i NEN mle Si = ¢ % 7, ¢ ¢ eS SU LY ES ea aN ain , * Ss NX ul XESS NS A U.S.GEOLOGICAL SURVEY 20° = ee “si a a \ JURATRIAS. 2 MILES. Ni iN ——— bo .S SS WN See NSS Sines 7 FOSS = WV) MS ———————< —— NG NON-CONFORMITIES. Scale Contour Interval 100 Feet. HT mrt : {hit in i N ‘ Xs <\ih eo ’ jee ingame IAAI SANTI pect HUH WU | wai it \ , CRETACEOUS BOULDER AND ADJOINING REGION, SHOWI PLEISTOCENE —_—_ LLUVIU ies Vere: rs i fae 5 i ; ‘ ; d : = 5 3 rd & a 8 2 © rt = HI] ry ye { | | | } MONOGRAPH XXVil. PLXI. . by LOWER WYOMING AS. LO NFORMITIE ATRL ‘O 2 MILES Re JL t ral DODO One, NON-C a ys ~ K GA Z say: a A — ian am “i o a Bentz r 2 Spt | le! 1s] SAS N o) a. E l8 Shs S'S sels |e " \ os) a 1a jaw, S| o ide = » , a 57) oe y Zz Z, 2 |: 5 ° | a a) a) laa a“) Be] 000 , . read before the Colorado Scientific Society in Denver, July 2, 1888. This ‘Annual Report of the U. S. Geol. and Geog. Survey of the Territories for 1877, p. 192. 2Third Ann. Rept. U. S. Geol. Survey, 1883, pp. 26-27. ®Sixth Ann. Rept. U.S. Geol. Survey, 1885, pp. 405-557. #The Artesian Wells of Denver—a report by a special committee of the Colorado Scientific Society, p.8. This report was reprinted in Vol.I of the Proceedings of the Society, issued in 1885, D: 19. ; THE DENVER FORMATION. 159 was published in the society’s “ Proceedings,”’ in association with a paper by Mr. Eldridge in which the Arapahoe formation was first described and named. In revised form the same communication was later published in the American Joumal of Science? Since the first published descriptions of the Arapahoe and Denver formations a number of articles have appeared in discussion of their age or describing fossils found in them. Reference will be made to these publications in the following chapter. From the foregoing historical sketch it appears that Golden has been visited by a goodly number of geologists and paleontologists who have paid more or less attention to Table Mountain and its strata. It is true that no very detailed work was attempted by any one of the gentlemen cited, yet several of them made extensive collections of fossil plants in the strata of the coal measures and of Table Mountain, and from the weight that has been attached to the determinations of these plant remains it would seem that the collectors must have had full confidence in the accu- racy of their knowledge concerning the relations of the various horizons from which the fossils were obtained. DESCRIPTION OF THE FORMATION. THE BASE OF THE SERIES. Character of the first sediments——The following description of the lowest deposits has special reference to the section more or less plainly seen along the western line, at the bases of Green and Table mountains, for here the sediments are more clearly typical than in the exposures apparently at the base of the series which are found north of Clear Creek. Here, too, the transition into higher horizons can be followed more con- nectedly, and thus a characteristic section established with which other outcrops may be compared. The actual contact of Denver and Arapahoe beds is seldom well exposed, hence a complete description of the change from one to the other can not be given. The boundary line of the Denver formation as drawn upon the map is approximate, being clearly established at but few points. The explanation of this inability to definitely locate ‘Proc. Colorado Sci. Soc., Vol. Il], pp. 119-133. 2The Denver Tertiary formation: Am. Jour. Sci., Vol. XXXVII, 1889, pp. 261-282. 160 GEOLOGY OF THE DENVER BASIN. what is probably in reality a very sharp line, lies in the soft and easily destructible character of the clays and friable sandstones constituting the strata of both formations near their contact. Even along the western border, where the edges of the strata are upturned, it is difficult to find good outcrops at this horizon. The lowest beds seen which have been referred to the Denver series are sandy layers consisting of a mixture of quartz and feldspar plainly derived from Archean rocks, with other minerals, such as augite, horn- blende, biotite, and feldspar, which exhibit properties belonging to the constituents of eruptive rocks. Except, however, occasional pebbles of distinct andesite, which sometimes occur in these layers, there may be nothing to suggest the character of these mineral particles until they are subjected to microscopical examination. It is then found that at least a portion of the feldspar contains ore grains or microlites of zircon, apatite, and augite, with more or less typical glass inclusions, all these indicating an eruptive origin. As to the hornblende and biotite, it is not always possible to determine their source so clearly, but the augite is, like the feldspar, plainly to be considered as a former component of some eruptive rock. Although much of the lower part of the Denver series is almost if not entirely free from Archean débris, the actual base contains a mixture of materials. A consideration of the facts leads to the belief that all mineral particles of noneruptive origin contained in the lower layers of the series belong to the movable sands of the sea-bottom upon which the new forma- tion began. The base of the Denver series is thus determined by the first appearance of eruptive material among the particles derived from the erys- talline or older sedimentary rocks. This line could be determined with accuracy were there sufticient outcrops along its course. Strata of Section Ravine—The single exposure along the western line which shows clearly and sharply the transition from the beds of the Arapahoe to those of the Denver formation is that in Section Ravine, upon the south- western slope of Green Mountain. The Arapahoe formation ends with a series of clays, which are succeeded at a certain point by a hard, coarse- grained sandstone or grit layer, in which quartz grains are very prominent, THE BASE OF THE DENVER. 161 but which contains in addition some augite and some clear plagioclase particles with glass inclusions. The strata are here vertical, with a strike N. 4° to 5° 30’ W., and the contact plane of sandstone and clay is wavy and irregular. At some 10 feet above the contact the sandstone is finer- erained, softer, and contains abundant glassy plagioclase grains, with horn- blende, augite, biotite, and some rounded quartz particles. Above this horizon the section is incomplete, but sandy beds or fine conglomerates appear at intervals, with a rapidly decreasing dip, and in these the particles are all derived from andesites. It is probable, then, that we have here exposed the actual contact line of the Denver and Arapahoe formations. Outcrops in Kinner Run— ‘he small water course called Kinner Run, which enters Golden from the southeast, near the base of Table Mountain, has cut down into the lower strata of the Denver formation at several points, though it may not have penetrated to the very base. The banks of the rayine east of the court-house show horizontal, crumbling gravel and sand strata in which minute pebbles of andesite are more abundant than quartz grains. Below these layers are clays. Also near the forks of the run, a little south of town, there is a similar contact between sandy beds con- taining small andesitic pebbles and pure clays below. THE STRATA OF SOUTH TABLE MOUNTAIN. General statement—F'or a distance of 300 or 400 feet above its base the Denver formation is prevailingly made up of fine-grained, friable sand rocks, of clays, and of intermediate mixtures, which are not sufticiently coherent to form good outcrops except under the most favorable cireum- stances. In consequence of this character no complete section can be given of this part of the series. The basaltic sheet of Table Mountain which was poured out upon the floor of the shallow Denver sea has preserved the underlying strata from complete destruction, and it is upon the steep slopes below this lava flow that all the best exposures in this part of the series are now to be found. While continuous sections are of very limited extent, a good idea of the character of the entire thickness of the strata represented may be obtained by correlating, as far as possible, the scattered outcrops of ‘Table MON XXVII——11 162 GEOLOGY OF THE DENVER BASIN. Mountain, and while the slopes of North Mountain are steeper and higher the most instructive exposures are all upon South Mountain, or adjacent to it. Exposures upon the western slopes— Were the rocks under discussion a little better adapted to resist the disintegrating tendencies of ordinary weather- ing, there would probably be continuous sections at many points. For instance, on the ridge running west from Castle Rock toward Golden, the coating of débris upon the slopes is very thin and the presence of Denver strata can be demonstrated at almost any spot, although the character of the stratum may not be well shown. As it is, the study of the footpath leading from Golden to Castle Rock, following in the main the ridge just mentioned, will give a very good idea of the essential characteristics of the formation. The lowest stratum clearly shown is that exposed by the ditch at the end of the ridge nearest town. ‘This is perhaps 75 feet above the outcrop in Kinner Run which has been mentioned as the possible base of the formation. The rock is yellowish in color, consisting of small pebbles, gravel, and sand with some clay as matrix, and crumbles easily. Some pebbles reach an inch in diameter and prove to be hornblende-andesites of fresh condition and typical structure. One of these contains 61.25 per cent SiO,, and in this rock tridymite is very abundant. With a hand lens one can recognize glassy feldspar, augite, biotite, and hornblende particles, and a few rounded quartz grains in the finer material. About 300 yards south of the footpath and 75 to 150 feet above the ditch level there is a succession of conglomeritic and sandy beds exposed by a small gully. The greater part of this exposure consists of more or less friable sandstone or tuff composed entirely of débris of andesites. The microscope shows glassy feldspar, biotite, augite, hornblende, ore, and small, worn particles of corresponding andesites. The cement is partly fibrous and partly isotropic, and is described in detail in another place. Pebbles of typical andesite are scattered through nearly all layers, but are most abundant near the top of the section here exposed, there forming a normal conglomerate. Nearly all the pebbles are small and there is a great variety of types represented, all belonging in the andesitic group. DENVER OF SOUTH TABLE MOUNTAIN. 163 At horizons corresponding to this outcrop there are in many places indications of the presence of similar strata, through small exposures or surfaces strewn with pebbles. On following the footpath up the ridge a number of outcrops of sandy strata may be found, though seldom of an extent deserving special mention. The beds shown are often so fine-grained that their composition may not attract attention. At the northern base of Castle Rock the contact of basalt and sandy strata is shown at several points near the path, and other outcrops reveal the character of the adjacent beds very well. About 250 yards southeast of Castle Rock there is a fine outcrop of one of the most persistent beds shown in Table Mountain, and one that conveys most readily and clearly an idea of the characteristic composition of the strata of the formation. It is a dark conglomerate, 15 to 20 feet in thickness, which here forms a small cliff at about 20 feet below the basalt sheet. It is chiefly made up of dark andesitic pebbles of many types, varying in diameter from 5 inches downward. The matrix is a coarse sand of eruptive origin, prominent among the particles being augite in isolated prisms with terminations, the edges not having been sufficiently rounded to obliterate the form. The sandy parts of the bed develop in places to wedge-shaped masses exhibiting in their relations to each other and to the conglomerate a very marked cross-bedding. Above this conglomerate are dark sandstones continuing upward to the basalt, which are characterized by the abundance of distinct augite crystals. Below are similar sandy strata. The variety of andesites represented in the conglomerate is great, as will be seen by a reference to the petrographical chapter. Most of them are augite-andesites with varying amounts of hornblende and biotite, but no type here found contains hypersthene, while such rocks are common in the higher strata shown in Green Mountain. One variety is very light colored, aside from the sparsely distributed augite prisms which correspond closely to those mentioned as occurring free in the sandy matrix of the conglomerate, and in the sandstones above. The variation in silica is great, some being very basic, while others contain free silica in the form of 164 GEOLOGY OF THE DENVER BASIN. tridymite. Compact porphyritic structural types prevail, although even- grained and porous rocks are also abundant. This stratum, while the most persistent known in Table Mountain, may, nevertheless, be also used as an excellent illustration of the lateral variation in Composition Common to all or nearly all the conglomeratic and sandy beds of the formation. On following this conglomerate toward Castle Rock, it is found to decrease rapidly in thickness, and at the same time the size of the pebbles becomes less. The horizon can be traced for some distance uninterruptedly, is then covered for an interval, and it can scarcely be identified with the equivalent sandy layers shown on the south face of Castle Rock. There are in the latter outcrops some sandy beds containing small pebbles, but not in sufficient quantity to constitute a conglomerate, nor in so great a quantity as may be found in many other horizons where no conglomerate proper is ever developed so far as known. In spite of this described variation the horizon is exceptionally well marked as a conglomerate, and at many points on both Table Mountains, and on all sides, the clear equivalent of this stratum is to be found at or within a few feet of the contact with the basalt sheet. Section at the northeastern point of South Table Mountain — While the exposures already described give a clear idea of much that is characteristic in the series under discussion, the lack of continuity in outcrops fails to bring out other impor- tant peculiarities of the formation. This lack is in a measure supplied by the succession of strata shown on the eastern slope of the northeastern extremity of the mountain. There is here a continuous section of 170 feet of strata, from the basalt downward. No other outcrop exhibits so great a thickness of the lower beds of the formation. The succession of beds here seen gives a very good idea of the constitution of all the Denver beds, except the heavier conglomerates. Below this section there are several small outcrops at points down to the ditch level. They are either of clayey beds, with but little sand, or they represent layers corresponding to 1 or 2 of the section. In the sand rock included under 1 of the section, black biotite leaves are very prominent; pebbles are very rare here, and most of the mineral grains are angular. The section is as follows: DENVER OF SOUTH TABLE MOUNTAIN. 165 Basalt, at top. Feet. 0. Dark-brown clays and fine gravel in alternating layers. 11 9. Coarse gravel and pebble-bearing bed..........--- 25D 8. Gravel layer at bottom, passing gradually into a dark, reddish-brown clayrabetopee s----- te ee ee 18 7. Conglomerate of small, dark andesite pebbles.....-..-- 2 6. Fine-grained, light-colored sand rock or tuff.......-.-- 5 5. Fine-grained rock, like 6 at base, passing into clay at 10) QS emer romn aS conch abel ce Sco aS ee NaS OA eeE EO 20 4, Dark clays, often mottled, containing small pebbles of light-colored andesite and tuff...................--- BA 3. A series of alternating light-colored clays and semicon- glomeritic or tuffaceous layers -......-...- SA eer 52 2, Eriable sand rock and sandy clays--.....-...------.-- 18 I, Sand rock, more compact than 2.-.......-.--..-.---.-. 6 Mota s Cec eS yoann ee ees ee nia Sees eer 171 In division 2 of the section there is a general distinction to be made between the lower 8 feet, which are dark-colored through the abundant admixture of vegetable matter, and the upper 10 feet, which are firmer and contain less carbonaceous substance, though stems and imperfect leaves of plants are common. Some of the lower layers are chiefly made up of plant remains. Under division 3 are included strata of peculiar constitution which are well developed in some places not far from this exposure, but do not appear at many other points where it seems likely they would be distinguishable if of the character here presented. In the section the division begins with some almost conglomeritic pebbles of a light-yellow or straw-colored andesite, and others of reddish or brownish colors. The latter are merely decomposed rocks similar in kind to the darker ones of other horizons, while the light andesite is of a variety found only in 8, 5, and 6 of the section. Passing upward in the series there is an alternation of clay, or what may be more expressively termed mud layers, with gravel or sand rock. The strata of this complex vary in thickness from a few inches to 6 feet or more and are laterally variable in this respect. The coarser-grained beds show cross-bedding. The mud layers are easily removed by water on any exposed surface, and so leave projecting shelves of the intermediate 166 GEOLOGY OF THE DENVER BASIN. strata, producing very jagged outcrops. Short tree stumps are common in the gravel layers, their roots penetrating the mud beds below. The repetition of this feature shows the conditions of deposition of these beds. The gravel layers ordinarily contain many small pebbles of the light andesite, while darker varieties are in some places almost excluded. The lighter-colored rock is augite-andesite with a very small amount of augite, carrying some hornblende and biotite; the groundmass is often largely eryptocrystalline. The upper layer of 3 is more nearly a sandstone in composition than the lower beds. It contains very many small pebbles, but the main substance is ash-like and the rock is thus similar to that of higher layers, designated tuff. The strata included under 4 are chiefly clayey in character, but there is really no sharp line at the base, for the upper stratum of 3 differs from the lowest of 4 in being firm and coarse-grained, and the latter passes through admixture of clay and increasing fineness of grain into the beds which are more properly called sandy clays, and so on to quite pure clay. Upward there is a change in color, the upper clay being dark reddish- brown with spots of lighter color. Pebbles of andesite occur sparingly all through, and a few angular fragments have been found. Tree stumps occur near the middle of this division. The darker clays are full of sand, as is shown by washing them, and if the sand thus purified be examined under the microscope it will be found to consist of augite, hornblende, biotite, and feldspar. With these minerals are small, round grains representing pebbles and fragments of andesite now almost completely destroyed. A sand of similar composition will be obtained on washing any clayey stratum in the Denver formation. Plant remains are abundant in the division 4, though seldom well preserved. ; The beds of No. 5 of the section are most typical. Deposited upon the hard, brown mottled clay of 4 is a stratum of quite compact sand rock of light-yellow or gray color. This is even-grained and uniform except for pebble-like masses or concretions of a material extremely like the matrix in which they lie, but as a rule somewhat coarser in grain. These masses are sometimes 3 or 4 inches in diameter and in certain layers are thickly DENVER OF SOUTH TABLE MOUNTAIN. 167 crowded together like the pebbles in a conglomerate. By the aid of the microscope it appears that both matrix and pebble-like masses are made up of angular mineral particles, chiefly glassy feldspar, with some hornblende, biotite, and augite, and also of particles representing the dense groundmass of corresponding andesite. But few true pebbles are present, and these are of the general type mentioned as occurring in the beds of 3. The rock is then made up chiefly of particles which are not extreme results of abrasion, and they are plainly derived from a certain type of rock. It is thought entirely appropriate to call it a tuff. The pebble-like masses are to be considered as masses of a tuff probably rounded up by wave action while yet semiplastic. In proof of the correctness of this deduction from their mineral constitution and structure it is to be mentioned that plant stems have been seen in the pebbles, but they are never found extending outward into the surrounding matrix. The lower 8 feet of division 5 are of the character above described, while in the upper 10 feet there is a gradual transition through sandy clays to a pure clay at top. In division 6 there is almost a repetition of the preceding beds with a development of friable sand rock or tuff in some layers, of about the composition described. Number 7 of the series represents a decided and sudden change in the character of the materials deposited. It is practically a fine conglomerate in which dark and comparatively basic andesites prevail, the lighter ones of the lower horizon being very subordinate in quantity. No pebbles seen in this stratum are over 5 cm. in diameter. This conglomerate may be the representative of the thick, dark bed near Castle Rock, but that is hardly probable, as it is here too far below the basalt, and such local development of sandy or gravelly layers is entirely possible at almost any horizon. The beds included under 8, 9, and 10 of the section are really not separable except for this particular section. Their differences are such as may arise in any complex of strata at about this general horizon, in different order and manner from that here found. The specially marked conglomerate (7) may also be included in this statement, but the change from 6 to 7 is probably always marked by the development of a coarse- 168 GEOLOGY OF THE DENVER BASIN. erained bed which may or may not be easily separated from the stratum next succeeding. In the seetion there is merely a marked line between the conglomerate (7) and a sandy or gravelly bed of practically the same composition but of finer grain. This is followed upward by sands and then by sandy clays, and these by darker, purer clays, to which succeeds again a coarse gravel layer (9), followed once more by transition beds. The bed at 9 is more probably the equivalent of the darker conglomerate usually found at about this horizon than is the better-defined conglomerate below (7). The detailed description of the foregoing section is intended to show the peculiar composition normal for the strata of the Denver series in those horizons and also to bring out clearly the conditions which must have attended the deposition of such fine-grained sediments. It is clear that the small pebbles so universally found have been greatly reduced in size through continued abrasion; that the transition from the coarsé layers at the base of a certain series upward through the finer and finer sediments to clays, which are as a rule sueceeded by a sudden change to coarse materials, must indicate the periods of comparative rest or disturbance of the waters of these seas. Furthermore, the presence of considerable tree stumps in erect position with roots in mud layers and broken trunks in sand or gravel, shows that the water was shallow or even that low-land masses alternated with shallow seas. Probably the latter was the case. The accumulation of these strata was slow and they represent a long epoch of sedimentation. Outcrops at the southeastern point of South Table Mountain Opposite the middle of the foot-like extremity of South Table Mountain is a knoll connected with the slope of the mountain by a ridge, and on the south side of this knoll is a little basin. From the basalt contact down to the bottom of this basin and on the ridge and knoll are good outerops of horizontal strata which again illustrate the occurrence of the beds found in the preceding section within 100 feet of the basalt. Here the section is not quite continuous, and the fine grain of most of the gravelly layers makes a direct comparison difficult, but it is still plain that the sections are equivalent so far as they go. At 60 feet below the basalt appears a light-colored layer, with pebbles of DENVER OF SOUTH TABLE MOUNTAIN. 169 straw-colored andesite. This represents the base of 5 of the section given. All below this point can be brought in comparison with the beds of 3 and 4, but erosion has not yet cut down enough to show the characteristics of bed 3. Fossil wood, leaves, and stems are abundant. Irregular fragments of andesite were found in layers at about 100 feet below the basalt. At this point the basalt capping is scarcely more than 10 feet in thickness; below it the contact with strata is plainly shown. As a whole it seems plain that the strata of this section are finer grained than those of any corresponding outcrops farther west. Cross-bedding is very beautifully shown in the sandy and gravelly rocks on the knoll and connecting ridge. Outcrops on the northern slopes of South Table Mountain—Qn the north slope of South Table Mountain there are prominent outcrops caused by the appearance of beds shown in division 3 of the section. One of these exposures is on the north side of the northeastern point. This shows nothing of special note. The other outcrop is below the indentation in the center of the northern face. Other outcrops on South Table Mountain—Beyond the chief outcrops mentioned there are none of special importance in determining the succession of strata present. All over the slopes are scattered eroppings of minor extent which can not be directly correlated with definite strata of the large sections, but none is found which does not have its equivalent. The leaf-bearing beds which have been visited by those in search of fossil plants are situated chiefly on the southern and southwestern slopes of the mountain and are in minor outcrops. Possibly none of them have been associated in consecutive outcrops with the darker conglomerate beds. Asa rule the best-preserved leaves oceur in fine, yellowish-brown sandstones or clayey strata in which the nature of the constituent mineral particles is not always plain to the unaided eye. One of the horizons which has furnished many of the fossil leaves both of the earlier and of the most recent collections is on the south slope of ‘the southwest point of the mountain, opposite the Reform School. These strata, at about 100 feet below the basalt, are water-bearing, and the Reform School authorities have dug into them, making an artificial spring which furnishes a water supply for the school. The rock taken out here is full 170 GEOLOGY OF THE DENVER BASIN. of leaves and stems of plants. A collection was made at this place in 1883 and the specimens were placed in the hands of Prof. Lester F. Ward for identification. EXPOSURES ON NORTH TABLE MOUNTAIN. The slopes of North Table Mountain are both higher and steeper than the average slopes of South Table Mountain, but there are no outcrops of Denver strata here that are of special importance. Near the basalt there are frequent exposures of a dark conglomerate bed of very variable thick- ness. On the west side of the southeast gulch an outcrop at the contact with the main basalt sheet shows the contact surface cutting down obliquely from the north across about 10 feet of strata, but whether this is due to a plowing action of the basalt or to earlier causes can not be determined. The horizon of the lower basalt streams is situated quite uniformly at 100 feet below the capping sheet, and is at the top of the beds included under 3 of the section already given. These basaltic streams, which are described in detail in the chapter on eruptive rocks, must have been poured out upon a sea bottom and quickly covered by sediments, as is plain from their texture and the fact that they are overlain by sandy beds of material simi- lar to those upon. which they rest. The best locality for examining the relations of these streams to the Denver strata is on the southern slope of the mountain, midway between the two large gulches. Here the upper and lower contacts of a small basalt stream, as well as those of the upper sheet, may be clearly seen, and also a number of characteristic beds of the Denver series. In the sandy strata of a horizon slightly above the basalt streams are found isolated angular specimens of pale bluish-gray augite-andesite, in structures varying from the massive to the very porous forms. The same rock is also occasionally found on South Mountain and on Green Mountain. It is not demonstrable. that all loose pieces come from a single horizon, but it is quite probable that the one above referred to contains nearly all of them. In the cavities of the porous variety are found chalcedony, quartz, and heulandite, and in some fragments the new zeolite species, ptilolite. A distinct spherical sundering was noticed in certain sandy beds not far below the basalt on the northwestern slopes of North Table Mountain. DENVER OF GREEN MOUNTAIN. ial GREEN MOUNTAIN. Position-and form——Green Mountain lies upon the plains between Golden and Morrison and 9 miles south of west from Denver. It is a bald, massive hill of smooth and gentle slopes, rising 1,200 feet above Bear Creek at its southern base, and nearly as much above the western and northern bases. Seen from Denver and corresponding points upon the plains it appears to be a part of the foothills of the main range, but it is actually separated from them by the zone, about 1 mile in width, in which the upturned edges of all the sedimentary formations below the Denver are exposed. The approaches on the north, east, and south are gradual up to the base of the mountain proper, where there is a rather sudden change to an angle of 15° to 20°. The massive appearance, as seen from a distance, is somewhat deceptive, for the mountain is deeply scarred on all sides by ravines which penetrate to its core, so that there is a system of smooth, narrow, branching ridges. These deep indentations are the heads of as many water courses, few of which are more than shallow drains when beyond the base of the steeper slopes. The surface of the mountain is thickly strewn with round bowlders, which have weathered out of the underlying strata. As will be seen by reference to the map, the entire mountain mass, with the exception of a narrow band at its western base, is made up of strata belonging to the Denver formation. Nowhere else is the thickness of these deposits approximately indicated, and even here it is evident that an unknown amount has been removed. The reason for the special preservation of Green Mountain is not apparent. The Green Mountain profle——The smooth slopes characteristic of the mass present few actual rock outcrops, and the deep ravines afford only limited exposures, but upon the western side of the mountain there is a hollow- ing out of the usually even surface, bounded on the north and south by minor ridges, and drained at the bottom by a little ravine. The steep face at the back of this hollow, the ridges on either side, and the ravine below, combine to give a practically continuous section of strata extending from near the summit down to the base of the steeper slopes, a vertical Wa GEOLOGY OF THE DENVER BASIN. distance of about 500 feet. In consequence of the near approach of Green Mountain to the line of the great fold, this exposure on its western face exhibits very clearly the extent to which its strata have taken part in that folding. At the top of the section the heavy conglomerates have a slight eastern dip; at its base a dip of 45° is shown, and at a distance to the westward but little greater than the known thickness of the intervening strata the beds are found in vertical position. In order to determine the total thickness of the Denver beds and the relative positions of prominent horizons, a straight profile line was surveyed across the mountain, passing up the outcrop described, as nearly parallel to the direction of dip as possible. The course of the section is N. 42° 30! E. and the direction of the dip is N. 74° 30’ E., hence there is a divergence of 82° between them. By projecting the outcrops of distinct horizons from the section line upon the line of dip, and by estimated average dips for certain subdivisions of the sections, an approximate determination of the total thickness of the section has been made. Estimated thickness of Denver formation. Average dip Length of | Equivalent ’ Saba | within | outcrop on | thickness | subdivision. | line of dip. | of strata. Feet. Feet. | a Vertical] ..-.. 250, 250 | b ibe 1807 9) 5 | C 55° 250, 205 d 42° 205 187 P| € 35° 170 98 ne 25° 275 116 q 15 465 120 h | 5 2, 145 239 | i IPE onizontalle=|e eee ee 100 In regard to the average dips assumed in the above table it should be explained that at various places on the line of the profile the dip can be determined approximately, though seldom very accu rately. The strata of the Arapahoe conglomerate horizon are found to be vertical. Above this to the dark andesitic conglomerate at the base of the section no dip can be DENVER OF GREEN MOUNTAIN. WS measured, but as.the latter has a dip of 45° and as it is shown by adjacent outcrops that the fold is here sharp and that the transition from vertical beds to those having a dip of 45° is sudden, it seems highly probable that some of the lower beds of the Denver series are vertical on the section line. From the dark conglomerate the exposure is almost continuous, and although sharp bedding planes are so rare as to make accurate dip meas- urement difficult, still the gradual changes are visible and they agree closely with the drawn section. Dip measurements were made as follows: 42° between d and c; 18° ing; 10° inh. For convenience in discussing the section, four divisions are made—A, B, C, and D. Division A, 58 feet-—This embraces all up to the dark conglomerate. No outcrops of importance occur on or near the profile line of division. Its base, which is also the base of the formation, is an assumed point deduced from the known thickness of the Arapahoe beds in the vicinity, their known strike, and vertical position. A few yards south of the line of profile is a smooth ridge showing here and there crumbling, sandy strata of the Denver formation, and also farther west the upper Arapahoe clays. The outcrops here are not numerous enough to permit a determination of the line between the formations with accuracy. This division must include strata corresponding to those described in Table Mountain, but the detailed constitution can hardly be the same here, or else more distinct outcrops would have resulted. The strata occurring about 100 feet below the basalt forming an alternating series of tuff, gravel, and clay layers of light color, are apparently not represented here, as such material would surely have been discoverable in the ridge mentioned near the profile line, or in some others on the western slope of the mountain. The few exposures in Section Ravine to the southward, which have already been mentioned (p. 160), show that the lower horizons are composed of very loose and friable clays and sand rocks, and the absence of firm outcrops on the section line is not at all remarkable. Division B, 50 feet——'Ihis division embraces the dark conglomerate and the gravelly transition beds immediately above and below it. Without being able to trace the actual connection on the surface this conglomerate is 174 GEOLOGY OF THE DENVER BASIN. thought to be the equivalent of that found just below the basalt of Table Mountain. ‘The reasons for this are the apparent correspondence as to horizon and the great similarity of composition, although it is recognized that the latter fact would prove little in the light of the known variability of the strata of such constitution. If this conglomerate is the equivalent of the Table Mountain bed we have a nearly complete section of the forma- tion by combining the two exposures. The stratum on the line of profile has a strike N. 15° 30’ W., with an easterly dip of 45°. It occurs along the entire western base of the mountain at or a little below the steeper slopes, and it can be traced continuously in either direction until the low approaches to the mountain are reached, At its base this division is composed of dark sand with a few small andesite pebbles. In the central and upper parts it shows pure conglom- erates with sandy layers interstratified. The latter are seldom continuous in the body of the conglomerate, their form being that of wedge-shaped masses with marked cross-bedding, shown both by their stratification and by their relation to the conglomerate proper. A similar cross-bedding is also apparent in the conglomerate itself, so that determinations of dip and strike ‘an be made only by observing the course of the bed as a whole. The pebbles are mostly small, but few reaching a diameter of 4 to 5 inches, the majority being less than 2 inches. The rocks represented by the pebbles are nearly all dark, and there does not seem to be as much variety here as in Table Mountain. Few of them are porous. They lie in a scanty matrix of sand composed entirely of débris of andesitic rocks, while the actual cementing material is chiefly zeolitic in nature. In the small, irregular spaces between larger pebbles may occasionally be found cavities lined by small crystals of chabazite or stilbite, or in other cases filled by yellow calcite. Isolated augite crystals are abundant in the sandy matrix, but they are less perfect in form than those of the Table Mountain conglomerate. A few Archean pebbles were found in this conglomerate after careful examination of a considerable area. The distinetly conglomeritic portion of Division B is actually 25 feet in thickness. The remainder of the 50 feet assigned to it is chiefly at the DENVER OF GREEN MOUNTAIN. 11 2(/55 base and represents the change from the friable semi-clayey rocks of A to the distinct conglomerate. Veinlets in this sand rock are usually filled by zeolite or Galcite and the actual cement is doubtless of the same substances. Division C, 285 feet—The strata included under C are probably very much like those of Division A, but they are much better exposed than the former on the line of profile. The three divisions (A, B, and C) really constitute a single division standing in contrast to the strata above. Nearly the entire thickness of C is made up of yellowish-brown sands and clays with intermediate members. The little ravine coming from the hollow in the face of the mountain above exposes nearly the entire thick- ness in a most excellent manner. Few horizons stand out so plainly that they can be specially designated. Immediately above the conglomerate of B come about 30 feet of pure sandy strata; there is then a layer of pebbles 4 inches thick, succeeded by yellowish-brown clays containing varying amount of sand. The small conglomerate layer is frequently broken up into a series of lenticular masses arranged one after another. At about 100 feet above B is a development of 6 to 8 feet of dark, sandy clays with light spots very similar to certain layers noticed in the section at the northeast point of South Table Mountain. Above this comes a crumbling semiconglomerate or gravel layer 3 feet thick. This stratum has a strike N. 15° 30’ W. and a dip of 42° easterly. Only eruptive material was noticed in this conglomerate. This bed is followed by fine, yellowish, sandy beds, soon passing into clays of buff, lavender, and dark-brown colors, also somewhat arenaceous. Some of the dark-brown clays are mottled by light spots, as in the stratum already mentioned. In such clays occur concretionary masses of hardened, light-colored clay with an outer zone of darker material in which a rude cone-in-cone structure is visible, the apices pointing inward. These masses are ellipsoidal and up to 2 feet in diameter. Search for fossils in them proved fruitless. These concretions are apparently not confined to particular layers, but may be found anywhere in clay beds of the character mentioned. The upper portion of Division C is composed chiefly of friable sand rock with clay, and it is not so continuously exposed as are the lower portions. 176 GEOLOGY OF THE DENVER BASIN. Division D, 525 feet-——This stands in contrast to lower divisions both in structure, being composed of coarse conglomerate beds, and in material, through the appearance and general increase in importance of Archean débris with the eruptives. The actual base of D is not shown by the outcrops, although it is approximately indicated by the exposures in the small ravine just south of the section. It is in all probability a sharp line. The series begins with a conglomerate which is less compact and firm than that of B. At the base the pebbles are not very large, but they vary in character, showing for the first time a decided admixture of Archean with the andesite. There are also distinct clay bowlders mingled with the others im these lower horizons. Above the first 25 feet of conglomerate there comes a gap which evidently represents a return to clay or fine, sandy deposits. Above this gap come still coarser conglomerates with a greater admixture of Archean than before. Bowlders here vary from 1 to 2 feet in diameter for the large ones, and from that limit downward. The bowlders lie in a matrix of coarse, crumbling sand in which angular Archean particles are predominant. Cohering outcrops are naturally rare, and smooth, steep slopes covered with round pebbles and bowlders are usually presented, but a few strokes of the pick reveal beneath the surface a mass of varying-sized bowlders embedded in crumbling gravel. The relative proportion of Archean and eruptive materials in these strata is hard to determine without close scrutiny. The andesites are much decomposed as a rule, and on exposed surfaces crumble away quickly, while the Archean remains comparatively fresh, and is thus more prominent. In the lower 100 feet of D the eruptive material is nearly equal to the Archean in quantity. Large bowlders increase in number upward in the series. At about 100 feet above the base of D there appears on the line of this section a stratum with an easterly dip of 18° which contains so much iron oxide as its cement that it is hard and forms a projecting outcrop. At about the same horizon, too, there is a decided gain in the amount of Archean relative to the eruptive bowlders. The iron is noticeable through about 10 feet of strata, though present in large quantities only in the stratum at the base, about 1 foot in thickness. DENVER OF GREEN MOUNTAIN. tig About some pebbles in this heavily iron-bearing stratum there is a shell of limonite nearly an inch in thickness, in which an imperfect radiate structure an be seen. Archean strongly predominates over eruptive material in all the strata from this horizon upward. Dakota conglomerate bowlders were noticed at and near the iron-bearing horizon. One of them was 2 feet in diameter. The sandy matrix of the crumbling conglomerates is chiefly composed of quartz and red feldspar. Besides the stratum especially mentioned there is a slight appearance of iron oxide in the cement at many places, the result being a firmer rock in all instances. Large bowlders, with an average diameter of from 1 to 2 feet, are thickly piled in together in some layers, with smaller ones in the interstices. While conglomerate rocks strongly predominate in all of Section D, fine-grained beds appear in places, though perhaps with but limited lateral development, much as the conglomerates appear in the fine-grained beds of the lower portion of the formation. Thus, a few feet below the point / on the profile, there appears a dark, arenaceous clay stratum, containing much vegetable matter. Seen on the line of profile it appears to be a well-defined horizon occurring as a break in the conglomerate series, but this stratum can not be identified as such at a distance of but a few yards to the north on a ridge where there are good croppings. Probably the conglomerate succeeding it was deposited in turbulent waters, which locally or perhaps generally destroyed the fine deposit of a period of quiet. The study of the conglomerate series made it evident that fine-grained beds of local development might occur at almost any horizon, probably representing in all cases remnants of a layer of former continuity. In subdivision / no continuous bed of fine grain was found, but one noticeable local stratum was observed a few yards north of the line of section on the northern ridge. Here is a dark clay stratum, 18 feet thick, with coarse sandstone above it. The clay is dull-purplish in color and has a few gravel stones mixed in with it. One foot more or less below the sandstone is a local layer of lignitic material, a part of which is changed into true jet of great brilliancy. In part of this layer the woody fiber and toughness are still preserved. ‘Tracing these beds southward they wedge out rapidly, and on MON XXVII 12 ; 178 GEOLOGY OF THE DENVER BASIN, the line of profile are represented by almost normal conglomerate, vertically and laterally, with the appearance of bowlders in increasing quantity. The dip of the sandstone above the jet layer is 18°. Approaching the summit, the bowlders are larger and the conglomerate of which they are a part becomes looser and looser, until it is difficult in places to see that regularly stratified deposits are to be recognized in the heap of bowlders. On a glance at a considerable face of the exposure, however, the stratification is always plain. Andesite continues to be a marked element of the conglomerate to the very top of the series, but the variety formerly noticeable is now no longer a feature. The rock prevailingly represented here is a dense pyroxene- andesite in which the microscope shows both augite and hypersthene. The amount of Dakota conglomerate pebbles and bowlders is larger than below, while red and white sandstones are also present in considerable number. Just on the edge of the exposure is a large, white bowlder, 6 feet in diameter, of a felsitic eruptive rock not identified in any lower horizon. This may be an erratic, and it is also possible that glacial bowlders may be mingled with those of the coarse conglomerate on the upper slopes of the mountain. Distribution of particular beds ——]n speaking of the form of the mountain it was said that at a certain level the low, gentle slopes or approaches gave way to steeper ones. ‘This line is approximately the base of the heavy conglomerate series. On following up almost any one of the ravines penetrating the mountain, outcrops representing the lower 100 feet of this conglomerate may be found in the stream bed or on the adjacent slopes, and the larger ridges practically terminate at the same horizon. ‘Those ravines leading south into Bear Creek usually show the dark conglomerate of Bas soon as they have cut down sufficiently to bring it to the surface, and this occurs half way or more down to the creek. The gentle slopes about the mountain are then, for the most part, in the series of fine-grained sandstones or clays of the Division C, above described, and, except for the local development of a conglomeritic layer, there are few conspicuous outcrops, although the beds of almost all water courses reveal clayey strata ® DENVER OF THE PLAINS. 179 or sands here and there, whose characters, as members of the formation, would not be at all clear were it not for the continuous section which has been described. THE STRATA OF THE PLAINS. The horizons represented—'The bed of the Platte at Denver is vertically 500 feet below the horizontal strata thought to be at or near the base of the formation in Kinner Run at Golden, and 600 to 800 feet below the more or less uneven horizon at which the Denver beds disappear under the Monument Creek to the south and southeast. From Table Mountain toward Denver there must, then, be a slight dip, enough at least to carry the bottom of the formation under the city, and from the known limit of the strata along the northern line, and from the data of the artesian wells, it seems that the base of the series is, in fact, but little below the river level This slight dip is not recognizable in outcrops, for the frequent local variations in planes of sedimentation are greater than the dip in question. For the same reason one can not determine accurately the thickness of strata actually exposed between Denver and the southeastern limit of the formation, but here, too, it seems most likely that it is much less than the vertical interval above mentioned, owing to a slight northerly dip. All the strata of the plains are considered to be equivalents of the lower 400 feet of the series represented in Table Mountain. General characteristics —Although the strata of Green and Table mountains have been described as very fine-grained in the horizons corresponding to these deposits of the plains, the latter are as a rule still finer. This is shown chiefly in the simall size of the andesitie pebbles found in the grit or conglomerate layers. Near the western line it is common to find some few larger pebbles, 2 inches or more in diameter, in almost any bed containing pebbles, but beyond the Platte such pebbles are very rare. Coarse-grained beds may be searched in vain for any worn fragment a quarter of an inch in diameter, while smaller ones may be abundant. This fact is of course a natural sequence of the conclusion adopted that the source of the eruptive material was entirely on the western shore. A feature of the strata found on the plains is the abundance of nodules in certain sandy layers. These are often 3 feet or more in diameter, 180 GEOLOGY OF THE DENVER BASIN. . dark in color, and very hard, while lying in a soft, friable sand rock, easily disintegrating and of yellowish-brown color. In form the nodules are more frequently lenticular than round, but almost perfect spheres have been seen in some cases. As a rule certain layers carry many nodules, and oceasionally they adjoin each other so closely as to make up the greater part of a given stratum, or they may be united, and as an extreme of this development a given layer may be found to possess the color and hardness of a nodule for several yards with quite uniform thickness, though always ending with round outlines showing the real formal relation. A complex of layers in which such lenticular masses are abundant rarely fails to make itself known by good outcrops. On Murphy Creek are several particularly fine exposures of such strata, and on the High Line ditch, between Cherry Creek and the Platte, such outcrops are numerous. In the latter case, the bed of the ditch fora short distance below an exposure of nodular masses is covered by the nodules washed out. Similarly, on the dry creeks of the region south and southeast of Denver, the appearance of dark nodular fragments in the stream bed surely indicates good expo- sures not far upstream, cut out by the floods of the rainy season. Good examples of the nodular masses may also be seen on the west bank of the Platte opposite Overland Park. One feature of many outcrops of the Denver strata seen upon the plains is likely to be at least temporarily misunderstood by anyone not thoroughly acquainted with the developments in Table Mountain. This feature is the irregular, unconformable contact so frequently seen to exist between a conglomerate or grit layer above and a clay or shale below. Often the distinctive eruptive character of some of the pebbles and frag- ments of the coarser bed may be seen at a glance, while in the clay below nothing showing its true position is visible, and one is inclined to wonder if the actual base of the Denver may not be exposed, the clay belonging to the Arapahoe or Laramie. Often the unconformability is very marked, and unless adjacent exposures show similar relations at other levels, or reveal characteristic Denver sandstones below the clay, the true relationship may not be clear. The changes in conditions of sedimentation which give rise to such stratigraphical relations of consecutive beds were, liowever, common DENVER OF THE PLAINS. 181 in both Denver and Arapahoe epochs. Fine sediments were often disturbed and locally removed at the beginning of periods of rapid deposition of coarser materials. Such alternations of sediments were especially mentioned while discussing the exposures of South Table Mountain While the sediments of the plains are usually finer grained than those of the western border, and hence are distinguishable with less readiness, there is another element at first contributing to the difficulty in the appear- ance of quartz and feldspathic grains plainly not of eruptive origin. This feature will be discussed more fully in a following section, and it is only necessary here to mention the conclusion reached that such materials came from the Arapahoe shores on the north and south. Hints from topography— W hile the area of the map east of the line of Green and Table mountains is essentially plain, there are many diversities found o solid rock formation D> to bear more or less direct relation to the underlyin in spite of the numerous Pleistocene divisions which play the chief réles. A thorough acquaintance with the region immediately underlain by the Denver beds shows several peculiarities to be named. The strata have been described as prevailingly loose and friable, yet they resist erosion in a way of their own. A clay matrix for many beds causes the disintegration to proceed slowly and only on the very surface. Water may remove the outside layer easily, but the wet clay below holds the loose grains together for some little time. In many strata, too, a still more powerful agent exists in the secondary cementing substance of zeolitie character which has formed sometimes very abundantly. The dark, friable sandstone exposed in a surface quarry on the south side of Bear Creek contains 54.59 per cent of substance soluble in hydrochloric acid, and a fine-grained, brownish bed shown in the ravine near the old St. Luke’s Hospital, Highlands, was found to contain 38.22 per cent of soluble matter, a large amount of gelatinous silica being formed in each case. The result of this resistance to erosion is to produce rounded bluffs on all considerable streams, as Bear, Clear, Van Bibber, and Coal creeks, and along the Platte. Such lines of bluffs generally represent practically con- tinuous outcrops, while solid rock appears only in gullies or on actual stream banks. The Arapahoe and Laramie do not produce such forms, 182 GEOLOGY OF THE DENVER BASIN. though the differences are perhaps not very pronounced to the unaccus- tomed eye. Fossil remains—As far as present experience goes the strata of the plains are richer in animal remains than the better-exposed strata of Table Moun- tain, and this conclusion does not rest solely on the experience of the writer and of the earlier explorers of the flora at Golden. The exposures of Table Mountain have been studied for years by students from the School of Mines at Golden, alone and under the guidance of Prof. A. Lakes, who has collected so many fossil plants from this locality, and in all this time but a single vertebrate fossil has been found (a tooth of a dinosaur) and no invertebrate remains whatever. In the vicinity of Denver, however, mainly through the careful searchings of Messrs. G. L. Cannon, jr., and 'T. W. Stanton, a considerable number of vertebrate fossils and some shells have been found in place in typical Denver sandstones and clays. THE SHORE-LINE DEPOSITS. Besides the present limited extent of the Denver beds there are certain indications that the sea in which the beds were deposited was quite circumscribed, at least in its earlier stages. On the west the continental land mass could not have been far away from the present foothill line, though as a matter of fact there is no known evidence to show how far back any of the sedimentary formations of the district may have gone. But on the north and south there is evidence indicating that during the first deposits of the Denver period the shore-lines were not far distant from the present boundaries. The evidence in the one case lies in the stratigraphic relation to the Arapahoe and in the other in a combination of lithologic and stratigraphic relations. Within the Denver period, however, there may have been considerable changes of level and correspondingly of extent covered by the sea, so that the observations to be mentioned do not apply with any certainty to the later stages in the history of the sea. The northern shore-line—F'yom North Table Mountain eastward to the hills north of Arvada, erosion has removed everything which might have THE DENVER SHORE-LINE. 183 indicated the shore-line; but in the angle between Clear Creek and the Platte River, in the low hills a mile or more back from these streams, remnants of the Denver are found resting on the Arapahoe in positions showing a very uneven sea-bottom, and suggesting the proximity of the northern shore-line of Arapahoe land. The details of this region are derived chiefly from Mr. Eldridge’s notes, for these Denver exposures were discovered by him while studying the Arapahoe. It is impossible to indicate upon the map the details of the relation between the two formations as it is here known. ‘The generalization of the map may be explained in words by saying that these hills are capped by thin remnants of the Denver beds at varying altitudes, through which many of the small drains have cut into the underlying Arapahoe; and that, as practical horizontality exists for both formations as a whole, the elevation of the Arapahoe floor at this ‘point, beg some 200 feet above the known Denver beds on Clear Creek near its mouth, necessitates the conclusion that the north shore of the Denver Basin was rapidly ascending in the area in question. Moreover, the still higher ground occupied by the Arapahoe to the northward seems to show the form of the Denver sea-bottom in this direction. The southern shore-line deposits, East of the Platte River and near the ascend ing line of the Arapahoe as it approaches the Monument Creek there are some exposures of Denver strata which clearly show the immediate prox- imity of the old shore-line. On one of the creeks, just about the High- Line Canal crossing, are several good outcrops on sloping gulch banks, of very light-colored sandstones or grits, coarse and crumbling, in which no eruptive material can be found. These light-colored beds lie in isolated patches, while above, below, and, in some cases, between them, are typical Denver beds, rich in dark, eruptive-rock pebbles. Taken by itself this group of outcrops would be accounted for with difficulty, but a short distance up the same creek on the north bank is a bluff outcrop in which the explanation for this occurrence is well shown. Here are seen grits composed of quartz and feldspar, overlain and underlain by normal Denver beds, and these grits are seen to pass laterally in continuous outcrops into beds of nearly the same grain in which andesitic pebbles are abundant 184 GEOLOGY OF THE DENVER BASIN. and soon predominate. Such a change takes place within a distance of 50 feet, there being continuous stratification from one end to the other of the variable beds. The Denver strata in this place are therefore capable of being locally developed as nearly pure sandstones of Archean débris, and this possi- bility indicates most plausibly that a local source for such material is near at hand. The loose sandstones and grits of the Arapahoe are distant but a few hundred yards from this spot, where they seem to have constituted the shore, although actual exposures of the shore-line could not be found. Such local variations of the Denver sandstones were found in less marked degree throughout the southern part of the area represented as Denver, from the Platte to Coal Creek, and it seems probable from this fact that the early Denver sea was quite circumscribed in this direction, where the boundary of present exposures is set by the overlapping of the Monument Creek. OCCURRENCE OF THE FORMATION. As the strata of the Denver formation are, almost without exception, soft and friable and thus easily disintegrated, and as they have been especially exposed to erosive agencies, their identification as a distinct formation would have been very difficult were it not for the protected outcrops of Table Mountain and the section shown in the mass of Green Mountain. Without these it would have been almost impossible to correlate the scattered out- crops in obscure ravines and drains upon the plains, and many exposures of clay or fine-grained sandstones would have long gone unrecognized. Now that the relations are clear it is seen that characteristic outcrops are really quite common over the greater part of the field. It is the aim of this section to describe the localities and special features of the principal exposures known, that it may be easy for others to test the conclusions reached in regard to this interesting series of rocks, the identification of which bears upon several questions of Rocky Mountain geology and raises, on the other hand, so many new problems for future solution. Table Mountain—'The lower portion of the Denver strata is much better exposed on Table Mountain, near Golden, than at any othér place. Here, EXTENT OF THE DENVER. 185 at the northwestern extremity of the present area of these rocks, the lower 450 to 500 feet of the formation is now protected by the basalt sheets which were poured out upon them during the Denver epoch. North Table Mountain possesses an average elevation above the adjacent country of 650 to 800 feet; its basalt capping presents cliffs varying in height from 100 to 200 feet, and below them are rather steep slopes upon which here and there over the entire extent of the mountain are outcrops of Denver strata. So far as known they are horizontal or have possibly a very slight dip to the southeast. The soft, easily disintegrated sandstones and clays afford projecting outcrops only under the most favorable circumstances, hence the slopes are unusually débris-covered, but where smooth, a blow of the pick will commonly expose the sandy or clayey strata. The contact of the main basaltic sheet with the Denver strata is exposed in many places at the base of the cliffs, and the relation of the smaller, earlier basalt streams to the inclosing sandstones is clearly shown. The former contact is particularly well exhibited at the head of the south- ern gulch which cuts into the mountain; also on the southern face between the large gulches, on the western side of the larger gulch, and on the northwestern face above the eastern end of the lower basalt flow. Below the last contact and at several places upon the southern slopes are outcrops of strata at various horizons. The actual line between the Denver and the underlying Arapahoe beds is nowhere visible. From the data soon to be given it is estimated that the thickness of the Denver strata present below the basalt can not be far from 450 to 475 feet. The base of the formation along the northern foot of Table Mountain is drawn according to the estimate, with the assumption that there is a slight easterly dip to the strata. Directly west of North Table Mountain the rising ground: and nearness to the great fold require that the Denver beds should take some part in the fold, and the yellowish, sandy strata which are imperfectly seen in the southern end of the railroad cutting do appear to have an easterly dip of 20° to 25°. These strata are thought to be near the base of the Denver series, although by no means of characteristic Composition. South Table Mountain, although lower, exhibits much better exposures 186 GEOLOGY OF THE DENVER BASIN. of these Denver rocks than are found upon North Table Mountain. At its northeastern extremity there is a continuous outcrop, from the basalt down- ward, of about 170 feet of horizontal strata, a description of which has been given. Adjacent to the southeastern point is also a fine exposure of strata below the basalt, and at several places upon the northern and western slopes the more prominent horizons are well shown. Southeast of Castle Rock a few hundred yards there appears a particularly fine exposure of the most typical conglomerate of the series, at the horizon immediately below the basalt. The footpath from Golden up to Castle Rock also passes over characteristic outerops, both near the basalt and on the ridge below, at about the 5,950-foot contour. Lower horizons of typical composition are shown at various places along the irrigation ditch, especially at the western base of Castle Rock. Plant remains oceur quite abundantly throughout the sandy strata within 300 feet of the basalt and have been found particularly well preserved along the southwestern slope of South Table Mountain. All outcrops on South Mountain indicate a practical horizontality for the formation as a whole, although there are some minor irregularities. The basalt sheets, whose source is in dikes between Ralston Creek and Table Mountain, evidently flowed in a direction south-southeast upon the inclined sea-bottom, and the surface upon which they rest corresponds approximately to a definite horizon in the formation. Owing to the greater height of North Table Mountain it seems at first glance as if the present surface in contact with the basalt was more inclined than is actually the case. A careful estimate of the total fall of the surface upon which the basalt rests, from the northern edge of North Table Mountain to the southern edge of South Table Mountain, makes it 375 feet in a distance of 20,500 feet, which corresponds closely to a dip of 1°. The western base of the formation Irom Clear Creek southward to Bear Creek the base of the Denver formation has been traced by the following data: In the bed of Kinner Run, the small stream which enters Golden from the southeast, there are several outcrops of importance. The first of these is in the town, just east of the court-house, where loose, crumbling beds are shown which contain much quartz in company with the eruptive material. EXTENT OF THE DENVER. 187 These strata are underlain. by clays like those of the upper portion of the Arapahoe series, and it seems quite probable that the actual line of separation between the two formations is here shown. The rising ground to the southward carries the junction line to the westward, and outcrops of decided Denver strata are found due west of the Reform School in the bed of Kinner Run just below the ditch and old railroad grade crossings. In all intervening portions of Kinner Run the stream bed is excavated in alluvial deposits. At the point just mentioned, however, friable, leaf- bearing sandstones are shown with a dip of 27° to 30° in an easterly direction, and a strike about N. 20° W. Microscopical examination reveals beyond a doubt the characteristic composition of Denver beds. Less than 500 yards to the eastward the cut made for the ditch in the north edge of the terrace upon which the Reform School stands exposes horizontal beds of fine-grained conglomerate or grits of the same formation, while less than 300 yards to the westward the Arapahoe conglomerate is found with an easterly dip of 70° or more. The relationship of the Denver beds to those of the Arapahoe series, and the position of both with regard to the great fold are still more clearly shown in the abandoned railroad cutting southwest of the Reform School and 450 yards south from the outcrop in Kinner Run. The cutting is in the terrace, and shows in its western portion Arapahoe beds from the upper part of the conglomeritic section, with very steep easterly dip. Succeeding these is a gap corresponding to the clay strata of the upper Arapahoe, and then brown and yellowish friable sandstones of the lower Denver formation, the latter having a dip of 20° to 30° eastward. The exact line between the formations is not visible, but can be located with sufficient accuracy from the approximate thickness of the Arapahoe clays above the conglomerate. It is here plain that the sharp bend in the fold, as expressed in the outcrops of the present surface, occurs in the upper part of the Arapahoe series, and the Denver strata are affected only in their lower members. From the Reform School southward to the base of Green Mountain proper the dividing line between these two formations is necessarily drawn with reference to the conglomerate horizon of the Arapahoe, as that alone is well shown. Its strata can be traced for a mile or more, with a constant 1&8 GEOLOGY OF THE DENVER BASIN. steep dip, beyond which there is a gap of nearly a mile. The strike and dip must be continued unchanged in this covered area, however, as the point at which the beds reappear, at the northern base of Green Mountain, is in the projection of the last known strikes to the north. The form of Green Mountain is characterized throughout by smooth slopes, but there is a certain horizon traceable around its entire mass at which the steep upper slopes give way to more gentle ones. At this general horizon the deep excavations in the mass of the mountain give way to com- paratively shallow ravines in the lower slopes. On the western slope the line between Denver and Arapahoe strata can be approximately determined from the persistent outcrops of the conglomerate of the latter formation, which can be found in vertical position, crossing the ridges between the ravines and minor drains. East of these outcrops there is usually a space in which at a few points only are found small clay outcrops. All ridges are covered by débris of bowlders from the mountain above, and the drains are mostly shallow and grassed over along this line. As all strata of the Arapahoe and the lower ones of the Denver series are vertical or dip very steeply all along the western base of the mountain, it is evident that the base of the latter formation must come at a distance above the Arapahoe conglomerate equal to the estimated thickness of the intervening clays In one of the ravines running west in the north part of the mountain distinct outcrops of Denver sandstones were found about 100 yards from the Arapahoe conglomerate in a direction normal to the strike of the latter. At this same outcrop the clays underneath the sandstone seem to be of the Arapahoe. Above this horizon are several outcrops of undoubted Denver strata, and at the base of the steeper slopes there appears the conglomerate bed made up of dark andesite pebbles, which can be followed along the whole western slope of the mountain, bearing always the same relation to the Arapahoe conglomerate. These two horizons run nearly parallel at a distance slightly greater than the thickness of the intervening strata, with occasional outcrops between them, and continue to maintain this position until the erosion of the southern slopes of the mountain cuts sufficiently down into the Denver beds to cause the outcrops of their less steeply dipping members to diverge somewhat from the line -of the lower EXTENT OF THE DENVER. 189 conglomerate, which continues with a gradual eastward curve in its strike to the banks of Bear Creek, opposite Mount Carbon. All outcrops which are found along this general line prove the presence of the Denver beds at the points where the deductions from the known dip and thickness of the lower formation would naturally put them. As has been shown by Mr. Eldridge in the preceding chapter, the thickness of the Arapahoe strata in this region, as well as the location of its upper limit, can be most beautifully shown in one of the ravines upon the southwestern slope of the mountain. Hence the deduced position of the base of the Denver formation with reference to the very persistent exposures of Arapahoe conglomerate must be considered as nearly correct. The Arapahoe conglomerate can easily be traced to the bank of Bear Creek, opposite Mount Carbon, where it is very clearly shown, in vertical position, close to the wagon road on the north side of the creek. A few hundred yards down the creek is the mouth of Coyote Gulch, whose banks show horizontal Denver strata. The fold is therefore very sharp here as well as along the base of Green Mountain. In following up the bed of Coyote Gulch many most characteristic exposures of the Denyer beds are seen. Green Mountain—"["he massive hill bearing this name is almost exclusively made up of sandstones and conglomerates of the formation under diseus- sion. Only at its western base, as described in a preceding section, are other series represented. Its peculiar form and gentle slopes result from the ready disintegration of the loosely cemented strata. A casual observer would scarcely think of referring the large, loose bowlders scattered over its surface to immediately underlying strata, and outcrops which clearly reveal this fact are rare, at least for the upper part of the mountain. Upon the western face is the only cliff-like exposure of any extent occurring, and this has been described in foregoing pages in detail. The ravines heading in the mountain mass all show minor outcrops in their beds, but the smooth slopes are usually débris covered. After leaving the upper slopes these ravines cut down into the finer-grained sandstones and con- glomerates, giving evidence of the rock formation underlying the low, grassy country about the mountain on the south, east, and north. 190 GEOLOGY OF THE DENVER BASIN. South of Bear Creek—])enver beds are well shown in the little knoll on the northern slope, near the east end of Mount Carbon. The strata exposed are conglomerate and grits in typical development, with plain cross-bedding and having a slight dip to the eastward. Everything is concealed on the flat, drift-covered top of the hill, and the northern slope has few distinct outcrops. The proximity of the vertical Arapahoe conglomerate to these nearly flat Denver beds shows that the fold is very sharp and its axis proba- bly lies below the Denver. South of Mount Carbon the line of the Arapahoe is clear, but it is no longer followed so closely by the Denver. This is shown by exposures about one mile southeast of Mount Carbon. Here is a distinct ridge of the Arapahoe conglomerate in vertical position, beyond which to the east is a shallow lake whose banks exhibit sandy and clayey strata of the Arapahoe in horizontal position. A county road runs due east from the north side of this lake and for about 3 miles traverses a cultivated district, in which there are a number of ponds formed for irrigation purposes, and no rock outcrops were found. This road is 1 or 2 miles south of Bear Creek and between them runs a low ridge bounding the valley proper, upon which there are several knolls. Denver strata form this ridge and may be seen in small outcrops. Along the southern base of the ridge are fields and several ponds. As far as can be determined at present this ridge represents the south line of the Denver here, the ponds and cultivated areas being most probably underlain by Arapahoe clays. A small quarry has been opened in cross-grained and rather friable Denver sandstones on the ridge a quarter of a mile north of the road and about 25 miles east of Mount Carbon. About here the ridge swings around to the southeast, uniting with similar banks of the Platte River. Denver beds are found in many places along the line of the road above mentioned as it crosses this ridge and descends to the Platte Valley. An outerop of Denver beds appears at the water level at the western point of the great bend in the Platte north of Littleton. From here south no exposures have been found on the west side of the Platte, though the strata must curve in this direction to a point several miles south of Littleton. The flat west of Littleton and the drift-covered banks above were searched for outcrops in vain, though they may exist in undiscovered places. EXTENT OF THE DENVER. 191 Area between Bear and Clear creeks—'This entire block of country east of Green and Table mountains is underlain by Denver strata which appear in numer- ous places, as a rule either in the rounded knolls or in ravines and gullies. Even in some cultivated or grassy fields any slight excavation, as small irrigation ditches, will reveal crumbling brown sandstones or clays, which a microscopical examination shows to be composed largely of andesitie débris. The soil from disintegration of such material has a characteristic dark yellowish-brown color. The northern bank of Bear Creek is abrupt, yet the slopes are usually smooth and rounded, with numerous little drains cutting into them. Though projecting rock faces are rare, this entire bank is practically one continuous outcrop of Denver beds, and illustrates admirably the way in which the strata resist erosion, while seemingly so easily attacked. A prominent conglomerate horizon is often identifiable by the black pebbles strewn abundantly over the surface. About one mile west from the Platte a dark conglomerate forms an unusually prominent outcrop. It is evidently a local development, in much reduced thickness, of some bed elsewhere found as a conglomerate. Peb- bles 6 to 8 inches in diameter are not uncommon, though the greater number do not exceed 2 inches. The variety is great, though all seem to be andesites. Heulandite and calcite in plain crystals or grains compose the cement of these pebbles. A total thickness of about 15 feet is mainly conglomerate, and it grades off to sandstone or grit, with few pebbles both above and below, and probably laterally in a similar manner. The western bank of the Platte, from Bear Creek to Clear Creek, is formed by a line of low bluffs, along which, in many places, good outcrops of the Denver beds are exposed. The Platte cuts into this bank at some of its bends and lays bare dark concretionary sandstones, as at a point opposite Overland Park. Small irrigation ditches and the cuttings on the Denver, Leadville and Gunnison Railroad also reveal the characteristic strata at many points. In addition to these outcrops the small water courses often cut into the solid rock, and the edges of the bank, or knolls upon it, frequently show rock in place in the same manner as along the Bear Creek bluffs above referred to. 192 GEOLOGY OF THE DENVER BASIN. The best point to see the Denver beds on the Platte banks is opposite Overland Park. The river itself, a ditch, the railroad cutting, and a ravine coming from the west all have cut into and exposed some yery typical sandstones, and semiconglomerates. Tere some large dinosaur bones were found in the dark, hard nodules of one horizon. In the interior of the area under discussion the principal drainage courses show more or less frequent outcrops. One heading in Green Mountain, and entering the Platte at Denver, has a great number of exposures along its bank. The south bank of Clear Creek also presents a good series of exposures. Under the bowlder beds of this valley the Denver beds are shown by railroad and wagon-road cuttings at several places. The banks of the Platte in the immediate vicinity of Denver are espec- ially treated in a succeeding section. North of Clear Creek—Tast of Table Mountain the northern limit of the Denver beds follows quite closely the south bank of Van Bibber Creek to its junction with Ralston Creek near Arvada. Here again the formation is betrayed by a line of distinct low bluffs, and outcrops and evidences of the peculiar strata may be found along the whole line. ‘The rather level tract between Van Bibber and Clear creeks is chiefly due to the bowlder beds of the latter valley. On a small hill half a mile west of the mouth of Van Bibber Creek these bowlder beds may be seen resting on Denver strata. At about the mouth of Van Bibber Creek the Denver beds cross Ralston Creek and run up on the prominent hill north of Arvada. On this hill are several outcrops, and a prospect shaft is sunk some 15 feet into the sandstones on the eastern point of the hill. Only the capping of the hill is made of Denver strata, the Arapahoe bemg shown not far below the top and in the drain on the northern side. The northwestern limit is quite well defined where Mr. Eldridge found Denver strata near the top of a ridge, with Arapahoe not far below on either side. As has been explained, the area of the Denver strata between this hill and the Platte is to be con- sidered as part of a shore-line deposit wpon an uneven surface, and the border lines of the Denver are therefore only approximately correct, for EXTENT OF THE DENVER. 193 outcrops are rare and many possible variations may be concealed by the heavy Pleistocene deposits of the region. The south slope of the hill mentioned above is covered and the Arapahoe may run from the west farther down Ralston Creek than has been indicated; indeed, the hill may be capped by an entirely isolated patch of the Denver strata. This does not seem at all probable, however. Denver and vicinity.— Within a radius of 4 miles from the city hall in Denver there are a great many spots where the strata of the formation bearing the name of the city are well shown, and while one should go to Table Mountain to gain a first acquaintance with these horizons some of the exposures within the city limits are very good illustrations of some of the chief features of the series. The best exposures are on the west bank of the Platte at numerous points in the small drains which enter the river between Overland Park and Clear Creek. In the largest of these tributaries which heads on the eastern slope of Green Mountain and enters the Platte near the Larimer street bridge there are many good outcrops within a mile of the river. In these rocks Mr. Cannon found the skull of Ceratops alticornis, the first of the horned dinosaurs described by Professor Marsh, associated with other vertebrate remains. Still nearer the city, in the ravine by the old St. Luke’s Hospital, too) Highlands, there was formerly a very good outcrop of Denver sandstones and clays, with cross-bedding structure, and full of plant remains in certain layers. Here, too, ocewrs a thin local seam of coal, which has from time to time been rediscovered and announced in the papers as indicating the presence of valuable coal in North Denver. In these same strata Mx.'T. W. Stanton found some molluscan remains, associated with plants, and a small but perfect crocodile tooth. This ravine has now been filled in grading streets. At various excavations and cuttings at and about the Globe smelter the Denver sandstones have been well exposed. On the western bank of the Platte a good outcrop occurs at water level, at the foot of Thirty- seventh street, and another at Twentieth street. Close by the waterworks MON XxXVII——13 : 194 GEOLOGY OF THE DENVER BASIN. im West Denver and along the western bank of Lake Archer the Denver strata appear distinctly. On the south bank of Cherry Creek, in Shackleton Place addition to Denver, a small outcrop occurs. Wells in Ashley’s addition and at other places on Capitol Hill prove the presence of beds there, and the excavation for the reservoir on Capitol Hill has also disclosed Denver strata under the Pleistocene. North of Sand Creek—T'he area of Denver beds north of Sand Creek is very small and no good outcrops are known. Proc. Colo. Sci. Soc., Vol. III, Part III, 1891, pp. 359-458. 2Geologic Atlas of the United States, folio 7, Pikes Peak, U.S. Geol. Sury., 1894. Geology by Whitman Cross. SOURCE OF DENVER MATERIAL. 205 is to be mentioned that in the neighborhood of Idaho Springs, Georgetown, Empire, Central City, and other points on the eastern slope of the mountains, there are numerous dikes and massive bodies of igneous rocks which, while of granular or porphyritic structure, and usually coarsely crystalline, are in chemical and mineralogical composition very similar to some types of the andesites found in the Denver beds. They exhibit the characteristics of intrusive or deep-seated eruptives, which cooled far from the surface, and none of them, so far as known, can be thought of as a surface flow. The rocks of these bodies were first known to the writer from their appearance in the bowlder beds of Clear Creek; subsequently through a few specimens kindly furnished by Mr. J. 5. Randall, of Georgetown, and, finally, through personal visits to the localities mentioned. The rocks of the Clear Creek bowlder beds do not come from the foothills, for no such rocks are known in them, and Bear Creek, Ralston Creek, and other streams whose sources are in the foothills, do not show such rocks in the material which they bring down. The direct equivalents of many of them occur as dikes and stocks near Georgetown and Empire. It is entirely in accord with the views of the writer to suppose that the diorites and diorite-porphyries of these dikes and irregular intrusive masses are the deep-seated equivalents of the surface flows represented as andesites in the Denver conglomerates. It is also true that the great outpouring of various lavas which must be assumed according to the foregoing considera- tions, necessitates an assumption that the channels through which the molten magma ascended still exist somewhere. But there is as yet no positive evidence connecting the dikes and irregular masses referred to with the surface flows assumed. The establishment of such a connection will carry with it a complete confirmation of the opinions which have been expressed as to the geological importance of the lithological characteristics of the Denver beds. Hence, while the possibility of this connection must not be overlooked in judging the opinions which have been expressed, no attempt will be made to elaborate this theoretical defense of the deductions which the facts are believed to warrant. The western land masses adjacent to the Denver sea have alone been directly considered in the foregoing remarks. The only sections of any 206 GEOLOGY OF THE DENVER BASIN. considerable thickness now exposed are those of Green or Table mountains, situated on the western shore-line. From the exposures on the plains we learn that while other shores did contribute noneruptive materials during the earlier part of the Denver epoch, the eruptive débris from the western area was at all times strongly predominant, and nothing appears, on extend- ing the view, to invalidate the conclusions reached. At points on Coal Creek farthest removed from the western shore-line, and on Cherry Creek, near the Arapahoe beds of the southern shore, are strata consisting exclu- e@ admixture of Archean débris is sively of eruptive materials, and a stron surely accompanied by evidences of its secondary and local source in the Arapahoe grits. While the influx of eruptive materials was always strong enough in these earlier periods to make itself everywhere predominant, it is not found that the sand washed in at times from the northern and south- ern Arapahoe shores ever became noticeable in contemporaneous deposits of Green or Table mountains. SECTION I11.—_AGE OF THE ARAPAHOE AND DENVER FORMATIONS. By WHITMAN CROSS. STATEMENT OF THE QUESTION. Former classification of the formations— Until the discoveries which are described in preceding chapters were made the strata of the Arapahoe and Denver formations had been uniformly assigned by geologists to the Laramie, under the accepted definition of the latter as the uppermost division of the conformable Cretaceous series; and not only had they been assigned to the Laramie, but no characteristics of any kind had been mentioned, or apparently observed, by which these upper beds might be even locally distinguished from the lower, coal-bearing horizon. This correlation was based on the presence of the true Laramie below the beds in question, on the failure to notice their peculiar and distinguishing characteristics, and on assumptions regarding the unity of the fossil flora, whose species were, however, collected from widely separated horizons in the Golden section. The assignment to the Tertiary——In the earliest descriptions of these forma- tions by Mr. Eldridge and the writer they were assigned to the Tertiary. AGE OF TAKE ARAPAHOE AND DENVER. 207 The reason for this assignment was the discovery that between the Laramie and Arapahoe epochs there had occurred an orographie disturbance whose magnitude was measured, for this locality, by the presence in the Arapahoe strata of pebbles of highly indurated clastic rocks, sandstones, conglom- erates, etc., clearly belonging to various geological horizons as far down as the ‘Trias, representing erosion of 14,000 feet of strata, according to the section of the formations in question in the Denver region. The lithological character of the Denver beds showed that the interval of unknown dura- tion between the Arapahoe and Denver epochs had witnessed the occurrence of voleanic eruptions on a gigantic scale, and also subsequent local erosion. Up to the time when these formations were thus identified, great orographic movements in the Rocky Mountains had been commonly sup- posed to mark the ending of Mesozoic time, and to be in great measure the cause of the wonderful changes that took place at this period, especially in vertebrate life, as shown by the remains in the earliest known Eocene deposits. The beginning of Tertiary time was also known to be widely characterized by great volcanic outbreaks, recorded in the sediments of the Green River, Florissant, and other Eocene basins. Hence it seemed natural to place the Arapahoe and Denver beds in the Tertiary, as, perhaps, the earliest lake deposits of Cenozoic time. Examination of the paleonto- logic evidence available at the time showed either that it did not controvert the assignment, or, as in the case of the fossil plants, was entirely untrust- worthy because the floras of the distinct horizons involved could not then be compared. The recent discoveries of fossil vertebrate remains are said by paleon- tologists to show that the life of the epochs under discussion was much more nearly allied to Mesozoic than to Cenozoie types, and in deference to this opinion the post-Laramie formations are classed in this report with the Cretaceous. But such a course raises at once the question as to the nature and position of the boundary between Mesozoic and Cenozoic time in the Rocky Mountains, and broadens very materially the treatment which must be given to the problem. Discoveries of allied formations Within the past few years a number of local formations have been discovered in Colorado and Montana which are more 208 GEOLOGY OF THE DENVER BASIN. or less clearly equivalents of either the Arapahoe or the Denver beds. Few of them have been examined as yet in any detail, but all furnish valuable evidence bearing upon the question under discussion in this chapter, and this evidence shows that the stratigraphic and lithologic facts observed in the Denver Basin can not be lightly regarded as of purely local importance. On the contrary, these formations are so widely distributed and agree so per- fectly in the trend of the evidence they afford that the consideration as to the age of the formations of the Denver area plainly involves a logical and consistent treatment of a most important period of Rocky Mountain his- tory. The question as to the weight to be given the stratigraphic and lithologic facts of the Denver area broadens at once to a discussion of the dynamic history of the interval between Mesozoic and Cenozoic time, or of the transition from one to the other. Questions of paleontology. Since the identification of the Arapahoe and Den- ver formations there haye been many important discoveries of fossil remains in the Laramie or in the younger group of local formations to which the Denver and Arapahoe belong. The most remarkable of these fossils are vertebrates, and in another chapter Professor Marsh briefly out- lines the character of those most directly connected with the formations of the Denver Basin. The interpretation of this new evidence in its bearing upon the subject of this chapter necessitates the discussion of our present knowledge concerning the occurrence and distribution of these fossils, and also of the relation of paleontologic to other kinds of evidence, for there is not perfect harmony in the conclusions drawn from weighing the various kinds of evidence. The fossil floras of the Laramie and post-Laramie formations have been recently revised by Mr. Knowlton, and a concise summary ot his results will be found in a later chapter. The conclusions drawn from this revision illustrate the self-evident fact, too often disregarded, that the occur- rence and distribution of fossils must first be ascertained and accurately recorded before they become of value in the classification of allied and associated formations. In applying the data of paleontology to the present question it is nec- essary to review both present knowledge of the fossils and the general AGE OF THE ARAPAHOE AND DENVER. 209 relation of such evidence to the problems of stratigraphical and historical geology. Phases of the problem—'[‘he discussion of the age of the Arapahoe and Den- ver formations is thus seen to involve many points of interest which may be summarily stated as follows: 1. Age of the two local formations. 2. Age of numerous allied formations. 3. Relations of, and weight to be assigned to, various classes of evidence. : 4. Condition of knowledge of the faunas and floras of the formations in question. 5. History of the period between Mesozoic and Cenozoic times in the Rocky Mountains. 6. Desi Rocky Mountains. gnation of the boundary between these great divisions in the 7. Relation between physical changes and faunal modifications in the general period involved. 8. Relations between the chronological systems necessary to express the history of physical changes in the earth, and of the development of life upon it, i. e., the desirability of a dual nomenclature. It would be manifestly out of place to discuss all these broad questions in detail in a volume of essentially local character, and such a discussion is in this instance at present impossible, because of imperfect knowledge in many directions, but it will be the aim in the following pages to present the evidence in the special case in such a way as to indicate or suggest its value in respect to some of the broader problems which naturally develop from the one of local importance. EVIDENCE OF LITHOLOGIC CONSTITUTION. Conglomerates of the Arapahoe beds —'I"he conglomerates of the Arapahoe contain evidence of events in the long interval that separated their deposition events which can from that of the sandstones and clays of the Laramie not be disregarded in spite of the apparent conformability of the two formations as now exposed in many places. Mr. Eldridge has described the MON XXv1i——14. : 210 GEOLOGY OF THE DENVER BASIN. various materials identified by him in the beds, but a brief recapitulation of the facts may not be out of place. Aside from the débris of granite and gneiss, which must be expected in coarse sediments adjacent to a continent largely made up of those rocks, the Arapahoe conglomerate contains pebbles from various Mesozoic forma- tions. Pieces of coal and friable sandstone were found in a few places. These may be considered as derived from the Laramie, which also seems the probable source for pebbles of fossil wood. Clay ironstone and certain dense earthy limestones are most plausibly from the Colorado and Montana Cretaceous, while hard, almost quartzitic, white sandstones may be referred to the Dakota. That the latter contributed to the Arapahoe sediments is most conclusively established by the pebbles of the very characteristic fine- grained conglomerate at its base. Red sandstone and certain limestone pebbles of the Arapahoe are probably derived from Jurassic strata, as they are not known to occur in any higher horizons. There are also present pebbles apparently of the ‘Triassic sandstones and of cherty masses containing Carboniferous Beaumontia. In distinction from the Denver beds the Arapahoe strata contain no pebbles of volcanic rocks, and by their constituents above mentioned they are markedly different from any other horizon below them in the foothill section. This lithologic character is evidence of important stratigraphic relations to be further considered in a succeeding section. Volcanic materials of the Denver beds ——T'he sharp distinction to be drawn between the lithologic characters of the Denver and earlier sediments has been fully described and emphasized. While the voleanic types represented in the pebbles of the Denver beds all belong to the andesites, as far as observed, it is to be pointed out that the variety within that very large group is great, and indicates a source of supply which must have been long in accumulation. Whether from a single great voleano or from sev- eral different centers these various andesite lavas may safely be considered as the products of a very long period of voleanic activity which did not begin until after the close of the Arapahoe epoch. The basaltic magmas of the Denver epoch, preserved in Table Mountain, may belong to the close of this cycle of eruptions. AGE OF THE ARAPAHOE AND DENVER. 211 It will scarcely be questioned by anyone that the Denver epoch deserves to be recognized in any adequate chronology of geologic events in this region, but the full importance of the time in which these volcanic outbursts took place appears only after considering the facts presented by other deposits allied to those of the Denver sea. The great extent of the voleanic products, which must be assumed from the practical exclusion of other material from the Denver sediments, has been dwelt upon, and any claim that this feature is only of local importance is further answered in the deposits of other contemporaneous seas. Texture of the Denver strata—T'he fine grain of the Denver clays, tuffs, sand- stones, and most of the conglomerates shows that they were accumulated very slowly and that the epoch of sedimentation was of no mean importance. The strata of very fine grain, such as the clays, tuffs, and sandstones, which may be compared in this respect with the beds of the Laramie, exceed the latter in thickness, and indicate that the Laramie and Denver epochs of sedimentation may have been of nearly equal duration. The Arapahoe strata are likewise of very fine grain, and in neither case can it be assumed that these deposits were much more rapidly accumulated than those of the Laramie. Conclusions from lithologic evidence —'T'he pebbles of the Arapahoe formation show that in the interval between Laramie and Arapahoe deposition an oro- graphic movement took place in this vicinity, so great that all the formations of the Cretaceous, and some still older ones, were elevated to form land masses adjacent to the Arapahoe sea or lake. As the Arapahoe conglomerate does not present a record of progressive erosion of these thousands of feet of Mesozoic beds it must be inferred that a long period of degradation not represented in known sediments preceded the Arapahoe and prepared the land surface which contributed from many horizons of older strata to the early conglomerates of this formation. The Denver strata are most clearly characterized in their lithologie constitution and bear witness to important events in the preceding interval. The volcanic phenomena of this epoch, while affording criteria for distin- guishing it from the preceding, do not bear upon the question of geologic age so strongly as the facts of the Arapahoe. 213 GEOLOGY OF THE DENVER BASIN. The strata of the Arapahoe and Denver have a total thickness much greater than that of the Laramie, and by their texture speak for epochs of sedimentation presumably equal in duration to that of the Laramie. EVIDENCE OF STRATIGRAPHIC RELATIONS. Stratigraphic break between Laramie and Arapahoe.— Erosion of the upper limb of the monoclinal foothill fold has destroyed the zone where the Arapahoe beds must have originally overlapped the Laramie and some older deposits with angular unconformity, unless it be that the Arapahoe basin was entirely excavated out of Laramie strata. Minor unconformity of this latter kind is shown in the plains area, as described by Mr. Eldridge. The evidence of the lithologic character of the Arapahoe beds has been given, and the following deductions from that evidence seem unavoidable. Somewhere tributary to the Arapahoe sea many thousand feet of the Mesozoic strata were upturned and had already been greatly eroded prior to the new epoch of sedimentation, so that widely separated horizons contributed simultaneously to the basal conglomerate of the Arapahoe. It is most natural to assume that the present foothill fold, which is known to mark the line of progressive or repeated movement, elevated the western shore-line deposits of the older formations, and that the erosion of this western area produced the various pebbles of the Arapahoe. But the position of the land mass of these eroded Mesozoic sediments is of secondary importance compared with the facts of a great orographic movement which terminated the long succession of conformable | Cretaceous sediments at the close of the Laramie. As the Arapahoe deposit seems to have been local in character, it was at first possible to assert that this orographic movement was not of importance in the broad area of the Rocky Mountains. But the facts of the Middle Park and Livingston beds, and the inferences present knowledge justifies in other described localities, all tend to show that this stratigraphic break was extensive, and recent discoveries have uniformly increased its importance. Relation between Denver and Arapahoe beds—As far as known the Denver lake basin was eroded out of the Arapahoe strata, and to the northeast of Denver it would seem that the preceding formation was entirely destroyed, AGE OF THE ARAPAHOE AND DENVER. 213 allowing the Denver to rest on the Laramie. There is no known reason for the assumption that the Denver beds overlapped the Arapahoe on the west, but it is plain trom the fact that red and white sandstone and the Dakota conglomerate are prominent among the first noneruptive materials to appear in Denver sediments, that the western shore of the Denver sea consisted in part of the same stratified rocks that contributed to the Arapahoe, which had been long concealed with granite and gneiss beneath the volcanic flood. Suggestions from other facts—While the equivalent of the Arapahoe has not been identified in the sections on the Animas River, on the Grand River, or in Montana, where direct equivalents of the Denver beds lie between the Laramie and the lowest recognized Eocene, it is a noteworthy fact that the Laramie is not especially thick in these places. In other words, it does not appear that the Laramie epoch of deposition continued in these localities during the long tinte represented by the Arapahoe and the preceding interval of elevation and erosion. As far as inference may be drawn from this fact, it is to the effect that the pre-Arapahoe uplift terminated Laramie deposition throughout the mountain district of Colo- rado, and at least locally in Montana. Subsequent deposition would at present seem to have been more local during the Arapahoe than in the Denver epoch, in Colorado at least. EVIDENCE OF ALLIED FORMATIONS. The formations to be mentioned as allied either to the Arapahoe or Denver occur chiefly in Colorado, on all sides of the mountain area, and in its larger elevated basins or parks. One of the newly differentiated series of strata occurs in Montana, and indications point to the presence of similar formations in the intermediate area of Wyoming. Valuable information concerning several of these formations has been given by R. C. Hills in a presidential address before the Colorado Scientific Society.". The present writer has personally examined several of the deposits, and in 1892 published a general review of what was then known ‘ Orographie and structural features of Rocky Mountain geology: Proce. Colo. Sci. Soc., Vol. III, Part III, 1891, pp. 359-458. 214 GEOLOGY OF THE DENVER BASIN. of the various formations in question.’ Little can at present be added to that summary, which is much more complete in details than that which is to follow. The Middle Park beds——Across the Colorado Range from Denver is the elevated basin of Middle Park, containing a great series of strata repre- sented upon the Hayden map of Colorado as Laramie. These beds were of special interest at the time of their discovery by Marvine, in 1873, by reason of the marked angular unconformity with the underlying Cretaceous section, which was taken as confirmatory evidence that the “ Lignitic” beds were Eocene. When the Laramie was defined as conformable with the Cretaceous series below, the facts of the Middle Park beds were conven- iently regarded as of local importance, a procedure commonly resorted to when new discoveries controvert old ideas. As Marvine’s description” of the Middle Park beds showed them to be not only unconformable with the Cretaceous section, but also lithologically similar to the Denver beds, a correlation with the latter was at once sug- gested, and the region was visited in 1889 by Mr. G. L. Cannon, jr., of Denver, and in 1891 by the writer, both under direction of Mr. Emmons The result of these visits was published in 1892.° The strata in Middle Park designated as Laramie by the Hayden survey extend from a point south of the Grand River northward, forming the high divide between Middle and North, Parks, and occupying a large area in the latter region. According to Marvine these strata are 5,500 feet in thickness. And Marvine did not include with them an underlying bedded formation, called by him ‘‘doleritic breccia,” to which he assigned a maximum thickness of 800 or 900 feet. In the article above cited the writer has reviewed in detail the facts and descriptions of Marvine, and in the following paragraphs will be given the facts concerning these strata as now understood. The “doleritic breccia” of Marvine is a series of dark tuffs, conglom- erates, and breccia beds, made up of a large series of andesitic fragments, } Post-Laramie deposits of Colorado: Am. Jour. Sci., 3d series, Vol. XLIV, 1892, pp. 19-42. “Seventh Ann. Rept. U.S. G. and G. S., for 1873. ’The post-Laramie beds of Middle Park, Colo., by Whitman Cross: Proe. Colo. Sci. Soc., Vol. III, 1891. AGE OF THE ARAPAHOE AND DENVER. Pps) of types identical with those in the Denver formation. These beds are coarser in texture and are laterally more variable than the Denver strata, but resembie them very much in many details. The sharp line drawn by Marvine between the ‘“ breccia” and his ‘ Lignitic” series does not appear justifiable. While Marvine does not refer to volcanic materials in the upper series, there is in fact a gradation between the lower, dark, almost purely andesitie strata and the lighter-colored beds above, in which granitic débris usually predominates, although micaceous and hornblendie andesites are abundant for more than 2,000 feet upward in the series—as far as the writer’s observations go. Plant remains are the only fossils as yet known from the Middle Park strata. These were found by the Hayden survey party in the “ Lignitic” series only, but they occur also in the dark tuff layers of the lower beds. A number of the species described by Lesquereux as from Middle Park are now known to have come from the Eocene lake bed at Florissant, Colo. The entire known fossil flora of the Middle Park series has been studied by Mr. Knowlton, and the result will appear in his forthecomig monograph on the Laramie and allied floras. It is sufficient to say here that twenty-five satisfactory species are known from these strata, and that by far the strongest alliance is with the flora of the Denver formations. Along the Grand River near Hot Sulphur Springs the stratigraphic relations of the Middle Park beds are clearly shown. They here rest upon the upturned and eroded section of the Mesozoic series, from the Jura to oranite. No Laramie is here to) the Fox Hills, and overlap the former to the seen, whether from erosion or nondeposition is not definitely shown, but from the fact that coal-bearing strata are present in North Park it may be inferred that the Laramie once existed in Middle Park, but was entirely removed by the erosion preceding the deposition of the Middle Park beds. It is not definitely known at present whether the entire thickness of 6,400 feet of strata—taking Marvine’s measurement—belongs in fact to one formation, but no reason for doubting this has appeared as yet. For nearly 3,000 feet the series is certainly one, lithologically, stratigraphically, and in its fossil plants. From the foregoing statements it appears that in Middle and North 216 GEOLOGY OF THE DENVER BASIN. parks there exists a formation’ very similar to the Denver beds, which has also been classed with the Laramie. It seems probable that the two forma- tions were laid down almost contemporaneously. If the two series be con- sidered as of the same age, the evidence of the Middle Park beds strongly confirms the deductions made from the Arapahoe and Denver formations concerning geologic events in the epochs immediately succeeding the Laramie. No strata corresponding to the Arapahoe have yet been found beneath the Middle Park beds, but they may well exist in places not yet carefully examined. The folding and great erosion which preceded the Arapahoe on the eastern side of the mountains are paralleled by the elevation and degradation which succeeded the uppermost member of the conformable Cretaceous series in Middle Park, preparing the surface on which the tuffs and conglomerates rest. The material of the latter was manifestly derived from a great series of volcanic eruptions similar in character and variety to those assumed for the slope adjacent to the Denver sea. Whether the thick- ness of the Middle Park beds be taken at 3,000 or 6,400 feet, they testify to a long epoch of gradual subsidence contemporaneous with deposition. Strata near Canyon—In his cited address Mr. R. C. Hills referred to rem- nants of a formation resembling the Denver beds that occur south of the Arkansas River in the vicinity of Canyon, Colo. Directed by information personally given by Mr. Hills, Mr. Eldridge examined the district and found apparent equivalents of both Arapahoe and Denver beds. The ana- logue of the former is a heavy conglomerate, resting upon the Laramie, upturned with it in the foothill section, but exhibiting among its pebbles fragments recognized by Mr. Eldridge as derived from the Niobrara and Dakota Cretaceous and from the Jura. These tell the same story of uplift- ing and erosion which is contained in the Arapahoe conglomerate. The formation resembling the Denver beds is made up of a variety of andesitic rocks and is lithologically almost identical with the formation named. No fossils have been found in either of these formations, but their occurrence is noteworthy in connection with others. If they belong to the AGE OF THE ARAPAHOE AND DENVER. 217 post-Laramie group they testify to the extent of the orographie disturbances and volcanic eruptions of the time immediately succeeding the Laramie. Formations of the Huerfano Basin—()n the eastern side of the mountains, in Huerfano Basin, Mr. R. C. Hills has found and described! a series of strata which he subdivided as follows: ( Huerfano beds, 3,300 feet = Bridger group. Huerfano series. ... 4 Cuchara beds, 300 feet. nee: 1 ow aa ' Lower Eocene. ( Poison Canyon beds, 3,500 feet. 5 Great angular unconformity exists between the Laramie and the Poison Canyon beds, but none has been detected between the several members of the new series. The Huertano beds are assigned to the Bridger Eocene from the presence of Tillotherium, Hyrochyus, Glyptosaurus, Paleeosyops, and other vertebrates. The Cuchara and Poison Canyon beds are separated on lithological grounds, and no fossils have been found in them. The former formation consists of ‘‘pink and white massive sandstones;” the latter of “soft sandstones and fine conglomerates of a yellowish tinge, with occasional bands of yellow clay or marl.” It is believed by Mr. Hills that the Cuchara and Poison Canyon beds are probably contemporaneous with some of the other post-Laramie formations here referred to. The Animas River beds—In the article already cited on the post-Laramie deposits of Colorado, the writer referred to a series of strata occurring on the Animas River below Durango, which had been visited by Mr. T. W. Stanton and found to be very similar to the Denver beds. In the summer of 1894 the writer was able to hurriedly examine this series of beds as exposed on the railroad below Durango, and found them to resemble the typical Denver beds in a very high degree. These strata occur above the Laramie and below the Puerco, and, as far as the present meager observations show, are conformable with both of them where now preserved. The beds are some 760 feet or more in thickness, and are composed of yellowish-brown clays, tuffs, sandstones, and conglomerates, in which andesitic material greatly predominates, and present a variety rivaling that in the Denver beds. ‘The recently discovered Tertiary beds of the Huerfano River Basin, Colorado: Proce. Colo. Sci. Soc., Vol. III, 1888, p. 148. Additional notes on the Huerfano beds: ibid., Vol. III, 1889, p. 217. Remarks on the classification of the Huerfano Eocene: ibid., Vol. IV. 918 GEOLOGY OF THE DENVER BASIN. A few fossil plants occur, but those found thus far are poorly preserved, and the only identifiable species collected is Magnolia tenuinervis Lx., a com- mon Denver bed species originally described from Table Mountain. No invertebrate fossils have been found as yet, but it seems probable that a number of vertebrate species, described by Cope as from the Laramie of the Animas River section, came out of the strata which so closely resemble the Denver formation. In an article discussing the relations of the Puerco and Laramie deposits! Professor Cope refers to the succession of beds on the Animas River, saying: ‘According to the observations of Mr. David Baldwin the Laramie beds succeed [the Puerco] downward, conformably it is thought by Mr. Baldwin; and have a thickness of 2,000 feet at Animas City, New Mexico [Colorado?]. A few fossils sent from time to time by Mr. Baldwin identify the Laramie. This is especially done by the teeth of the dino- saurian genus Dysganus Cope, which is restricted to the Laramie formation elsewhere. Also by the presence of the genera Lelaps and Diclonius, which in like manner do not extend upward into the Puerco beds.” According to the statement of Professor Cope, made personally to the writer and quoted with his permission, these fossils were collected incidentally to the investigation of the Puerco fauna and for the purpose of identifying the underlying formation. He believes it most probable that they came from what are here called the Animas beds, which extend for several hundred feet below the Puerco. Professor Cope now regards the Dysganus and Diclonius as closely allied to the horned dinosaurs (Ceratopsidze Marsh, Agathaumidee Cope), which, as will be shown, form the most characteristic element of the vertebrate fauna known in the Arapahoe and Denver beds. The Animas beds, as this post-Laramie formation may be called, are to be regarded as a most direct equivalent of the Denver beds, identical in peculiar lithologic character, lying between typical Laramie and Puerco, and containing fossils which, so far as known, indicate a similar fauna and flora. This occurrence on the border of Colorado and New Mexico, and south of the San Juan Mountains, is evidence of the great importance 1 Am. Naturalist, Vol. XIX, 1885, p. 985. AGE OF THE ARAPAHOE AND DENVER. 219 of the long-continued volcanic outbursts which the formations thus far mentioned show to have occurred at about the same time in widely separated districts. The Ohio Creek and Ruby formation —In the area covered by the Anthracite sheet? in the West Elk Mountains, Colorado, occur two formations which are analogous to the Arapahoe and Denver beds. One of these, the Ohio Creek formation, is known only in two small isolated remnants, resting on an eroded surface of the Laramie. The strata are loose, friable sandstones, grits, and fine conglomerates, and contain unidentifiable plant remains. The conglomerates contain many chert pebbles carrying crinoid stems and other apparently Carboniferous fossils. From this fact it is argued by Mr. R. C. Hills, who personally vestigated both the formations in question, that between the Laramie and Ohio Creek epochs there was great erosion, cutting through the entire Cretaceous section, at least. In the Ruby Range of the Anthracite sheet is a formation 2,000 feet in thickness, resting with apparent conformity on the Laramie, but consisting almost entirely of andesitic débris, forming tuffs, sandstones, conglomerates, ete., of purplish color, and much indurated in this locality through numerous dikes of igneous rock. The ‘Ruby formation,” as it has been called, has been traced by Mr. R. C. Hills continuously for more than 80 miles to the northward, beyond Grand River, where it decreases in thickness to 300 feet. At this point the formation is overlain by the Wasatch Eocene and underlain by “200 feet of soft, white sandstones and yellow clay,” below which are the firm, gray sandstones of the Laramie. Mr. Hills suggests a correlation between the soft sandstones and the Ohio Creek beds. No fossils are known in either of the new formations aside from carbonized plant stems. It thus appears that in the region between the Gunnison and the Grand rivers there is an extensive formation, consisting, like the Denver beds, of fine andesitic débris, and occupying a position between the Wasatch and the Laramie. The Ohio Creek beds are less distinctly an equivalent of the Arapahoe, the principal evidence in this direction being that while these strata are not definitely known beneath the Ruby series the basal 'Geologic Atlas of the United States, Anthracite-Crested Butte folio, No. 12, 1895. 220 GEOLOGY OF THE DENVER BASIN. conglomerate of the latter usually contains chert pebbles with crinoid stems, which may very probably have come from the erosion of the Ohio Creek beds in the vicinity. The latter formation is certainly preserved only in small remnants. Northwestern Colorado—The Ruby beds do not seem to be present in the northwestern part of Colorado, where the Laramie coal measures are well developed. But in his often-cited address Mr. Hills states his opinion that the upper portion of what has been called Laramie, on the Yampa River and elsewhere, will finally prove to be a distinct formation, and in that case analogous with the Arapahoe and identical with the soft, yellowish sandstones occurring on the Grand River between the Ruby beds and the Laramie. The only evidence adduced by Mr. Hills in favor of this view is the fact that the lower part of the questionable series of strata consists of normal Laramie beds, firm, even-grained sandstones, clays, and coal seams of excellent quality. Such normal beds are present on the Yampa in a thickness equal to their usual development elsewhere in Colorado. Above them occurs a series of ‘soft sandy strata with some shales and clays,” containing beds of impure lignite, and of general different physical appearance from the underlying unquestionable Laramie. The Wasatch Eocene overlies this section. This opinion, although unsupported by definite evidence, is worthy of much consideration, coming, as it does, from the geologist who is far more familiar with the Laramie and its associated formations in Colorado, Wyoming, and adjacent territory, than anyone else. The Livingston formation, Montana——T'he recent investigations of Mr. W. H. Weed and Dr. A. C. Peale in the district covered by the Livingston and Three Forks atlas sheets show that a formation directly analogous with the Denver occurs in Montana. ! The Livingston formation, as it has been called by Mr. Weed, occurs in typical development in the Bozeman coal field, where it overlies the Laramie. It had been classed with the latter previous to the investigations 1 Geologic Atlas of the United States, Folio 1, Livingston sheet, 1894; Folio 20, Three Forks sheet, by A. C. Peale, 1895. The Laramie and the overlying Livingston formation in Montana, by W. H. Weed and F. H. Knowlton: Bull. U. 8. Geol. Survey No. 105, 1893. AGE OF THE ARAPAHOE AND DENVER. 221 of Mr. Weed, largely owing to the same error which was committed in the Denver field, namely, of grouping together as one flora the plants from the coal horizon in the Laramie and those from a somewhat higher horizon in what now prove to be Livingston strata. As the ‘Laramie flora” of 1877, when these plants were determined by Lesquereux, consisted very largely of the Denver plants from Table Mountain, it is natural that the fossil plants of the two horizons in the Bozeman field should have seemed to belong together. The Livingston formation is about 7,000 feet in thickness and is over- lain by the Fort Union beds. — Its lithologic character is almost identical with that of the Denver beds, the sandstones, conglomerates, and tufts consisting chiefly of andesitic débris of great variety. In consequence the strata are dark-brown or yellowish-brown in color, and contrast distinctly with the quartzose sandstones of the Laramie with which they had been previously classed. Mr. Weed and Mr. Peale both found the Livingston beds transgressing the upturned edges of the underlying Mesozoic, although in less marked ‘degree than is the case for the Middle Park beds of Colorado. The fossil floras of the Bozeman coal beds and of the Livingston beds have been carefully revised by Mr. Knowlton, with the result that a marked difference appears between them. The Livingston flora contains 29 species, of which 22 have a known distribution in other places; 12 of them occur in the Denver flora, 6 in the underlying Bozeman coal horizon, or in other undoubted Laramie beds, and 4 in the Fort Union. This shows a strong affinity with the Denver and a less marked alliance with the Laramie and Fort Union floras. No vertebrate fossils have as yet been discovered in the Livingston beds, but two horizons within the division called the “leaf beds” by Mr. Weed have yielded invertebrates. At one of these horizons a few fresh- water forms, including Goniobasis tenuicarinata, were found, all having a resemblance to species known in the Fort Union beds. The species named occur also in the Denver beds. The other invertebrate fauna consists of brackish-water forms, viz: Ostrea subtrigonalis, Corbicula cytheriformis, Corbula subtrigonalis, and C. subtrigonalis var. perundata. hese forms occur 222 GEOLOGY OF THE DENVER BASIN. below that at which the fresh-water mollusks were found. In deseribing the Judith River beds it is stated that Messrs. Weed and Stanton have found these same brackish-water species on Judith River, above fresh-water fauna. ‘This fet shows, then, that these brackish-water forms occur above the stratigraphic break at the base of the Livingston, and hence they are not competent to decide whether strata containing them are Laramie or post-Laramie, the main question under discussion in this section. EVIDENCE OF FOSSIL PLANTS. Historical statement.—'["he typical, Denver beds of Table Mountain contain one of the largest and most fully described fossil floras known in this country. Nevertheless, until quite recently, this flora has been of little value in discussing the age of the formation containing it, or even the question as to the separation of the Denver and Laramie. In 1889, while describing the ‘ Denver Tertiary formation,” the writer gave facts showing that while the fossil plants collected at Golden formed a large part of the so-called “ Laramie flora,” it was impossible to ascertain from the published descriptions, from the labels accompanying specimens in the National Museum, from catalogues, or other published sources, whether the larger part of the species described come from the Laramie coal measures or from the Denver beds. It was then evident that until a very thorough revision of both old and new material, from the Laramie and Denver alike, had been completed, no safe conclusions could be drawn from the fossil floras upon the question of distinguishing the two formations. This revision has now been nearly completed, as appears in the see- tion by Mr. Knowlton, in Chapter VII, and it is unnecessary to repeat much of the detail showing how the published data concerning the Laramie and Denver floras became so lamentably inaccurate. But this history of the fossil plants is so applicable in its moral to the present condition of other fossil evidence that the leading facts will be given. More than 160 species of fossil plants from the vicinity of Golden were described by the late Leo Lesquereux, and assigned to the ‘“ Laramie flora.” These plants were collected during several years, by many persons, chiefly by Prof. Arthur Lakes, of Golden, and Professor Lesquereux himself, and AGE OF THE ARAPAHOE AND DENVER. 223 by various members of the Hayden survey. Although it is expressly stated by the geologists in their reports, and by Lesquereux in his mono- graphs, that several widely separated plant horizons existed, as, for example, the vertical coal beds and the horizontal strata of Table Mountain, the published descriptions in many cases do not specify locality further than as “Golden, Colo.” Nor do the original labels or catalogues contain more. Thus at the time the Denver beds were first described the published fossil flora of Golden consisted of over 100 species. In his monograph “Cretaceous and Tertiary Floras” (1883) Lesquereux specifies the coal measures of the Laramie as the “habitat” of but 3 species and only 13 are referred to Table Mountain. By searching the original descriptions and noting incidental references in the various reports it may be ascertained that 9 species came from the coal measures and 16 from Table Mountain. Of the remaining 75 per cent of the Golden flora, as known in 1888, the writer has been unable to find published or otherwise recorded evidence of the horizons from which they were obtained. In 1888 Lesquereux added 68 species to the flora of Golden, without specifying the horizon of a single one of them. In 1886 Prof. L. F. Ward published his synopsis of the Laramie Flora and in his table of distribution included all the species of Lesquereux from Golden, most of which were from unknown horizons. At various times in the course of the investigation of the Denver beds, fossil plants were sent to Profs. L. F. Ward and J. 5. Newberry, with the request that if possible they should discriminate between Denver and Laramie floras. In every case the reply was that no difference could be seen. The evident explanation of this inability to distinguish the two floras was that what was by them considered the “Taramie flora” embraced also all known Denver plants, and the data did not exist in the published record by which the species from the two formations could be separated for comparison. From examination of the published data concerning the Middle Park, Bozeman, and other local floras, especially those of Wyoming localities, it appears that an entire revision of the earlier publications is necessary before a correct list of Laramie plants can be compared with that of post- Laramie species. 224 GEOLOGY OF THE DENVER BASIN. The explanation of all this early laxity in the records concerning the localities and geologic horizons from which so many fossil plants were obtained is apparently twofold. No doubt the prime cause of the trouble is to be found in the conditions of the reconnaissance work during which the early collections were made. A flood of new material from various collectors, in many different localities, was poured upon the paleobotanist each year, and he was expected to determine the species and correlate the formations at once. The recording of locality and horizon, even when known, was to a great extent impossible without the aid of a large clerical force. But it is also clear that the necessity of such a record was not duly appreciated either by geologist or paleontologist. Both placed a value upon fossils, a large proportion of which were new to science, which could legitimately be given to them only by a series of accurate observations, accurately recorded. ; Present knowledge of the fossil plants—In Chapter VII Mr. Knowlton states the extent of the revision of the Laramie and allied floras which has been attempted, and gives in summary form the result for the formations of the Denver Basin. Owing to the vast amount of material to be examined and the necessity for new collections and observations in some important localities, this revision is still far from complete, but as regards Colorado, where the stratigraphic relations of the plant-bearing horizons are best known, the revision is most advanced and the results most satisfactory. The Denver flora, as at present known, embraces 150 species, the Laramie flora of this district contains 98 species, and but 15 species are common to both formations. For this district, then, the floras are very markedly distinct. If the entire known Laramie flora of Colorado, embracing large col- lections from the coal fields of Canyon, Walsenburg, Raton, and the West Elk Mountains, be compared with the Denver flora, it is found that out of more than 100 species in the Laramie of Colorado only a very few have been found in the Denver beds. Further comparisons, as with the Laramie floras reported from Wyoming and farther north, are as yet impracticable. AGE OF THE ARAPAHOE AND DENVER. 225 The floras of the Middle Park and Livingston formations have already been stated to resemble the Denver very closely. The Arapahoe formation was found by Mr. Eldridge to contain poorly preserved fossil leaves at a number of localities within the Denver region, but none determinable was collected. But in the course of the recent collections made for Mr. Emmons by Professor Lakes, a leaf-bearing stratum was found near Sedalia, south of Denver, in what are apparently Arapahoe beds. This local flora contains 19 species, 3 of which are new; 2 are known elsewhere only in the flora of Carbon, Wyo.; and all of the 14 remaining species have been found in the Denver beds. Of the 14 Denver species, 7 have no other known distribution, but 5 are found in the true Laramie, 1 in the Livingston beds, and 1 on Sand Creek, east of Denver, in beds whose exact locality is not known. This analysis brings out a strong relationship to the Denver flora and also an affinity with the Laramie, seemingly stronger than that of the Denver flora for the Laramie. The presence of Carbon species is in the line of the suggestion made elsewhere that the beds of Carbon and some other localities in Wyoming may prove to be of Arapahoe age. In his summary Mr. Knowlton distinguishes the locality of Sand Creek with 10 species, for the reason that these plants, all collected by the members of the Hayden survey, come from a locality where the Arapahoe rests on the Laramie (see map), and where both carry fossil plants. The specimens preserved in the National Museum do not satisfactorily indicate the horizon from which they came. It seems probable that a part of them came from the Arapahoe beds and a part from the Laramie. Conclusions from fossil plants——F'rom the large Denver and Laramie floras, containing so few species in common, it would appear that the two epochs were separated by an interval in which the change in vegetation was very important. During this interval the Arapahoe beds were deposited, but present knowledge of the plant life of that epoch is too meager to be of value in correlating that formation. The most important conclusion to be drawn from the consideration of the fossil plants applies with equal force to other classes of fossils, namely, that conscientious observation and record of stratigraphic facts concerning MON XXVII 15 226 GEOLOGY OF THE DENVER BASIN. beds in which new fossils are found are necessary to give those fossils value as evidence in geological correlation, especially in the case of allied or adjacent formations. The disregard of this self-evident proposition has already delayed an understanding of some important problems of Rocky Mountain geology, and has caused a great deal of labor in clearing away a confusion which need never have existed. EVIDENCE OF INVERTEBRATE FOSSILS. The only invertebrate fossils thus far found in the Denver beds were obtained by Mr, 'T. W. Stanton in 1886 from a ravine near the old St. Luke’s Hospital, in Highlands, Denver. These were submitted to Dr. C. A. White for determination. Viviparus trochiformis and Goniobasis tenuicarinata are the only specifically identifiable forms, while imperfect shells referred to Corbicula, Physa, and Unio are also present. These species of Viviparus and Goniobasis are common in the Laramie and are also known in the Fort Union, and in what Dr. C. A. White has called the Wasatch Eocene, in Utah. The other forms also possess a wide range, and it is clear that these fossils are without value in the discussion to which this chapter is devoted. EVIDENCE OF VERTEBRATES. Known vertebrate fossils of the Denver and Arapahoe —Ag far as is known to the writer, no vertebrate fossil had been described or positively identified from the Laramie, Arapahoe, or Denver formations of the Denver Basin before the beginning of the investigations recorded in this volume (1881).’ A few had been found by Prof. A. Lakes, of Golden, and during the progress of the work a considerable number of bones were discovered by Mr. G. L. Cannon, jr., of Denver, and by Mr. Eldridge and the writer. The collec- tion of this material extended over a number of years. The fossils obtained were for the most part isolated bones or fragments, and all or nearly all of them were sent to Prof. O. C. Marsh for examination. Owing to the fact that few connected parts of skeletons have been found, and because the bones belong for the most part to new types, many of them have as yet been identified only in a general way, yet a number 1 As mentioned in a later section of this chapter, it is possible that certain dinosaurs and other vertebrate fossils from Bijou Creek, described by Professor Cope, came from strata of Arapahoe age. The locality is beyond the limits of the Denver Basin as the term is here used. AGE OF THE ARAPAHOE AND DENVER. PPA | of genera and species have been based on some of the best preserved specimens trom the Denver Basin. A large number of the remains belong, moreover, more or less distinctly, to dinosaurian forms which have been discovered in much greater perfection in other localities within the last few years, so that a review of these other discoveries is necessary in discussing the geological importance of the fossils of the Denver area. Of the vertebrates from the formations now under discussion the first to be described by Professor Marsh was found in place in the Denver beds near the Platte River, in Highlands. It was at first described as Bison alticornis, and assumed by Professor Marsh to probably indicate a Pliocene formation.’ In 1889 this fossil was recognized by him as belonging to the new family of horned dinosaurs, the Ceratopsidee, which he had mean- time founded on specimens discovered in Montana.’ Other remains from the Denver Basin were at this time referred to the same family, and in more recent years, as various representatives of dinosaurian types have been described from the collections made in Wyoming or Montana, some of the less perfect bones of the Denver region have been referred to the new species. Some other vertebrate forms have also been identified, and at present the list of species known from the post-Laramie of the Denver region is as follows: Arapahoe formation : Dinosaurs: Ceratops montanus. Triceratops galeus (type). Claosaurus annectens. Ceratops alticornis. Denver formation: Dinosaurs: Ceratops alticornis (type). Ornithomimus velox (type). Triceratops horridus. Claosaurus annectens. Turtles: Compsemys victus. Trionyx foveatus. Crocodile: Crocodilus humilus. Fish: Lepidotus occidentalis. 1 Notice of new fossil mammals: Am. Jour. Sci., Vol. XXXIV, 1887, p. 323. S24 new family of horned Dinosauria-from the Cretaceous: Am. Jour. Sci., Vol. XXXVI, 1888, p. 477. Notice of new American Dinosauria: Ibid., 1889, p. 334. 228 GROLOGY OF THE DENVER BASIN, It is probable that the last four species of this list occur also abun- dantly in the Arapahoe beds, From this list it appears that very few species are restricted to either formation, even with the present imperfect knowledge of the fauna. Horizons from which the fossils were obtained —All the determinable species of verte- brates referred to above were found either in the Arapahoe or Denver strata. The better preserved ones came from the Denver beds exposed in the ravines tributary to the Platte River in or within a few miles of the city of Denver, Others were found on the slopes of Green Mountain in the outerops of the Denver beds, and one at least came from Table Mountain. The fossils assigned to the Arapahoe were nearly all obtained by Mr. Kldridge, either directly embedded in the grits of this formation or in the gravel resulting from the surface disintegration of these beds. The prinei- pal localities are north of Clear Creek and west of the Platte River. | The Ceratops fauna of other localities, Since the description of the first repre- sentative of the Ceratopsidee by Professor Marsh, in 1888, a large number of allied dinosaurian forms and many associated vertebrate fossils of other types have been discovered by him and grouped as the “Ceratops fauna.” It also appears that a number of forms previously described by Cope belong to this family of horned dinosaurs, According to the statements of Professor Marsh, Wyoming and Mon- tana have yielded great numbers of fossils assigned to the Ceratops fauna, In Wyoming most of the species were found in Converse County, near the eastern border of the State, and in Montana the Judith River Basin has produced a large number of the forms described by Marsh and Cope. As mentioned in describing the post-Laramie beds of the Animas River, this region has yielded several dinosaurian types from strata not far below the Puerco. ‘These were described by Cope, as well as some similar forms trom Bijou Creek, about 40 miles east of Denver. A representative of the horned dinosawrs was also described in 1872 by Cope, from Black Butte, in Wyoming, under the name Agathaumas. This is now regarded by both Cope and Marsh as allied to Ceratops. In association with the Ceratopsidse in these various localities are representatives of other dinosaurian families, and also crocodiles, turtles, AGE OF THE ARAPAHOE AND DENVER. 229 fishes, birds, and a remarkable mammalian fauna. There are also mollusks and fossil plants. Geological significance of the Ceratops fauna—T'he vertebrate fossils grouped by Professor Marsh in the Ceratops fauna have hitherto been referred to by him as coming “from the Laramie of Wyoming,” of Montana, or of Colorado, and no doubt has been expressed by paleontologists of this country concerning the reference of the strata containing these remains to the Cretaceous rather than to the Eocene. The fauna is said to exhibit pronounced Mesozoic affinities and remote connection with the earliest Tertiary forms. In the closing discussion of evidence further reference will be made to the basis for this opinion, but it is desired to show in this place that present knowledge does not permit the use of the Ceratops fauna, extensive and remarkable as it is, in distinguishing the post-Laramie from the Laramie proper. The cause of the inability to use this remark- able fauna lies in the fact that the distribution of the species within the series of formations in question is not satisfactorily known—precisely the difficulty hitherto experienced with the fossil flora. A review of the facts concerning the leading localities makes this clear. The Ceratops beds as a “horizon."—The strata in which the Ceratopside and associated fossils occur have been grouped by Professor Marsh as the “Ceratops beds,” and frequently referred to by him as constituting ‘a well-marked horizon.” Although such a use of the term “horizon” may be satisfactory to the vertebrate paleontologist, it is clear, on the grounds already presented, that it does not adequately express the facts of stratigraphy. The Ceratops “horizon” in the Denver region embraces the Arapahoe and Denver formations, and if the Laramie proper of other regions contains the same fauna, the “horizon” really embraces three stratigraphically distinct formations. Aside from its inaccuracy, it seems to the writer that this use of the term “horizon” is quite unjustifiable, for it can not but be misleading to those unacquainted with the regions involved. The first Ceratops (C. montanus) was described by Professor Marsh in December, 1888." The locality and horizon in which the new fossil was Y ‘Am. Jour. Sci., 3d series, Vol. XXXVI, 1888, p. 477. 230 GEOLOGY OF THE DENVER BASIN. obtained were given only in general terms. It was said that it was found ‘in place, in the Laramie deposits of the Cretaceous, in Montana * * *,” “The associated fossils found with the present specimen are remains of other dinosaurs, crocodiles, turtles, and fishes, mostly of Cretaceous types. The mollusks in the same beds indicate fresh-water deposits.” It is also remarked that ‘ remains of the same reptile, or one nearly allied, had previously been found in Colorado, in deposits of about the same age, by Mr. G. H. Eldridge.” In fact all the remains found by Mr. Eldridge came from the Arapahoe strata, so that the horizon of this species in the Denver Basin is indicated. In April, 1889, Professor Marsh described Ceratops horridus,’ the local- ity and horizon being stated as follows: ‘The present specimen is from the Laramie formation of Wyoming, but fragmentary remains, which may be referred provisionally to the same species, have been found in Colorado.” The latter reference is to material from the Denver beds. The species ras later assigned to the new genus Triceratops. The genus Triceratops was established by Professor Marsh in August, 1889,° with three species. The type ‘was discovered in the Laramie formation of Wyoming.” ‘‘A much smaller species is represented by various remains probably from the same horizon in Colorado.” The type of this smaller form, 7. galeus, was in fact obtained by Mr. Eldridge in the Arapahoe beds of the Denver Basin. In December, 1889, one year after the description of the first recog- nized Ceratops, Professor Marsh gave a description of the skull of the Ceratopsidee, prefacing it with some remarks on the geological occurrence.’ He asserted that ‘the geological horizon of these strange reptiles is a distinct one in the Upper Cretaceous, and has now been traced nearly 800 miles along the eastern flank of the Rocky Mountains. It is marked almost everywhere by remains of these reptiles, and hence the strata may be called the Ceratops beds. They are fresh-water or brackish deposits, which form 1Am. Jour. Sci., 3d series, Vol. XXXVII, p. 334. °Am. Jour. Sci., 3d series, Vol. XX XVIII, p. 173. %Am. Jour. Sci., 3d series, Vol. XXXVIII, p. 501. AGE OF THE ARAPAHOE AND DENVER. 231 a part of the so-called Laramie, but are below the uppermost beds referred to that group.” The statement that ‘‘a distinct horizon” has been “traced nearly 800 miles” and that “it is marked almost everywhere” by certain fossils would imply either that actual continuity had been proved or that the stratigraphic position of the fossil-bearing strata had been found to be clearly the same at numerous localities not far apart. But when Professor Marsh made the above assertion the Denver region was the only one in which the position of the Ceratops-bearing beds had been established in complete sections, and here they were found to be separated from the typical Laramie below them by a great stratigraphic break. Moreover, none of the described fossils was found east of the mountains between the Denver Basin and Converse County, Wyo., a distance of 200 miles. As far, then, as the new fossils themselves are concerned they prove either a great extension of the Arapahoe and Denver (post-Laramie of this report), or a distribution of the fossils in question beyond the limits of what may properly be termed one formation or horizon. It is plainly of primary importance to ascertain whether any of the strata containing the so-called Ceratops fauna really belong to the true Laramie, as distinguished from the Arapahoe, or whether they all belong to the latter formation. A review of the known tacts concerning the published localities will now be given. The Ceratops beds of Converse County, Wyo— The most important locality for the Ceratops fauna as yet discovered is that of Converse County, Wyo., for the reason that every species belonging to that fauna thus far described by Professor Marsh trom Wyoming was found there, and these species form much the greater part of the total vertebrate fauna as now known. This statement is made on the authority of Mr. J.B. Hatcher, now of Princeton College, under whom, as Professor Marsh’s assistant, all the fossils in question were collected. In the original descriptions by Professor Marsh the fossils were said to have been obtained in ‘the Laramie of Wyoming” or “the Ceratops beds of Wyoming.” It is important to emphasize the fact that not one of the déscribed species came from the typical Laramie strata of southern Wyoming or from their demonstrated equivalent. It ~~ Baw GEOLOGY OF THE DENVER BASIN. becomes manifestly of interest to analyze critically the grounds upon which the strata of Converse County containing the Ceratops fauna have been referred to the Laramie, and to compare them with the Ceratops- bearing formations of the Denver Basin, whose stratigraphic relations to the Laramie have been described. While Professor Marsh has himself given no details of locality in describing species, there appeared in 1893 an article on “The Ceratops beds of Converse County, Wyo.,” by J. B. Hatcher,’ prepared and published with Professor Marsh’s approval. The article gives many valuable data concerning the character and position of the strata in question and specially states the reasons for believing them to be true Laramie. f Converse County lies on the eastern border of Wyoming, as shown in fig. 24 of this volume. The beds are best exposed on the eastern and south- ern borders of a synclinal basin. Near the southeastern limit the Ceratops beds dip to the northwest at angles varying from 16° to 29° and rest with apparent conformity on Fox Hills strata, identified by their marine inverte- brate fauna. To the northwest the Ceratops beds become nearly hori- zontal and pass under strata of more recent age, referred to below. It is important to notice that according to Mr. Hatcher, “the eastern shore of the fresh waters, in which the Ceratops beds were deposited, was nearly that of the present border of these beds. ‘The eastern limit of the fresh waters was confined to the western slope of the Black Hills and that chain of minor uplifts connecting them with the Laramie Range to the southwest.” If this be true it is plain that the Ceratops “horizon” has not been traced so continuously along the eastern base of the mountains as might be supposed from the statement of Professor Marsh, above quoted. The Ceratops beds of Converse County are 3,000 feet in thickness. At the base is a nonfossiliferous, fine-grained, white or yellowish-brown sandstone member, 400 feet in thickness. The lower division of 150 feet is well stratified; the upper 250 feet massive in texture. Above this sand- stone comes a complex of sandstones, shales, clays, marls, limestones, and thin, impure lignite beds. It would appear that this sandstone corresponds in general lithological character to the basal sandstone of the Laramie as V iam, Jour. Sci., 3d series, Vol. XLY, 1893, p. 135. AGE OF THE ARAPAHOE AND DENVER. 233 developed in Colorado, but, as Mr. Hatcher admits, its separation from the Fox Hills is arbitrary, and neither brackish-water shells, plants, nor coal beds are present to indicate its identity with the Laramie. The assignment of this sandstone to the Ceratops beds is supported only by absence of unconformity and the similarity of the heavy sandstone to the thin beds of the upper series. The fossil-bearing member of the Ceratops beds consists, in Mr. Hatcher's language, ‘‘of alternating sandstones, shales, and lignite, with occasional local deposits of limestones and marls. The different strata of the series are not always continuous, a stratum of sandstone giving way to one of shales, and vice versa. This is especially true of the upper two- thirds of the beds.” ‘The shales are quite soft and loosely compacted, composed mostly of clay with more or less sand in places. The prevailing color is dark-brown, but they are sometimes red or bluish.” ‘The lignites occur in thin seams, never more than a few inches thick, of only limited extent, and with many impurities. At no place in the ‘Ceratops’ beds’ of this region have workable coal beds been found.” ‘Intercalated with the sandstones, shales, and lignites, are quite local deposits of limestones, clays, and marls. The latter are composed almost entirely of fresh-water shells, fragments of bone, teeth, etc.” “All the deposits of the ‘Ceratops beds’ of this region bear evidence of having been laid down in fresh waters. Among the invertebrate fossils found in them, only fresh-water forms are known. There is no evidence that marine or brackish waters have ever had access to this region since the recession of the former at the close of the Fox Hills period.” This description of the Converse County Ceratops beds shows them to be quite different from the Laramie of Colorado, or of southern Wyo- ming, but similar in many ways to the Arapahoe beds, or to the strata of debatable age in other localities. It would not be justifiable to assert at present that the beds of the true Laramie never possess such a variable character and such a loose, friable texture as is shown by the strata in question, but it is certainly fair to point out that this constitution is met with in the post-Laramie and later fresh-water deposits, and that so eminent an authority on the Laramie and associated formations as Mr. R. C. Hills 234 GEOLOGY OF THE DENVER BASIN. has laid special emphasis on this different constitution of strata in discussing the formations of the Yampa district in Colorado. The invertebrates of the Ceratops beds, found often in intimate association with the dinosaurian remains, are thought by Mr. Hatcher to be evidence of the Laramie age of the beds. He mentions five species identified by Dr. C E. Beecher, and states that there are others. It is said that some of them are “known from the typical Laramie,” and some “are characteristic of it.” The weight of this evidence in the present discus- sion clearly depends upon what is considered “typical Laramie.” The known distribution of the five species mentioned by Mr. Hatcher is given in the subjoined table, prepared for the writer by Mr. T. W. Stanton, accompanied by’ some remarks on the localities, which are published with Mr. Stanton’s kind permission: | | | e acl le eens Sine PS) Se=} jee ° ad Solo = So conmuleeya. | el eS lee 4 eR bale ise ieee ans | ‘aI aS a a aa lee | Ra |B a ov | 4 | = =a /o8 |Oagle at aes So)... |, B g o Sass Ses) Pre s | Soo = Mo |} eq |od|ma $)/o8 |e] sa] & OlPa | ess ° + =! | AB Reg o Nese ROME || HS) ag l/=2 | sg) s¢g P=) Jae | wo | of | $4 | cad | sh | of | BS © By (eo lea ee iltey ot lee onl el eelteat ale | | Wnio couesii Wihite- 22.2. -2- ~~ ne Me wignsce me | Seer Sphzrium formosum M. & H........-..--.|-.----|------ \coaae x | Limnwa compactilis Meek...-..........-.-|...---|------ eaoctellsoase Campeloma multilineata M. & H.......-.-.|------ ee We Boe WeeESee Fi : | Tulotoma thompsoni White -..-.--..-..--. Pe Neescme) oceosellontioce Concerning these localities Mr. Stanton’s comments are as follows: The locality on Crow Creek, northern Colorado, seems to be in the true Laramie, and the Black Butte locality has usually been referred to the same formation. The invertebrates from Black Butte come from beds below the dinosaurian remains, according to Dr. White. The localities near Fort Union, Fort Clarke, Heart River, and in the valley of the Yellowstone are in the Fort Union beds. The two species from Lebo Creek are there associated with a number of other species that occur in the original Fort Union area. The Weber, Utah, locality is in the Wasatch Eocene, and that at Separation, Wyoming, is considered as probably Laramie by Dr. White. From the foregoing it is plain that the shells mentioned by Mr. Hatcher do not serve as evidence for the Laramie age of the Converse County beds as strongly as they show their intimate relation to the Fort Union formation, now commonly regarded as Eocene. This fauna can not be AGE OF THE ARAPAHOE AND DENVER. 235 characterized as a Laramie fauna except by including the Fort Union in the Laramie, a view formerly prevailing @, it is true, but now abandoned by all the recent writers on this question. As regards the reference of the Converse County beds to the Laramie or post-Laramie on the occurrence of these shells, their evidence would seem strongly in favor of the post- Laramie. In a succeeding section the evidence concerning Black Butte is reviewed and it is plain that reasonable doubts may be entertained in regard to the assignment of the saurian beds at that point to the Laramie. The formation overlying the Ceratops beds with apparent conformity on the west side of the Converse County basin is sajd by Mr. Hatcher to be of about the same thickness (3,000 feet) and constitution, but it is destitute of fossils except for an abundant flora. The large number of leaves sent by Mr. Hatcher to the National Museum have been examined by Mr. F. H. Knowlton, who has kindly authorized the statement in this place that but few identifiable species are present. The greater part of the material consists of one species, Platanus raynoldsii, a representative species of the Denver beds flora and also known in the Fort Union. Reviewing the facts given by Mr. Hatcher concerning the strata of Wyoming, which have yielded the Ceratops fauna of Professor Marsh, with regard to the assignment of these strata to the Laramie or Post- Laramie, it appears to the writer that the question is by no means settled, and that a reference to the post-Laramie has much in its favor. The inver- tebrate fossils would certainly favor such a reference should any weight be attached to the few species above named. And the vertebrate fauna shows strong alliance with that of the Arapahoe and Denver formations, while the other localities which have yielded these remains are all more or less open to the suspicion that they may also be post-Laramie. The lithological character of the fossil-bearing beds allies them rather with the post-Laramie than with the Laramie. The conformity of the series with the Fox Hills is considered the most weighty line of evidence by Mr. Hatcher, but in the light of the cireum- stances of this particular case it is clear that too much weight may easily be laid upon this fact. The Arapahoe and Laramie beds seem conformable as far as they have been traced along the line of the foothill fold, but the 236 GEOLOGY OF THE DENVER BASIN, conglomerate reveals the extent:of the stratigraphic break really existing between the formations. The pre-Arapahoe uplift seems to have been greatest in the mountain areas of Colorado and Montana, but a study of the various known facts leads the writer to the view, presented in more detail below, that large areas adjacent to the mountains were raised some- what above sea-level at the time of the more pronounced mountain uplift, and that in the interval before subsidence caused the lakes of the Arapahoe to be formed the land surface of the Laramie sediments on the plains may have been very little modified while great erosion took place in the moun- tains. Or the loose and unconsolidated Laramie sediments of the plains, being not much elevated above baselevel, may have wasted so evenly that subsequent deposits upon them now seem conformable. The beds of Converse County, of the Judith River Basin in Montana, and the area between them, the Animas River beds below Durango, and the post-Laramie formations of other districts, occupy positions removed from the regions of greatest disturbance, and it is natural that they should seem conformable with the underlying strata, whatever they may be. In the Livingston region the post-Laramie formation of that name lies with apparent conformity on the Laramie, as seen in the Bozeman and other sections, but to the westward its base transgresses the edges of the Laramie, and in the Three Forks area a great angular unconformity is seen at the base of the Livingston. There are many places in the West where the section of visible sedi- mentary formations from the Cambrian to the Cretaceous seems a conform- able one, and it has frequently been spoken of as such. But the researches of the last two decades have proven the existence of many important strati- graphic breaks in this series, which are in certain places shown as great unconformities but can not be identified at other points. Especially in the plains country adjacent to the Rocky Mountains conformity of formations can not be assumed to prove continuity of sedimentation. The visible conformity between the Ceratops beds and the Fox Hills in Converse County can not be accepted, contrary to other evidence, as proving the former to have been deposited in the epoch next succeeding the Fox Hills. AGE OF THE ARAPAHOE AND DENVER. pei ( Ceratopsida from Black Butte, Wyoming—While none of the types deseribed by Professor Marsh come from the typical Laramie of southern Wyoming, a representative of this family was described from Black Butte by Professor Cope, in 1872, under the name Agathaumas,’ which is now recognized by both paleontologists as belonging to the horned dinosaurs. ‘This occurrence does not, however, prove the extension of the Ceratops fauna downward into Laramie strata equivalent to the Laramie of the Denver field, for the Black Butte locality is one concerning which geologists have differed in their observations, some thinking to have found evidence of a true break, others not, and it is clear that much more field work in that region is necessary to harmonize the evidence of fossil plants, vertebrates, and the published stratigraphic data. The dinosaurian horizon at Black Butte is above a line at which Major Powell observed what he has described as an important physical break® in the series of 5,000 or 6,000 feet of strata assigned by others as a whole to the Laramie. The saurian layer is also very near the top of the series and is hence in the part which must correspond with the Arapahoe or Denver in case any subdivision of this section is carried through. The fossil flora of Black Butte is neither distinctly Laramie nor Denver according to Mr. Knowlton, having the intermediate character which might be expected of the Arapahoe flora. The invertebrates of Black Butte are partly brackish and partly fresh water forms. The former are known in other deposits supposed to belong to the lower part of the Laramie, while the latter range upward into post-Laramie and Fort Union strata. It can only be said that none of the lines of evidence is competent to satisfactorily decide the position of the Black Butte deposits. Other Ceratops localities in Wyoming —T'hrough the courtesy of Mr. J. B. Hatcher the writer is able to state that representatiyes of the Ceratopsidee have been discovered on the south side of the Seminole Mountains, and on the west side of the North Platte River directly opposite the mouth of Medicine Bow River, Wyoming. This locality is 20 miles north of Fort ‘Am, Nat., Vol. VI, 1872, p. 669. Proc. Acad. Nat. Sci., Phila., 1872, p. 279. Proc. Am, Phil. Soc., XII, 1872, p. 481. 2Geology of the Uinta Mountains, p. 72. 238 GEOLOGY OF THE DENVER BASIN. Fred Steele, on the Union Pacific Railroad, and 28 miles northwest of Carbon Station. As a glance at the atlas of the Fortieth Parallel Survey will show, it is very probable that this dinosaurian locality is in the same formation which oceurs about Carbon. This latter locality has yielded a number of fossil leaves which have been ineluded in the Laramie flora by Lesquereux, Ward, and others. The paleobotanists have, however, always recognized that the Carbon flora was similar to that of Table Mountain, at Golden, and this similarity is confirmed by Mr. Knowlton’s recent review, although a table of species is not yet ready for publication. It will be remembered, also, that two species of the Sedalia flora, supposed to be Arapahoe, are known elsewhere only at Carbon. In closing his discussion of the Middle Park beds’ the writer pointed out that while no equivalents of the Arapahoe were known between the eroded Cretaceous section and the Middle Park equivalent of the Denver, the Carbon and Black Butte floras were so much like the Denver flora, and the localities so situated with regard to the mountain area of Middle and North Parks, as to make natural the suggestion “that the plant-bearing beds of these two localities (Carbon and Black Butte) may possibly repre- sent the deposits contemporaneous with the erosion preceding the Middle Park period.” This suggestion seems to become more and more plausible as the distinction between the Laramie and Denver floras becomes better known. If the new Ceratops locality is actually in the same formation with the plant beds of Carbon an important field for investigation is certainly indicated. Mr. Hatcher also states that remains of the Ceratopside have been found on the eastern slope of the Big Horn Mountains, about 40 miles south of Buffalo, Wyo. The Ceratops beds of Montana—Next to the locality of Converse County, Wyo., that of Judith River Basin, in Montana, is the most important known locality for the Ceratops fauna. It was here that Prof. E. D. Cope discoy- ered several representatives of this fauna in 1876, the genera Dysganus and Monoclonius of Cope being now recognized as horned dinosaurs. While the original description of Professor Cope was in an article entitled 1 The post-Laramie beds of Middle Park, Colo.: Proc. Colo. Sei. Soe., Vol. III, 1891. AGE OF THE ARAPAHOE AND DENVER. 239 “Descriptions of some vertebrate remains from the Fort Union beds of Montana,”' the localities and stratigraphical position of the strata yielding these remains were very clearly given by Professor Cope® in the next year, and it then appeared that the ‘Judith River beds,” one of the local divisions of Hayden’s ‘‘Lignitic” series, was the immediate formation from which the fossils were obtained. According to the personal communication of Mr. J. B. Hatcher, nearly all of the forms described by Professor Marsh “from the Laramie of Montana” were obtained by Mr. Hatcher in the same series of strata in the Judith River region which contained the fossils described by Professor Cope. Without reviewing in detail the literature of these beds it is desired to point out the fact that the Judith River strata may perhaps represent the Arapahoe or some other post-Laramie formation, and not the true Laramie of Colorado and Wyoming. The explorations of the Hayden survey and other later examinations of the Judith River and adjoining districts have shown a complex of some- what variable sandstones, shales, clays, and lignites containing in various horizons fresh and brackish water shells and the vertebrate fauna of Cope and Marsh. Below this complex is the Fox Hills Cretaceous and above it the Fort Union formation, the latter boundary not being as yet well established. On account of stratigraphic relation to the Fox Hills, and from the evidence of the faunas mentioned, the reference of these strata to the Laramie has not previously been questioned, so far as the writer is aware. Instead of reviewing past descriptions of the Judith River country the following notes by Mr. T. W. Stanton, who visited the region in 1894 in company with Mr. W. H. Weed, are given with his permission. They pertain only to the lower part of the series, but the main question under discussion is as to the lower known limit of the Ceratops fauna. Mr. Stanton gives the following description of the section near the mouth of the Judith River: The fresh-water Judith River beds are well exposed in bluffs on Dog Creek, 4 or 5 miles from the mouth of Judith River, and also on the north side of the Missouri within 3 or 4 miles of the same place. The section in this neighborhood shows about ) Proc. Acad. Nat. Sci., Phila., Vol. XXVIII, 1876, p. 248. ? Bull. U.S. G. and G. 8., Vol. III, 1877, p. 565. 240 GEOLOGY OF THE DENVER BASIN, 650 feet of marine Cretaceous strata overlain by 300 to 350 feet of fresh-water beds. The succession of strata and thickness as estimated by Mr. W. H. Weed are as follows, beginning at the base: 1. Soft, dark clay shales. 2. Band of ferruginous sandstone with Avricula linguiformis, Inoceramus cripsii, Baroda wyomingensis, Placenticeras placenta, ete. 3. Shales like No, 1. 4, Coarse gray laminated sandstone. . Carbonaceous shales with bed of lignite at base..............-. Deca jore Sra soe a, 52 100 6. Brown sandstone with great numbers of Cardium speciosum and a few other SPeCleS' s.-sSes Merwarssieis aks oS = ES er Ae ee 30 a, (Sandy shalesinsew-2 4: eee cee midttebis « doa. ge abe oe cel aera eee eee 25 ie) Jark clay shales with coneretions containing Paculites ovatus in lower portion and sandy bands and coneretions near the top with a characteristic Fox Hills fauna ineluding— Nucula sp. Liopistha (Cymella) undata. Clisocolus cordatus. Pholodomya subventricosa. Callista nebrascensis. Mactra formosa. Tellina equilateralis. Lunatia suberassa. Tanecredia americana. saculites ovatus. The total thickness of this bed was not seen at any one place, but it is at least 350 feet. Immediately above these dark shales is a bed of greenish-yellow sandstone which occasionally forms bluff exposures 50 or 60 feet high, but usually only slightly exposed in steep slopes and largely covered by wash from the softer and lighter-colored beds above. This was taken as the dividing line between the marine and fresh water beds, though no fossils excepting silicified wood were found in the lower 200 feet of the latter. The remainder of the section, about 300 feet in thickness, is apparently conformable with the underlying beds, but is quite distinet from them in color and texture. It consists of alternations of light-colored, soft, friable sandstones, clays, and marls, with some seams of lignite and purplish carbonaceous bands. Fossils are abundant in the upper 200 feet, consisting of fragments of silicified wood, bones, and numerous invertebrates. The latter include the following species: Spherimn recticardinale. Goniobasis sublevis. Spheerium planum. Goniobasis subtortuosa. Unio dane. Goniobasis sp. closely related to Unio eryptorhynehus. G, tenuicarinata. Anodonta propatoris. Jampeloma vetula. Viviparus conradi. Vetrina? obliqua. Helix veternus. Physa copei. AGE OF THE ARAPAHOE AND DENVER. 941 At the top of the exposure above these fresh-water beds there is a band of brackish-water fossils, reported by both Meek and Hayden and by Cope, which con- tain Ostrea subtrigonalis, Anomia sp. Corbicula occidentalis, Corbula cytheriformis, Goniobasis convexra, ete. This band was not seen by me in the neighborhood of Judith River, but I afterwards saw it near Havre, Mont., holding the same position above the fresh-water beds. These brackish-water shells are specifically identical with those found by Mr. Weed in the Livingston beds; hence they do not indicate the Laramie age of the Judith River beds. The same considerations concern- ing apparent conformity with the Fox Hills which were urged in discussing the Converse County beds apply to the Judith River beds, and it seems to the writer that they are not shown to be typical Laramie by the evidence at present available. Dinosaur-bearing beds near Castle Gate, Utah——In the summer of 1894 Mr. T. W. Stanton found bones of a dinosaur near Castle Gate, Utah, in sandstones ocewring above the Laramie coal beds of that region and below the Wasatch strata of the plateau. The remains in question were submitted to Prof. O. C. Marsh, who identified them as belonging to Claosaurus annectens Marsh, first found in the Ceratops beds of Converse County, Wyo., and afterwards identified in the Arapahoe strata, near Denver. It is therefore of much interest to compare the Utah section with the others in which the same dinosaur has been found. Mr. Stanton has kindly offered the following notes upon the Upper Cretaceous section of Price River Canyon, near Castle Gate, and of the series up to the undoubted Wasatch Eocene: The lowest beds exposed in this neighborhood are dark clay shales, with occasional bands of sandstone, in which a few specimens of Inoceramus proximus, which is characteristic of the Montana formation, have been found, Fartuer south- west, in Castle Valley, a much lower horizon in the same series of dark shales yields Prionocyclus wyomingensis, Scaphites warreni, Inoceramus dimidius, and other characteristic fossils of the Colorado formation. Above the horizon at which Jnoceramus proximus was found the strata consist of alternations of shale and irregular, heavy beds of brown and gray sandstones, the latter greatly predominating and forming probably four-fifths of the entire thickness of 500 feet up to the principal Castle Gate coal bed. About 200 feet below the coal there is a fossiliferous band of shale in which a few braeckish-water Laramie MON XXVII 16 242 GEOLOGY OF THE DENVER BASIN. fossils were found. The species are Ostrea glabra M. & H., Corbula subtrigonalis M. & H., Modiola regularis White, and a few other indeterminate forms. Specimens of Ostrea were obtained to within 100 feet of the coal. Above the Castle Gate coal mine there are about 300 feet of alternating brownish sandstones and shales, with several seams of coal and some thin, calcareous bands, in one of which, about 150 feet above the main coal bed, a few species of fresh-water mollusca were found. ‘These include Viviparus panguitchensis White, Viviparus trochiformis M. & H., Goniobasis tenuicarinata M. & H., and indeterminate species of Bulinus, Physa, Limnea, Planorbis, Unio, and Spherium. They show rather close relationship with the fauna that occurs at a much higher horizon. Next in ascending order is a series of heavy-bedded, brownish-gray sandstones usually forming vertical cliffs and having an estimated thickness of 800 or 1,000 feet. At the foot of one of these cliffs, just north of Castle Gate, some bones of a large reptile were found in a mass of sandstone that had evidently fallen from the cliff. These were submitted to Prof. O. C. Marsh, who reports that “they agree in essential particulars with the type specimens of Claosaurus annectens, which occurs in the Ceratops beds of the upper Laramie of Wyoming.” Overlying this massive sandstone is a series of similar sandstones in beds 20 or 50 feet thick, alternating with shales, and having a total thickness of about 300 feet, and these merge into a series containing a greater proportion of shale and some bands of fresh-water limestone in which invertebrate fossils are very abundant, including the following: Unio mendax White. Goniobasis filifera White. Physa pleromatis White. Limniea tenuicostata M. & H. Viviparus trochiformis M. & H. Hydrobia utahensis White. Viviparus leidyi M. & H. Cypris saupetensis White. Goniobasis tenuicarinata M. & H. These all oecur in the lower portion of the series referred by Dr. C. A. White to the Wasatch formation (Bull. U. S. G. S. No. 34, p. 10) on the higher hills near Castle Gate and at Pleasant Valley Junction and other localities in that region. Several of the species are identical with forms that occur in the Fort Union beds on the Missouri River, and some of them also occur in beds believed to belong to the true Laramie of Colorado and Wyoming. The entire series from the marine Cretaceous up into the fresh-water Eocene seems to be conformable, and there are no sudden changes in the character of the sediments. The close relationship, and in some cases specific identity, of the fresh- water mollusca in the coal-bearing series and in the Wasateh also favor Dr. White’s view that sedimentation was continuous from the one into the other. From Castle Gate the Laramie coal beds may be traced with practical continuity to Grand River in Colorado, about 180 miles, and at the latter AGE OF THE ARAPAHOE AND DENVER. QA3 locality the Ruby formation, one of the apparent equivalents of the Denver beds, oceurs between the Laramie and the Wasatch. As stated in an earlier part of this section, the Ruby formation reaches a thickness of 2,000 feet in the West Elk Mountains. It is also to be borne in mind that in the Anthracite district of Colorado there is a formation—the Ohio Creek—which is the probable though not demonstrated equivalent of the Arapahoe. If the Castle Gate section be assumed to be the product of continuous sedimentation from the Fox Hills to and including the Wasatch Eocene, as advocated by Dr. C. A. White for certain regions, it is still true that the several time-intervals of the Laramie, Arapahoe, Denver, and also the Puerco, must be represented in that section, and the Claosaurus, having been found in the upper sandstone member of the group of strata referable to the Cretaceous, is most plausibly of one of the post-Laramie epochs. This is rendered still more plausible by the character of the invertebrate fauna occurring below the vertebrate horizon, which is much more closely related to the Fort Union or Wasatch faunas than to any known from unquestion+ able Laramie. If the sedimentation was not continuous in the Utah seas, then there are stratigraphic breaks in the series of apparently conformable beds. As to the equivalent of the Puerco in the section, it remains to be demonstrated that the Wasatch of the New Mexico section, which rests on the Puerco, is the same as the Wasatch of Utah, as identified by the invertebrate paleon- tologists. Furthermore, it is well known that while Professor Cope, who has defined and studied the Puerco, refers it to the Mesozoic, as ‘‘ post- Cretaceous,” Professor Marsh includes it with the lower Wasatch Eocene. Bijou Creek, Coloradéo— A bout 40 miles east of Denver is the valley of Bijou Creek, one of the typical streams of the plains, rising some 25 miles north- east of Colorado Springs and coursing a little east of north to the Platte River, a distance of about 80 miles. The upper portion of its course is in the Monument Creek strata, while near its mouth are true Laramie beds, according to Dr. C. A. White,’ who, in 1877, collected such common Laramie shells as Ostrea glabra, Anomia micronema, Corbula subtrigonalis, elania wyomingensis, and several species of Corbicula. From the locality Mel [ 4 1 1 sy f Corbicul I \ ‘Eleventh Ann. Rept. U.S.G. and G. Survey of Terr., p. 189. 244 GEOLOGY OF THE DENVER BASIN. where the above fossils were found to Bijou Basin, at the head of the creek, Dr. White was unable to find any other fossil-bearing horizon, and but few outcrops of strata were seen north of the Kansas Pacific Railroad crossing. In Monograph II of the Hayden survey, Prof. E. D. Cope described two dinosaurian fossils from Colorado under the names Polyonax mortuarius and Cionodon arctatus. These forms are now regarded by Cope! as belong- ing to the Ceratopside, Polyonax corresponding to Triceratops Marsh, while the position of Cionodon, which is known only from teeth, is not certainly established. Marsh also considers Polyonax to be a horned dinosaur, though not surely separable from Agathaumas Cope. With the above forms occurs a Hadrosaurus, and the turtles Compsemys and Trionyx. So far as the writer is aware, Professor Cope has not given the localities of these fossils in connection with descriptions of them, but on personal inquiry he kindly stated that they were obtained on Bijou Creek, about 40 miles east of Denver, but he could not specify the exact locality. The locality of Bijou Creek is interesting, as its general position im certain important particulars is much like that of the Converse County, Wyo., locality, and that of the Judith River Basin in Montana. In all these cases the beds lie some distance away from the main mountain range, and the connection with the more complete sections commonly found in the foothill region is interrupted. As in Wyoming and Montana, so im Bijou Valley, the first natural assumption of the collector would be that the strata containing the vertebrates belonged to the Laramie. But as the Laramie of the Colorado foothills is not known to contain these or allied remains while the Arapahoe beds do contain them, and as the latter, in all probability, once extended much farther out into the plains area than the eastern boundary of the Denver map, it seems strongly probable that the dinosaurs described by Professor Cope came from Arapahoe beds reappearing from beneath Denver and Monument Creek sediments in the valley of Bijou Creek. CONCLUSIONS FROM EVIDENCE. Individuality of formations established— The” facts of stratigraphy and lithology seem to the writer much more than sutlicient to prove that the Arapahoe and Denver formations are entitled to recognition as distinct formations. ‘Am. Naturalist, Vol. XXIII, 1889, p. 906. AGE OF THE ARAPAHOE AND DENVER. 245 The only question is as to the extent of the separation from the Laramie, and more knowledge in various directions is necessary to determine this point. It is-noticeable that the developments of the past few years have steadily increased the importance’ of these formations. The evidence of fossil plants has been shown by Mr. Knowlton to confirm that of stratig- raphy as to the distinctness of the Laramie and Denver formations, and this result leads to the hope that when the Laramie fauna has been carefully examined in respect to the distribution of its members all lines of evidence may be found in accord. A few years ago the Laramie flora was deemed as indivisible as is the fauna to-day. A study of the facts which have been presented brings out certain features of the time-intervals between the close of the Laramie and the close of the Denver epochs which it may be well to recapitulate in this place. The pre-Arapahoe uplift—-The Arapahoe sediments of the Denver Basin testify beyond question to a preceding uplift which terminated Laramie deposition in this vicinity. By this movement some adjacent area of Mesozoic rocks was greatly elevated and was eroded to an unknown but considerable extent before the beginning of deposition in the Arapahoe lake or sea. This latter conclusion follows from the fact that the early conglomerates of the Arapahoe contain pebbles from yarious horizons. The Arapahoe sediments do not record a progressive erosion, cutting deeper and deeper into the uplifted Mesozoic area, so much as they teil of an already eroded surface. In Middle Park the entire Mesozoic section was upturned and eroded prior to the Denver epoch. It is reasonable to suppose that this elevation was contemporaneous with that of the Denver Basin. In the mountain area tributary to the Canyon district the apparent evidence of the strata described above indicates an uplift similar to that near Denver. In the West Elk Mountains an uplift is less certainly proven. From the general character of the Laramie in Colorado, and its observed thickness, it may even be inferred that the pre-Arapahoe uplift terminated Laramie sedi- mentation throughout the mountain district. No evidence is known to the writer showing that Laramie deposition was ended by any other orographic movement within the area of Colorado. 246 GEOLOGY OF THE DENVER BASIN. In the Livingston area of Montana the known facts speak for an order of events similar to that of Middle Park. The Arapahoe epoch—As pointed out in the preceding section, the Arap- ahoe epoch of sedimentation represents only a part of the time of erosion which followed the pre-Arapahoe uplift. When further identifications and correlations of formations have been made, it may be possible to extend the scope of this epoch to cover the entire period of erosion and contempora- neous sedimentation. In many ways the Arapahoe epoch was probably much more impor- tant than the Denver, though its deposits are less widely identified at present. It is plain that far from shore-lines the sandstones and shales of the Arapahoe might readily be lithologically indistinguishable from those of the Laramie, and if it shall be proven that the pre- Arapahoe uplift extended through Wyoming into Montana, the deposits of the Arapahoe epoch are to be sought for in these States in the upper portions of the great sections which have been referred to the Laramie, provided these sections are not incomplete through removal of the Laramie before the Arapahoe deposition began. The influence of the great pre-Arapahoe uplift upon life existing at the close of the Laramie is unfortunately not yet known to a degree allowing much discussion. ‘The fossil plants found must be intermediate in character between those of the Laramie and Denver, and allied to both. The verte- brates of the Arapahoe are highly modified and specialized types, but when and how they acquired their remarkable characters is not known. If it be true that the Ceratopsidse were not modified by the climatic and other changes of the pre-Arapahoe interval it may well be wondered what caused their sudden extermination. Pre-Denver volcanic eruptions—T'he numerous deposits of andesitic tuff, sand- tone, and conglomerate, which have been described, testify to ernormous outpourings of andesitie lavas all over Colorado and extensively in Mon- tana, and at nearly the same time. It will be evident to all that this is a very remarkable and important event in the volcanic history of the Rocky Mountains. But in judging of this epoch as an element in the general history, it is probable that the chief data available for forming that judgment can be fully appreciated only by petrologists. ; AGE OF THE ARAPAHOE AND DENVER. 24.7 It is a striking feature of all the deposits of andesitic material that they show a great range of andesitic types, making it probable that a very long series of eruptions occurred, in the course of which marked changes in the composition of the volcanic products took place. To the petrologist this variation from basic to acidic extremes within the andesitic group means a long period during which chemical differentiation went on, producing magmas of widely different constitution. The variation of lavas in the Denver beds is much greater than that shown in many of the largest known voleanoes, such as tna and the Hawaiian Islands. The andesitic eruptions of Colorado can not be regarded as mere interruptions of Arapahoe sedimentation, for the reason that in Middle Park, on the Animas River, and in the Elk Mountains, the beds of volcanic material rest directly on the Laramie, or on an eroded surface, not on any equivalent of the Arapahoe. It is quite possible that no sedimentation took place in these regions from the close of the Laramie epoch until the time of subsidence which led to the Denver and equivalent deposits. To one appreciating the enormous amount of molten material extravasated in this period it must suggest itself that the subsidence which so generally followed the eruptions was in some measure a result of the enormous outpouring. The Denver epoch— I'he importance of the Denver epoch is to be measured by the thickness and character of its sediments and by its fossils. The strata show the epoch to have been one of subsidence in several localities and to an extent making it probable that large continental areas were involved. In the Denver Basin the remaining beds of this epoch are 1,400 feet in thickness; the Ruby beds of the West Elk Mountains are 2,000 feet thick; and Marvine assigns 6,400 feet of strata to his ‘Lignitic” and “doleritic breccia.” These thicknesses are equal to or exceed those of the Laramie in the same districts. The materials forming the Denver beds are softer than those of the Laramie sandstones, and much less abrasion, and hence probably a shorter time, is represented in the accumulation of Denver sandstones than in the case of texturally similar rocks of the Laramie. But the time represented by the Denver and equivalent formations is one of much importance. 248 GEOLOGY OF THE DENVER BASIN. The plant lite of the Denver epoch was materially different from that of the Laramie, as shown by Mr. Knowlton in Chapter VII; but until the flora of the Arapahoe is much better known we can not tell how much of the modification was produced during the interval immediately preceding the Denver, and how much during the earlier intervals. The vertebrates of the Arapahoe and Denver beds thus far identified are so few compared with those of Wyoming and Montana that until the distribution of the latter in the several fossil-bearing horizons has been clearly established it can not be known whether the Denver fauna has peculiarities distinguishing it from that of the Arapahoe. Post-Denver interval——he Denver formation may be probably considered as the uppermost member of the Cretaceous, now that the Fort Union beds of Montana have been recognized as Kocene from their rich fossil flora. If the Denver beds are so regarded, the time-interval succeeding their deposition is a most important one as marking the boundary between Mesozoic and Cenozoic times in the Rocky Mountains. In the Denver Basin no undisputed Eocene strata have been preserved, if, indeed, they were ever deposited in this region. But there are three known localities where formations apparently the equivalents of the Denver beds rest upon the typical Laramie, and are overlain by the lowest Eocene deposits of the respective regions. These localities are: On the Animas River, in Colo- rado and New Mexico, where the Puerco formation overlies the Animas beds; on Grand River, in western Colorado, where the Ruby beds are overlain by the Wasatch, and in Montana, where the Livingston formation is overlain by the Fort Union. In these three localities one may hope to find some evidence as to the orographic or other dynamic disturbances which are commonly assumed to have characterized this interval. Field researches in these regions have not as yet been sufficiently thorough to demonstrate the absence of phenomena indicating orographie disturbance, but no evidence of important movements has been announced. In fact, as far as the writer is aware there is no described case of unconformity or dynamic disturbance which has hitherto been supposed to belong to the post-Cretaceous interval which may not as well be referred to the pre-Arapahoe movement. In all cases where the lowest recognized AGE OF THE ARAPAHOE AND DENVER. PAI Eocene formation is found resting unconformably on the Laramie or older strata, it must still be a matter for proof as to whether the movement thus recorded took place before or after the post-Laramie epochs which have been described above. In other words it appears that at present there is no distinct evidence of an especially important earth movement succeeding the Cretaceous period, if the Denver beds are assigned to the Mesozoic, 3D while that preceding the Arapahoe seems from present knowledge to have been of the character and magnitude usually assumed for the disturbance closing the Mesozoic. As the stratigraphic data now available do not satisfactorily determine the preeminent importance of the post-Denver interval, the evidence of fossils must be relied upon to justify the reference of the Arapahoe and Denver formations to the Cretaceous. While an exhaustive discussion of this question can not be entered upon in this place, the writer wishes to briefly state the inferences which seem to him justifiable from a considera- tion of the facts already presented. Evidence of fossil plants—It is shown by Mr. Knowlton that the Denver flora is remarkably distinct from that of the Laramie of the Denver Basin, only 15 species being now known in both formations out of a total flora of 148 species. An equally satisfactory comparison of the Denver flora with that of the Fort Union ean not be made until the latter has been revised, but Mr. Knowlton informs the writer that at least 13 species seem common to the Denver and Fort Union floras. The Middle Park and Livingston floras are included with that of the Denver beds in these statements. These figures do not indicate a much greater break in plant life between the Denver and Fort Union epochs than is found between Laramie and Denver. In general character the Fort Union and Laramie floras are so closely related that for a long time these formations were assigned to the same geologic epoch. It does not appear then that the climatic influences of the post-Denver interval modified plant life to a superlative degree. Invertebrate fossils——[‘he Laramie seas have commonly been regarded as transitional in character between the true marine waters of the Fox Hills Cretaceous and the fresh-water lakes of the Eocene. In harmony with this idea the invertebrate fossils represent brackish-water or fresh-water forms, 250 GEOLOGY OF THE DENVER BASIN. with a few known in the Fox Hills. Many of the fresh-water species are also known to range upward into Wasatch or Fort Union Eocene beds. As mentioned in discussing the section at Castle Gate, Utah, the gradation in character of the invertebrate fauna from the Fox Hills to the Wasatch is so gradual and the section apparently so complete as to cause Dr. C. A. White to suggest that in this region sedimentation was continuous into Eocene times. But this view is opposed by the evidence offered by verte- brate paleontology and does not accord well with the facts of stratigraphy that have been set forth. It is at least plain that until the distribution of the invertebrate fossils through the various formations under consideration is much better known than at present the evidence of vertebrate animals and fossil plants is more useful than that of the Mollusea. Even the presence of brackish-water shells does not prove the strata containing them to be of true Laramie age, for My. Weed has found in the Livingston formation forms which, according to Mr. Stanton, are identical with some from the Judith River beds. Vertebrate fossils —As shown by Professor Marsh in another chapter, the vertebrate fauna of the Laramie and the formations here termed the post-Laramie is a very remarkable one, and the consensus of opinion among paleontologists that this great fauna is strongly Mesozoic in its affinities has determined the present reference of the Arapahoe and Denver to the Cretaceous. As has been shown, it is only in this broad way that this vertebrate fauna can now be used as evidence in the question under discussion in this chapter, and it will be well to examine the grounds upon which the positive opinion as to the geological significance of these fossils rests. The leading elements of this fauna are the dinosaurs and the mam- mals, both represented by many genera and species, nearly all of them new. The mammals are considered by Professor Marsh as mainly allied to Jurassic types, but it is believed by Professors Cope and Osborn that they are intimately related to the mammals of the Puerco. The latter is referred by Professor Marsh to the base of the Eocene (lower Wasatch), while Professor Cope classes it with the Mesozoic. The wide differences of opinion thus brought out clearly deprives these mammalian remains of much of their value in the present discussion. 3 AGE OF THE ARAPAHOE AND DENVER. 25t The dinosaurs of the Ceratops beds are highly modified and special- ized forms unknown as yet in other parts of the world, except, perhaps, in the Gosau formation of Austria,“and the conclusion that they necessarily indicate a Mesozoic age implies some reason why they may not have survived into the early Tertiary. In the light of the facts which have been presented concerning the several epochs succeeding the Laramie, it is not clear to the writer why this belief that the dinosaurs, or, indeed, the whole vertebrate fauna, surely indicate a Mesozoic age should be so positively maintained as is done by the vertebrate paleontologists. If the dinosaurs of the Ceratops fauna did actually live in the Lara- mie epoch of Colorado they survived a great orographic movement and its accompanying climatic changes, and continued through the Arapahoe and Denver epochs so little modified that Professor Marsh has not detected any changes corresponding to the stratigraphic time divisions. This is all the more remarkable since the fossil plants show a great modification during this time, and it has been commonly claimed that enormous and highly specialized vertebrate animals are particularly sensitive to conditions of environment. If the Laramie vertebrates were unaffected by the known dynamic phenomena of the Colorado region in post-Laramie times, it may well be asked what caused their extermination in the post-Denver interval, where as yet no evidence of orographic movements comparable with that of the pre-Arapahoe have been feund. And if their extinction was due in large measure to other causes than those associated with dynamic phenomena, may that extinction not have been deferred until the Eocene? These considerations seem to the writer ample ground for the demand that the causes leading to the extinction of the Ceratops fauna should be definitely connected with some orographic disturbance at the close of the Denver epoch before their presence in the Arapahoe and Denver beds can be admitted as full proof of the Mesozoic age of these formations. 7? Is a dual nomenclature desirable2—'T'he vertebrate fauna of the post-Laramie beds is said by paleontologists to be strongly Mesozoic in its affinities. The post-Laramie formations are later than the beginning of the great Rocky Mountain revolution which has heretofore been considered to mark 252 GEOLOGY OF THE DENVER BASIN. the close of Mesozoic time in the mountainous regions of western North America. It has been generally assumed that the revolution caused in some way the great change in life observed in the fossils of the earliest Kocene deposits as compared with those of the Cretaceous. But recent discoveries show that the gap in life grows less as knowledge increases, while accu- mulating evidence continually enhances the importance of the orographic movement occurring between the Laramie and Arapahoe epochs. Applying the criterion of continental development as it has been applied in the past, the pre-Arapahoe movement, which terminated the long series of conformable Cretaceous sediments, marks the end of Mesozoic time. Applying the criterion of life, and especially of vertebrate life, the post-Laramie epochs may be assigned to the Cretaceous. It is not the writer’s desire to advocate the establishment of a dual nomenclature for the case under discussion, but simply to recognize this aspect of the question as the logical deduction from the evidence that has thus far been presented. Investigation must be carried on in many direc- tions before the relative importance of the various factors of this problem can be established. The importance of the pre-Arapahoe movement in comparison with later ones which are not as yet so clearly defined must be sarefully demonstrated. The distribution of all classes of fossil remains through the series of formations must be studied, and the extinction of the Mesozoic types of vertebrates must be connected with the great movement of orographic importance by something more tangible than mere assumption SECTION IV.—MONUMENT CREEK FORMATION. By GrorGe H, ELDRIDGE. STRATIGRAPHY. The name “ Monument Creek” was first applied by Dr. F. V. Hayden to the series of strata which forms the prominent divide between the Platte and Arkansas rivers, extending from the base of the Colorado Range east- ward. This use of the term is provisionally accepted in this report. About the middle of the series is a well-defined break in deposition, the divisions above and below which may, upon systematic study, be found to be distinct formations. THE MONUMENT CREEK FORMATION, 253 The Monument Creek formation occurs along the southern edge of the Denver field in the steep slopes of a high mesa and also stretches from its base prairieward in thin sheets. The floor of the lake in which the Monu- ment Creek was deposited was more or less irregular from erosion, and in one part or another consisted of the clays and sandstones of the Laramie, Arapahoe, or Denver formations. In the foothill region the Monument Creek lies in contact with the Arapahoe; between Platte River and Cherry Creek a few hundred feet of Denver beds exist, which further to the east disappear. North and east of Coal Creek, on the eastern edge of the field, both Denver and Arapahoe are wanting and the Monument Creek rests directly upon the clays of the Laramie. The Monument Creek consists of conglomerates, sandstones, and bright, vari-colored, arenaceous shales. These alternate with one another, but the conglomerates are especially prominent in the upper division, while the sandstones and shales are about equally distributed throughout the whole formation in beds from 20 to 40 feet thick. Only a portion of the lower division of the Monument Creek extends within the Denver field. This displays marked regularity in the succession and composi- tion of its beds, except at the very base, where, owing to the uneven floor, the material varies from conglomerate through sandstone to arena- ceous shale. ® , v - IGNEOUS FORMATIONS. 287 small spheres, with a radiate structure, which are either loosely aggregated or form walls. They are sometimes embedded in another zeolite of similar color and granular structure. The former mineral proves to be thomsonite, the latter laumontite, with stilbite as a common associate. These zeolitie deposits are noteworthy because they frequently color this zone so that it is distinguishable at a considerable distance. Débris of basalt accumulates at the base of the cliffs so that the lower contact is usually hidden, and the loose material may rise so that the foot of the bare cliff is at or near the top of the lower sheet. The débris piles consist of large blocks formed by the jointing, which is very pronounced. A good columnar structure characterizes the cliffs in many places, while the spherical sundering and other forms produced by weathering are also locally well developed. The photograph reproduced in Pl. XIV shows the perfection of the spherical sundering as exhibited on the south face of North Table Mountain near the main zeolite locality. The larger spheres are 2 or 3 feet in diameter. North Table Mountain—'The upper surface of North Table Mountain is by no means so flat as it appears at a distance. Numerous shallow drains lead into the two large gulches and toward the minor indentations. There are, too, a few knolls rising to greater elevation than any point upon the cliff line. In one or two depressions are stagnant pools. The highest point of the mountain, 6,599 feet, near its western edge, is a rather sharp knoll surrounded by minor points with connecting ridges, producing a series of semibasin-shaped areas, to which the fanciful name of “The Craters” has been locally given. Marvine, too, lends his support to the idea that they indicate the points at which the lava welled up and whence it spread out as a sheet. In point of fact the area about this knoll represents the basalt of a higher horizon in the upper sheet than is now left at amy other spot. The rock, which is here much bleached, is more coarsely crystalline than elsewhere, and the surface configuration, which alone suggested the term “crater,” is due entirely to a peculiar development of jointing and to the manner in which a rock thus jointed is naturally affected by erosion. The accompanying illustration, Pl. XV, shows the structure about the highest 288 GEOLOGY OF THE DENVER BASIN. point. Within certain large and rudely spherical:spaces the basalt seems to have undergone a more perfect tabular jointing than in the areas between these spheres. The tablets of the spheres are smaller and thinner, and hence lighter and more easily removed than those of the intervening rock, and the result is that basin-shaped depressions are produced, separated by curved ridges that are sometimes quite narrow. None of these basins is entirely surrounded by a wall, as that would naturally prevent the removal of the rock fragments. Neither spherical sundering nor disintegration of the basalt occurs here, and the only visible reason for the ‘‘ crater” forms lies in the variable jointing within and without these imperfectly spherical areas. The basalt at this point is quite compact and contains no more small vesicles than may be found in the most massive parts of the lower flow. From analogy with the lower sheet this highest point must be at least 50 to 75 feet below what was formerly the upper surface of the flow, and it is probably 200 feet above the bottom. The cliff at the southwest point of North Table Mountain shows fully 140 feet of the rock belonging to the upper flow. Ascending this cliff, through a crevice near the point, it will be noticed that the rude columnar structure prevailing in the lower parts passes by a horizontal fissuring of the columns into a tabular jointing which is most pronounced at the highest horizon remaining. The rock of the tablets is also much lighter in color and apparently coarser in grain than below, and thus approaches in char- acter that found some 50 to 75 feet higher in the flow, at the poimts shown in Pl. XV. South Table Mountain — W hile the basaltie capping of South Table Mountain is, at some points along its northern face, resolvable into two flows, like those of North Table Mountain, this is by no means commonly the case. Instead of the regular structure described, there are many places along the cliffs of the northern, western, and eastern faces where the basalt from the tuff upward consists of solid and compact rock mixed most irregularly with very porous matter, producing a breccia-like structure. Sometimes uni- formly compact rock appears above or below such breeciated material. Castle Rock, the projecting point above the town of Golden, exhibits NIVINNOW 378¥L HLYON ‘1qvS¥8 NI ONILNIO? YvINEVL AX “Id HIAXX HdWYDONOW ABAUNS 1WDINO1039 "Ss ‘Nn IGNEOUS FORMATIONS. 289 this irregular structure very well, as will be seen from the illustration, PI. XVI, taken from a photograph, which represents the composition of the southern face of this point. The high cliffs running east from Castle Rock are of quite massive rock until near the indentation at about the center of the northern face. For some distance east of this place the cliffs show a very irregular relation between compact and porous rock, and the fully normal status is not again shown on the northern face. This prominent northeast point of the mountain is likewise more or less irregular in its presentation of massive and porous rock. The explanation of this structure consists in assuming that the earlier basalt sheet did not extend so far to the south as the northern line of South Table Mountain, but that it sent off arms or tongues of the common lava- like character, with high walls of rough, scoriaceous fragments cemented by parts of more compact rock. Over these arms and walls, filling the spaces between them, came the second sheet, itself broken and irregular in structure from the unevenness of the surface. The present cliff lines of the mountain give profiles across or parallel to these arms. The confused mingling of structures is most marked along the northern face of South Table Mountain. The northern cliffs correspond nearly in height to the opposite ones of North Table Mountain, but the thickness of the capping decreases southward, until scarcely more than 10 feet remains in some places on the south line. This thinning is evidently in part due to erosion—for there is no amygda- loidal zone left when the basalt is very thin—but as this may be regarded as having been approximately equal over the whole mountain, it is plain that the lava itself thinned out. On the north slope of Green Mountain, 2 miles south from the southeast point of Table Mountain, there is a knoll, upon the summit of which loose, angular fragments of porous basalt lie in such quantity as to suggest that they represent a remnant of the sheet broken up in place. As no outerops of basalt appear to the south, on the rising slopes of Green Mountain, it must be assumed that the lava did not extend so far in connected sheet form, but angular fragments strewn over the surface of several little benches of about the same elevation indicate the former presence of a thin sheet or of outlying arms of basalt. MON XXVII 19 : 290 GEOLOGY OF THE DENVER BASIN. THE EARLIEST BASALTIC OUTFLOW, The capping sheets of Table Mountain do not represent the earliest sur- face flows of basalt in this region, for at various points on all sides of North Table Mountain, at a horizon which is quite uniformly 100 feet below the capping sheet, there appear typical streams of basalt. The present outcrops of these bodies form little cliffs on the edges of the benches occasioned by the presence of the eruptive rock, and all exposures seem to be sections of small streams. None of them reach as far south as South Table Mountain, and no dikes of exactly corresponding rock are known, for although a thorough examination of this type shows it to be petrographically identical with that of the more recent sheets, still it can always be recognized at once by its numerous augite crystals, which lie ina much darker and denser ground- mass than is presented by the other type. The map shows the position and relative size of the six outcrops of this earlier rock that have been examined. By far the largest of these bodies appears below the high cliffs at the north- east extremity of North Table Mountain, forming itself a bench and minor cliff, distinguishable even from Denver. The southern end of this outerop is well exposed by a gully which cuts across it. On the northern side of this small ravine the mass is shown as it rapidly thins out. The whole face here exposed is rough and very irregularly porous, with an almost scoriaceous crust of the same structure that is shown in the upper sheets. South of the gully a thin arm, 2 or 3 feet thick, consisting entirely of very vesicular lava, extends for some distance. To the north the body thickens rapidly and soon reaches a vertical thickness of 50 to 60 feet, which is maintained nearly to the extremity shown by the map on the north side of the mountain. When it reaches this thickness the mass has a structure much like that of the higher sheets. A porous zone adjoins the strata below, succeeded by massive rock in the center, and a thicker porous zone comes at the top. There are, however, no large cavities in these bodies, and they seldom contain other minerals than calcite and a green substance allied to delessite. There is a rude, vertical, columnar structure on the cliffs, and locally a spherical sundering. On the western slope of the northeastern portion of the mountain is a small outerop of similar character, whose extremities are covered by débris. U. S. GEOLOGICAL SURVEY MONOGRAPH XXVII_ PL. XVI SOUTH FACE OF CASTLE ROCK, GOLDEN IGNEOUS FORMATIONS. 291 The stream of the greatest extent after that first described occurs on the northwest slope of the mountain. It has caused a bench large enough to be indicated on the map. The eastern portion of this stream is thin and very lava-like in structure, with distorted and long-drawn-out pores. Under the bench its thickness increases somewhat, but probably does not reach that of the first mass, although an exact estimate is here impossible, owing to the débris. A small knoll north of the eastern end is capped by a remnant of this stream. On the south slope of the mountain, between the two large gulches, are sections of two small streams, the western one being so exposed as to give amost excellent opportunity for the study of these masses. Approach- ing the outcrop from the west, on the same level, a thin sheet is first met with, consisting of very porous lava. This rapidly thickens eastward, and at its maximum the body is 50 feet thick. On the western side, where the mass is about 20 feet thick, the upper surface presents a mixture of scoriaceous basalt fragments with sand, and the whole structure reminds one most forcibly of the appearance of the walls of modern lava streams, which often consist of fragments of the quickly cooled crust broken and pushed along by the molten lava within, gathering up sand and foreign matter as the movement progresses. The eastern side of this section is covered by débris, but the mass certainly does not connect on the surface with a-similar stream but a few hundred yards farther east. This latter body is not well exposed. The last stream to be mentioned occurs on the southeastern slope of the eastern point, and is of a size corresponding with the preceding. The body is a surface flow, as is shown very clearly in this case. « The structure of these simple lava streams has been repeatedly brought out in order that no doubt may arise, owing to incomplete statement, of their actual nature as surface flows, for this fact is important in explaining some problems in the geology of the district. The structure of the lower lava streams and their uniform appearance at certain horizons may be affirmed as sufficient evidence of their nature as surface flows. They were covered by sediments of the same character and formation as those beneath them, and it is therefore quite probable that the 292 GEOLOGY OF THE DENVER BASIN. streams were poured out upon a shallow sea bottom near the shore-line. As for the larger streams above, their structure is also emphatically that of surface flows, and no reason is known for objecting to the view that their time of eruption followed that of the lower streams by the period occupied in the deposition of the intervening 100 feet of Denver strata. It is true that there is nothing in the present position and appearance of the upper sheets of Table Mountain which might not be satisfactorily explained upon the supposition that the surface upon which the basalts were poured out corresponded to a comparatively recent surface of erosion, But when a lava of almost identical composition and so related in occurrence can be shown to be of the Denver epoch, there would seem to be no reason for claiming two widely separated periods of eruption of the same rock in the same small area. At least the burden of proof would seem to rest with any supposed advocates of the latter view. As a matter of fact, nothing is known incompatible with the former. The fact that the basalt sheets are not known in Green Mountain at a horizon corresponding to that of Table Mountain is simply explained by assuming the sheets to have been of very limited extent, an assumption also necessary in case the second view is adopted. THE ZEOLITIC MINERALS OF TABLE MOUNTAIN. General oceurrence—The minerals occurring in the amygdaloidal zone of the Table Mountain basalt have already been described in some detail by Dr. W. F. Hillebrand and the writer,’ chiefly from the purely mineral- ogical standpoint. In this place some further facts of the mode of oceur- rence will be given, with suggestions as to the genesis of certain unusual to) ae. A varieties. The list of species identified embraces the following: but seems at first glance less distinctly porphyritic, because its augite crystals are smaller, and the darker, fine-grained mass of the rock obscures them. On closer examination, however, the structure is seen to be more nearly the normal porphyritie than in the first case, for augite, olivine, and a considerable number of plagioclase tablets are distinct phenocrysts im an evidently crystalline but megascopically unresolvable groundmass. The largest phenocrysts are tablets of labradorite 0.5 em. across. By microscopical study it is found that the Ralston dike rock has the same mineralogical constitution as the Valmont dolerite, and the difference between them lies chiefly in the development of the plagioclase. Here there is a decided contrast between the phenocrysts of probable labradorite and the smaller crystals of the groundmass. The latter are short and stout, and nearly all have an irregular zone of apparent orthoclase about them. This is much more distinct than in the Valmont rock and its identification as orthoclase seems unquestionable. There are many irregular grains of orthoclase independent of the plagioclase. The augite phenocrysts are very irregular in form, and seem either to have suffered resorption which destroyed their symmetry or they never possessed sharp crystallographic boundaries. There are many very small irregular grains, as in the preceding rock. Reddish-brown biotite shows a very strong tendency to form a ragged 304 GEOLOGY OF THE DENVER BASIN, fringe about magnetite. Apatite has a phenoerystic development in large, short, more or less rounded prisms with axial inclusions, and also in clear needles of later formation, TABLE MOUNTAIN BASALT; CAPPING SHERTS. Occurrence—Tho basaltic flows of the capping sheet of Table Mountain rest upon Denver strata, and they are believed to have been poured out upon the sea-bottom at a time in the Denver epoch corresponding to their present horizon in that formation, ‘The reasons for this conclusion have been given in full, the most weighty of the considerations being the existence of the earlier smaller basalt bodies which are undeniably of Denver age. ‘The capping is made up of two flows, the lower of which has a maximum thickness of about 125 feet, while the upper one has been so subject to erosion that its former thickness can not now be estimated, though from the remnant on North Table Mountain it can safely be considered to have been much greater than that of the lower flow. In the lower sheet may be found almost all the struetural phases of basaltic lava flows, and the upper body corresponds to the same standard as far as can be seen, ‘The upper surface of the lower flow is very porous, or even scoriaceous, with a cracked and broken appearance in places. For 40 feet below the surface the rock is vesicular, some cavities being several feet in diameter, "The central portion is uniformly massive and compact, while a narrow porous zone is found adjoining the lower contact. Conditions of consolidation——It is not deemed of great importance to the present discussion to know whether these basaltic lavas were poured out into shallow seas or not. The water could not have had great influence upon the course of consolidation after a firm crust was formed, though the pressure of confined steam assists in explaining the occurrence of very large vesicles at rare intervals among the smaller ones. Whether cooling. in water or not, the earlier flow was certainly covered by the lava from a second eruption before its jagged surface was appreciably modified by erosion and before either sediments or otherwise-formed deposits could lodge upon it. The identity of lavas and their juxtaposition suggest that very possibly the first lava was covered by the second before complete consolidation. In such a case the rate of cooling in the two bodies was IGNEOUS FORMATIONS. 305 mutually affected to some degree. In any case it is certain that the crust formed upon a basic lava stream of such thickness could effectually retard the consolidation of the interior portion and give time for the formation of a highly crystalline mass. Description —'The constituents of the Table Mountain basalt are the same as of the rocks above described, but there is naturally a great differ- ence in their development in different parts of the sheets. Porphyritic structure is more pronounced than in the dike rocks, yet the interior parts of both flows are highly crystalline. The grain of the groundmass produces the chief difference in outward appearance. In the central portions of both sheets the groundmass is clearly a crystalline mass of dark-gray color. Approaching the surface the rock is darker and the groundmass becomes aphanitic. In the inner parts of the vesicular zone ¢ lens still shows a fine crystalline groundmass, and it is only in the outer, more or less scoriaceous crust that the microscope reveals a globulitic, glassy base. The phenocrysts of these surface flows are nearly the same in develop- ment in all parts of the masses—plausible evidence that they existed in the magma at the time of eruption. They are also very similar to those of the dike rocks. Olivine is less abundant, and its crystals are now almost wholly decomposed, yielding a dark-green serpentine. Augite has usually a very well-developed crystal form, a fact which seems to indicate that the phenocrysts of the Ralston dike became irregular through resorption. Plagioclase tablets like those of the dike rocks are present here, but less abundantly. Biotite appears in the vesicles of the lower sheet in hexagonal leaves attached by the edges and inclosed by zeolitic deposits. The groundmass minerals of these sheets are plagioclase, orthoclase, augite, magnetite, and apatite. Plagioclase occurs in stout little crystals or staves. In the coarser-grained parts orthoclase is found in an irregular oriented zone about many plagioclase crystals, as well as in irregular grains. With a decrease in size of the plagioclase staves the oriented growth diminishes, and in the denser parts one can only see that a colorless, apparently feldspathic substance occupies the main space between staves of plagioclase. Irregular grains of augite and a magnetite dust are sprinkled MON XXVII 20 306 GEOLOGY OF THE DENVER BASIN. abundantly through the feldspathic mass. This magnetite dust and the clear apatite prisms are plainly iater in formation than the larger grains and prisms which are often included in the augite phenocrysts. The rock of the highest point on North Table Mountain represents the inner part of the upper, thicker sheet. Its constitution is not very different from that of the Ralston dike rock. By a kind of oxidizing process to which it has been subjected all iron-bearing minerals— magnetite, biotite, the alteration product of olivine, the small grains of augite, and even the outer zone of the augite phenocrysts—have been changed into an opaque, dark, reddish-brown substance (limonite), and this eives the rock a reddish tinge in mass. At Castle Rock was found a dark nodule in the basalt, about 14 inches in diameter, which is an irregular aggregate of imperfect augite prisms similar to the phenocrysts of the rock but of a decided green color, with very little magnetite and plagioclase. This mass seems to be a segre- gation of augite in the magma before eruption, and bears a significant resemblance to the common olivine nodules in basalt, or to those of amphibole in diorite or andesite. Chemical composition The typical massive rock of the lower sheet was sub- jected to analysis by Dr. W. F. Hillebrand, with the following result: Analysis of basalt from Table Mountain, Colorado. Sk 0 eS ae Se eo Sa eee Re OR eieeio a Ree Gomer Sor E ems ose a ees Secon 2e5= 52. 59 TOs, 2222225 sees cs eat ds ses een eee Sein cee ee See eee eee ee 84 5.0 SO ee 8 See Se ee Sa eae memes ieee ne Sein Soo 17.91 WesOgnais daesceec cance - esse eGe ee cce acts scien 7a ae See ee eee see 3.81 WOO weet ac aes See elses esse ent ore SED Se nee ate ee tee ees 5.18 Mn’. ..o2s22 cedeens cco sses esi eclees ee gee on ae eee eee ee eee Trace. OF: 0 ee ee eo See Pe ee es eas SrmNI= Saas AOSe oO. MAAC 7. 24 MeO sls ksh ee ee I oe Se 4.11 1 0 Ee ee See eee iss I Oe ee ee nine See See ree s te 3. 83 1. (Oe Stee Ses ESE Er ARR Oar Paes SEEM EMO DaeERecnas gests 2.94 PiOg.a< obec dosed ss ese vin Sacks 2 se rose ceeeas op eka see Sees tee eee 14 OD nog cee cecese nde: Sesok. cscs See ease sea e er =e ee eee eee 05 gO. oss n secs cate 5 opicidn cc Sei dclee eel eeine sole cioe oe nena DE ebeascs see clmin Rene: SRO cae eine eee eee = ils} CaO) son om soto Rect seRe eS oe on tne otis Bae: Sie eee nee ere eet irs 8. 24 MeO ¢occ0en Pa SE en ck Sens Fae ee ee IO Re ores eet eC 5. 73 Ke Owsset nite nscee rota. Sco eee io eee cei he =e Ab Sc ee eee 3.90 NiO) oe spe Ss chee ae ftns Srey roa eae emcees Per we ne eer Sf ene 2.99 Pg Q§. ccc scinte Siew mame we csc aPe SS ee oo eee eEre Ca AEE Sick ee CD eee .81 OQiecsace oid wheeecsent ask Geteotns yieceteeie- eee p ace oe eeeeee eae .15 PO lees ee Sct) Se ec eine eile = ate ete moe SVS erate Se ees ee ee 91 100. 27 A comparison of the analyses of the three basalts shows them to be very similar, even to the abnormal ratio of the alkalis, and this fact is used elsewhere as strong evidence that all belong to the same period of eruption. AUGITE-MICA-SYENITE. This rock occurs in small, irregular masses in the Archean near the head of the north fork of Turkey Creek, Jefferson County. Description—The rock is dark-brown in color, quite compact on the outer edges of the masses, while in the inner parts it is sometimes rather IGNEOUS FORMATIONS. 309 coarse-grained. In the latter case brown biotite leaves and green augite prisms are quite’ easily distinguished by the naked eye. The feldspar is of a gray or bluish tint, and except where the cleavage faces show distinetly it is scarcely recognizable. Biotite and augite seem about equal in quantity, and their predominance gives character to the mass, which has macroscop- ically a decided resemblance to some minettes. In the compacter parts of the mass the augite is much restrained in its development, while biotite is still prominent in minute flakes. Microscopical examination of both coarse-grained and_ fine-grained types shows them to possess the mineralogical composition shown by many European ‘“minettes” or “augite-syenites.” The more important constituents are orthoclase, augite, and biotite, with rhombic pyroxene, hornblende, plagioclase, quartz, apatite, and magnetite. The structure of the coarser type is granular, while the more compact form shows something of an approach to porphyritie structure through the prominence of the pyroxene crystals. Orthoclase, the most abundant mineral, is developed in irregular grains, which in the coarser rock are quite uniformly of a pale, smoky-brown tint, owing to a cloud of extremely minute inclusions, the character of which could not be determined with a power of 1,200 diameters. They do not seem to be fluid, however. These dust-like particles are often so evenly distributed that the tone of the entire individual is uniform, while in other vases they seem to be arranged in more or less parallel rows or streams, as though on certain planes of growth; and certain portions, usually the outer borders of each grain, are clear. Minute fluid inclusions and particles of other minerals are common. Carlsbad twinning is frequent. In the coarser rocks no plagioclase is visible, but in the finer-grained there are some small stave-like crystals, of many laminze, which are probably oligoclase. Augite, the next constituent of importance, occurs in numerous crystals and in small, round grains. The erystals in the coarser type are 1 to 3 mm. in length, and reach 1 mm. in thickness. In color they are pale-green and perfectly nonpleochroic. They usually swarm with inclusions of magnetite, biotite, and especially apatite. There is also some glass. In form and in abundance of mineral inclusions these augites resemble very closely the 310 GEOLOGY OF THE DENVER BASIN. crystals commonly found in the minettes of Germany. In the finer-grained rock augite is less frequently developed in good crystals, and there especially is the appearance of a rhombic pyroxene noticeable. The latter is faintly but distinctly pleochroic, exhibits parallel extinction, possesses pimacoidal as well as prismatic cleavage and cross-fissures from which a fibrous alter- ation product extends, and is more nearly free from inclusions than the accompanying augite. The rhombic mineral is not abundant in any case. It is presumably less rich in iron than the hypersthene of the andesites, and is hence to be considered as bronzite. Green hornblende occurs in small, irregular grains in the compact rock and also intergrown with augite in the outer zone of a few crystals. Occasionally it surrounds the augite. The biotite is of dark-brown color and occurs in small, irregular leaves often attached to pyroxene grains and containing many inclusions of magnetite. Apatite is very abundant. Its prisms included in augite and the stouter free crystals are decidedly pinkish in color, with distinct pleochroism. The smaller prisms are often colorless. Magnetite is present in small amount in the usual grains. Quartz appears only in the finer-grained rock, in very small, angular particles as the last product of consolidation, and in very insignificant quantity. Chemical composition —The compact rock, in quite fresh condition, was analyzed by Mr. L. G. Eakins, with the following result: Analysis of augite-mica-syenite from the north fork of Turkey Creek, Jefferson County, Colo. STO SR pap ose mononaso soeecoodeedsc case qdonono poems shoctbooascedheces 56. 90 TiO} 2 ie ceca) Sec delsncs aces Sadacs see swcinnes ose ease ae epee eaee eee .19 IN ICO BSS 9 Sian eerriyinns A eicIS werale Tinta ae Synch alate w/a \nleige ols Mei e sisisiaeiahtellarcs nies iebsia ae 18.50 1 oy 0 Pa eee Ia Ae SSS Ona eeo Emenee OBrien Scone arore st Sactearite: ke 1.) 0 IRS OnE IS Sent obme 8. 83 INEM OS aes Hse aoa eSB are caocidn Sood asco ser dme Yanera Pade OGRA Oo Ao Gada 3.06 10 0 ee Pay eS Ree eee SORES a is ee SO mito Cor cence. Dena OcrCoo an aS ao 1.59 There are small quantities of MgO, H,O, ete. The tuff, as a whole, contains SiO,, 58.45; Na,O, 1.69; KO, 0.66. TUFRFACEOUS BEDS IN GENERAL, Those beds in which rounded particles appear in very small quantity are naturally of almost identical habit with the type described, and there is nothing new in them excepting the particles of andesitic rocks, usually reddish or brownish in color, and often quite decomposed. The cementing substance seems, in many cases, to be identical with that already referred to. Many beds deserving the name of tuff are coarser-grained than the above type, and consist of fragments and worn particles in about equal parts. In many of them the eruptive character of the material is obscured by decomposition, and especially by the hydrated iron oxide to which the beds owe their distinctive yellowish-brown color. The cementing material of various tuffaceous beds has been found to be zeolitic, as might be expected, and from frequent occurrence of heuland- ite in many places it seems probable that this species is more commonly IGNEOUS FORMATIONS. S15 the cement than any other substance. By tests made upon brown semituff from various places on the plains, it appears that a very large amount is sometimes soluble in hydrochloric acid, with gelatinization indicating the zeolitic character of the main substance dissolved. In one case 54.59 per cent was dissolved. ANDESITIC PEBBLES OF CONGLOMERATES. The eruptive materials of the Denver beds represented by tuffs and by pebbles of conglomerates form one of the most characteristic features of that formation. It has been shown that a large part of the series is almost exclusively made up of these materials, and they clearly represent a very large amount of rock destroyed. The source of these pebbles is not known, and they give the only known clues as to the events of one of the important epochs in the geological history of this region. As pebbles of unknown origin, they are not worthy of detailed description, but it is desirable to record their character with some definite- ness, because this information makes evident the extent of the volcanic eruptions of the time between the Arapahoe and Denver epochs, a period which seems to have been characterized by similar outbursts along the whole Rocky Mountain region from New Mexico to Montana. Mineralogical composition —Hxamination of hundreds of pebbles in the Denver beds, from the bottom to the top of the section, has not definitely revealed the presence of any other eruptive rock type than andesite, but the mem- bers of this large family are nearly all represented. At the siliceous end of the series is a light-colored rock, with sparing phenocrysts of augite and plagioclase in a microlitic groundmass of thoroughly trachytie habit. It is possible that this rock might properly be called a trachyte, but only a single pebble was observed. Mica-andesite, generally rich in tridymite or quartz, is abundant in the lower beds of Table Mountain. These and some of the more siliceous hornblende-mica rocks may be called dacites. Hornblende and mica are present in varying amounts in many pebbles, and augite is often associated with them in subordinate degree. Classify- ing andesites by the relative abundance of the constituents biotite, horn- blende, and pyroxene, there are all manner of varieties to be found in the 316 GEOLOGY OF THE DENVER BASIN. pebbles, the transition stages being those familiar to students of general petrography. With the increase in augite comes greater basicity in general, and the extreme is a rock closely allied to the basalts, though none distinctly referable to the latter type was found. Apparently magnesia was not strong in these lavas, for while augitic varieties are so numerous, hypersthene was found in but one typical pyroxene-andesite from the summit of Green Mountain. This also explains why basalts are not found. The basalt-flows of Table Mountain are apparently contemporaneous with the Denver beds, and it is therefore probable that some basalt exists in the conglomerates. No especially interesting mineralogical types were observed. Structure——A]] of the rocks are porphyritic, and possess the further characteristics of lavas rather than of intrusive masses. Plagioclase and augite phenocrysts commonly contain abundant glass inclusions, and a zonal optical structure is frequent in the former. Hornblende and _ biotite are ordinarily much resorbed, and augite sometimes shows the same action. The groundmass naturally varies very much, but is usually to be described as microlitic, with fluidal structure in most cases. In the hornblende- and mica-andesites the groundmass is often cryptocrystalline or partly isotropic, with yellow or brownish globulites and patches of tridymite. In the more basic rocks minute feldspar microlites are often found in a dark base, usually obscured by hydration of the iron oxide. None of the structures observed are such as are found, according to the writer’s experience, in intrusive rocks. Texture—The great majority of the pebbles are naturally dense, but in the coarser conglomerates there are vesicular pebbles in great variety, representing nearly every observed massive rock type. In some of the pores are zeolites. One rock, an augite-andesite, was found in many places in angular fragments, from apparently a single horizon, and of all textures, from the compact to the extremely vesicular. The porous modifications of this rock contain heulandite and the new zeolitic species ptilolite? The angularity of the fragments and the various textures seem to indicate a local source for this rock. 'On ptilolite, a new mineral, by Whitman Cross and L. G. Eakins: Am. Jour. Sei., 3d series, Vol. XXXII, 1886, pp. 117-121. 5 CeEAGE ebay Vale: ECONOMIC GEOLOGY. By GrorGr H. ELDRIDGE. SECTION I.—COAL. DEVELOPMENT OF THE BASIN. The date of the discovery of coal in Colorado can not be authenticated, but it was probably prior to 1860, actual mining operations having been traced back to this year. The location’ is said to have been on Coal (upper Sand) Creek, in T. 4 8., R. 65 W., at a prominent lignite outcrop still visible in the north bank of the creek between 1 and 2 miles above the mouth of Murphy Creek. Among the earliest producers were the Marshall mines, in operation in 1863, assuming importance in 1865; the Murphy mine, on Ralston Creek; and the mines at Golden, discovered in 1861-62. The product of these mines for the years 1864-1869, inclusive—the earliest record attainable——is reported as follows: Coal product of Marshall, Murphy, and Golden mines, Colorado, 1864-1869, Short tons. SUG E SUI RR Baas et a Gy Re SR ee 500 LISS aie SN SR RN 8 2 6 Py rr ca) SEN, MRSS RE eR el ee 1, 200 EERE epee epee eS oF ct 2 ae eS a 6, 400 TRG TEEN tow tos eee ey SE ee ans athe ic! Myers liseli 3 foo oe 17, 000 A SGR Sees ks - Sey eee eee ae 8 RE oe i te be 10, 500 SCO See era: i a) 5 ee he sae sc See Se eS ee 8, 000 ‘ Geol. Survey of the Territories, F. V. Hayden, Report for 1873, p. 121. 317 318 GEOLOGY OF THE DENVER BASIN. The increase for the years 1867 and 1868 and the subsequent decrease is attributable to temporary activity in metallic mining and milling during those years. In the summer of 1870, upon the completion of the Denver Pacific Railroad from Cheyenne to Denver, the Kansas Pacific, and the Colorado Central from Denver to Golden, the demand upon the mines of the foot- hills in Jefferson and Boulder counties largely and permanently increased, their annual production for 1870 and 1871 being respectively 13,500 and 15,860 tons. In 1872 the completion of the Boulder Valley Railroad from Brighton to Boulder brought into market the product of the mines—already well developed—at Erie and Canfield. In consequence of this accession the coal production for the Denver fields for the seven years 1872-1878 rose to the following figures: Coal product of the Denver fields, 1872-1878. Year comprare | Short tons. | Short tons. | Short tons. IT PRE Soko oosoe | 14, 200 54, 340 | 68, 540 | AB @scsoco.osesc0s55¢ 14, 000 43, 790 | 57, 790 ies Ky (Le eeees 2 ena ee 15, 000 44, 280 59, 280 STS 5 ec ncaa te © 23, 700 59, 860 83, 560 LSIGY cise saaoow eg 28, 750 68, 600 97, 350 ASTI S. .ocidesese Shee ee Seee ne oe eee 130, 000? | IST iso cd Se ceases ele eee eee eee eee 87, 825 The rapidly increasing demands of railways, furnaces, mines, mills, towns, and cities in 1879—the year of great mining excitement at Leadville and Silver Cliff—caused another notable increase in the coal product of Colorado, the output of the Denver Basin at this time amounting to 182,630 tons. This constituted the bulk of the inerease for the entire State, the mines of southern Colorado being the only others that showed advance, and these rather in the line of development than owing to conditions of trade. From 1879, however, the latter mines largely increased their capacity and output, while the mines of the Denver Basin temporarily fell COAL. 319 off in product, owing to diminished demand for their coal. For the next few years the product of the Denver Basin stood as follows: Coal product of Denver fields, 1880-1884, Short tons. UC OS Beers SO SAE Sei R Jae RIE esis Ole Co n UaSecte ites ac eOS G5 aaa aes 123, 518 Lobe eS AOC ere Ce Dac eI ONC Cea Seto cannot Sia Maa oane San a 156, 126 Dee ken mak ame aren care eon eee rie se eis Ue aeien ee oben suis ou seats 300, 000 HBR S ees soe nee eee ae ete ee een aE ie eee eee on OA ANF In 1884 strikes among the coal miners, extending from August 4 to the end of the year, greatly lessened the coal production of the entire State. Although the yield of the Denver Basin showed an increase over that of the preceding year, it fell far below what it should have been, considering the growth of Denver and of railway and manufacturing interests generally at that time. In 1885 the production of the Denver Basin amounted to 244,346 short tons, and in 1886 it reached 260,145 short tons. Coal product since 1887 of counties included within the Denver Basin. A y County. | 1887. | 1888. | 1889. | 1890. | 1891. | 1892. | 1893, | 1894. Short tons. Short tons. Short tons. | Short tons. Short tons.| Short tons. Short tons. Short tons. Boulder ..---.-- | 297,338 | 315, 155 | 323,096 | 425, 704 | 498, 494 | 545,563 | 663, 220 419, 734 VOLUN eeesca | 39,281 | 28,054 | 28,628 | 46,417) 22,554] 2,205 | 35,355 | 42,818 Arapahoe ...-.. | 16,000 1, 700 823 700 | 1,273 654 633 559 Jefferson ..----. | 12,000 9,000 | 10,790 | 10,984 | 17,910] 21,219; 1,895 | 34,108 Douglas........ 3, 500 400 260 | 700) e--aneee= 200 200 eee eee The increase in the yield of the mines of the Denver Basin in 1887 is largely traceable to the opening of new mines in the Erie-Canfield district, and to the prosecution of mining in all districts, particularly the Marshall, on a more extensive scale and with greater energy than had hitherto been shown. The increased output was required to meet the domestic and manufacturing demands created by the particularly rapid growth of Denver at this time. In the other fields of Colorado there was likewise a large increase in the output of this year, the product being consumed both within 320 GEOLOGY OF THE DENVER BASIN. the State itself and beyond, along the lines of the great railroads to the east of the Rocky Mountains as far as the Missouri River. In the fall of 1888 still another area of coal was opened up, in the valley of Coal Creek, 24 miles below Louisville and 17 miles north- northwest of Denver. The output of this district for that year was 11,726 tons, of which the Simpson, which began operations in September, yielded 11,126 tons. The town of Lafayette is now located here. In this year, also, the Standard mine, in the Erie district, was put in condition to produce heavily, and late in the season the old Welch mine at Louisville, after a long period of comparative idleness, was reopened by a new slope about 800 yards to the southeast of the old shaft. Since then several new mines have been opened in both the Lafayette and Louisville districts. List of mines in the Denver Basin, including worked and abandoned. Name. Location. Condition. ! | = = = = -| = E ee Sand! Creelkc.o-.2 e.- 2-6 1D 4 SARE GDaWie ase ae Abandoned. Said to have been first discovery of coal in Colorado, Mwio ishattseecoesee see T. 3 S., R. 65 W., sec. 28. | Abandoned prior to 1872. | Tonsland. | NCLantON)e- aeeee = Scranton: = 22-25 -7325-05--=- Produced prior to 1889. Since aban- doned. Platteville - ~-<. 5-2-2. T. 3.N., R. 66 W., sees. 17-20... Worked occasionally for loeal de- mands. | Stoner, Hopkins and | Near Platteville ...---...---- | Abandoned. others. Bxcelsion, soc23 ese Near “Evans 22S. se eee ese eee For local demand. Hatonio25 saesc2see a" Near: Hatone. ase ss ce eer Do. BLOWN Soe North of Golden 2 to 3 miles. North of Golden 2 miles North of Golden 1 mile | One-half mile north of Clear Creek. Golden; immediately north of Clear Creek. T.48., R. 70 W., sec. T.48., R. 70 W., sec. 23. | Abandoned. T.458., R. 70 Wi, sec. 24. --.--- for local use. Opened in 1860. Abandoned. Abandoned. Abandoned sometime prior to 1870. Was working in 1879; since abandoned. Abandoned (a prospect). Abandoned in 1887. | | Mentioned by Fossett as yielding in 1879. Long closed. Rocky Moun- tain 1 and 2 reopened in 1890. Abandoned. Opened in 1884. Worked. Opened in 1890. Worked. Abandoned. Abandoned about August 20, 1889. Cut into water of old workings. Discovered in 1861-62. Abandoned. Worked in 1873. Hayden refers to it in 1873 Report. Abandoned. ‘‘Has been worked; coal not very good.” 1873 Report. Opened prior to 1868. 1872. Hayden in Abandoned in Abandoned many years ago. 1 Some of the mines noticed as worked may be at present temporarily closed. COAL. ooo List of mines in the Denver Basin, including worked and abandoned—Continued. Name. | Location. Condition.! | | -| eraiiiieeee eee eer HAS ee eres ee vied. | Abandoned. Opened in 1882 or 1883 | by slope, and some coal taken out. | Wilkoneeeseee = oo [UR S., R. 69 W., sec. 31 --. ...- Abandoned prior to 1873. Mount Carbon-..-.--.. Mount Carbon! e--s----) -2e- | Worked. Local trade. An old mine. Wenrich (Gilpin). .--- i cLeensey LV OOMW is RECO se =tarts Abandoned long since. May have | Deen worked in 1873. GMOS 625 noesegosdede Northof Deer Creek between | Abandoned long since. Worked in | 1873. Opened in 1866 (?). Abandoned. | 1 and 2 miles. | Deer Creek (mouth | West of Platte ..-........... | of). INCL Soo 5 pace eCEOSS | A prospect on east side of | Coal shipped to Denver in the seven- | Platte, opposite Archer. ties. Hayden's Report gives 1866 as date of opening. Abandoned. Weightman ...-....-. | Wallow, Creélos. -2-c-2- eect Abandoned several years ago. Douglas (Cannon, West of Sedalia 4 miles --... Worked. Development commenced Pearl Ash, Lehigh). in 1883. Production variable. —— —— —_—_—__— — ——— ee 1 Some of the mines noticed as worked may be at present temporari-y closed. GEOLOGICAL OCCURRENCE OF THE COAL. With the exception of the coal on the eastern border of the basin, in the vicinity of Scranton and Coal Creek, which is upper Laramie, the workable beds of the several fields about Denver are confined to the series of sandstones and shales that constitute the lower 200 feet of the formation. They probably attain their maximum depth of about 2,300 feet on the line of Section V, shallowing north and south of this. They are sharply upturned along their western border; along the northwestern, dipping gently to the southeast; while beneath the prairies they are probably thrown into a great number of minor folds and faults. The transverse sections of the field show the general configuration of the coal horizon to be that of an unsymmetrical shallow basin, the western and northern limits clearly defined, the southern and eastern indefinite. The coal beds of the Denver Basin occur under three different conditions: (a@) That in which the beds belong to the lower division of the Laramie and are steeply inclined along the foothills of the Colorado range; (b) that in which the beds are also of the lower Laramie but occupy an approximately horizontal position beneath the prairie; and (c) that in which 324 GEOLOGY OF THE DENVER BASIN. the beds are horizontal but belong to the upper Laramie, occurring well up in the series of clays, a single locality presenting this condition, the Seranton area, 20 miles northeast of Denver. The workable coal beds, whether of the upper or lower division of the Laramie, occur as deposits of irregular outline and distribution from one to several miles in area and of a thickness of from 3 to 14 feet. Their presence or absence, even in localities where the measures have been subjected to combined structural and erosive influences, as along the northwestern edge of the formation, depends upon original conditions of deposition, and in consequence of the uncertainty of these, actual coal- bearing areas at depths beneath the prairies obviously can not be deter- mined except by exploration with drills. That such areas exist can not be doubted, for their presence along the exposed margin of the coal meas- ures is repeatedly proved, and the sequence of events and the conditions of deposition must have varied but slightly for any part of the basin. Along the western rim of the basin the coal measures suffer no visible interruption by reason of dynamic agencies, though the continuity of the beds is frequently broken by nondeposition. In the northern portion of the basin, where the measures occur at comparatively shallow depths and for a zone of 5 or 6 miles within the periphery of the Laramie are within easy reach of the surface, combined disturbance and erosion have caused ereat irregularity of occurrence. The coal of the lower Laramie belongs eminently to a period of sandstone deposition; that of the upper Laramie to a period of clays with but slight association of sandstone. The heavy sandstones that do occur just above the Scranton seam are not of Cretaceous but of Tertiary age, and lie unconformably upon the coal and other beds of the Laramie. Coal occurs at eight or ten horizons within the 200 feet of strata constituting the lower Laramie coal measures, but of these five is the maximum number showing a workable thickness in any one locality in the basin, the usual number being two or three. On account of the covered outcrop and the variability of the measures above the basal sandstones, both in number of seams and character of sedimentation, the identity of a seam can not ordinarily be determined beyond the comparatively limited > COAL. one area in which it is specially developed. The basal sandstones, A and B, and the bed of oysters, about 12 feet above their summit, are the chief horizons of reference in the series. Notwithstanding the indefinite position of the coal seams in the lower Laramie, their occurrence for the basin in general points to certain horizons at which a workable thickness seems to be more frequently developed than at others. These horizons are: Between sandstones A and B, immediately above sandstone B, just below sandstone C, and at one or two points between the two last mentioned. Those most commonly developed lie in the shaly beds between the B and C sandstones. The productive area of coal depends upon the thickness of the seam and the purity of the coal. Either of these varies independently of the other. In thickness the beds occur from a knife-edge to 14 feet without parting. In quality the coal may pass from the state of the highest purity for the field to one in which there is a large proportion of earthy matter, to a carbonaceous shale, or even to a shale wholly argillaceous. A combination of the two methods of variation is of frequent occurrence. Partings of a more or less carbonaceous clay or quartzose sand-rock, varying from a line to a foot or two in thickness, are frequently present and may extend through an entire district: Nearly all carry plant remains— leaves, bark, ete. COAL AREAS. INTRODUCTION. The coal fields of the Denver Basin are the Foothill, Marshall, Davidson, Louisville, Lafayette, Baker, Erie (embracing the Canfield and Mitchell), White Rock, and Scranton. Beyond the basin as mapped are the McKissic and Platteville banks, several miles to the north, and the Douglas or Lehigh bank, 6 miles southeast of Platte Canyon. With the exception of the Scranton the areas are all geologically related, but have become distinct from one another through faulting, folding, and erosion. The mines of the Foothill region lie along the upturned portions of the measures. The Marshall district adjoins the northern end of the foothill area, its mines confined to the region of gentle dip immediately east of the latter. The Louisville, Lafayette, and Baker 326 GEOLOGY OF THE DENVER BASIN. areas are faulted from one another, and the Canfield and Mitchell areas are separated by the eroded crest of an anticlinal fold or by a local cross-fault. Faults and folds separate the Davidson from the Louisville and Marshall districts, and the White Rock is severed from the others by erosion and a succession of faults. The Scranton is geologically distinct from all others by its higher horizon. Structurally the coal areas may be grouped under the Foothill region, the Davidson syneline, the Coal Creek syneline, the area east of the Coal Creek syncline, the White Rock field, and the Seranton field. FOOTHILL AREA. EXTENT. The foothill area includes the highly inclined strata of coal measures alone the base of the Colorado Range, and extends from the Marshall area in the north, with which it is continuous, to the vicinity of Wildeat Mountain in the south, 10 miles beyond the limit of the basin as mapped. South of this latter point no coal is opened or known to exist in workable thickness until the vicinity of Colorado Springs is reached, a distance of nearly 50 miles; north of the basin openings occur here and there as far as Greeley, 50 miles from Denver; in both directions, however, the beds are nearly horizontal and belong to the prairie class. East and west the foothill area is limited, if the outerop alone is considered, by the confines of the Laramie—indeed, by the confines of its basal series of sandstones, coals, and shales, which is alone productive in this portion of the basin. The width of the area is therefore only about 250 feet. The beds doubtless extend beneath the prairie upon reaching the flexure in which the measures are involved, but they then lie too deep—at least 1,200 to 1,500 feet—tor present profitable mining. COAL. 327 PRODUCTIVE LOCALITIES. Following is a list of mines and prospects opened at various times on the vertical beds of the foothills: Mines in the foothill area of the Denver Basin. Name. Locality. The Coal Creek minesa............--- ona aia to ot = Upper Coal Creek. MEME y AEN MING d= 2 50-2 - vo aniston enna ses T.258., R.70 W., sec. 28. | MurphyIine:a ~~ jg. - saersaceecers + seeeeeece see T.258.,R.70 W., sec. 33. | Chumeh prospectia: --=\Sa-c0 ences sess ee soo wecic sa == Near Ralston Station. | RABIN Were coss Ssasen eased eae soe bee Do. Goloradormind:a' ss. 2 Sons 2.25 Foca oe eee RUG SPOTS TING Con < owls nam see seen eee oe ee | ROGKVAMOUNtAING NO: Lene 22 oe oe aes emer eee North of Golden 2 to 3 miles. MOOK MOUNTAIN NOs seve 6, cbse omer aeee es eee LiSWO) it iri ie ee REET ae ta toe meen ey SY Mineral sand Cos minesids.. 56 ---sen oe eee ec eee ee North of Golden 2 miles, GOO ODI SAT scans wana a)eoela ala cesiccneseececmeeemee North of Golden 1 mile. INGi WLtG ABN 2 531.3 deatsectede ee teeter eee Bee Half mile north of Clear Creek. DOReLaM Wai aos nae Sce tte eke steince choles seeders Golden, north of Clear Creek. A\WMLOTIUCY JANG NY eS A eee ere be sae Golden, south of Clear Creek. clo] 102): (0 90) Sa a eRe aE IS een meee T.4S., R. 70 W., sec. 3. | Welchiand Loveland minea.+.:.---.--e-cc22ce5 cases. Do. pevuteeimebtimngis. 5°. 20. = ce ey mee T.4S.,R.70 W., sec. 14. Rowe; 0c; Or Rooney Gs -<-.1.!22 sone as eeve ae eset T.458., R.70 W., sec. 23. MEINE s, S ao eo we ida teks seen eee Le Oees T.48., R.70 W., sec. 24. Fen AIA Sette os sino oe as a ial aim oa eee ee se aels sees WOON es aa 2 ceo Sts ces aa eee eee nae ee oe T.458., R.69 W., sec. 31. Monntre anon 2... => cones san Soee eee -eeee eee ea: Mount Carbon. MMos tli ( Crt tI Ben matss er aeeem cmon ona Ree ee T.5.5S., R.69 W., sec. 9. DOD GS hae Serck ewe aialsa2 cin we cle amiaipee emo aid tare ae ata aos North of Deer Creek. SEM CTEGKK 0 .- 6-5 sleiiee Sos eee eine ae ee mem aa e's *Mouth of Deer Creek. J TUNIS Seep enone ap Bence sec. Se aeaae eno ore East of Platte, opposite Archer. MUGHAL Oe eae Sep mees Peach cot sack eeracu Deeeone Willow Creek. eg (Cannon, Pearl Ash, Lehigh) .....-....--.-- West of Sedalia 4 miles. a Now abandoned, January 1, 1890. b Abandoned since the study of this field was begun. Of the above openings the Douglas, Mount Carbon, New White Ash, Golden Star, and Rocky Mountain Nos. 1 and 2 are still (October, 1890) in operation; the Old White Ash and Ralston mines have been closed since the inauguration of this work; the Murphy and Loveland were originally important producers, but have long been idle; and the remainder, idle and 328 GEOLOGY OF THE DENVER BASIN. never important, varied from pits with an annual output of a few hundred tons to mere prospects. The history of these mines and the extent to which they have been worked are, in a measure, indicative of the conditions of the beds in their respective regions, but abandonment of mines once well established does not signify that they have been worked out, but rather that they can no longer produce economically in competition with the mines of the prairies, where the coal is horizontal and within a compara- tively short distance of the surface. For much of the distance along the foothills the coal measures are barren, and the extent of the productive portion can be determined only by most careful prospecting; and it must always be borne in mind that, through the irregularity of outline of the original deposit, a bed continuous in outcrop for a considerable distance may extend to but slight depth, or vice versa. Thus far, continuity in depth has been found to exceed the limits of economic working. STRIKES AND DIPS. The strike of the coal measures in the foothill region is, with the exception of the portion included within the area of the unconformity about Golden, parallel with the range (N. 15° to 18° W.) in the southern portion of the field, and north or a little west of north in the northern. Between Bear and Coal creeks, however, the coal measures lie in a broad, westward-sweeping curve, without crumple or fracture of importance. The dip of the beds varies from 10° to 15° on either side of vertical. The depth at which the dip of the overturned beds becomes vertical and then normal (easterly) varies from point to point, but in the lower levels of the White Ash mine at Golden between 700 and 850 feet would consti- tute approximately their vertical portion. Below this the eastward dip may generally be expected, and at 1,200 or 1,400 feet the shallow dip charac- teristic of the prairie occurs. The curve from overturn to normal is of such a long radius that probably but slight fracturing of the coal seams has resulted. The seams of workable thickness along the foothills are apparently wholly confined to the series of strata between sandstones B and C, that between A and B being nowhere with certainty recognized as important. COAL. 329 SEDALIA DISTRICT, The Douglas mine—This is opened on a small creek about 6 miles south- east of Platte Canyon and 4 or 5 west of Sedalia. Two coal seams are mined, the linear extent of which is undefined, but, from their thickness and strong development in the pit, there is probably at least one-half or three-fourths of a mile of workable coal. In depth it doubtless extends far beyond economic mining limits. The measures here have a strike N. 24° W., with an easterly dip of about 73°. The horizon of the two coal seams is probably between sandstones B and C. The upper, easternmost seam is 8 or 9 feet thick where opened; the other, 4 feet; a heavy sandstone, varying from 10 to 20 feet in thickness, separating them. (Fig. A, Pl. XVIII.) The seams were clear of partings and bone within the limits of work at the time of examination. The coal is jet-black, lustrous, hard, square-jointed or nearly so, and of good resist- ance to atmospheric influences. Its structure and friability have been but little influenced by the bending to which the strata were subjected in the general uplift of the range. A slight amount of pyrite and resin is scattered through it. SEDALIA DISTRICT TO MOUNT CARBON. Various coal openings have been made between the Douglas mine and Mount Carbon, the locality next north now worked. These are, in succession, the Weightman, Archer, Deer Creek, Jones, and Wenrich or Gilpin, all of which are reported, as having occasionally yielded a slight product, chiefly between the years 1870 and 1880. All are now caved and beyond examination. The horizon, thickness, or character of the coals can not be given. The strike of the measures included within this distance is N. 15° to 18° W., or with the trend of the range, and their dip is from 75° east to overturned. MOUNT CARBON DISTRICT. This designation applies to a short stretch of coal measures within a half mile north and south of Bear Creek, including the prominent flat- topped hill known as Mount Carbon. The valley of Bear Creek is here 300 GEOLOGY OF THE DENVER BASIN. a third of a mile wide and is bordered by bluffs 50 feet high. The confluence of Turkey and Bear creeks is in the middle of the bottom lands northwest of Mount Carbon, upon Laramie beds. Mount Carbon is a remnant of the early terraced bench lands now separated by the valley of Turkey Creek and adjacent lowlands from the main body of uplands nearer the foothills. Its altitude above the creek is between 300 and 350 feet. The shape is elliptical, the longer axis east and west. South of Mount Carbon the conglomerates at the base of the Arapahoe form a line of prominent combs from which the distance to the coal measures, about 900 feet, may be closely laid off—the basal sandstones of the Laramie forming no outcrop along here. North of Bear Creek the prairie is cut by a number of shallow coulées, one of which enters the creek valley along the base of the Laramie, the coal having been opened both in this coulée and upon the prairie to the east. In the northern face of Mount Carbon is an excellent exposure, in section, of the great fold along the front of the Colorado Range. ‘The vertical portion of the beds appears in the coal-measure sandstones and the heavy sandstones and conglomerates at the base of the Arapahoe, and occupies the western half of the hill; the eastern half of the hill exposes the remainder of the Arapahoe and the basal members of the Denver formation, the latter forming a conspicuous knoll of gently dipping strata a little northeast of the main elevation. The fold itself shows in section in the eastern third of the hill, and is traceable by occasional visible outcrops of the more resisting beds and by a searching examination in the slightly covered strata by means of the pick. This structure continues both north and south of Mount Carbon, but the actual flexure is no longer visible owing to the planing down of the prairies and the comparatively insignificant height of the bluffs bordering the streams. Both the prairies to the north of Bear Creek and the cap of Mount Carbon consist of a light but uniformly distributed Quaternary gravel, of rather coarse material, through which the harder beds of the vertical portions of the underlying formations occasionally outerop. The strike of the beds in the Mount Carbon region is a gently varying curve from N. 20° W.2 miles south of Bear Creek to N. 60° W. immedi- ately north of the creek, the region lying just within the southern confines COAL. ool of the great unconformity extending northward past Golden to Coal Creek. The dip of the highly inclined strata is between 75° and 90° eastward, that of the beds of gentle inclination, near the eastern base of Mount Carbon about 15°, shallowing still further to the east of this. A cross-section of the more important part of the coal measures, taken on the northern face of Mount Carbon, is given in Fig. B, Pl. XVIII. Four seams are moderately developed, only the eastern two being of workable thickness. All lie in the horizon between sandstones B and C, the sketch involving about 43 feet of strata. This series of coal beds, from indications afforded by old workings and prospects along their outerop, may extend with short interruptions for a mile and a half both north and south, but that any single bed holds its width for this distance is extremely doubtful. The depth to which they maintain their surface width is entirely a matter of conjecture; it may be less than in the regions of more strongly developed beds, as, for instance, the Douglas; or, on the other hand, their width may increase with depth, the present outcrops being perhaps near the periphery of the original deposits. The coal of the Mount Carbon mine, so far as exposed at the time of examination, was quite free from partings; a single narrow but persistent streak of bony material occurred in one seam about 6 inches from the top, in the other at the top. The coal is, like other foothill coals, bright, jet-black, comparatively little fractured in the upheaval of the range, square-jointed, and is said to contaim very little sulphur, in the form of pyrite, and but a slight amount of resin, which is uniformly distributed. The present opening upon the coal is by drift, about 125 feet below the top of the hill. _ Leaves, bark, and other vegetable débris occur in abundance in this locality. Considerable coal was formerly shipped from a shaft at the foot of Mount Carbon and from the Wilson shaft immediately north of Bear Creek, both mines having been equipped with primitive hoisting plants. MOUNT CARBON TO GOLDEN. Northward from the Mount Carbon district occur, in the order named, Mann, Roe, Wheeler, Welch and Loveland, and Johnson. The Roe mine lies at the western base of Green Mountain, about the abandoned mines aoe GEOLOGY OF THE DENVER BASIN. 3 miles north of Bear Creek. Following is a summarized account of this opening, taken from the reports of Hayden’ and Marvine.” The entrance to the mine is by a slope of 45°, 170 feet in length, through 141 feet of sandstone. The strata are here nearly vertical in position, though at neigh- boring points often with an easterly dip of between 70° and 80°. There are three seams of coal, 4 feet each, with 34 feet of clay intervening. Below the coal there is a bed of clay 5 feet thick, and above 34 feet of arenaceous clay. The coal is com- pact, makes an excellent fuel, and leaves a white ash. The mine had shipped 250 tons up to 1868 and was abandoned in 1872 for want of good communication. The Wheeler mine,’ 1 mile north of the Roe, was formerly worked to a depth of 40 feet, furnishing coal of inferior quality; thickness of seam, 7 feet. The Roe and Wheeler openings are still visible, but the surface in their vicinity affords no ready section of the coal measures. A section of the measures at the Welch and Loveland mine,* by Captain Berthoud, C. E., from notes on an open cut, follows: Feet Coal No: od) easternmost)bed\t 22 | 32 2 ase eee ee eee 5 Sand'stonevand clayo e222. eee see ses eee Steere 16 Groen Fas eee: tee On Reed coaa ee - or ansdooonsot onsec 1 ENO 2 occ sbe chs ascasss shogne osc esescessoseseccecec 64 P@=Claiy clases cpa Se he ces papal eee race gio tey = este rate eee tate acs eae rove a ete erate 3 Goal, No: So No: 1, westernmost bedi cer == s-ae eee ae a ie eae 3 Shale and ‘sandstone. 2: s 2.22552 See ode hae ees Bee ee 3 The general strike of the measures in the vicinity of the above mine is N. 30° to 82° W., with a dip westward (overturned) of 70° to 80°. At the Johnson mine, one-fourth of a mile north of the Welch and Loveland, the coal is said by Mr. Marvine to be from 7 to 9 feet thick. It ‘U.S. Geol. Survey of the Territories, Report on Colorado and New Mexico, 1869, p. 35. 2U.S8. Geol. Survey of the Territories, Report an Colorado, 1873, p. 127. 3U.S. Geol. and Geog. Survey of the Territories, Report on Colorado, 1873, p. 127. 4U.S. Geol. and Geog. Survey of the Territories, F. V. Hayden, 1873, p. 127. > Mined. ‘Not mined. COAL. 333 strikes about N. 31° W., and dips to the westward (overturned) about 80°. It was opened by a shaft 90 feet deep. The mine has long been abandoned. Between the Mount Carbon and Golden coal districts the Laramie measures lie in the southern half of the great inward-sweeping arch resulting from the Golden unconformities. Although the strains to which the strata, both here and along the northern half of the arch, were sub- jected in the general uplift of the range and the concomitant adjustment of the beds were doubtless considerable, but slight fracturing resulted. The coal horizon may easily be traced from surface particles and from the frequent comb-like outcrops of either the Laramie sandstones themselves or the conglomerates and sandstones at the base of the Arapahoe a few hundred feet to the east. From the succession of strata and from the thick- ness and character of the sandstones occurring west of the coal, it is prob- able that the seams formerly opened along here belong to the horizon between sandstones B and C. Leaves and other plant remains occur in abundance. GOLDEN DISTRICT, General description his district includes the Old White Ash, Loveland, New White Ash, Golden Star, Excelsior, and Rocky Mountain 1 and 2 mines. It occupies the valley of Clear Creek and its tributaries directly west of the Table Mountains. The strike, for a long distance south of Clear Creek, is N. 30° to 35° W.; north of the creek it rapidly curves to N. 7° 30’ E., increasing to N. 17° E. in the vicinity of the Ralston mine. The measures have a dip at the surface of between 65° and 80° westward, reaching vertical at a depth of between 700 and 900 feet. Below this depth their dip is eastward, lessening in amount until probably between 1,200 and 1,500 feet the beds have assumed the gentle dip underlying the eastern portion of the valley and the Table Mountains. The distance on the trend of the strata for which the measures are productive is difficult of approximation, but, with short interruptions, is probably 1 mile south of Clear Creek and 3 miles north of it, reaching in the latter direction to the disturbed area just north of Van Bibber Creek. In depth, the beds undoubtedly hold their width far beyond the limits of economic mining. 334 GEOLOGY OF THE DENVER BASIN. The general stratigraphy of the region about Golden is given in the 5 Oo } J f=) fo) preceding chapters. The details of the coal measures are shown in figs. C to F, inclusive, of Pl. XVIII, and in the following section. Section, in part, of coal measures at Golden. Thick- No. | Nature of strata. ness in feet. IRS Ge CEO aomeomSooe cose orneaend eae | 6 FN One sese a sks as Sees saa Semele 2 | SulvGlay Sao. ccekecisiacspecteldsare cmceer sess ameter 8 AN GOR) Se 2 ocr: fac saraente es oeeie aiae ld aotastomieve ates | 2 | By) (One = se eeagieesed abe9 Cans ame cOnsacanese 2 6 | 4Sandstonetec Sces.2cat eee scene eee sees | 3 NOL mone 20 sc = Says Ee eee eee acre ee | 4 8) Blackislatess- -..22 5-2 sss eee eee eee 3 CIM) bn eae ES Se mes ee AR ase | | LOMWWGSandstone..2- t= 42 oteoenite ee eee aeeae 7 11 | (ON hy Seco cdecaaseanenosbocosp anes HeeacoGsce | 3 12 | Sandstone? ass e< ncee Satie seine cet ame 12 13° | Coal <4-Ga eee. a eee ee eer eee oy | 14 \Sandstone==ens . esac eee aa ee eee 4 | Gy MOI pe Se eaasoecee sacs Sao Jeno coca cee enorac 4 | Motalstoum~ain coalibed \.sseee eee oes 70 Much variability is shown in these sections, not only by the measures themselves but by the coal beds as well; it is therefore impossible to identify the seams in the several mines without exploration of the entire width of measures at each shaft. From the character and succession of the exposed strata, however, it is probable that the coals are those between sandstones B and C. The fossil flora of the Golden coal measures is abundant, though of little variety. Oysters also were discovered by Captain Berthoud, of Golden, in the vicinity of the coal beds, though no note was taken of the precise horizon. The Old White Ash mine—'This is in the south bluff of Clear Creek at the upper edge of the town of Golden. It was among the first worked in VII, On the Geology of Colorado, 1873, p. 126. COAL. 335 Colorado and was abandoned about August 20, 1889, by reason of accidental flooding, supposedly from the Loveland mine to the north. A longitudinal cross-section of the Old White Ash mine at the date of abandonment is given in C, Pl. XVIII. The collar of the shaft is 135 feet west of the main worked seam, and at the 600-foot level is still 39 feet to the west. Below this the strata become vertical, with indications of an easterly turn, so that the shaft will nowhere cut the main seam. The seam is opened from the shaft by cross-cuts, levels being driven from these. A second seam, 3 feet thick, lies from 10 to 20 feet west of that worked. This has been found to vary considerably in thickness, but has never fallen below workable limits. The floor and roof of the large seam is either a black or gray slate or clay, or a gray sandstone, the former more commonly occurring. The entire seam is generally free from partings, and the coal from below the 200-foot level is extremely hard and bright. The Loveland mine —'This is just north of Clear Creek, on the same seam as the Old White Ash. The width of the seam is reported between 9 and 10 feet, with occasional partings. The coal ranks with that of the Old White Ash. The smaller seam, west of that worked in the Old White Ash mine, again appears here with a thickness of 4 feet. The New White Ash mine—(F ig. O, Pl. XX.) This mine, opened in 1890, is located about a half mile north of Clear Creek. There are two workable seams, 32 feet apart, having in the portion opened a N. 7° W. strike, and a dip of 75° to 80° west (overturned). The strike gradually changes to north as distance in this direction is gained, while the dip slowly approaches the vertical in depth. The west seam is 3 feet 6 inches to 4 feet wide; the east 4 feet, but indicating an increase below present levels. Both seams are worked from the same shaft, which, in October, 1890, had reached depth of 317 feet, with cross-cuts to the beds at 17 3, 245, and 317 feet. Boe Recently a a fate a) accident occurred, causing re death of ton) men she were working at the end of the lowest level of the White Ash mine in the direction of the Loveland mine. The latter mine has for years been full of water. Oneof the upper levels of the White Ash, which if protracted would have made connection with the lowest level of the Loveland, has for a long time been on fire, and it is supposed that this at last burned through into the Hoveiadd! letting in the water, which ran down the White Ash shaft and drowned the men working in the levels below. The bodies of the men have not been recovered, and the mine has been closed down since the accident.” (Ann. Report Golden School of Mines for 1889, p. 60.) 336 GEOLOGY OF THE DENVER BASIN. The collar of the shaft is 25 feet west of the outcrop of the western seam, but at the 245-foot level the shaft is 4 feet east of the seam, and at the 317-foot level, 12 feet. Sandstone forms the west wall of the western seam. Slate with sandstone forms the eastern. The eastern seam was but little exploited at the time the mine was examined in the fall of 1890. The Golden Star mine——his is located about a mile north of Clear Creek, a half mile distant from the New White Ash mine. Two seams are worked, the shaft, which is sunk to the depth of 160 feet, cutting the western at 30 feet and at the 130-foot level standing about 15 feet east of it, the distance between the seams being about 33 feet. Fig. F, Pl. XVIII, gives the relation of the beds to each other. The strike here averages N. 8° E., with some very slight local deviations, the dip being 80° west (overturned). The walls are of slate or a kind of fire-clay, and sandstone. Both beds are said to occur in the nearer abandoned mine to the north of the Golden Star, but to be entirely wanting opposite the end of the Dakota hogback, a few hundred yards still farther north. The coal of the Golden Star mine is clear, hard, and bright; it is rectangular jointed and shows but a slight amount of pyrite or resin. The Rocky Mountain mines Nos. tand2—(H jo. P, Pl. XX.) These shafts are 950 feet apart, No. 1 being the southernmost and located on the northern slopes of the divide between Clear and Ralston creeks, a few hundred feet east of the Dakota hogback. Two coal seams are present, their strike being about N. 7° E, their dip 80° to 85° west in No. 1 and 85° to 87° east in No. 2, both observations being taken on the 175-foot level. The east vein in No. 1 shows 3 feet of clear coal in one body, the west 4 feet, the distance between the two being 29 feet. In the No. 2 mine, to the north, the east vein is between 3 and 4 feet wide, the west 6 to 7 feet, both of clear coal. In this mine but 7 feet of rock separates the two seams, while in the old Pittsburg shaft—700 feet to the north of No. 2—the two seams are reported to come together, forming one of 11 to 12 feet without parting. The coal of these mines is very clean and bright, and but slightly fractured. COAL. 337 RALSTON CREEK DISTRICT. Ralston Springs mine—'This is located in the valley of Van Bibber Creek, and until_ recently was a large producer. The shaft lies immediately north of a sharp Laramie knoll in the middle of the valley, and is sunk to the west of the overturned strata. The strike of the measures along here is very constantly N. 17° E., the dip being 65° and 80° westward, becoming vertical in depth. The seam is reported 8 feet thick and without partings. South of the shaft, in the vicinity of the Laramie knoll, the coal is said to be so interstratified with sand and clay as to be worthless. Northward it has been extensively developed and, it is stated, promises to hold width and quality to the end, at the east and west fault marking the southern line of the disturbed area about the Ralston dike. The disturbed area east of Ralston dike——J"he coal measures between the two east and west faults bounding this area have been horizontally displaced con- siderably over half a mile. Owing to this and the nonrecognition of the faults, early explorers were led to believe that the coal was Fox Hills, since characteristic Mactra had apparently been found above it. In reality, how- ever, the Mactra occur in the dislocated beds north of the southern fault, beneath the coal, but east of an extended line of coal outcrop from the Ralston Springs mine; hence the error. The fault remained undiscovered until the present explorations. The coal measures of the disturbed area dip in a general way eastward, but where they have been exploited, at the southern end, are greatly fractured. North of the area, beyond the influence of disturbance, the region of the Ralston Creek mines is reached, these having been among the greatest producers in early times. The Ralston Creek mines——These embrace two shafts, one in the northern bluff of the valley, the other on the prairie a short distance to the south. They attained a depth of 112 feet below creek level, but have been closed for many years. There are said to be two workable coal seams: the western, near the base of the measures, 9 feet thick; the other, 25 feet to the east, from 14 to 18 feet thick." The measures have a N. 4° W. strike, continuing between this and north for several miles northward, and to the south nearly to the fault along the northern edge of the disturbed area 1U. S. Geol. and Geog. Survey of the Territories, F. V. Hayden, Report on Colorado, 1873, p. 125. MON XXVII 22, 338 GEOLOGY OF THE DENVER BASIN. east of the Ralston dike. The dip is vertical, or nearly so, for the entire distance given. The old Murphy shaft on the north side of the creek, said to be sunk on the eastern of the two workable seams, is about 125 feet east of the base of the Laramie, which would place the seams in the same general horizon as the others along the foothills; that is, in the zone between sandstones B and C. From surface relations between points along a gradually shallowing dip, from the thickness of the formation, and from the recognized horizons, it is estimated that the coal measures would be found under the region of slightly dipping’ strata, between a quarter and half a mile to the east of the old mines, at a depth of 1,200 feet beneath the creek level, the fold by which the beds are upturned being sharp and pronounced. The extent of the productive measures of this locality can not be greater than half a mile to the south of Ralston Creek, while to the north it may extend with some interruptions as far as Leyden Guleh, a distance of about 2 miles. In depth the beds probably extend far below economically workable limits. Fic. 12.—Section showing coal measures of lower Laramie at Coal Creek. 1. Sandstone, white, heavy-bedded. 2. A suecession of sandstones and shales with occasional ironstones. 3. Coal; brown shale just above and below. 4. Sandstones, white, heavy-bedded. 5. Coal. This seam has been slightly worked for local trade. 7, Sandstone B. The coal of the Ralston Creek mines, of which the principal one was the Murphy, is reported hard and lustrous. This mine has produced as high as 50 or 60 tons per day, and a possible total of 25,000 tons. RALSTON CREEK TO MARSHALL. The Leyden mine—This is located in Leyden Gulch. It was originally a small producer, but has long been abandoned. From reports it is believed that the beds have changed in their character, forming, perhaps, the northern limit of ‘the Ralston area. From Leyden Gulch to Coal Creek the basal sandstones of the Laramie outcrop in low combs at a number of places. At Coal Creek the measures afford the above section (fig. 12). COAL. 339 The two coal seams now showing are: The upper, 24 feet thick; the lower, 34 feet. This statement differs from that of Marvine, in the Report on Colorado for 1873, page 124, in which six beds are mentioned; the one worked, of a thickness of 7 feet. The measures at the creek strike a little west of north and dip east about 50°, although north and south of this they become vertical. It is probable that from Coal Creek northward the strata bordering the great fold on the east were considerably elevated, producing a gradual diminution in this direction in the depth at which the coal is to be found. The coal measures outcrop along the southern bluffs of South Boulder Creek, there being here only a slight southeasterly dip. THE BOULDER COAL FIELD. The entire coal field in the northwestern part of the Denver Basin is in ‘the main confined to the two great synclines of the region; the one known as the Davidson lying diagonally across the Davidson and Lake mesas, its axis about a mile east of the town of Marshall; the other lying along the valley of Coal Creek, designated the Coal Creek syncline, and including the Louisville, Erie, and other subdivisions. THE DAVIDSON SYNCLINE. General features — The Davidson syncline in its greatest extent embraces the Davidson-Lake mesa and its slopes from South Boulder Creek on the north to upper Coal Creek on the south. The areas mined at the time of examination were two, the Marshall and Davidson, but there are several long-abandoned openings and prospects scattered over other por- tions of the region. The western rim of the syncline as it appears in the Laramie formation coincides with the western line of outcrop of the basal sandstones; the eastern rim crosses the Davidson-Lake mesa in a northeast-and-southwest direction, very near the summit of the divide between Coal Creek and the drainage to the South Boulder through the town of Marshall. The southern end of the syncline is in the ridge separating Coal Creek from the Marshall Lake basin; the northern end lies midway between the fortieth parallel and Boulder Creek. The syncline is longitudinally crumpled by gentle yet clearly defined anticlinal rolls at 340 GEOLOGY OF THE DENVER BASIN. two points, the region of the Burnt Knoll and the southern rim of the Marshall Lake basin; the axial extent of these rolls is between a half mile and a mile, the transverse extent about a half mile. The axis of the general syncline suffers the greatest depression between the Burnt Knoll and the southern side of the Lake basin, a maximum depth of 500 to 700 feet being attained by the coal measures.’ The continuity of the strata involved in the syneline is frequently interrupted by faulting. The outcrops of coal are broken and irregular, and the region is cut into small areas of various opposing portions of the Laramie and Fox Hills. Instead, therefore, of comparative geological sim- plicity there is great complexity, beds terminating abruptly beneath the surface or, by reason of gentle dip, having been brought into such positions that erosion has easily removed them in part or in whole. The western rim——Beginning at the vertical beds at the northern end of Ss the foothill district, a little southwest of the town of Marshall, the exterior ) MILE SS See Fic. 13.—Section of coal benches, Davidson mesa and northward, east of Marshall. A, B, C, Sandstones. F, F, Faults. X, Pleistocene cap of mesa. =< = coal. periphery of the coal passes directly down the steep slopes of South Boulder Creek to the low line of bluffs immediately above the bottom lands. In this distance the strata change from their highly inclined position to one of gentle dip to the southeast. The outcrop of the coal beds follows the crest of the lower bluffs to a short distance east of Marshall, after which, gradually receding to a half mile from the valley bottom, it follows the low rise constituting the second terrace, the lower terrace along here being formed of the basal sandstone A of the Laramie, with occasionally a portion of the B stratum, the second terrace including the B sandstone and the overlying coal measures. The trend of the coal outcrop in the second 1In the test boring at the outlet of the Lake basin in the SW.4 sec. 14, T. 1 S., R. 70 W., Mr. R. C. Hills, of the Colorado Fuel Company, reports adepth reached of 645 feet, with the base of the Laramie still below. In drilling, one 3-foot coal seam was passed, and artesian water was obtained. COAL. 341 terrace is northeast. The second terrace disappears a half mile west of Burnt Knoll, the coal outcrop shortly passing into the northern or main branch of the Marshall fault system, which a little beyond occupies the bed of Dry Creek. Southeast of the fracture, for the greater portion of its length, beds of a much lower horizon, even the upper portion of the Fox Hills, occupy the surface of the country. Only near the western end of the fault has the coal on the south escaped erosion. The coal of the interfault block, between the middle and southern branches of the fault system, rapidly rises from west to east, occupying the northern blutts of the Davidson mesa as far as the steep-dipping portion of the syncline. Here its outcrop turns northward across the low country, paralleling the main Marshall fault at the distance of about a quarter of a mile. On this portion of the trend is opened the Allen-Bond mine. The south branch of the Marshall fault system is also of especial importance in its influence upon the coal outcrop. In effect this fault has determined a line of bluffs to the south of a topographic depression at Marshall, lowering the beds on the north and leaving the outcrop of the coal in those to the south. The western end of the break probably lies in the main Marshall fault, at the point of the bluffs where the vertical dip becomes shallow, the outcrop of the coal here splitting, one portion passing into the lower blutis along the periphery of the field, the other along the higher bluffs to the south of the Marshall depression. In the latter blutts the coal outcrop extends eastward in a nearly horizontal line, midway their height, for nearly a mile, where, at the steeply dipping portion of the Davidson syncline on the eastern edge of the district, it turns down, crosses the gulch, and on the opposite side again rises, only to immediately sink in the depression which occurs on the northern side of the fault. The eastern end of this fault is in the fold just mentioned. The possible bluff fault, a few hundred feet south of the foregoing, if present, is repetitive, the downthrow being on the north, the displacement slight. The eross-fault at the west end of the Marshall mesa is also one of slight throw, appearing in cross-section in the north and south bluffs of the mesa. 342 GEOLOGY OF THE DENVER BASIN. In the vicinity of Burnt Knoll, which is the center of the slight anticlinal rise in the axis of the Davidson syncline, the trend of the coal outcrop becomes indefinite, but probably passes to the south of the knoll and thence onward to the region of the Davidson field just east, the old Allen opening lying a short distance south-southeast of the knoll. At the northern or northwestern point of the Davidson mesa is a small flat in which lies the coal of the Dunn and Davidson mines. The eastern rim—I mmediately east of the above mines the outcrop of the coal measures, as an effect of erosion combined with a rise of the beds against the southern arm of the Davidson fault, turns back upon itself, taking on its southward trend along the eastern rim of the syncline. The basal sandstones of the Laramie boldly outcrop a short distance southwest of the mines, in the northwestern face of the mesa, their general strike being N. 15° E., their dip 25° W. The coal outerops a little to their west. coAl << ABOUT 1% MILES. SD ee Fig. 14.—Section across coal benches of the Davidson mesa, Davidson district. A, B, Basal sand- stonesof Laramie. ——= coal. A short distance southwest of the sandstone outcrops, on the eastern side of the syneline, borings, of depths unlearned, are said to have revealed the presence of 3, 4, and 5 foot seams of coal. From the above region the coal horizon enters the mesa on the eastern side of the broad indentation in its northern face, reappearing to the south, about the head of Marshall Gulch. The basal sandstones of the Laramie form strong horizontal outcrops in the railroad cuts and ditches just east of the divide in this vicinity, on the west slope assuming a western dip of 5° to 12°, which increases somewhat as the center of the syncline is approached. Together with the coal measures at the western end of the railroad cut, they now pass beneath the surface into the valley below. In the ditches between the divide and Marshall Lake, in the eastern half of the distance, the basal sandstones of the Laramie repeatedly outcrop in practically horizontal position about 40 feet below the top of the mesa; COAL. 343 the western half of the distance is occupied by the coal measures, which dip to the NNW. 5° to 10°, occasionally returning to a horizontal position. Only traces of coal seams appear along here, and it is possible that the series entire is not present; on the other hand, the coal beds themselves may have decreased in thickness. Beneath Marshall Lake the strata are horizontal, but im a deeply cut ditch to the north they assume a south- easterly dip of 5° to 10°, with a general strike N. 30° E. Southeast of the lake, in the gentle rise of the mesa, the coal measures dip northwest. Along here the Ostrea bed is traceable for several hundred feet; it also appears flat in the bottom of the lake basin, the two outcrops, except for surface débris, being doubtless continuous. On the eastern side of the basin the northwesterly dip continues to the southern end of the depression, but, as will be shown beyond, the rim here becomes somewhat irregular in trend, and the trough itself is considerably modified. From the horizontal outcrop of the Ostrea bed in the center of the Lake basin the measures rapidly steepen southward, entering the bluff southwest of the lake with a strike of S. 30° W. and a dip of 24° NW. The outcrop of the coal measures in the western walls of the basin con- stitutes the eastern rim of the Marshall coal area in this part of the field, the sharper and deeper portions of the syncline now lying entirely to the west, beneath the prairie uplands. The synclinal depression, except for the modification of its southeastern lip, is here, doubtless, at its narrowest point, since, at the head of a gulch entering the basin from the northwest, the beds have a southeasterly dip of 25° to 35°, their strike being N. 60° E. and the distance across the trough not over 1$ miles. The coal measures entering the steep slopes at the southwest corner of the basin pass through the mesa, to again outcrop in the bluffs on the north side of a short tributary of Coal Creek. Coal is here reported 35 feet thick, but the drift is now closed, an outcrop of little promise alone showing. The strike is approximately N. 60° E., witha well-defined dip of about 6° NW. A short distance down the gulch, on the opposite side, the basal sandstones are seen, while above the higher coal-measure sandstones outcrop, succeeded in turn by the upper Laramie shales and ironstones. | Asmall seams, 0a A 3 c041. E* rene |~ Sandy Stat Need COAL | Shate ant Sandstone | | Gurlletd Mine.) 86 Midas, 2 art G" Slate | = Jas | | weet ae | ia - | mai 6 (wo ~ Clay State | q Sandstone x Shale #4 2erCOAL 2nd 8° COAL @COAL | | Shale and Sandstone an or | EIGOAL iWork) S al Probably't Sandstone no Coal reported | 3 \ ] | — COAU (worked) on al 2 | wd t four! 4 and Clay [ /| pas at | ee Toil Mine sk 109't fi} \ | fe COAL | nel Shale andl Sandstone on die T6COAL roo‘ De Sima Van | | ‘i 5 } : Sandstone ani Shate me 7TFCOAL rf ~-Hard Sandstone / zh \\ | | are @°COAL ; | u 02 0200 } — sons t Ma ‘ Se " \\ | | | hia Mt o (opener another He Se Meyeuainous COAL | | | ve | MM con anote Jf / ~ Clay | " | \ | | | | |~— Shale and Sandstone 205] | ZL ms | / | | | kee } | | | 9s AMM, COM Simp Vein | ~ Fire Clay / Satdetons | | |~ Sandstone BY... | | / / 2 | \ | | za COAL reported Sandstone B i. ff y, een i | | | | a = Sundstone B M0 ue = / S | na 26°COAL | I | | a {| = Y COAL g: 94, i {6 Lock | | Slate Clay and | | 15 o \ / mitiioee a | | | i iCOAL cs one Reeonatavonie | COAL | | | | Sandstone uy L be Sih Sandsténe 8 x COA y | sandstone and 3 6 L “ : | | 5° COAL | 165i V3'PireClay ~ Sandy Clay | | | sandy Shale $ | | 08 SPure Clay and \ a 159 1 v0 Sandstone rere \ | | | | | ' | fee! =] = COAL W6 Clay and. 7 (#COAL Sump Vein | | en =f Sandstone | | i | | tne 1 © COAL | we) \ | | | ! | tay WO COAL,; Varies | | | |- Sandstone A Sandstone B. ~ | Sandstone and State | | | \ Decreases to to 1200' \ | | | S Of Shatt | | | | | 5, Mmmm 26° COA. ret) | \ 3 widens tot \ | | | | | | cy 8°) COAL VS COAL Sump Vein \ | | | - hp ty 7 r ‘ Slate and Sandstone F 226 4 se s= #6°COAL, Fire Clay ~~ Sandstone 100 ? or 2 COAL : | \ 74° COAL | | Lea : ee vinta , Sand Shale | | andstone B cy =o 5 2° ire Clay Va Male | Sandstoneand Shaul 3 | b innew anid Slade 43 | | sundstones nd / \ with [ronson 3s 0" #° Fire Clay. COAL z 2 | 2s — COAL Sump Ver = 4 COAL = 26n S 2 SBI iil Ieal | | Sandstone 8 a zs 2m | mar vy COAL yo Sandstone Sandstone B | | | = t | | | | | rH Uisyy ie Z ae ; aps OF’ : e < ees SUCC ISSION OF STRATA IN THE SHAFTS LAFAYETTE AND ERIE DISTRICTS. COAL. 361 area it is generally found too narrow to open, prospects nowhere showing a thickness greater than 3 feet 6 inches. The character of the seam is given in several of the sections of Pls. XIX and XX. It often carries a parting of variable width, which may, however, entirely disappear over extended areas—particularly the case in the Lafayette mines. Seams Nos. 3 and 4— "These will be considered together on account of their union over certain parts of the Coal Creek syncline. The general character of each is shown in sections on Pls. XIX and XX. No 3 is that usually developed to a workable thickness, No. 4 being only occasionally so, except in the central portions of the syncline—in the Lafayette and a part of the Louisville fields—where it is united with No. 3 to the complete exclusion of the parting, the two forming a single bed of an average thickness of 14 feet. The approach and divergence of the seams to each other are distinctly shown in the Lafayette field. Along the axis of the syncline in the Simpson mine, and in the northern entries of the Cannon and the southern of the Excelsior, the two beds are united in a solid mass of coal; north and south of this, however, a parting of interbedded sand and clay appears, rapidly increasing, particularly to the north. In this direction the first appearance of the parting is at a point 223 feet north of the Excelsior shaft. From here the line of separation has an east-southeast trend—east of the shaft—the more southern the entry the farther the point from the main entry at which the appearance of the parting is encountered. In the Gladstone shaft, 500 feet north-northeast of the Excelsior, the parting has increased to 10 feet and shows evidence of a continued gain until its normal thickness of 25 feet is attained. In this direction both seams, 3 and 4, hold to a clean condition, only occasional thin and nonper- sistent partings showing in either. South of the axis the only opportunity of observation at the time of examination was in the south entries of the Cannon mine, where the parting had increased to 4 feet, and where, moreover, the lower seam, 3, had become so split with slate partings as to render it unprofitable for mining. This thickness of the main parting and the slate in the lower seam may increase or diminish to the south. In the Louisville district there is neither the regularity in occurrence nor the freedom from partings in seams 3 and 4 and their resultant 362 GEOLOGY OF THE DENVER BASIN. composite seam that there is in the Lafayette region. The individual seams, 3 and 4, are still generally identifiable by position, but vary in thickness from 1 foot to 3 feet and 3 feet to 6 feet, respectively, and this with rapidity and comparative frequency. The parting separating the two seams varies from 1 inch to its maximum width, 20 feet. A study of the sections of the Louisville district, Pl. XVII, clearly illustrates these features, for, from the nearest approach to the union of the seams—in the Acme, Caledonia, and Welch mines—and from a comparatively well-developed thickness, every gradation is observed to the opposite extremes, which occur in the vicinity of the Ajax and Marshall Consolidated shafts, where thickness of seams is reduced to a minimum and the mass of separating rock has increased to the maximum. It is a condition worthy of note in the mines of the Louisville and Lafayette districts that where the individual seams are no longer closely united in a mammoth seam they are inclined to become divided by partings, and that they are the more divided as separa- tion of the primary seams increases. The manner in which the component seams, 3 and 4, and their asso- ciated sandy and clayey beds sometimes vary is well illustrated in the Marshall Consolidated mine. (Figs. F, G, H, Pl. XX.) In passing from the shaft westward along the main entry, the lower seam is found to have widened from 2 feet 6 inches to 3 feet 6 inches in a distance of 1,200 feet; the upper seam, after a decrease from 3 feet to 1 foot, 400 feet west of the shaft, has again widened to 2 feet 6 inches at a point 1,200 feet west; while the intervening series of sandstone and slate, which is 20 feet thick at the shaft and without coal, has at a distance of 400 feet west decreased to a total width of about 3 feet 6 inches, with one or two impure coal streaks, and at 1,200 feet is only 1 inch thick. Regarding the identity of the component seams in the Marshall Con- solidated and Ajax mines, no question exists as to that of the lower; the correlation of the upper, however, is in places attended with some uncer- tainty on account of occasional partings that exist in the mdividual seams which can not be traced through from mine to mine, and so lead to a confusion between the beds. The Ajax presents the most serious difficulty in this respect. COAL. 363 An important consideration regarding the Nos. 3 and 4 seams in the Louisville-Lafayette field is the greater range in width of the lower, No. 3, and its tendency to maintain a superior thickness; the No. 4 seam rarely reaches a width greated than 4 feet, while the No. 3 frequently attains one of 6 feet, or even 7 feet 6 inches. THE AVAILABLE COAL AREA OF THE COAL CREEK SYNCLINE. It is impossible from surface and present mining data to estimate the productive area of the Coal Creek syncline, but the region is the most promising of the several coal areas of the Denver Basin, both in the thickness and in the continuity of its seams. A considerable tract at the northern end of the syncline in the vicinity of Erie, Canfield, and Mitchell has already proved of value; the region of Lafayette is now yielding from a large area, with evidence from drill holes of much greater extent; the vicinity of Louisville is rapidly developing still another valuable area of production. Southwest of Louisville about 2 miles, the syncline has again been prospected and, according to best accounts, proved to carry at least one seam of workable thickness. At several points in the outlying area north- west of the Harper and Sand Gulch faults there exists a seam which ranges from 34 feet to 5 feet in thickness at depths varying from 48 to 154 feet. On the summit of the high ridge north of Coal Creek, near the old stage well on the Denver and Louisville wagon road, 6 feet 4 inches of coal is said to exist at a depth of 60 feet; and again, just west of the point where the former Denver, Utah and Pacific Railroad crossed Coal Creek, 3 feet 10 inches exists at a depth of 93 feet. These points of exploitation are well distributed and indicate for the syncline productive measures of broad extent and exceptional continuity. This, together with the number of seams which are of recognized workable thickness in one area or another renders the entire region one of great importance. The mining areas within the Coal Creek syncline conform to the several subbasins described. There are five in all, the Superior, Louisville, Lafayette, Mitchell, and Canfield-Erie, though perhaps the area immediately about Erie may more properly belong to the Mitchell basin. Following is a list of mines in the several districts, including both worked and abandoned. The names are those in existence at the time of 364 GEOLOGY OF THE DENVER BASIN. the examination by the Survey, 1890. Changes have occurred with the advent of other owners, and so far as possible all later names are added in brackets. Superior district—Superior mine (abandoned). Louisville district—-The Welch (abandoned), Marshall Consolidated, Marshall Slope, Caledonia, Acme, Ajax [Leader], Hecla Nos. 1 and 2. Lafayette district—The Cannon [Colorado Smokeless], Simpson Nos. 1 and 2, Spencer, Excelsior, Gladstone, Ottis. Mitchell district——The Mitchell, New Mitchell, Cleveland, McGregor, Stewart, Garfield Nos. 1 and 2, Longs Peak, Boulder Valley (abandoned), Northwestern. Canfield-Erie district—The Jackson [Chase], Star, Progress [Stand- ard], Northrup (closed), New Boulder Valley, Superior (abandoned), Deitz (abandoned), Briggs (abandoned). THE SUPERIOR DISTRICT. This includes the portion of the field lying on the northwestern rim of the Coal Creek syncline, separated from the main body of coal to the east by the Harper and Sand Gulch faults. The confines of the coal and the general geology were given in defining the syncline’s rim. A single opening of importance, the Superior shaft, is located on the northern slopes of the Davidson mesa, a seam 3 feet 6 inches thick being reported as cut at a depth of 90 feet. The coal is said to be clear and hard. The mine has long been abandoned. THE LOUISVILLE DISTRICT. The structural limits of the Louisville district are the Louisville fault on the southeast and the rim of the syneline and the Harper fault on the northwest; southwest, also, it extends to the rim, while to the northeast it merges with the Lafayette area. That the entire district is underlain with coal of workable thickness is doubtful, but the distribution of the mines and prospects indicates that a large portion of it may become productive. The region varies widely, however, in the value of its lands for coal- mining purposes, for beneath a part certain of the beds are thick and consolidated, while beneath the remainder they are thin, separated, and COAL, 365 considerably split by partings. These areas are not delineated, but from observation in the mines open at the time of examination it was inferred that the northwestern half of the syncline would prove the more important. Here the Nos. 3 and 4 seams locally approach within an inch of each other, and over a considerable area are sufficiently close to be mined together. Where separated the No. 3 was more commonly mined, although No. 4 might also be workable both at the same point and at others. The sections A and B, Pl. XIX, and F to L, inclusive, Pl. XX, afford a general view of the character and relations of the several beds. The hollow of the Louisville syncline is apparently broad and gently rolling, with local faults of light throw, encountered in mining. In the northwestern half there is a gentle rise of the strata toward the rim, increasing from 1° or 2° to 10° or 15°. ‘The rise to the southeast is much sharper, 15° to 20°, increasing locally to 25°, 30°, or even 40°. The heavier coal seams are quite free from dirt and the smaller layers of sand or clay, the only impurities of this nature being the partings of slate or bone from 1 to 6 inches or a foot thick, prevalent throughout the district. The coal is in the main bright, black, and square-jointed. Its texture is homogeneous, laminated, or fibrous, the last character being due to maintenance of the original wood structure in certain of the layers. It mines either in dicy blocks or with a conchoidal fracture in irregular lumps. The hardness varies, being perhaps the greater in the lower portions of the seams. With the exception of a single slope on the southeast outcrop of the coal, the seams are all worked from shafts. THE LAFAYETTE DISTRICT. This district includes the half of the Coal Creek syncline east of the Louisville fault. The precise limits are as yet undefined. To the northeast it is possibly continuous with the Mitchell area. On the northwest the line of demarcation is probably a little below the crest on the northwestern slope of the ridge separating Sand Guleh from Coal Creek, until the Louisville fault is reached, when this becomes its limit. To the southwest and south it probably continues beyond the rim of the syncline, although interruption may occur in this direction by the possible prolongation of the Baker fault from east of Coal Creek. On the east the Coal Creek 366 GEOLOGY OF THE DENVER BASIN, fault cuts the field from the productive area of the Baker mine and the coal lying to the east-northeast. The axis of the Lafayette trough lies a little south of the Simpson- Spencer shafts; the drainage being from the mines north and south to the Simpson-Spencer. The hollow of the trough is somewhat corrugated with minor flexures of gentle rise, but its general shape is broad and rather flat, the dip steepening only at the limits of the area now opened, on the northwest, very gradually. The mines show several faults with various inclinations for their planes, some of which are at a very acute angle with the planes of stratification. The throws rarely exceed 8 or 10 feet. The coal of the Lafayette field is of the general character of the Erie coals. It is jointed and works in large blocks. A woody structure of the coal is frequently encountered, as though carbonization had taken place in solid blocks, fiber for fiber. Silicified trunks of trees, knots, and branches are here and there found, but so far as observed they lie in no definite direction. The mines are all opened by shafts. Sections of the two seams at several points in the district are shown in Figs. A to E, inclysive, Pl. XX. MITCHELL DISTRICT. The productive portion of the Mitchell area is defined on the west by the crest of the ridge separating Coal Creek and Sand Gulch, on the east by the sharp rise of the beds to the surface along the Coal Creek fault, and on the north by the Erie fault, against which the coal measures probably abut with little bending. 'T’o the south the field is apparently continuous with the Lafayette. The limits thus defined form the exterior periphery of the field. It is hardly possible that the coal seams should hold workable within the entire area, since their tendency to a rapid variation in thickness is well established. Their general relations are shown in the sections of shafts, and their structure in the plate of individual beds. The coal of the area is bright, hard, and dicey. Both laminated and homogeneous varieties occur. ‘Mother of coal,” pyrite, resin, and white sulphate of lime are present in variable but never high percentages. The mines are all worked from shafts. Figs. E to J, Pl. XIX, show the general character of the seams. COAL. 367 THE CANFIELD-ERIE DISTRICT. The western and northern limits of the Canfield-Erie district are the outcrop of the measures beneath the Quaternary of the Boulder Valley, contracted somewhat by the natural deterioration of the coal under decrease of cover, and perhaps also, at the northern end, by a thinning of the coal below workable thickness. The eastern limit of the field lies a few hundred feet west of the lower Coal Creek fault; the southern, a little north of the Erie fracture, the beds on this side of it being turned up against the plane. Near the western edge of the area, where this fault has apparently disappeared, the Canfield-Erie and Mitchell fields become continuous. The disturbed condition of the strata in proximity to the faults has been noted in the preceding pages. In this portion of the district the measures lie in the gentlest possible curve consistent with a synclinal structure, approximating the horizontal over much of the area. Local flexures occur, and also faults of small throw. Of the latter the Jackson- Star is the most important thus far encountered. The throw is but 30 feet, however, not enough to have exposed any of the beds to the influence of erosion and so brought about their removal. The stratigraphy of the Canfield-Erie region is somewhat less clearly defined than that of the Mitchell, chiefly because of the less satisfactory data afforded by the mines. The succession of strata in the Progress shaft (Pl. XVII) and in another prospect a short distance from this indicates that the seam worked in the former is the No. 2, or that above sandstone B. No reliable section of either the Star or Jackson shafts is attainable. From the depth of the Star, which is about that of the Progress shaft, it is probable that the same bed (No. 2) is worked in both. From the depth of the Jackson shaft, the difference in altitude between its collar and that of the Progress, and the dip of the beds in the latter mine, it is probable that the seam worked in the Jackson is likewise the No. 2. Whether the No. 2 or No. 1, could readily be decided by boring or by an examination of the strata in the shaft. In the old Boulder Valley mine, also, the seam worked was probably the No. 2. The new Boulder Valley mine is located on the eastern edge of this 368 GEOLOGY OF THE DENVER BASIN. field. At the time of examination, in the fall of 1890, it was too slightly developed to afford more than a section of the seam worked. (Pl. XX, Fig. N.) The measures were then reported to rise rapidly to the south and west, and the coal to deteriorate. The 5-foot layer was the only one worked. It lies at a depth of 70 feet below the collar of the shaft. Regarding the Northrup and other abandoned mines no data are attainable. The coal of the Canfield-Erie area resembles in every detail that of the Mitehell basin. The sections of the seams worked in the Erie-Canfield district are shown in Figs. K, L, and M, Pl. XIX, and Fig. N, Pl, XX. THE AREA EAST OF COAL CREEK. General description— The western slope of the high ridge between Coal and Dry creeks is underlain by strata that were at one time directly continuous with those of the Coal Creek syncline. The measures out- cropping are the upper and lower Laramie and the highest layers of the Fox Hills. The area is one of gentle flexures, with a balance of dip to the east or southeast and a strike varying between north and northeast. Occasionally a curvature is sharp, but the displacement is always limited. The chief faults are the Coal Creek, which limits the area on the west, and the Baker, which passes just east of the Baker mine, with a trend N. 61° 35/ KE. The eross-fault in the angle between the two is of little importance. Other faults may have been developed from some of the sharper folds, but they can not be detected at the surface. The coal measures extend northeast, east, and southeast an undeter- mined distance beyond the map limits. With the exception of an outcrop on the west side of the Baker fault, along its southwestern half they lie at a considerable depth beneath the surface. Along the Baker fault they rise in outcrop between 14 and 2 miles northeast of the Baker mine, followed by the basal sandstones of the Laramie in a gulch but a mile north of the mine, these by the Fox Hills in one only half or three-fourths of a mile north. Between the last point and the mine the coal measures with the lower beds of the upper Laramie have escaped erosion and oecupy the COAL. 569 area westward to the Coal Creek fault. The maximum distance which the coal horizon has receded from the fault is about one-fourth mile. The strata on the northwest of the fracture dip 10° to 15° NNW.; on the southwest they have been but little disturbed. Along the short cross- fault west of the Baker mine the coal measures on its north dip 3° to 10° NNE.; they abut against upper Laramie on the south. The triangular interfault block, considerably fractured, is of no economic importance. Excepting at the Baker mine, the only outcrops of coal on the exposed line of coal measures are one mile northeast of the mine, immediately over sandstone B, where the thickness is apparently not over 2 feet, and several small seams in the bluffs of Coal Creek just west of the mine. East of the Baker fault the depth at which the coal measures lie is unknown. The stratigraphy of the coal measures, as shown in outcrops in the vicinity of the Baker mine, is merely a variation of that in other regions— rapidly alternating sandstones, lignitic clays, and occasional narrow coal seams; low down in which, in this instance, is the heavy seam worked at the mine, the basal sandstones lying still beneath. A sandstone a short distance above the Baker seam is locally developed to a thickness of nearly 20 feet, and, according to Mr. Davis, the manager, in places lies directly on the coal. A thin bed of fire-clay usually underlies the coal. The Baker mine—(Tjo. D), Pl. XIX.) This is located on the bench land east of Coal Creek, about three-fourths of a mile north-northeast of its confluence with Rock Creek. The mine is opened by a slope on the seam, the strike of which is about N. 37° E., the dip averaging 15° NW. The seam worked is probably either No. 2 or 3, possibly botia, consolidated. The coal lies in two benches, the lower about 4 feet thick, the upper 7 feet, separated by 7 or 8 inches of slate, which is persistent throughout the mine. Of the lower bench the lower 36 inches is a fibrous coal, the 6 inches above a “curly” coal, while the remainder has a con- choidal fracture and is clear and bright. The upper bench carries rather more iron than the lower, and is a softer coal. The coal of both benches is of considerable solidity and works in large blocks. MON XXVII——24 Fie. A. Douglas (Lehigh) mine...... Nels to oer eee te seid XLIV, XLV B. Mount Carbon measures .............--.-..--. SEV Rha HS . 4. 44o.ins. 2 See eee Ce ee eee se = SUT VeVi EB: s@levelandimin 694522 72s Sa eS arepeeeesee aee eeeo XVI, XVII FsMieGrecor min Ons: cs 6240 fe oe Sterne = ahaa Ee Oe XX, XXII G.Stewartwmines.. 2S... hehe oS ee cea ohare ee XIX, XX, XXI Garfieldemin@z.2..2) ss. sso ccc eee ae ee ee eee XXTV I. Garfield mine (specimen from Garfield mine for special analysis, ROR VA see Sats Soha: SSA et ee SEAS ee ee Se ONG VIPNONG VET J. Mitchelll'mine:..<.. 5: 2 2 2 2 xv 164 > xivé | i } : 3 I a1) 2? 37) xii 2 ea i G H J = 4 xx KIX 3 3 eit x Be | 3 be XXVil \xxVIN AKIX XXI 16 2 > KAR tS I B i) K L M } XXXIV ly XXXIK ) 2 2 | 2 SXVII = =o XXXII eX | pen eh [xxvii mn } | | | Xu COAL SEAMS OF THE DENVER BASIN. Figures A-B LOUISVILLE FIELD. Figure D BAKER FIELD. Figure Cc DAVIDSON FIELD. Figures E-M ERIE FIELD. Figures N-O SCRANTON FIELD. Seale °o r 2 3 4 5 e 7 8 [9-6 —— es ——— es es | COAL BONE SLATE SANDSTONE Zl lCL | | COAL. 373 past. Chief among these are the McKissic, about 10 miles northeast of Erie; the Platteville, near Platteville; the Excelsior, near Evans, and the Eaton and Brown, near Eaton, all stations on the Union Pacific Railroad, from 25 to 60 miles north of Denver. The measures are in direct con- tinuation with those of the Denver field. THE SCRANTON COAL FIELD. This field lies at the head of Second Creek, about 20 miles east of Denver, the meridian of 104° 40’ passing just east of the settlement. It is impossible from surface outcrops to define the limits of the field, but there is believed to be an area of at least 15 to 20 square miles over which a workable thickness of coal, except for local thinning, is likely to be found. A seam is opened at Scranton which has a thickness of over 10 feet, includ- ing partings; coal is also reported, 6 feet thick, at a depth of 90 feet, in a prospect shaft about 3 miles south-southwest of Scranton, just north of the Kansas Pacific Railroad; again, in two other shafts between the latter and 5D the Seranton mines, of about the same thickness; in traces, also, about 34 miles southwest of Scranton, three-fourths of a mile north of the railroad; and, finally, in the bluffs of Sand Creek, where a 6-foot bed may be seen in arenaceous clays and sandstones. In the last locality, 10 feet above the coal, is a heavy bed of Monument Creek sandstone, a marked line of unconformity showing between it and the underlying series. The Monu- ment Creek formation is also found directly over the coal in the immediate vicinity of the Scranton mine. Eastward from these localities it extends far beyond the area mapped. The coal of this field is all of the upper, shaly division of the Laramie. The structure of the Scranton coal field is apparently that of hori- zontal or slightly undulating strata for the area in general, with sharper folds and steeper dips in particular localities. Faults have not been detected, although it would be surprising if an area of this extent and character should be entirely devoid of them. The strike of the beds at the Scranton mine is N. 15° W., the dip 3° to 5° E. The general character of the bed mined at Scranton and the strata with which it is associated are shown in the following section (fig. 16), reported DESCRIPTION OF PL. XX. Samples. HiG..-A. ‘Cannon mines. 2 ee: is elas cc cee see er Oe ee eee eee OI, CII har etek, S.C loo sceseaseaseusdnc sans sa tocssneesesos: CIIl B. Simpson mine ih Pcs te ee CLV @, Excelsior: inine-\c 4e.9-.=/'3 2580 so. Seats ee ee eee ee CV, CVI 1)! Excel siorsmin@S: 22 eee cis oa - a= oo ee eo ee oee ereee CVII He iGladstone mine.2a+,4-54.2--5208 se tee ce ee eee eee ne CVIII F. Marshall Consolidated mine, 1,200’ W. of shaft...............- CXV, CXVI G. Marshall Consolidated mine, 400’ W. of shaft................-... Hi. Marshall Consolidated mine, at shaft’--.----2------------- = Te AGM eM G) sc. See oe ee Se ee eee es ee ee ee ee Cee ee CIX, CX Ji Acme smin@=2e sass doo eee eee eles eC Ree eee eee CXI Ke Called omnia min Crete ete SED Ree, - CXVII, CXVIUIL CXIX 15 UNjobs TN) G pena Sdd sash eos Ssacds oo eaabUSode Secs: OXI, CXMM, CXTV IW, ANGE TBYO TNE) os ces Sa oe oc osseous te shee sae ee NSS core Soclsesoes CXXVI NS Wen onieie Walley wie. ososbas socoscacdseessesnssueby scars CXXV OssNiewaiWihuibewA's hianain © reat eee eee eee ee eee CXX, CXXT P. Rocky Mountain mine { Now ose amc" Se gaan rr ana psi ra as INO} 2)d eis bee to ecto ee ee ae eee CXXIV d74 U.S.GEOLOGICAL SURVEY MONOGRAPH XXVIII PL.XX CxVil cxvill CxIx CxxIl JULIUS BIEN & CONT. COAL SEAMS OF THE DENVER BASIN. Figures A-E, LAFAYETTE FIELD Figures F-L, LOUISVILLE FIELD Figure °M ALLEN-BOND 5 Figure N ERIE FIELD ” Figures O-P, FOOTHILL COALS Scale 3 oY 2° <6 6° 7' 8° COAL BONE SLATE SANDS TONE | COAL. 5i7(5) from an old shaft of the mine, and in Figs. N and O of Pl. XIX, obtained in the mine at the time of examination. 39 ‘Gray Shale The prevalence of clay partings in the coal 6loal worked is very noticeable, these and the bone about the 3 Coaland Shale middle of the bed being remarkably persistent. The bands of coal, clay, and sandstone, imme- \516 Gray Shale 16 aixr 7A A sas ] "AR CO YTYNT ee ar with diately above the seam, in fact forming a part SP el ae oe of it, are irregular in occurrence, their lines of deposition rising or sinking, the individual strata thinning to a knife-edge or attaining almost the maximum thickness of the series. fae a, : 5? bee 0 Sandstone The coal itself is a thoroughly representa- c=) « tive type of the lignite of the plains; its streak of YI ‘2 ] 76 Coal Several is brown; it weathers rapidly and disintegrates eae 7 Scale 40FT | ee ey ey en Y= 5 i EA poh, Mecha Ue completely; it contains 25 per cent water, yields ,., 75 section of col measures a large amount of ash, and burns with evolution — °f "pPer Laramie at Seranton, of comparatively little heat. THE COAL. CHEMICAL ANALYSES OF INDIVIDUAL SAMPLES. The analyses in Table I may be regarded as representative, both of the mines from which they were obtained and the areas in which the mines are. With the exception of Nos. 37, 127, 128, and 129, all are of com- mercial mine samples, but owing to the ease with which the slate or other partings are separated in mining, and the freedom of the coal itself from impurities, the analyses are available for scientific discussion of the coals as well as for their commercial comparison. The samples were taken directly across the seams from fresh surfaces cut for the purpose. Rooms long abandoned, and others newly opened, were sampled to show the degree of deterioration on long exposure underground or to illustrate possible variation in the physical and chemical conditions of the coal. Such portions of the bed as were excluded in shipping—partings, pyrite balls, silicified roots, and bone—were also excluded from the samples, but 376 GEOLOGY OF THE DENVER BASIN. in all cases where feasible the coal ordinarily left for support to the roof was included. Whenever the coal appeared to show physical differences in its natural position in the mine, or where statements as to supposed or actual differences were made by the mine superintendent, separate samples were taken for purposes of study. To obtain a fair average the samples were equably distributed throughout the mine. CHEMICAL ANALYSES OF THE COALS OF THE DENVER BASIN, ARRANGED ACCORDING TO MINING DISTRICTS. TABLE I.—dAnalyses of coals of the Denver Basin. Ree Fixed ea | Sui Specific aan District. ber. | Mine. ‘carbon. busti- Water.| Ash. |phur. Color of ash. sae a “e | te. v-Cc. | Per ct. | Per ct. | Per ct. | Per y) rr et.| | ° Erie-Cantield .| 125 | New Boulder Val- | 42. 34 37.81 | 14.90| 4.95 | 0.49 Yellow-gray ---..- 1.35 @19.4) 1.12 | ley. | | 18 | Progress..-.------ 44.73 | 33.57 | 16.64 | 4,51 | .55| Lightyellow.....- 1.341 @13.0| 1.33 43.86 | 32.10 | 18.54] 4.99 | .51 | Yellow-white....- 1.386 @12.0 | 1.37 44,51 32. 51 17.03 5. 35 .60 | Reddish-white. ...| 1.335 @ 10.0 1.37 45.15 33.37 | 16.04) 4.86 D8) White --- <=. 22 - 1.350 @ 10.5 1.35 44.69 | 32.34 17. 61 4.70 .76 | White, slight red | 1.386 @14.0| 1.38 tint. | 45.55 33.01 | .77 Yellow-white 1.333 @ 15.0) 1.38 44.62 | 31.35 | 79 a UT | eee do 1.338 @ 17.5 1.42 Mitohellic eee 47.86 | 30.82) 17.25 | 3.55] .62 | Red............... 1.345 @12.0 | 1.55 | 44.63 | 34.51) 16.80] 3.52) .54 | Lightyellow...... 1.381 @12.0| 1.29 43,89'| 34.14.) 17.03'} 4.53] 141 | Mellow -.------..- 1.336 @ 11.0) 1.29 44. 68 34. 59 17. 66 3.25 .42.) Red-yellow -. - 1.336 @ 15.0 1.29 44.97 | 33.43) 17.25} 3.79 .56 | Light yellow | 1.328@12.0| 1.35 45, 62 | 34.54 | 15.44 | S76) AC40 GNWibate senate men 1.389 @ 12.0 1.32 | 44.43 | 32.63, 18.32) 3.97 | .65 | Slightly reddish --) 1.331@19.0 | 1.36 | 44.74 | 34.98 | 16.38) 3.38 | .52| Reddish .......... | 1.330 @ 19.0 | 1.28 44.55 | 33.85 | 17.58) 3.48 54 lpatse GW) etneac EACe=E, 1.334 @ 18.5 1.32 43.77 83.84 | 18.07 | 3.84 .48 | Reddish-white.-...) 1.331 @ 12.5 1, 26 45.30 | 33.81 16, 76 | 3. 60 .53 | Light red .....-..-. ) 1.334 @ 14.0] 1.34 44,86 | 33.42| 17.01} 4.22] .49] Reddish .......... | 1.350@12.0| 1.34 44.16 34.03 16,96 | 4.39 | .46 | Lightred......... 1.335 @ 13.0 1.30 45.261) (39/80) Sl 701)| (S259) all Red Gee aces aoe | 1.339 @16.0| 1.34 | 43.06 | 32.53) 16.84 6.47 | 1.10 Yellowish-gray..., 1.352 @13.0| 1.32 Area east of | 44,08 | 33.28} 18.38) 3.72) .54] Reddish - -- 1.324@ 5.5) 1.32 Coal Creek. 45.05 | 34.67 | 16.38 | 8.46 | .44|..... doce: Seen | 1.341@ 8.0| 1.30 43.90 34.41 17.75 | 3.45 49 | Yellowish-white..| 1.329 @ 13.0 1.28 Lafayette... . 108 | Gladstone ......-- 44.98 | 36.70 | 13.72 | 4.65 | 1, 22 105 | Excelsior ......... 45.10 | 387.82) 13.42) 3.66 1.19 106 |. 45.16 | 37.81 | 13.04 | 3.99 | 1.19 107 | 44.56] 98.13) 19.47 | 3.84 1.17 103 | Simpson-Spencer -| 43.85 | 37.09 | 14.74 | 4.32 1.18 104) )en-es Gb) -ssesthocoods | 44.34 | 37,87 13, 57 4.12 1.17 127 |....- donee | 46.15 | 34.65 | 16.27 | 2.98 | 1.33 COAL. 377 TABLE I.—Analyses of coals of the Denver Basin—Continued. ] ie Volatile fee ' ihe ; fete | Specific | Fuel | District. > ae Mine. Peaeed bust Water.) Ash. phir. Color of ash. ated a ae | : | | v-e | | Per ct.| Per ct. Per ct. Per ct. \Pr ct.| | ° Lafayette..... | 128 | Simpson-Spencer -| 26.87 | 28.18 | 11.43 | 33.52} 55 | Pink.............. 1.61 @ 16.6 +95 101 Cannon. | 37.78 | 19.26) 4.82| .46| Lightgray......-: [1.36 @22.4) 1.17 LORWMecocdoW s-2 sees -70| 39.61| 11:85 | 3.84] .46|..... do to. 1.36 @20.2) 1.13 | Lonisville ....] 115 | Marshall Consoli- i 37.05 14.53 4.70 .33 | Yellowish-gray...|.-............ 1.18 dated. | | 86.98) 15.12] 5:14) .71 | Pale yellow....... [oevee stesso 1.16 | 97.20] 12.87| 4.65] .28| Dark gray........ | 1.38 @18.9| 1.21 | 88.09] 13.93 | 4.36] .38| Gray .. 1.37 @19.3} 1.15 | Beso Net Oeil) Paedadlly dT! doko. eed | one nee 1.16 33.94] 16.39) 4.75] .42/|..... Fi Lie ae bie ae 1.340 23.5 1.31 33.99 | 17.04) 4.35 .62) Light yellowish- | 1.329@24.0 1.29 gray. | | 93.92 | 17.34! 5.58) .40 Light gray | 1.343 @ 23.0 1.30 33.44 | 16.73 | 5.11 | .35 | Yellowish .. 1.354 @ 23.0) 1.3% 38.79 | 14.98) 4.16 | .29 | Yellowish gray...) 1.355@ 21.7 1.10 | | 38.27] 12.95] 5.26] .37| Lightgray........ .387 @16.8| 1.14] 87.88 | 14.07} 5.10 | .82 | Reddish-yellow...|..-........... 1.13 35.30 | 15.63) 2.61| .30 1.34 20.2) 1.32 36.39 | 14,11 | 4,59 | .34 | Reddish-yellow...}.............. 1.23 86.08 | 14.10 | 4.26) .34 |--.-. (iti a ae | 1.37 @19.6, 1.26 | 38,08 | 15.81 | 5.26] .85 | Yellow :.......... 1.35 @19.9| 1.07 Davidson ..... 33,87 | 16.49 | 3.95, .58 | Reddish-yellow ...| 1.324 @ 24.0 1.33 Allen-Bond ....| 37.57 | 12.45] 4.85] .33 | Reddish-gray..... 1.38 @17.8! 1.20 Marshall... -. | 32.60 | 16.24) 3.68) .86| White............ 1.343 @ 22.0) 1.43 | 33.85 | 14.37] 3.20 | .67 | Very light yellow.| 1.344@ 18.0 1.42 | 36.18 15.06 | 2.94 .74| Light yellow...... 1.315 @ 20.0 1.25 28.66 18.67 | 4.90) .41 | Reddish-yellow...) 1.371 @ 24.0 1.65 10 | Marshall No.5..-.| 43.08 | 35.44 | 16.12) 4.67| .69|..... don te 1.341 @ 25.0) 1.22 11 | Marshall No.3..../ 45.08 | 34.79 | 13.81) 4.71 | 1.61 | Gray ............. | 1.348 @ 25.0 | 1.30 Py bees doeesc eases 45.03 | 36.10) 14.79 | 3.07 | 1.01 |..... Doerr cost. 1.339 @ 16.0 | 1.25 Foothill ...... 124 | Rocky Mountain, | 41.26 | 37.87 14,40 | 6.47 | .45 | Dark PTA Vee sce 1.41 @21.4) 1.09 No. 2. | 123 | Rocky Mountain, | 38.89 36.65 | 14.13 | 10.33 .48 | Reddish-gray..... 1.44 @21.6 1.06 No.1. | } 14.09 | 8.51 98 | Yellow-gray...... 1.41 @ 20.2 - 98 19.46 | 5.71 .42 | Slightly reddish ..| 1.365 @ 11.0) 1.16 20.95 6.36) .56 | Greenish-gray..-.| 1.353@ 4.0 1.07 | 14.94 5.29] .28 | Darkyellow-gray.| 1.40 @19.4 |) 1.07 14.60 | 7.89 | .31 | Dark gray 1.10 18.36 | 3.14] .40] Gray... 1.20 19.02 | 3.68| .45 | Greenish-gray ....] 1.359@ 16.0) 1.25 OSU) EXCeH Lee) beet by eae aera 1.3620 17.0) 1.32 24.27 | 5.81| .47 | Reddish-yellow...| 1.329 @ 18.5 | 1.08 S270) 5.874) 51 |. 25. (yea aes 1.329@ 12.0) 1.03 22.94 | 4.92 | 1.07 | Light gray.......- 1.344@ 13.0 1.00 22.15 | 8.48) 1.68 1.388 @ 11.0! 1.05 Scranton..---! 26.08 13.55) .41 | Reddish-gray..... 1.365 @ 14.0) .90 26.37 | 7.57| .52| Gray.....--.------ 1.317 @ 13.5 7 26. 92 13.22 | .42 |) Greenish-gray .-..) 1.321 @ 16.0 -97 27581) | s8470'|| 47 || hac Mover eon 1.345@14.0 98 9895118, G0i|l5.\48)|*--do;-<.- =1.33),—This embraces the prairie coals of the lower Laramie. They resemble in their physical appearance many bituminous coals of the East, and in their fuel ratio certain coals in the lower portion of the Pennsylvania bituminous series. Their proportionately great amount of water, however, and the fact that the total percentage of fuel constituents is much less than that universally present in the Eastern coals, prevent a close relationship between the two series. The coal of Class I withstands weathering well and is the highest of the coals about Denver in economic value. class 1 (5°-=107).—This embraces the coals of the foothills to a depth of 300 feet. At greater depths they approach Class I. Class ut (,*,=09°).—This is characterized by the excess of volatile matter over the fixed carbon, by the extremely large proportion of water con- tained, and by the high percentage of ash. The coal of this class weathers most readily upon exposure to the atmosphere, its color becoming brown, its appearance earthy. It is a lignite, regarding this term as signifying a position above the peat and below a variety of coal commonly accepted as near the base of the bituminous series in the East. It is a class in which the fuel ratio oftener falls below than exceeds 1. SPECIAL SAMPLES OF COAL. The analyses of these coals are given in Table I. No. 37, from the Garfield mine, represents a hard, smooth, pitchy coal, somewhat resembling the higher bituminous and coking coals of the mountain regions. The analysis shows it to be somewhat higher in fixed carbon and lower in volatile-combustible matter than the general samples of the Erie-Canfield field. Its fuel ratio, 1.55, places it considerably above the associated coals. This variety is distributed without regularity through both the vertical and the horizontal extent of the seams, but does not enter prominently into their composition. No. 127, from the Simpson mine, is a typical specimen of conchoidal coal. It is of frequent occurrence in the prairie coals of the bituminous variety, forming prominent layers from 2 to 8 inches in thickness. It resembles in its fixed carbon and volatile-combustible contents the run of COAL. 387 the coal in the Denver field, and in its fuel ratio, 1.33, falls into the higher class. No. 128, from the Simpson mine, is a typical specimen of “bone coal.” Its fuel ratio is 0.95, approaching the lignites in this respect. Its ash amounts to 33.52 per cent. Its water content is 11.43 per cent. No. 129, from the Acme mine, is a specimen of fibrous, woody, lustrous coal. It is low in ash, of medium range among the prairie coals in its water content, and has a fuel ratio of 1.31, showing it to be somewhat below the variety of which 127 is a sample. SECTION II._THE CLAYS OF THE DENVER BASIN. The economic clays of the Denver Basin are derived from the Pleis- tocene, Denver, Laramie, Fox Hills, and Dakota formations, the Denver and Laramie, however, affording but a small proportion of the total yield. The clays of the Dakota are refractory and are employed in the manu- facture of fire-brick, crucibles, and allied products; those of the other formations are of the ordinary type, and are manufactured into brick and tile for general architectural purposes. FIRE-CLAY. The fire-clay of the Dakota occurs in noncontinuous bands, 5 to 15: feet thick and several hundred in length, in the argillaceous shales which separate the two or three heavy layers of sandstone that constitute the bulk of the formation. Numerous openings have been made upon them in the vieinity of Clear, Bear, and Deer creeks, and at other, less access- ible points along the foothills, within and beyond the confines of the Denver Basin. With the exception of those near Golden the openings are prospects. The mines near Golden are in the hogback about 2 miles north of the town. Several bodies of fire-clay here occur immediately beneath the upper heavy sandstone, access being by tunnel through this. On the next two pages analyses of several of the typical clays of this country and Europe are tabulated, together with the uses in which the clays are employed. 388 GEOLOGY OF THE DENVER BASIN. Analyses of fire-clays from the Denver Basin and from other Americun and foreign localities for comparison.* : | has = A = ; | tes ieee = lina Z | ” = | o¢g o a 2 5 a & ae oir = $- g x —Oo 2 eo é Locality. Ie) at of % ac] te) be | 2H a of ie a en 3 ae | 8 ry 83 2 = a ° & oe 2 a e = = 5 ‘Ss be S$ a £ | Bea een eae a a Biri ste 1 Denver Fire Clay Co., Denver, Colo...) 39.184 | 33. 64 11.75 84, 524 0.80 11. 216 12. 016 2 William B. Dixon, Woodbridge, N.J..) 31,12 26. 95 9. 63 67.70 1.90 28. 81 30,71 William IL. Berry, Woodbridge, N.J..| 40,50 35. 90 12. 80 89, 20 1,30 6. 40 7.70 Loughridge & Powers, Woodbridge, 42. 23 39. 53 13.59 95. 35 1.40 50 1.90 5 Crossman Clay and Manufacturing | Co., Raritan River, N.J..........--- 87. 85 35.75 12. 30 85. 90 1. 60 10. 50 12.10 6 KE. FF. Roberts, pits near Kagleswood, IN. OU temniceiais wane tute athe telsle tame nan a= 40. 40 38. 40 12.50 91.80 With Al,O, 5. 20 5, 20 7 | J. D, MHylton's fire-clay, Pensanken (CRN ES [ee ne ie Rocce nocnescosece 17.50 18.11 5. 50 41.11 With Al,O, 56. 80 56. 80 8 Mineral Point fire-clay, near Johns- Ua Moe Soc a8s Rscsicssosssesc? 44.95 88, 84 12.50 96, 29 1,55 380 1.85 9 Mount Savage fire-clay, Mount Sav- | apo, Ma---cc- ONG ee eet 39.90 | 30,08 7.60 | 77.58 1.15 16. 90 18, 05 ! 10 Seioto Star Fire Brick Co., Scioto- epleOLIbNceeroa tee eet 52.36 | 83.84] 12.28] 97.98 |............ With clay. |........... 11 | Portsmouth Fire Brick Co., Ports- |\Wromouth; Ohtopesscunswanee sys 50.95 | 39.49 O18) |), ¢O0N62) ene eee With clay. |........... 12 Fire-clay, Cheltenham, Mo..........-. 88.10 31.53 11.30 80. 93 1,50 12.70 14, 20 18 Glass-pot clay (strong), Stourbridge, | Worcester County, England........| 80.50 | 22,52 8.30 61,32 1,00 83.65 34, 65 4 Crucible fire-clay, Halifax, Yorkshire, | LON sans See CCR oS cOoOd 39.45 | 32.19 11. 80 CT Sd een ee aecroce 11. 80 11, 80 15 Fire-clay, Frankenthal- on - Rhine, | | CRB TIAN Wile mpleieetelt eee = eiere av aetet eaten 41,90 31. 69 9.45 83, 04 . 90 8.10 9, 00 16 | Vire-elay, Bolléne, Dept. de Vaucluse, DEAN ie eves eles eels ean clots ae 38, 20 28.19 10. 50 76.89 1.15 14.95 16.10 ~ The fire-clays from localities other than the Denver Basin are taken chiefly from ‘Report on Clays,’’ Geological Survey of New Jersey, 1878. Two, Nos. 10 and 11, are from Vol, V, Economic Geology, Geological Survey of Ohio. 1, Employed in manufacture of all refractory produects—tire-brick, chemists’ and assayers’ supplies, furnaces, ete. 2. “Of special value for making crucibles, glass pots, and the more siliceous fire-brick.” 3. “Used for making retorts.” 4. Por general refractory products, 5. Fire-brick and other highly refractory products. 6, For general refractory uses. 7. “Fire-brick, and for retorts and condensers in zine furnaces.” 8. For rolling-mill and blast-furnace brick. Best for steel works purposes cemented with Springfield kaolin. | CLAYS. 389) Analyses of fire-clays from the Denver Basin and from other American and foreign localities for comparison—Continued, | Sealers (heen lei | = i) ee el ae = P og Be iS | Locality. g % S = = 2 23 By = A ~— 3 fs ms te e he oa . | 4|4 g & | = z = aie ° = & & g ° fv 2 m a =) A a a = a 1 Denver Fire Clay Co., Denver, Colo...) 0.49 0,09 |.--.+--. Trace. 0.75 1,33 2.13 | 100.00 2,| William B. Dixon, Woodbridge, N.J..| Trace.) Trace.|..-..-.. 0.07! 1,24 1.31 157 | 100.29 3 | William H. Berry, Woodbridge, N.J... . 28 Pa Bee oe Eee 1.10 1.54 1.50 | 99.94 4 Loughridge & Powers, Woodbridge, DSA eee acees ee Abas ee 8 41 - 0B (hi Ch) aR ee 50 1.09 1.21 99.55 5 | Crossman Clay and Manufacturing | Co., Raritan River, N.J.--.-........ Ai) here a Boerne et Trace. 95 1.32 1.00 100. 32 6 | E.F. Roberts, Pits near Eagleswood, Lh AL Pee cca te ee ener eds Ao) Fae ee .22 25 1.20 2. 26 1.30 100. 06 7 J. D. Hylton's fire-clay, Pensauken | CORN al Ome ol at acta dee ee 76 -20 “ONE ease ee 1.09 2.16 40) 100,47 8 Mineral Point fire-clay, near Johns- eet pecan er EP acetate tr CE) eS es Lea ae Seer | 91 1, 26 .70 100,10 9 Mount Savage fire-clay, Mount Sav- | AGE, Md....-.--.s2ccacesecoccessoes PrP ee eee eeeetacisn 1. 67 3.97 90 | 100.50 10 | Scioto Star Fire Brick Co., Scioto- | —————--——_—_—_- AMANO INO seen oe eee teens ier atnen me 1.00 1.02 Zip eevae eee | 100.00 11 Portsmouth Fire Brick Co., Ports. | With | Mera These eee sue p Syl be ees .30 » 28 0; \ BO estes, 100. 51 12 Fire-clay, Cheltenham, Mo............ | elaicXO| sues ares Sava an Trace.) 1.92 2.32 2.50) 99.95 13 Glass-pot clay (strong), Stourbridge, | Worcester County, England........ | CN em es BAe Peer Laney sey: 1.43 1,93 2.10 | 100. 00 14 Crucible fire-clay, Haitifax, Yorkshire, | | NIPIRU Satie welve cenecasnasananancl tT OPS POE RAN er 2. 65 3.16 1.60 100.00 | 15 Fire-clay, Frankenthal-on- Rhine, WP Germanys soos. 2c... eaposenes PON ase ar [ee Se 2.54 4.76 3.20 | 100.00 16 Fire-clay, Bolléne, Dept. de Vaucluse, | | Framee...--------2eeeeeeeeeeeeeeeees) (MO | eeeeeee eeeeeeee ceeeee es 2.76 3.16 3.60 | 99.75 —— 2 ee 9. Used for fire-brick and other refractory products. 10. Fire-brick. 11. Fire-brick. 12. “Very refractory; used in glass pots and crucibles,”’ 13. Glass pots. ‘ 14. “‘High-class fire-brick, cupola linings, and Bessemer steel-makers’ requirements. The bricks are excellent.” 15. For high-class refractory products. 16. ‘Most noted avd most sought after of French clays for manufacture of refractory materials. Used one-third crude and one-third burnt, with one-third quartz sand for tuyéres of Bessemer converters and steel furnace hearths.” 390 GEOLOGY OF 'THE DENVER BASIN. The fire-clays of the Dakota are bedded, delicately laminated, and locally jointed. They are hard, compact, and fine-grained, the purer varie- ties being almost free from grit. The color is either a dark, bluish gray or a lighter shade, passing into a drab. The darker varieties contain a greater amount of carbonaceous matter, which is distributed either uniformly and invisibly through the mass of the clay or in more or less extended patches of lignitic particles on the planes of lamination. The impurities are a fine sand in thin layers, or oxide of iron, the result of the decomposition of minute grains of pyrite. The latter imparts to the clay a characteristic spotted appearance, sufficient to determine its Dakota horizon where other- wise—as in the area of the unconformity at Golden—through proximity to the dark Benton shales, it might be wholly misinterpreted. Following is an analysis' of an average specimen of Golden fire-clay, obtained from the mines north of the city: Per cent. SiOy Sos aviss seen ade be telat oho ee See ee ete er tata cares ait ne ee 50. 35 MIO ease ccicin ere nia Seals see iael slave aia wee epee ciate are ae ate ee ee ee . 80 Aly Og aia cintat| fevmials/ =isie(a elefmiatare slatatnls chats CeUeetase pa Sinn cliente re else cisterna ese rr eee 33. 64 10 He See oe coach GeO DEp ROaSreenOSARnce 2Stc sotyelttectce cations dete eee pre A (LOR a ee na Se CR Cn eRe secs DARE SORONSSS Megane oan" - scosomuaeesseotceséese = Trace. I EOF sam copanc TO OS ecan conor os caddnctiass adassh mocsono sane. ak5 soem Sods 09 100 Ano eas oR eeOn Bopminaoa acacia ceca one mcce canananaAasaconacddanseense 49 He OzandiOrgallicys=.n\s<.022-) ne =t-s-eee sees ccs eae cis tee cence eee ose 13. 88 100. 00 Much organic matter. The composition of pure kaolinite is: SiO,, 46.30 per cent; Al,O;, 39.80 per cent; H,O, 13.90 per cent. Computed on the basis of the kaolinite composition, the above analysis would become— Per cent. SiO;.combined §:2 sca. ei censors eee eeeeenenee Cee Seis eee 39. 134 G0 Papen ee SOE DooU CaGnG Sa eRe endo a rae aaon notocr cosadeenanegase Ses case 33. 64 HO Combined seer tecicesieins ate ee ie see ea ee 11. 75 2 8 Frece eR OOnS aa cap anaS Oe Cn ad cand a= mesons sorb Onaobbenance a cls acc 80 SOFUh E14 Ga aoemoomonq Osos osambeccinsod onaced coat oShermacatca sake eccsst 11. 216 LGN OF one eR AS OS BOE SOC CCE Aan bamease sede aoae cnc onasoe nace =—-——— ——_ — a | Motalssss-=-7--s-= 4.32 3. 82 SRoteal een ee 4.77 4.19 | ARTESIAN WELLS. 415 Arkansas, at Canyon, Colo. [Drainage area, 3,060 square miles. ] Discharge. | Discharge. | Month. Equivalent | Per square | Month. E uivalent| Per square depth on | mile | epth on mile basin. | drained. basin. drained. 1888. Inches. Sec. feet. | 1889—Continued. Inches. | Sec. feet. JPET A SoS nn ae a aees 0.15 | 0.13 | ATES Tee tsa es 0.13 | 0.11 1202) bt to ees ee Si le -16 || September ---........-| . 08 07 Meare eee oo" MBO (@pkober eA. o<.ee-- 08 07 JY OO a es ee - 36 | -33 NOVeMDeD = sa-525 an Sil} . 10 Bey shee steers 2s cic 54 wag DecCeMper=- 55502. 27 =55 13 reat UNG morse is ari tasiaj= = = 76 68 orale eee 1.92 L711 dhl 5 352 apeeease ome ee a eee a ie rere gee br fa ihe ate 5 INE Td i re ER ea Omg 6 tC Sah one DEAR OCe Bae a Eee ye eer ee rf J AV yal See eee cer Racor 6 ree sO nee ein in aie Baas Sones SoA Sere 8 WES ee. cea cot RRR et Sa se cease sen See oo ete Sass Se Sy Sato So ee ee 10 OO aes eee Pee ee oe pS en eI Oe ER Eee eC ae ao tee Se 15 AULA A pane soe eciee pease Dace cee ee too sto SSHCE Eee Goes abe con ne renee 12 PUP URGE oF ecccntt- sae e ne nee eee ose eoe nee oe aoe Ot Rae Se eae 12 Pe) ELEM OS Re BSE CREE SA aeme nn AP Re hare eee yr eee 9 Octoberzs2 22 2-2 25-22 ses - asc ae ae ae ese ee asad sade sjonse cpap oe essence 7 IN OMEM DONS = coro ste acs oe ee ee teeters ae elt ren oat 6 DE COMID ON a= a ale a sa” ape inet ee eee ae eae ean be maar eta 5 Combining in a proportion the total percentage for the months June to November and the actual evaporation as observed at Cherry Creek for this period, the result for the year is the 61 inches given above. In general, in considerations of the present nature, it is customary to allow for evaporation about one-third of the rainfall. For convenience in subsequent calculations, it will in this report be taken at 4 inches, or a little under one-third. The run-off and evaporation for the Denver Basin—4.113 inches and 4 inches, respectively—leave of the annual rainfall of 14.113 inches, 6 inches available for absorption by the strata upon which it falls. WATER AVAILABLE FOR ABSORPTION, From rainfall direct——The area of exposure of the lower water-bearing series—the Fox Hills sandstone, the basal sandstones of the Laramie, and Eleventh Ann. Rept. U. S. Geol. Survey, Part II, Irrigation, p. 34. MON XXVII 2 418 GEOLOGY OF THE DENVER BASIN. narrower bands of the coal series—available for collecting purposes for the artesian supply of the Denver Basin is approximately 51,480,000 square feet, based upon a width of 250 feet and a length of outcrop along the foothills of 89 miles, from Wildeat Mountain, south of the Platte, to South Boulder Creek. The extensive exposures of this series of sandstones in the northwestern part of the field are unavailable for the general artesian supply on account of intervening faults. The area of 51,480,000 square DoD feet covered to a depth of 6 inches, the available rainfall for absorption, affords 25,740,000 cubie feet, or 192,548,585 gallons, of water to be taken up by the underlying rocks each year. The area of the Arapahoe formation available for absorption, with reference to the Denver artesian system, and which, therefore, excludes the outlier northwest of Dry Creek, is about 1613 square miles, or 4,505,149,440 square feet. The amount of water falling upon this area for the absorption by its strata is 2,252,574,720 cubic feet, or 16,850,430,244 gallons, per year. The area of the Denver formation, disregarding the covering of loess which overlies it in many localities, is approximately 289 square miles, or 8,056,857,600 square feet, receiving upon it 4,028,428,800 cubic feet, or 30,134,742,207 gallons, of water per year, as the amount here available for absorption. The total quantity of water falling annually upon the artesian strata of the Denver Basin and available for absorption by them is 47,177,721,036 eallons. From water of irrigation—Prof, P. H. Van Diest,’ of Denver, has also esti- mated the quantity of water brought onto the absorbing areas by irrigation ditches. He says: But rainfall is not the only source of water within this basin. The many irrigation ditches bring a great amount of stored-up water from rainfall outside of the basin within its limits. It is estimated that of the amount of water brought in at the upper part of the High Line ditch, not more than 60 per cent is utilized for irrigation. This ditch has a flow of nearly 300 cubic feet per second during the thirty maximum days; of one-third of that flow during the thirty days before ' The artesian wells of Denver: Scientific Society, Denver, Colo., June, 1884, pp. 37 and 38. ARTESIAN WELLS. 419 the 10th of June, and of a half during the thirty days of irrigation after the 10th of July. The Table Mountain ditch has a flow of 1314 cubie feet per second during the maximum season, and the Rocky Mountain ditch of 189 cubic feet, and the same ratio before and after the maximum discharge as given for the High Line ditch. The flow of each of the two principal ditches heading in Bear Creek, not exactly known, will be very nearly equal to the flow of the Table Mountain ditch. The amount of water thus brought from outside the basin within its limits is, according to the above data, not less than 31,104,000,000 gallons [51,406,155,324: author], A good deal of this runs away in visible streams, as is amply demonstrated by the many gullies, ravines, and arroyos, which were before known as dry and now are little rivulets, also by the many springs that were formed in the neighborhood of ditches, some feeding lakes and increasing their extent. Another and large part of the above-named amount of water brought on by ditches is evaporated and consumed by vegetation, so that probably not more than 20 per cent percolates to the sub- soil. This 20 per cent would make an additional supply of 6,220,800,000 gallons (6,281,251,065: author}. Taking the author's figures, the total water available for absorption for the field from all sources would be 53,458,952,101 gallons per year. ABSORBING POWER OF STRATA. The absorbing power of the artesian strata involved in this discussion can, for the most part, only be estimated. A few determinations of this property were made for the lower Laramie and Fox Hills sandstones in the selection of building material for the State capitol at Denver,! which are as follows: l Num- Trac ality | Weight per | Absorption pi id ber. ocality. Age. | cubic foot. | in weight. | sorbed per | | | cubic foot. | | Pounds. | Per cent. | Pounds. | 15 | Beaver Creek .....| Niobrara ......-22.-----++- | s34.78| 5.98| 7.22 16 | Oak Creek .......- |, Fox Hills or Laramie..--.- | 119.81 BO) — ableaty/ 17 | Coal Creek .-..--.-- eee ae OOnasssteance aor cast 130. 42 6.05 — 7.89 PSO MOrinidad 222 <---> lessee Ueno ee ea 151. 01 3.12 5. 65 SOL Moro =o asss2eaIoene CON saetewlss scan secs - | 132,29 5. 89 7.79 ‘Second Biennial Report of the Board of Capitol Managers to the General Assembly of the State of Colorado, 1886. 490) GEOLOGY OF THE DENVER BASIN, The average weight of water absorbed per cubic foot of the above sandstone is 8 pounds, or a trifle less than 1 gallon per cubic foot. For the strata of the Arapahoe and Denver formations no determina- tions have been made. ‘The lower measures of the Arapahoe, about 300 feet, are of very coarse material, often conglomeratic, very friable, and porous, and consequently considerably more absorbent than the series at the base of the Laramie. The exposure of their vertical or highly inclined portion is one-fifth shorter than that of the Laramie, but their entire north- western edge is available at a low angle. of dip. The absorbing power of the sandy series constituting the upper water-bearing zone of the Arapahoe formation probably falls somewhat below that of the basal sandstone of the Laramie per cubic foot. Its material is generally a little finer, and it is of somewhat looser texture, but it carries some argillaceous constituents, which must lower the ratio slightly. Its absorbing area is rather greater than that of the lower sandstones of the formation, since it lies at a less angle of dip and often extends well out on the prairie. The shaly members of the Arapahoe, while distinctly argillaceous, have a considerable proportion of arenaceous material disseminated throughout their mass, and in addition not infrequently carry porous, lenticular bodies of sandstone at several horizons. Though their arena- ceous character doubtless renders the shales pervious and sometimes avail- able as water-bearing strata, and the bodies of sandstone increase still more their capacity for water, yet the ratio of absorption of the shaly member as a whole must fall far below that of the sandstones of the formation, The porosity and absorbing power of the Denver beds underlying the prairie is considerably below that of the older strata, their constitution being much less arenaceous. The formation is, however, sufficiently absorbent to play an important part in the artesian economy of the Denver region, especially in the matter of ground water. POWER OF TRANSMISSION IN THE WATER-BEARING STRATA. Closely related in importance to the absorbent power of different varieties of strata is the power of transmission. Like the former, this ARTESIAN WELLS. 4?1 ~ depends largely on porosity, but also on evenness of texture, and _ its influence upon the artesian flows of the Denver Basin may be gathered from what has preceded in regard to the composition and distribution of materials. It is not the same for all parts of a stratum, as is frequently instanced in the wells of Denver, and it may vary to such a degree as to completely shut off the flow in certain areas. The coarse sandstones and conglomeratic layers have the greatest power of transmission, and it is quite possible, also, that they act, when tapped, as an inspirator or injector, drawing water by greater freedom of flow from the less permeable strata into their own general current which is escaping by the artesian wells. CAPACITY OF THE STRATA, THEIR YIELD, AND THE RAINFALL, CONSIDERED RELATIVELY. It has not been possible to obtain data regarding the flows of the artesian wells of the Denver Basin within an interval of time that would permit determination of the total yield at a given instant. Furthermore, the influence of wells upon one another is a marked feature in all parts of the central portion of the exploited area, that is, within and in the immediate vicinity of the city of Denver. Large original flows in certain wells have seriously diminished upon tapping the same flow in a later well sunk in a more favorable position. Upon the establishment of this fact, to regulate the flows for mutual benefit and avoid waste, valves were applied to most of the wells. Again, flows have diminished from mechanical defects, either in sinking the well or in the packing employed, and such diminution may have been either temporary or permanent. Finally, in arriving at the y.ield of the system, individual estimates have oc vasionally been necessary, the parties drilling or owning the wells having neglected to record measurements. Had the original flows of all the wells been maintained, including also a few that from position or otherwise have always required pumping, with the most careful determinations possible from the data at hand, the maximum yield of the basin would not have fallen far short of 10,000,000 gallons per day, or 3,650,000,000 gallons per year, an amount that is only 21% per cent of the rainfall available for absorption by the strata 422 GEOLOGY OF THE DENVER BASIN. furnishing the bulk of the artesian supply of the field, that is, the Laramie and Arapahoe formations, the Denver formation being excluded from consideration on account of the slight extent to which it enters into the actual supply. The rainfall of the Denver Basin is therefore more than adequate to the absorbing and transmitting power of the beds. The failure of the wells to yield a larger supply is due to texture, by which these properties are governed. THE WELLS. The 394 wells along the Platte Valley may conveniently be considered in two divisions—-one comprising those within Denver and its immediate suburbs, 209 in number; the other, those in the country, 185 in number. Between the wells of the former division clearly developed relations exist, relations that can not be distinguished between the wells in the country or between these and the city wells. The interrelation between the city and its suburban wells is attributable to their concentration within a limited area which undergoes but slight change in the geological conditions of its strata; and the absence of such relations between the wells of the country is due to their wide distribution and the ever-recurring, though slight, changes in structural and stratigraphical conditions sure to exist over an extended area. THE WELLS OF DENVER AND ITS SUBURBS. In the following discussion the datum level to which depths of wells, flows, and stratigraphical planes have been referred is the general level of the Platte River at the foot of Fifteenth street, Denver. Where departures from this reference plane have been necessary they are specifically noted. In the consideration of the city wells the position of the strata under- lying the area including them has been regarded as horizontal, which is, at least, very near the actual case. There is, perhaps, a general dip of half a degree to the northward, or down the Platte. Flexures, if present, are insignificant. RANGE OF FLOWS IN STRATIGRAPHIC HORIZON, The thickness of the formations underlying the city of Denver and involved in the artesian supply is about 1,500 feet, of which 100 feet may be assigned to the Denver beds, 550 to 600 feet to the Arapahoe formation, ARTESIAN WELLS. 423 and 700 to 800 feet to the Laramie, making the bases of the formations respectively 100 feet, 650 to 700 feet, and 1,350 to 1,500 feet beneath the datum level of the Platte River. The vertical and lateral distribution _of the materials of this series of strata has already been discussed, but the distribution of the flows in connection therewith now requires brief consideration. Two broad water-bearing zones are clearly defined: A lower, corre- sponding in thickness and position with the lower zone of conglomerates and sandstones of the Arapahoe formation; an upper, between 100 and 200 feet thick, corresponding with the series of sandy strata in the upper part of the Arapahoe, a zone which at most points along the outcrop of the formation does not appear so distinctly developed as beneath the central portions of the field. Extending below these zones into the Laramie formation, there are but 10 wells, which vary greatly in depth and are widely distributed in location. They indicate little as to the value of the upper Laramie as a water-bearing series, but from the general succession of beds in the areas of outcrop it is doubtful if the formation becomes of economic importance until its lower, basal member of sandstones is reached. Moreover, the waters from the deepest wells in the Laramie of the Denver Basin are considerably mineralized. The flows recorded from within the Denver formation are but eight in number, only three of which are utilized. The irregularities which have been encountered in the vertical and lateral distribution of the several flows are not altogether indicative of equal irregularities in the water-bearing capacity of the strata or of the actual absence of flows in localities in which wells have not been sunk to the lowest horizon. On the contrary, it is highly probable that had all the wells been drilled to a uniform depth, as, for instance, to the lower limit of the Arapahoe, the water-bearing zones would have appeared much more unbroken. This is borne out both on natural grounds and from an exami- nation of the flows of the upper zone over those portions of the field from which the most complete information has been obtained. It has not always been possible to obtain data as to minor flows, these having often 424 GHOLOGY OF THE DENVER BASIN. been passed without comment in the search for greater yields. The value of their reeord, however, is established in such instances as are given, for not only is a greater persistency thus indicated for the flow, but the actual water-bearing character of the zone is preserved. A flow nonutilized in one locality or well may become of importance in another either through natural increase or through the diminution of other flows in this particular region. Examples of but two distinct flows, from different layers in the same water-bearing zone, either the lower or upper, are also not infrequent; in the vertical interval, however, at other localities, several flows may be recorded, This is in harmony with the distribution of the materials con- stituting the water-bearing zones, which is not everywhere uniform; the materials may vary laterally from an open, porous bed of sand or con- elomerate to one of much closer texture, almost impermeable to water, or, at least, by difference in texture and porosity, affording an easier channel in one direction than in another. The upper and lower water-bearing zones of the Arapahoe formation are separated by a more or less clearly defined body of shales of a general thickness of about 130 feet. Occasionally the lower zone shows a tendency toward an upward extension, while the upper one has here and there a flow slightly below its average lower limit. Within the median zone there are locally developed beds of a more or less arenaceous composition, which yield excellent flows to the scattered wells tapping them, and which apparently constitute an almost continuous water-bearing body beneath the lower or northern portion of the city, from Twenty-sixth street to the Grant smelter, and, again, in the vicinity of Argo. Considering the field as a whole, the midde zone may be regarded as in a measure blending the more highly developed upper and lower water-bearing series of the Arapahoe. Of the wells deriving their flows from the Laramie, the Evans is of particular interest as having alone reached in depth the upper portion of the coal series at the base of the formation. The details of this well are given in the chapter on individual wells. The other wells of the Laramie are sunk only to locally developed water strata in the clays of the upper member. ARTESIAN WELLS. 425 THE RELATIVE PRODUCTIVE POWER OF THE SEVERAL WATER-BEARING ZONES. The number of wells taking water from the Laramie and Denver formations is too small to afford material for reliable estimates of the productive power of these horizons. For the three zones of the Arapahoe, however, the number is larger, and approximate estimates may be inferred from the original yield of the wells, notwithstanding the fact that the actual number of wells drawing their supply from the several zones varies very considerably. The average daily yield of the Arapahoe wells, supposing them all to have flowed at the rate originally recorded, would have been— Gallons. Horthe mp per ZOne .Aon-. chap ee ee teen eae ee ete oes sear cee 64, 075 Hor the intermediate Z0n© . 555 sasoe se aae clase deena Saisie aes oss reese 45, 540 Forithe lower Zone. . jsds.0 Ceased flowing !...........- No decrease ncoececvece=—=- Flow not large........- Seccenieacee | Almost ceased '3...... fcnoss Largely decreased !5........ Ceased flowing!®...........- No decrease..... aceoSscacadt Ceased flowing !9............ No pressure; well pumped at present. Pres. head, 22’; 120,000 | Is pumped.............--.-- galls. per day. No decrease ...-.....--..--.| Pumping well”!..........-.. | | = Sood (0) snookdeeioecessicacasy Mace so sSsaagscoseessoce S009 45550 bo sconssbassscscd besed doyasce-.. eee nae Rate of discharge, head, or pressure December, 1890. longer in use.” Pumping; yield not known. Pumping well; yield 1,000 galls. per hour. 4 AtbandOnedienecamssasee se se Well plugged’. -.......2.-.. Pumping well®.....-....... Well plugged 2............. | Pumping; yield not known !? NOUMNOIA Ce scccesnecs cee Plugged! es ce..= Beeches Now pumping well; stopped flowing Nov., 1887.'7 Notdnmse ono oe oon cee Pom pede} sewecesesnso==- =< Undetermined, but very | Flow stopped in 1886; aban- much decreased. 5 galls. per minute ........-. Never doned; never pumped. Abandoned # ..............- used pumps. None since July, 1888; no |. Cost. |No. Tey eet $600 2 1,000; 3 1,200} 4 410 5 990 6 1, 600 vi nScastee!| 8 reese 9 8,090 | 10 3,000] 11 2,000 | 12 2,000} 13 2,250 | 14 Frc ase 15 Scanosee 16 mS SHOSE. 17 SAO SSS 18 eae 19 2,000} 20 Aoocoste 21 900 | 22 Water 434 No. Name of well. GEOLOGY OF THE DENVER BASIN. ARTESIAN WELLS OF DENVER—Continued, Location. Barkley Block........ Windsor.......----- on Mars, Middleton & Hunter. American House.....- Columbus House’..... Tremont House..--.--- Anheuser-Busch.....- Mon eit=saem ana macme) =9 Lindell Hotel ..-...... Milwaukee Brewery. - Spitzer ......- secancas Denver Brewing Co... Lion Brewery..-.---.. (Chit -esonasseqocds a Bomblitz............ ee Cor. 18th and Larimer sts. Cor. 18th and Larimer sts. Cor. 21st and Holliday (Market) sts. Foot of 17th st......-.- Wazee, bet. 17th and 18th. Cor. 16thand Blakests. Wazee, bet. 15th and 16th. 18th, bet. Holliday (Market) and Wazee. Cor. 10th and Wazee sts. Cor. 8th and Wazee sts. 8th, bet. Holliday (Market) and Wa- zee Sts. Cor. 6th and Wazee... Cor. 9th and Larimer. . Cor. 10th and Larimer. Cor. 10th and Larimer sts. Cor. 11th and Larimer. 12th, near Larimer. --- Cor. 12th and Larimer. Cor. 9th and Law- rence sts. Cor. 8th and Larimer sts. Cor. 11th and Law- rence sts. Cor. 4th st. and Grand ave. Date of sinking. Comp. Aug.4, 1883.| 602/430’ .. Comp. Aug. 30, 1883. Comp. Apr., 1884.. Dec,, 1883-.- <<. -.. Comp. July, 1884. - Comp. Aug., 1884-- Jan., 1886_..-..--. Heb: 1885--22-<.-< July, 1883.....--.. June, 1883...-..... July, 1885.......-- Latter part 1883... Comp. Oct. 7, 1884. OMY AS8se- saw cee Oct., 1884....20--.. Comp. Nov. 17, 1883. June, 1883 .....-.- Comp. Jan., 1885... Feb., 1885 Depth. Orig. 530’, now 900’ +30', 600'+27' .. 506410’ .. 520/410! .. 545/+20/ 356/+-10' .. 325/415! .. 314/412! _. 360’412' .. 333/415! 350/13! .. 330/+-15! .. 360/415 .. 354/418! .. 333/15! .. 585/422! .. 340/22! .. Orig. 360’, now 600’ +31. 294/425! .. 366/425" __ 315/--25! .. Depth below Casing used. surface of flows cut. 285! of 5§/ and 538! of |..........---. 4a", 10’ of 10’, 480! of 5§”, | 342/,515’,and 495! of 42”, 900’. 45! of 58’, 130’ of 43”, | 250/,335’,and and 460! of 3”, 585’. 362’ of 6” and 506’ of 4”.| 365’ and 506’. 14” nearly to the bot- | 315’ and 480’. tom. 25’ of 8”, 303’ of 58”, | 303’ and 491’. and 491! of 44”. OB ROti vee ao mem eee 185’ and 287’. 15! Of 31 ...cce-.ccncce|-s0cs-ceecoeee About 310! of 5g/..... 130’,190’,240', and 314’, 10) of BY). oa pr Sqr Eeaesosecoctes COM OP SG ce eeaesncscelte|~aracmsiancilan PAROS BI) TH eee ee ennc| Roc ores sccScce 37! of 34 .......-.----| 80/ and 300’. 4” nearly to the bot- | 180’ and 360’. tom. 286! of 4! ....0--enccee|---=- Sascescad 10! of 4” ........-200--| 130/,210/,and 300/. 40! of 7” and 550! of 5”.| 340’ and 530’. Cased with 3” gas pipe| 200’ and 315’. BNGUOnAUE=e. cece eee 150', 200’, 306’, and 358. DiS OF 2h eeeiaeeeaee eee ae BashOtl ale eaen nce omen cena eee 40’ of 2}/ and 260’ of 1}/ .| 200’ and 315’. 1 Decrease supposed to be due partly to filling with sand, partly to sinking of other wells. 2 At start rose above surface; at present about 15’ below. 3 Being sunk deeper. 4 At start had pressure of 50 lbs.; at present water 92’ below surface. 6 Pumped. 6 At start pressure was 20 lbs.; at present water level is 200’ below surface. 7 Now the Watkins Building. 8 Water 15’ below surface. ® Decrease supposed to be due to the sinking of other wells in the vicinity. 10 Much affected by the Mayer well. ul Water 15’ below the surface. 12 Has not flowed since Ice Co. completed their well, except at night for 4 months after Ice Co. started. 18 Diminished one-third in capacity; cause, too many wells. 14 Not flowed since June, 1888; at present 3’ below surface. Rose 7! above surface to July, 1885; then ceased flowing. ARTESIAN WELLS. ARTESIAN WELLS OF DENVER—Continued, 435 Depth of flows utilized. 342/,515/,and 900’. 585’ ....-200- 365’ and 506’. Cs =p se4 303’ and 491’. 287! caccces 314’ .....-26- eee 80’ and 300’. . 300! ...-..2-- 130/,210’, and 300’. 306’ and 358’. 366! ....ce0ee 315’ ........- Original rate of discharge, head, or pressure. 76,320 galls. per day ..... 300,000 galls. per day.........- conce Orig. press., 35 pounds............. 180,000 galls. per day; press., 20 pounds. Orig. head, 20'...2....-ccessssaccee- Press., 13 and 25 pounds.--...-....- 12 galls. a minute .........-........ Orig, head, 20 oon en eee eee nee ene 100,000 galls. per day....-.......... Not determined................ Sac Notestimated-..... cc. 2. oes oe aces Orig head |38!2ccesscceesacaneaeeees Not estimated.......... sscocteeeous 86,400 galls. per day; head, 15! to30/. Orig. head) 27! oo... Jo - =< aceee=- Asoc) 30 galls. per minute...-...----.. eee 150,000 galls. per day---.---...-. nse 2,500 galls. per hour.........-.-. oe 100,000 galls. per day.....---..-...- TEE EP acs LT SR Ae cisnsoccedaaas mec Orig. head, 40’........-.......... ase Rate of discharge, head, or pressure February, 1886. 77,581 galls. per day!........ 9) Not determined; flow still large. 100,000 galls. per day.® ...... No decrease............ saad Sree GO ees egene sss ae GID) ao co neioemececosconad a GIO s-po-osassceccosgacac Flow ceased; present.? pumped at About three-fourths of original.1° Not as large as at first...... Ceased to flow; pumped 20,000 galls. per day. Beye Ue Sareea 1 BU no =coee No decrease........- encceee- 3 galls. per minute...... meas Noldecrease--- oc. s oo saan Brac OBS ec ac55 enesccces Flow ceased; pumped at rate of 27,000 galls. per day.?! 1,102 galls. per hour? ....... No decrease.....-.... cosnse= Somes OOnewcacct ass ence sccam = 15 Diminished from 30’ above to 10’ below surface. 16 Flowed through 2-inch pipe 27’ above surface when first sunk; diminished to 3’ above early in 1889; ceased flowing July, 1889; now 6’ below surface. 17 Never used after flow stopped. 18 Water at present 18’ below surface. 19 Very much affected by well at Western Hotel until latter was cased. 20 Has not been used for 3 years. 21 Decrease due to the sinking of the well at the Lindell Hotel. 22 Flowed for one year, about 3’ above surface; at present 28’ below. Uses 200’ plunger. 23 Much affected by Spitzer and Lindell wells. %4 At present water is 8 feet below surface. 25 Flowed for one year about 8’ above surface; then used pump until April, 1890, when pump conld not raise enough for stable. Rate of discharge, head, or pressure December, 1890. Pumping well; pumps 3,000 galls. per hour.? Pumping well8............-. Abandoned ...........--..-- Pumping well; notin use at present.® No longer in use ...........- -| Pumping well............ ces IVE PEW) cemass cee tee c= Sevce Pumping well®.............. Pumped; capacity not more than one-tenth of original tlow. Pumping well; capacity not kuown.!! Pumping well !?.....20--.-.- Pumping well; capacity 50 galls. per minute.!3 Pumping well; water not diminished so far as known.!4 RON PI eS we asenaue oee- Pumping well!®_...........- Abandoned?” ............... Pom ping 18s eons cnn eesce Abandoned” ........... Pumpiug well®?......... aa Pumping well*............- Abandoned * .......... aaaa¢! Not in-use**-_-.-. .-.... es Cost. No. euseawet 23 $2,200) 24 Loess 25 sastdand 26 a 27 1,800 | 28 Sesseaes| noo) 300 | 30 700 | 31 700 | 33 400 | 33 Weaccees 34 400 | 35 Pe aesens| 0130 Nestbaze 37 600 | 38 1,500 39 600 | 40 s00| 41 510 | 42 400 | 43 150 | 44 26 Well flowed from time of sinking until Jan., 1889; water raised 40’ above surface for about 3 years; at present itis 4’ below surface. 436 GEOLOGY OF THE DENVER BASIN. ARTESIAN WELLS OF DENVER—Continued. Name of well. | Wells at waterworks on Vine street; sunk in 600’ deep. D. & R.G. No. 2....-.- A. E. Pierce.....------ Fleming ...-.--------- Miles ...-..----------- Bonesteel....-.------. KM0X « sccsnccene-s-=-- Stevens-.-...---...--- Home Artesian Well... Gas Works........--- Germania House.....- Steam Heating Co...- TGUNG wceneeeess= <=) EEICINAN ee ecmeleeln se diem os6c6S555sne54| INSorin ison seceeasec on Bareslaux -..-...---.- | MICE ee eee ene IMitae diewae sais ee aim = = Location. At Burnham..-.--.-.. Cor. Lincoln and Ells- worth sts. Cor. Sherman E}lsworth sts. Out beyond Expos. building. S. 15th, bet. Mooseand Deer sts. Cor. Deer and Alta sts. and Cor. Hallet and Pine sts. Riverside Cemetery. -. Cor. Waverly and 23d sts. Cor. Stout and 21ststs- Cor. Champa‘ and 23d sts. 25th, bet. Curtis and Champa. 10th ave., bet. Rogers and Gilpin sts. Near cor. Wewatta and 19th. Cor. 19th and Wanetta Cor. 19th and New Haven. On New Haven st..--- Wewatta. bet. 19th and 20th sts. Cottage Lane, Wewatta. Cor. Stanton and Me- Nassar aves. near 113 MecNassar ave..... On MeNassar ave....- No. 30 Argoltst..-.--- Date of sinking. 1883; from 568’ to 1883....--.---.---- Fall of 1884....... Fall of 1883. ...--- Comp. Mar., 1884. . Comp. Sept., 1885. . Comp. June, 1884. Ditall ses ieee Comp. June, 1885... Mar., 1884 ........- Comp. Aug., 1884. . Comp. Apr., 1885-. May, 1885 ..-.-.-:. Comp. June, 1884- Sept., 1885......-.- Dec, hesseeceeases Dec.; 1884 .....---- Comp. Nov., 1885... Winter, 1883 ...... Comp. Feb., 1885 .. Comp. Oct., 1885. - ‘ Depth below Depth. Casing used. surface of flows cut. BT sas 800/58! .. 720'+-70' 995'+-70! 670/+ 80! 610'4-45! 850/4-125! - Gli5! Of 54"... ...------ 5’ to first flow, 520! of 4”, and 720! of 3”. 100’ of 7’, 600! of 6”, and 700! of 4’. 64/ of 8',430' 0f 53”, and 655! of 43", 4} nearly to the bot- tom. 750’ of 4"; 3” nearly 520' and 720’. 685/ and 850" 300/,430',and 650. to the bottom. 725’, and 850! 500!-F- 130! -|. coe coco e Sacerce soos secrecy stetetae BOI 75! <4 -B1S cof Age oe ewes tenses | soso cere 860/+4-50! ..| 579! of 43” and 730! of | 310’,580/,and BEE 730". 670/445! ..| 570’ of 4” and 670! of 3.) 308/,590’,and 670', 700/4-45' ..| 580! of 5§/......ee-0ne-l pate oe 750'+-40' 550/ of 5g’ and 640! of | 550’.640’,and 40, 735!. 685’+150' .| 8” to bed rock; 670! | 600’ and 685’. of 54/, Orig. 500’, | 25’ of 10’, 300' of 6§”, | 320’ and 495’. now 610! and 449! of 53”. +0. S410 Dees | AMOR ae aceneenene eel eaas aS 325/+0.-...| 8” to bed rock and 248! !...... Cacose so of 6”. 309'4+0....| 13/’ nearly to the bot- | 300’......... tom. Orig. 310', | 24” nearly to the bot- | 190’ and 300/. now 535! tom. +0. 825! 0)... -|) 200! of 24 os Sseanciae|sacwaice nine nria 300'+0....| 3/ to near the bottom.|....-...-.+-.. 304/+0....| 38! of 24" and 265’ of | 150’,200',and 14". 265'. BPAUIEUE Aer FP ooseenecan sas Bel) oft ann sake! 300'40....| 150’ of 14”..-... AaCosoF 245' and 300. 1 Abandoned for the last three years; pressure diminished and the water level sank below the surface. 2 Cost about $1,200 each. 3 Decrease supposed due to filling with sand. 4 \t start rose about 30! above surface; gradually diminished, and stopped flowing in 1886. Pump put in, but abardoned in 1888. 5 Seemed to be affected by the sinking of the wells at the waterworks. © Decrease supposed to be due to defective casing. 7 Flowed 22! above surface at start, but fell off rapidly, and in 18 months was even with surface. Pump set and § Pumped until sammer of 1885, when it gave out. © Decrease due to the sinking of other wells and filling of this with sand. used until 1888, when, finding it difficult to pamp enough, well was abandoned. ARTESIAN WELLS. 437 ARTESIAN WELLS OF DENVER—Continued. nL epee ce Original rate of discharge, head, | Rate of discharge, head, or | Rate of discharge, head, or alized: or pressure. pressure February, 1886. pressure December, 1890. Cost. | No. 45 46 aconsccesceeny Flow from first well, 156,000 galls. | Total flow from 5 wells, (O) 2) 47 per day. 200,000 galls. per day. 48 49 Ser odeesaosoS Orig. head, 30/; good flow. .....---- Decreased 60 per cent’...... Abandoned‘............. ---| $1,800 | 50 a0 ieeerieaatan 100,000 galls. per day; press., 30 | Nodecrease.......... 51 pounds. 850'.........| Not estimated, but very large...... Ceased flowing; pumped at |..... Canc RSC SRB aS sci beer 52 present. Bavremtl Goll-| ahojaio Palla, Per ky... secon sameuu eNO COCLOSRG 6 occ cae wens [a nccc onavcatacecadacccecmssce. 2,500 | 53 600’ .........| Not estimated; good flow.....-.-..] Almost ceased §............. Abandoned ................- 1,100} 54 850!.........| 21,600 galls. per day ............ --.| Ceased flowing?.............]..... WO. eee ccaaascease esse 3,500 | 55 } aes Sone Bea ENON OWeracenaessaacseescdaa nena (Eis GON emanates ne anne | as one 56 Bibi oee sees == 500,000 galls. per day; press., 35 | No decrease ................ Stopped flowing ............ 1,000 | 57 pounds. ; MSU) ccccceces 140,000 galls. per day..-.......---.- Notestimated; still large...| Abandoned .............. e--| 4,300] 58 590/ and 670’.| 147,000 galls. per day..... Se noo-nesi- 75,000 galls.?................ (19) 2,000} 59 Soteneeaeeas=-|) OLS. DLESS., c+ POUNUS. -nncecnncces Very little decrease.........| Abandoned?! ..-............ 2,200} 60 550/,640',and | 80,000 galls. per day.-.....-.-..---- Wo decrease. .......... Rob ead Bates Cabseee re ceerseeoc eth ee 2,300} 61 735'. 685’ .........| 45,000 galls. per day.......-....-.-. Slightly increased; 50,000 |..-..........-----. 2.222.202. 2,500 | 62 now. 320! and 495'.| 300,000 galls. per day......--.-... --| 150,000 galls. per day '......| Pumping well! ............ meceaces| || Lik? cnebeecsseas ce 1,500 galls. per hour........---..-..| Almost ceased flowing......]| Abandoned June, 188814..... 350 | 64 ee 200,000 galls. per day...--.-..--.-..| 50,000 galls.!9...............| Abandoned !6...............!........1 65 | 800’ .........| Head, 25’....... BE caS CRA IOSCEEOS AIC BNO /d oprease j= tonwewe ce wner|wanmaclacpmns oem see tes scenes 500 | 66 anbree sees 29 galls. a minute ..................| Almost ceased.....--....... Pumping well!7.......2.... 465 | 67 SeOnodasctnsod Orig. head, 15’...........----- s=ssea|| NO decrease ....-.5-5...-.-0]--<-- (One -ckCESenop-cecocee BECHtseY 68 oo nee Water rose into second stories...../ Head, 2! or 3’................| Abandoned..-..............|........| 69 Dire 2 ae 24 galls. a minute .-.... stteserascse 10 galls. a minute ..-........ (Sy See Teeemoes 70 Jerre eee lac cad cniec cueneueeeee Sascccaa- 1S GUM oe escioe (Cao NIT gee ere 71 800) ~~ <=. - es Not determined...-....- weseercenn~ Decreased 4------......... ae (3) 200 | 72 10 Stopped flowing and abandoned in 1886. 11 Has not flowed since 1886. 12 Decrease of 50,000 galls. when neighboring wells are being pumped. 13 At start flowed 20’ above surface; at present 25’ below. 14 Never pumped. Water at present 12’ below surface. 18 Very much affected by Union Depot and Gas House wells. 16 At first rose 20' above surface, but at present 25’ below. Bored, near by, another well in 1889. 17 Tn 1886, upon stopping of flow, well was deepened to 535’ without obtaining additional water. At present, water Is 55’ below surface. 48 When drilled rose 15’ above; at present 8’ below surface. 4 19Tn the Italian quarters. Could get no information except that no artesian water is used now. 438 GEOLOGY OF THE DENVER BASIN. ARTESIAN WELLS OF DENVER—Continued. Name of well. Electric Light Co-.--- Schiindelholz....------ Belisle..--eeeese------ City Laundry....-.--- Universal Sampling Works. Swift ...ccc----0-0--- Bennett...------------ 30th and Arapahoe sts. Colorado Iron Works. A dairs.sccocceccovsa== Nichols ....ccecee----. Morrison Bros .....-.. U.P. Hospital .....-.. Grant Smelter.....--. MeNassor...... Morrison ..... D. U.andP. R. R...... Smedley .-....00...--- Jobn Mullen......-.-- Doyle & White.....-. Sanderson ..--ee----- Green. ...ccc--cee----- Location. Foot of 22d st.-..-.... Cor. 26th and Holliday (Market) sts. Lawrence, bet. 25th and 26th sts. 27th, bet. Larimer and Lawrence sts. Cor. 28th and Blake. -. 28th, near Champa .... Cor. California and 31st sts. Cor. 30th and Arapa- hoe. Cor. 35th and Holliday (Market) sts. 32d st., bet. Lawrence and Arapahoe. Cor. 33d and Wyn- koop sts. Blake, bet. 35th and 36th sts. Cor. 36th and Wazee.. Holliday (Market), bet. $6thand 37th sts. Near the Grant smel- ter. At smelting works --. At shops in North Denver. Cor. Clear Creek ave. and Backus st. 3ell ave., bet. Wan- less and Kent sts. On 19th, near Central. Cor. 18th and Boulder Sts. 18th, bet. Central and Boulder. Date of sinking. July, 1883......... Comp. Nov., 1883.. July, 1884....-.--- Oot., 1884 ......... Winter, 1883 ...... 1883... cccccccccce- Aug., 1883 .......-. July, 1884..-...... Comp. Mar., 1884.. Summer of 1884... Dec., 1883 .......-- July, 1885..-...... Comp. Aug., 1883... Comp. Feb., 1884 .. Latter part of 1884 1883..... ecooaccos4 Sept., 1885 ........ Feb., 1886; finish- ed 1887. Dec., 1885 ......... Apr., 1884...... sidc, Sept., 1885 ........ Saree (ig ssctcesonos Feb., 1884.... Jan., 1884 ........,| Depth. 330/414! .. 416'423" .. 420/480! .. 406/434! .. 400/40... 457/446! . 450/441! .. 620/420! .. 400/45"... 587'--5! ... 420’—10' .. 392'—5! .. 366/—5! ... 400'/—5! ... 675'—5' ... 620/—30! .. 300’—10' .. 400’—10! .. 306'4.0/ ... 647/450! .. 300/-+-25' 320'-+-25' .. 579/430! 500'+30' .. Casing used. 6" drive pipe and 285! of 44/, BOOUOLIS lanaeeeatslcsts Cased 50’.............. 5’ to bed rock and 300! of 33. 250/0F 5B... 2... conan aU OL Goatees 95’ of 6”, and 44” nearly to the bottom. 850” of 54" and 610! of 43M, 50/ of 4 and 339! of 2’. 90’ of 8//, 250’ stove- pipe, and 525/ of 5g”. Gi bly 2k Se ee .| 50! of 5§” and 340! of 3h". 316! OF 3! orc c ees canwes 60! of 6” and 350’ of 24”. 390! of 5§/’ and 640’ of 44", 387! of 7§ and 556’ of 5g! ay (ey gab SRR aac 23” for 200'.. 2” nearly to the bot- tom. 400! of 4! .cccecne---e- ee |d00to Fats eeeemeeeeeee ‘ S0Nofmi soso eeeeaee --| 23’ nearly to the bot- tom. . 24! for 18! os scenes eeu 1 Decrease supposed due to the sinking of the Gas House and Steam Heating Co.'s wells. 2 Water sank considerably below surface after 1887. 3 Decrease due to the sinking of other wells and the filling of this with sand. 4 Stopped flowing at a date not known. Depth below surface of flows cut. 448’, 100',375/,500', and 610’. 160/,220’,and 370!. 250/,375/,and 580/, 250! and 375’. 180! and 375’. 400’ and 675’. 130/,180',240', 325/, 575’. 188’ and 288’. 240', 375’, and 575!. 300/ and 500’. Small hand pump put in, but unsatisfactory; well abandoned. 6 Flowed for 6 months after drilling: then pamps used. In 1885 fine sand came up, but well pumps free now. 6 Failure supposed due to leaky casing. 7 Well has not flowed since 1888; water soon sank to 27’ below surface; pump was then put in. 8 Decrease supposed due to filling with sand. ®* Water at first rose 140’. 10 Flow stopped 1885. Never pumped. Flowed for 2 years 380 galls. per minute. Water now 6! below surface. 1 Stopped flowing in 1888, and pumps used. At present 20/ below surface. Two upper flows were shut off. 12 Never pumped. 13 Decrease supposed due to the sinking of other wells. 14 Plowed with great pressure at first, which gradually diminished, and ceased in 1888. Fell 12’ when Nichols’ well was finished. At present 25’ below surface. 15 Used two flows; upper one gave out in a year; lower one flowed till 1887. Well was cleared, but water did not come to surface again. ; ARTESIAN WELLS. ARTESIAN WELLS OF DENVER—Continued. 439 Dae Original rate of discharge; head, | Rate of discharge, head, or miitireds or pressure, pressure February, 1886. ---eeeeeee-e--| Orig. press., 30 pounds..........---| Press.,10 pounds! .........- caeetta eaeoes-| Orig. head, 40! .-. eeeeee-| 400'+100! - Summer of 1884...) 541/+75! .. Early part of 1884.) 700’-+60! .. 1883 Svaweceesesee 350'-415! .. AS OPS | 331/40... [agg eee eee 515!4.25" .. Tee ee near 375/412". TORBeee sa seeceee| i310 Oe aeset we eee 679/430! .. Winter, 1883...... 516/25! .. ieee ee ae 325'4436! .. 1885. ...| 325/435" 1SeStAeneaee 680/-+18" .. Depth below Casing used. surface of flows cut. S00UNOr CA and Sls |psenensaceeeee nearly to the bot- tom. BM) cecpnuscacccassse~=|-aenne eecmecie 3h’ for 100’, 24” for |........ sceene 100’, and bal. 14”. SBS OF Ig! ccenccasesoc|-eousescacazen 4'' nearly to bottom ...|.........-.... 23” to bed rock .....-- 170’, 180’, and 280. 5§” to bed rock and | 170/,180/,and 34! of 24/, 280/. 6” to bed rock and 70” | 170’, 180’, and of 3}. 280/, wee ee nnn ewww ewes e ee! sens enna eseees 750’ stovepipe 660/ and 750’. 90! of 44 and 350! of 3’’.|......-....... 100/ of 8’, 380! of 43, |...202--sseees and 650! of 3”. 621! Of 5§! .--eeeeeee--| 350! and 630/. 400/ of 5§/’ and 4” near- ly to the bottom. Deine De ees ee eel Ma See BM Lenwccccceccccnceas= sccccecccccccs| ‘ 775’. Stopped flowing when Zang wells were sunk. Abandoned in 1888. 4 Due to the sinking of a number of wells in lower ground. § Stopped flowing in 1885. Now 72! below surface. 6 Decrease supposed due to a rupture in the seed-bag packing. 7 Rose 32/ above surface at start; then slowly fell and stopped in Sept., 1890. Now 2' below surface. Fell 6’ when Zang well was used; also known as the Denver Land and Ice Co.'s well. 8 Had good original flow. Sunk to 525’ in 1887, Stopped flowing in summer of 1889. Windmill now pumps water. ® Flowed 30! above surface at start; stopped May, 1890. Ice Co.’s well is not pumped. At present 16’ below surface. Much alkali in it, which rendered it unfit for drinking purposes. Will rise to surface when 10Probably sunk deeper in fall of 1888. Flowed until June, 1890; at present water stands at 15’ below surface. Will rise to surface when Ice Co.'s well shuts down. 1! Rose 40’ at start, now 12/ below surface. Will rise to surface when Ice Co. shuts down. Occasionally overflows during the night. 12 Water at present is 30/ below surface. 13 Decrease due to bad casing. M4 Stopped flowing when Weir No.2 was sunk. Water now 50’ below surface. ARTESIAN WELLS. 443 ARTESIAN WELLS OF DeNvER—Continued. Depth of Orici . ; ; riginal rate of discharge, head, | Rate of discharge, head, or | Rate of discharge, head, or | « ot ies . & or pressure. ‘ : pressure February, 1886. pressure December, 1890. Cost. | No. sees aoss meeser|| OFS. Pregs., 30 108 -e.n--ccsecsces=- ae be one-fourth of origi- | Abandoned?.... -| $1,200 | 120 nal. ssenecceeese--| Yield 1,800 galls. per day *.........| No decrease........--------- Abandoned in 1887 ........- 500 | 121 brosotéoatance Orig. head, 20'....eeseccee-e---e----| Coased flowing 4.......----- Pumping well® ......s0.++--|----- oo.) 122 secccceneee--| Flow not i cea | Slightly decreased © ........|..... QU! aadennndeaene decease 200 | 123 panceawedes=-= | Not estimated ......cccce---------- Perceptible decrease!....... .---- (ors hoasaicaeans 500 | 124 hat and| Not determined............--....-- No decrease ....- aanas=asase 875 | 125 0. 170',180',and|.-... fi Asesso-eeeeacnascaasot8 ----.| 10 galls. per min ............ 525 | 126 280/. | 170’, 180’, and |..... GI) -sssenseecceteccconescensco: | No decrease ............---- 600 | 127 280', Unease Seteeaec|o send Ol=evancansee-ecesccatae sae LOCKS HOW Pi reaces sass c'= 2 sop emen fies (EE ese Asodsd pene Ol ecenn aacsseneo Aa Scoaa. Bases Hobe! do.13....... presd-ntonee Seep manned pecnscesecoe--| O1g falls. per Minute -....-..-...... iearoae dO ..-.0c--cccecce--2se0-| Abandoned 5............-..| 900 | 130 SEee eee crate Press., 55 lbs.....-.. Cero s=canshccr | No decrease ......... eo-e---| Pumping well!¢ ............ | 2,100 | 181 | 350’ and 630’.) 500 galls. per hour..........-.-.-.. Lessthan3ljgalls.perhour’| Pumping well!*............- sacecis --| 132 maseusecccace=|sosccnnscees enwesscee se sabres aces =a NOU OW eae aeleie ns ames! CORRAL Ret aoe ee eee Roce BRCe Ene eee Heeeeee 133 popscoocereced|| Ofori EU UE Rec coocceeceeee Ceased flowing "®......e0e.--| Pumping well” ............ 1,500 | 184 700" 205.2.) GOO fOW ...20--cscncc-ce--------- No decrease .-.... esncvecase|sadeonericer casas Bone Secon ees eee] 135 Rotten tae ae Good flow; head) 30) ono occ cacao cnenrceccccuacevecssecavesse== (74) pee second pets) sone asco-nass Water never came to surface...... Considerable decrease......| Abandoned™ ........ ...---|---+.---| 137 sence socodeed | Fair flow. .c.cccosse- ceca Savacat seesak ccavcacase wesnasinieass ck Pumping*’..... pactccsccence! Feccsns- 138 Pecetaacnscass| Hscet darts," 25,23 pee nme eee eee Se cactls seceedecesaco conse? bsceotad EEN) Sonneconeoocss Water never came to surface ......|..-...----=-.------+0«--------| Abandoned ™ ............-.-|..--.---| 140 Lasser bAcenecn Oniginally flowed “coc smessse sencecclonces=s ave =swee=eiveencowans ec HT) hee ees eee baecese | 141 eaten er: IKOnipt head 70'S... essa es eee eee ee ster a et ene Leg aie eats. oes 2 Pace Angee (Cont St Ar epee seSeecrc cones. cent hesesc hosted Lo sscesoncced PU an fot ee ase srarcasace| Flowed ... (79) Pomped ® ons. ccoscscs 15 Flowed till 1885, when another well was drilled. 16 Stopped flowing 1888; well cleaned and sunk 8’ deeper, but did not flow again; pumps then set. Water now 12! below surface. 17 Pumped at the rate of 500 galls. per day. 18 Cleaned and deepened (Oct., 1899) to 654’. 19 Pumped at present. 20 At start flowed slightly above surface. Stopped flowing 6 months after sinking. 21 Has nut flowed since 1888. 22 Fell considerably in 1884. At present 28’ below surface. 23 Well has not flowed since Jan., 1889. Water at present 16’ below surface. 24 When sunk water flowed above surface; it is now 12’ below. Flow suddenly ceased when Ice Co. well was sunk. Water considerably below surface at present. Now 60’ below surface. Water soft at first; now hard. % At present water 28! below surface. 26 Stopped flowing in 1888. Water now 25’ below surface. 27 Flowed at start 70’ above surface; gradually diminished; stopped flowing in 1888; now 12’ below surface. 28 Rises 4 feet above surface. Flows even with surface. ® Rose above surface at first, but gradually diminished in 1888 and stopped flowing. Water now 16’ below surface. 444 GEOLOGY OF THE DENVER BASIN. ARTESIAN WELLS OF DENVER—Continued. ayes Depth below No. Name of well. Location. Date of sinking. Depth. Casing used. surface of flows cut. KE. | Evans .--..- coganoeaee| 14thand Arapahoests-.| June, 1886-..-----. 1,140" ~ =<. 765' of 34" and‘1,100! |. 2-2-2... cne ot 24. 401 Comp. fall of 1885-| 300’....-.- 24) towped TOG Ke ecneo =| one see = mee 402 Fall of 1883 ....... 300’ ....... BUito ed rook ces sa 4st eee 403 Qot., 1885) -=2--- =<. 860! 222-22 3” to bed rock A04 || Se enor areca ase ese ees nee 0 = a eeineeeeees|aewe= (Gi socanecascoe 340! ....... 3” to bed rock 405 June, 1885- -| 360’ ...... 3’ to bed rock. 406 ULE eee ic opeancosne bse5 ease] Roc scoscdomee sashes 5 esesessosasss 301 | A.J. Konzie ......---- 15th and Wynkoop ..-} Jan. 15th, 1890....) 605/410! -.) 33” ....... 22... eee.) --- eee eee eee 302 | Broadway Theater. -..| 18thand Lincoln ave..| June, 1890...-..... CFB OM Serer Gy ee Pe ROSS Cee eo Ascad 303 | Wall & Purcell..-...-. 15th, bet. Wazee and |} 1888....--.-......-] S40! 10) oo nnn enn nen ew eeceumn Wynkoop sts. 304 | Armusson.........--- Cor. 12th and Wazee | 1887....-.....----- 536/415! ..| 34 to 2/....... sscbod possacans ee sts. 305 | Haller .-----.....---.- Cor. 12th and Larimer-| 1888......--......- 1,100’+20'.| 3” for entire depth. -..| 300’only low 306 | Electric Light Co ....| 6th and Lawrence ..-..| Oct., 1889 ...---.-. 630/425! ..| 59” to 44//.........2-2- assm acne wie 307 | Denver and South |.<.........----.-.- z=-=| Mar.) 18900 2--s--e5 6103/+25' _| 7 to 5”_.-... Rea |asdscaveeoases Park Shops. 308 | Tramway, Electric | Grand ave ---....-..-.. Apr 18902 sooo acm GPO Ep] ese |i Ua ar USe SoS s ea a Re eee Power House. 309) | Harman’---- 2... os} (UM) 2 Ass been ascasc TSR eee -| 350/+20' ..| 3” to 13” cece 310 | McClelland ..-.-...--.|.--.. Osos ee ~eee Mar J887-neasece | Gob o0 lee | pose sabes eee esas : 311 | Hanson Apr., 1887......... SUS ELA AC LE ee Ort 312 | Browne DE esc ce cas me pons OLE) CEM iy PU ae eee | RES erconas 313 | Artesian Ice Co ......|.-.-- GU} S-esecteeenacdes Feb., 1889. -.... -| 618!-+-20' ..| 68" tos”. - ee. 31L | Crisman.-............ 10th and Water sts...| July, 1889- 435/415! ..| 24’ for 400’. 315 | Excelsior Laundry ...| Arapahoe and 12thsts.| Apr., 1886...... 607/435’ ..| 4” to within 9! of bottom. off. 316 | West Denver Gas | Foot of 8th st.........]....- (UD a-nectece a8e 650/+10/ ..| Outside, 8” for 300’; |.-.-...-..-.-. Works. inside,’6” to bottom. 317 | Zang Brewery No. 3..| 7th and Water sts....| Aug., 1890 -....... 660'+50! ..| 7§” for 400’, 63’ for |.-.......... oe 100’, and 5g” for 160’. 318 New, Mining Ex- | Arapahoe and lithsts-| Oct., 1890 -..... ---| 608/440! ..| 53” to 44...... ececocd scrodonesecsae change. 319 | Denver City Cable Co.| Lawrenceand 18th sts.| July, 1890-.--.. «--| 650/436! ..| 78” for 220’, 6§” for |.--...... oaced 220', and 5§” for 210’. 320 | Mack Building ...-..-. oe 16th st.and Cali- | Oct., 1890 ......... A | ee oncce anos seapaoaconass ornia. 821 | Equitable Building...) Cor. Stout and 17th...| Sept., 1890 ..... e--| 610/445! ..| 69!" for 254’, 59” for |......... moter 189, 44” for 167’, and 3” for 60’. 322 | McPhee Block........ Cor. Glenarm and | Mar.,1890....-.... 750'+-45' ..| 7 for 250’, 5” for 250’, | Shuts off 3 16th sts. and 4” for 250’. flows. 323 | Kittridge Block ....-.|....- (tty Se amanenscacsses| Fall of 1889....... GE GA) Fs ese eyes sec bacremascosad5 324 | Railroad Building ....| Larimer and 15th sts..| Fall of 1887-...-.... G00!-1740 SiG ea ananneecacceavenalecaen sana ems 1Use of well stopped on account of bad water in lower part. Casing pulled as far as the 690’ flow, which alone is to be utilized. 2 Being sunk. 3 No increase or decrease observed as yet. 4Flowed for 9 months from date of sinking. At first 12’ above; now 4’ below surface. 6 At present water 12’ below surface. ® Distance from Western Hotel well only 8’. 7 Flowed 11’ above surface for 10 days, then sank below. § Never flowed. Water came within 18” of surface when first sunk; now 14’ below. ® Has not diminished. When sunk water rose within 12! of surface. 10 Well flowed 20’ above surface through a 3” pipe when first sunk; diminished to 10’ above surface April, 1890; now 4’ below. ~ | When first sunk water rose 30’ above surface, then gradually diminished to 10’ above. When Ice Co. began using their well, water sank to 6’ below; when Ice Co. shut down in Jan., 1890, well flowed as before for six weeks; then Ice Co. started again and flow ceased. pened Original rate of discharge, head, | Rate of discharge. head, or | Rate of discharge, head, or ifilizedo or pressure. pressure February, 1886. pressure December, 1890. socces Setenetee= |) CLODMMO Wrosonencoanea cap aceaen ene a2) sees scennerces= ce saceaetedat -| 40 galls. per minute. -| 50 galls. per minute ARTESIAN WELLS. 445 ARTESIAN WELLS OF DENVER—Continned. 120 galls. per minute ...... erenccess 20 galls. per minute 30 galls. per minute. 18 galls. per minute. 75 galls. per minute...............- Orig. head, 12' Raised 35’ above surface until 1888 . Never flowed....-.....---..- cocoa Yield, 700 galls. per hour......-.... Orig. head, 20/ | Orig. head, 30’ 60 galls. per minute No decrease Cost. Pumping welliaesscnssencee leases ems 301 /PROM PING Boo celwewe seriede elena aan 302 | Pumping well4.............|--..---. 303 nosed (OO -oAp soe Sscrenocnen sy caccses btn! ee OR eR OR erc coco seed Bemeccod | rth) oe HO berceesserenoponsascs Ceossoey (Cid baa: CO RSS Sse cee gaa seesed Eceoo sad fii Seiode Cl es Seto ssecace Poocemae | iit! weocooceecboee Onipahesd yo aeeceenese=aaeeee aes sendoenc ones Orig. head, 11’ TAU ea WE i ei he sees see] bees Sas Scoeeco se ccossacceee Orig. head, 4’.......... Orig. head, 20' scecaesseusined Orig. head, 25’ cenecinenecuaise | islowa,4/iabowe surface .<=<-0s<-=8|-saecee-saeeese es enone Pumps put in Nov. 22, |........| 317 1890.18 eee ana na ie idiot How <5 --eoeeeeses=eeeenee | eneteaee eee eee eaee | Pin ps motnet placed sss--s|s-20- 5-7] S16 Jghc5 =e 7530 ING ven MO Wed)-<<- cere. lem aoe oe mente erien seein ae ea enee ea aee | PROM PMP) advataccsnusccpstecesso~s| O19 sseda sep leased Hess ORR Se eras asasoscec See Scec a bsaccscosc: asgeccocse as sod Pumps not yet placed? .....ses-e-. 320 eee Seems aesia2 U0 <= sacbien vamcnn cen =nmenemaaan | ee eraen naan tee aaa af INDIE ecanecncscseeccsicamas sss (OOk ne acceenéas poss ies eee eS can ene sere Sse || - eos eSoonsss sactoccocescts| Ses a hace SS seccnesaeeascvepraaced |e) Scone Sacdeay SRE (iby ACES SaaS sono sass ano8 83} |sSot orcccen ae ssces toto sated bese (OS hea tes Soe emcees |p Laotécsossessy BOW: cocnclace she scone e eee eRe CS eee ae ae eee [ee ed O Arete cee es eho t ele Ts BOE 12Same remark as for McClelland well, but stopped flowing 1 month later than that. 13 Well flowed 11’ above surface until Ice Co. started; then sank to 4’ below. 14 Water came to surface when first sunk; at present 5’ below. 15 Well raised 4’ above surface when first sunk; stopped flowing Oct., 1887. 16 At start flowed 20’ above surface; at present, 9' below. 17 At start flowed 25’ above surface; gradually diminished, and at present a 75’ plunger is used. a considerable amount of soda. ‘8 Throws up considerable sand. Had quite an effect on Zang well No. 2. 19 At present water 60’ below surface. 29 At present water 45’ below surface. 21 At present water 50! below surface. 2 At present water 13’ below surface, At present water 20’ below surface, 2 When sunk came even with surface; at present 6’ above. (See No. 119.) Water now 7! below surface, Water contains 446 GEOLOGY OF THE DENVER BASIN, ARTESIAN WELLS OF DENVER—Continued. Depth below No. Name of well. Location. Date of sinking. Depth. Casing used. surface of flows cut. 325 Denier Tramway Co. Broaiway and 15th | June, 1888..... cong) TSE || GP ee en be in ae No. 1. st. 326 is Tramway Co. | Grand ave........---- Jan., 1890 .-.... ~--| 600'+-20! .-| 5§” 2.5 cee acemeeten|waeee = Ao Sere 0. 2. 327 | Stacy No.2 ..-.-... ---| 2lst and Market (or | 1887.......--...« ae C00 OTe oon reee Stacdscscsceaa|co canta oeeee Holliday). . 328 | Windsor Hotel No.2..| Larimer and 18th ----- 1g) ese 530/431" --|.-.... Oreo SccreR ast ic | eessac ccaeeoe 329 | Crescent Mills..-....- Stanton ave. and 19th.| Sept., 1889 ........| 393/40 ...| 43” for 200/............|....-..----.- 330 | United Oil Co. No.2..| Wewatta and 21st .--.| 1887......--... eo--| 668'40)-...] 5g” for 150! and 33/" |-..- 5. 22 for balance. 331 | Steam Heating Co....| 19th and New Haven..| 1889.......-..-.-.. 340/40... 332 | Monat No.1........... Blake and 26th....- woe] 1S8G-—neccs= 400'+20! ..) 34 .. ecg MonatiNOnenssacce ems |saeiee O Olena emlsiae n= --.| Nov., 1890 400'+-20! ..) 5§” for 320’. -..-....--- sab scs 334 | Harris.......------ ---| Delegany and 19thsts.| May, 1887 ........-] 100/+0....| 4” .......-.-.---.----- ¥ 335 .| Central and 18th...-.- Oct., 1890 ..... eee-| 463/430! .. 336) ||) Se@well\-cossseocosne ens Kent, near Witter -...} Sept., 1888 ..... ---| 425/450! .. 337 | Trowbridge.....-....- Witter and Kent......| Aug., 1888 ........ 473/430! .. 338 | Kountz--.---------.. .| Clifton, near Scott st..| 1889......-.... wee-| 900’+30/ - 339 | Wightman....-... acne. Dement ave., near | 1889.......---. eoe-| 340/465! ..| 4 2.222 RACHOOSeens se ws coeeeneee ert. B20) | Tatil eee ...| Howard, near Morri- | Spring, 1887----.-- 440/490! ..) 3” for 45’ and 13” for |...........--- son. balance. 841) ||Reabodyeeeceseseen === Howard st........ ese-| June, 1888 --......| 530/490! ..) 13! .. cee cennnne-e---- PeSCHEeaecoose 342 | Denver Brick Co. | River front --.-.-..- wePLSSGnaeeceen cn Spec cra e ts GWE AD aps saacpsescosasc| easoasaace aS. No. 3.1% 343 | Denver Brick Co. | South of Jacobs addi- | 1887......---.. eee ee| (480! 26023 bine co ase semnno ocean | Seema stne ieee No. 4.'8 tion. 8344 | Grant and 6thave.....|.......-.-.- fcgosoncens BN USE Gr ee aesoeonodonece qsoos|Sancrnossaeseo- 345 | Villa Park No.3...-.-| Barnum’ssubdivision- 300’+100' . _ 346 | Fletcher.......... -.--| Garden Place......... Nov., 1889. ........] 500’/—30'.. 347 | Boivier------------<-- 5th, near Central......|....- GO) a-cresca senn| 000'—39'. | owe 348 | Morrison.. eee.) Near Argo seceee=| 400’—20'_-| 241 for: 200"... 2. SOR Ace| boone tinastoces 349 | Anderson..........2../+---- domenser PANNE PAN Ga) | OAL! 5 - ne eco ea seoace) Heoaesecsnase7 350 | Norber Ice Co......... 275/— 20! .. 351 | Gunter ........ccceee- Sept., 1889 ........| 580/+-30! - 352 | Boston and Colorado 1887...0-.----22---| 800/430! - Smelting Co. Dd) | MHISNOL = nistelasse al ee--| Cor. Goss and Pros- | 1886......-..... See eZ 00) DOC ta tineeertarete PCOS EE Comte) reeinnnocoacd pect aves. Bods (eDAvise-setescesoes ----| Goss and Greeley sts-..| 1889..... SSaccaae --| 557/450! ..| 5§” for 300’ and 44” |....... Seasons for balance. 355 | Richardson No.1..... (2) Spring, 1§86....... DAU ann 3” for 515’... 356 | Richardson No.2-.... yy se GO Seccec ceecne PN ean BY os sateen conoccaose| Has 357 | Globe Smelter ........] Globeville ........ eoe-| 1887....-.2..000---| 505/—35! ..| 5§/” for 425’ and 44! | 175/,425',and for balance. 505" 1 Water at present 30’ below surface. 2 At presentl0/ below surface. 3 Had a good flow at first, about 30’ above surface, but in 1888 fell below. In an attempt to drill deeper, drill broke, tools wedged, and well was given up. 4 Drilled near the old well, which was abandoned. 5 Never rose above surface; at presentl0’ below. 6 Plowed at first, but gradually diminished. At present water 30/ below surface. Bored to replace No. 1, which was abandoned. 7 Never flowed; at present 20’ below surface. 8 Had considerable flow at first; gradually decreased; stopped flowing in 1888; now 18’ below surface. ® Never flowed; now water is 18’ below surface. 10 Never flowed; at present 12’ below surface, 11 Never flowed; at present water 20’ below surface. 12 Water about 60’ below surface—considerable iron in the water. 1 Water at present about 60’ below surface. 4 When sunk water came to within 5! of surface; now about 18’ below. ARTESIAN WELLS. 447 ARTESIAN WELLS OF DENVER—Continued. Depth of pe A F . _ Original rate of discharge, head Rate of discharge, head, or | Rate of discharge, head, or 5 ntilined = or pressure. 5 | pressure February, 1886. pressure December, 1890. Cost. | No. eae Rens :chod | AGGRO Ga asses AS cance sass Kaos Se coascossssesencacscaascd [entity ee ee 8) a | pa) sence catered) sc OO Asceccacteassattesassttnaced ick seca enseeneeasccacnecd RES Adi ass eRe ees eed a EP moos Seeeseeret OFie. NERC, OU 7) F000 NO We nase an=on| cneenneenenepaaacecencenscgee-} A UANOONES. cence sencese=n-=|amna-nes|)Ger arid ns SS SSS ee Fe eoreeeneectnc arse bese acdatebées=tosssseececodd Pumping 4...... Sceccboseess Sone e<--| 328 ----«--| Never flowed ..... SOS ASOT OTE RESSS pono Tao SS asabeicessaancr ose) Raa HN ee a ROR ea ARH ERS or) Sacco se-| 329 se-|)90) galls: peri minute, With) 28 LDS.) jescceceecceec es see ans acceen d= |) Recent. | Tapir, Peecary, Bison. 4 Quaternary. Bos, Equus, Tapirus, Dicotyles, Megatherium, Mylodon. Equus Beds. Equus, Tapirus, Elephas. Pliocene.) ,: 1: § Pliohippus, Tapiravus, Mastodon, Procamelus, Pliohippus Beds. 2 Aceratherium, Bos, Morotherium, Platygonus. is) ‘ohi Miohippus, Diceratherium, Thinohyus, Protoceras. = Miohippus Beds. RUS, , yus, S Z . mee \ Oreodon, Eporeodon, Hyanodon, Moropus, Ictops, (o) p: | Miocene. Oreodon Beds. 0 Hyracodon, Agriochwrus, Colodon, Leptocherus. A a Brontotherium Beds|)Prontotheriwm, Brontops, Allops, Titanops, Titano- iS) = | 2 therium, Mesohippus, Ancodus, Entelodon. im | is) | Diplacodon Beds. Diplacodon, Epihippus, Amynodon, Eomeryx. ae \Dinoceras, Tinoceras, Uintatherium, Palceosyops, Dinoceras Beds. 2? Orohippus, Hyrachyus, Colonoceras, Homacodon. Heliobatis Beds. Heliobatis, Amia, Lepidosteus, Asineops, Clupea. ‘ 2 \ Coryphodon, Bohippus, Eohyus, Hyracops, Parahyus. Coryphodon Beds. |)Lemurs, Ungulates, Tillodonts, Rodents, Serpents. Eocene. F Ceratops, Triceratops, Claosaurus, Ornithomimus. Ceratops Beds. Mammals, Cimolomys, Dipriodon, Selenacodon, Nanomyops, Stagodon. Birds, Cimoloptery. | Montana Group. Cretaceous. —— eS : | Birds with Teeth, Hesperornis, Ichthyornis, Apatornis. 5 | Pteranodon Beds. | Mosasaurs, Edestosawrus, Lestosawrus, Tylosaurus. 5 Pterodactyls, Pteranodon. Plesiosaurs, Turtles. 8 — ES = cS) Dakota Group. | 4 = Ss y Atlantosaurus Beds.| / Dinosaurs, Brontosaurus, Morosaurus. Diplodocus, Jurassic. Buptanodon Beds. Stegosaurus, Camptosaurus, Ceratosaurus. Mam- Hallopus Beds. | mals, Dryolestes, Stylacodon, Tinodon, Ctenacodon. Virst Mammals, Dromatherium. First Dinosaurs, Otozoum, or Anchisaurus, Ainmosaurus, Bathygnathus, Clepsy- Conn. River, Beds.| __saurus. Many footprints. Crocodiles, Belodon. Fishes, Oatopterus, Ischypterus, Ptycholepis. Triassic. Fig. 23.—Section to illustrate horizons of vertebrate fossils. Cenozoic, as developed in the Rocky Mountain region, and includes every- thing above the Paleozoic, although all the subdivisions are not indicated. In addition to showing the principal horizons from the Triassic to Recent deposits, it gives under each formation the genera of the most characteristic vertebrate fossils found in each. HORIZONS OF VERTEBRATE FOSSILS. 475 PART I. GEOLOGICAL HORIZONS. The base of the Mesozoic in the Denver Basin appears to be repre- sented by certain red sandstones, which rest directly against the Archean. No characteristic vertebrate fossils have yet been found in these strata, and they have usually been classed with the overlying Jurassic beds, under the general name of Juratrias. Farther to the south in New Mexico, and especially to the southwest in Arizona, this series of sandstones or their equivalents in position contain vertebrate fossils, and among these the “writer has recognized both dinosaurian and crocodilian remains of Triassic types. Future exploration in the Denver region and in the same horizon farther south may bring to light characteristic fossils and determine more accurately the age of these interesting deposits. JURASSIC, HALLOPUS BEDS, Immediately over the sandstones above mentioned is a large series of strata which are undoubtedly Jurassic in their upper portion, and perhaps throughout. Near the base of this series in the Canyon region are the Hallopus beds, as indicated in the section in fig. 23. These beds are of special importance biologically, owing to the fact that they contain remains of the smallest, and in many other respects the most interesting, dinosaurian reptiles yet discovered in any part of the world. The beds are of very limited extent, so far as now known. They have not yet been detected in the Denver Basin. BAPTANODON BEDS, The Baptanodon beds placed next in the section have also not been recognized near Denver, nor along the same horizon to the south, but are strongly developed in Utah and Wyoming on the flanks of the Uinta and Wasatch mountains, on the Laramie Plains, in the Big Horn and Wind River ranges, and at various other points farther north. They consist of marine strata, inclosing many typical invertebrate fossils in addition to the large reptile, Baptanodon, from which the horizon takes its name. As these 476 GEOLOGY OF THE DENVER BASIN. beds occur just below the Atlantosaurus series near the Black Hills and at other points in the north, it is possible that they may be represented in the same position in the Denver region. A typical exposure of the Baptanodon beds may be seen near Lake Como, Wyoming, where the writer, in 1868, first recognized this horizon and determined its Jurassic age. ATLANTOSAURUS BEDS. The most important geological horizon in the Denver region is the Atlantosaurus beds, which are here extensively developed. They may be seen to good advantage in passing from Golden to Morrison, where they will be found as a series of shales and sandstones resting below either upon- the red sandstone already mentioned or the succeeding strata, and covered above by the characteristic white Dakota sandstone, which m many places has protected them from erosion. These Atlantosaurus beds are of fresh- water origin, and their softer portions have suffered great denudation, thus leaving extensive valleys. This horizon is one of the most distinet and important yet found in this country, and it has now been traced, mainly by the bones of the gigantic reptiles it contains, for about 500 miles along the eastern flank of the Rocky Mountains. In the vicinity of Denver these deposits are gray or ash colored, while both to the north and south, and especially west of the mountains, they are usually variegated im color, red and yellow tints predominating. To the south of Denver the Atlantosaurus beds may be seen at various points for a hundred miles and more—at the Garden of the Gods, also near Canyon, and still farther south, at Webster Park, beyond the Arkansas River. The main Oil Creek locality, about 14 miles north of Canyon, known locally as the ‘Bone Yard,” has long been famous, as here and in the surrounding region were found some of the most remarkable fossils of this horizon. These remains, and others from near Morrison, in the Denver Basin, will be discussed more fully later in the present chapter. CRETACEOUS. PTERANODON BEDS. In the Dakota sandstone of Cretaceous age, next above the Atlanto- saurus beds, only a few fragments of vertebrate remains have yet been discovered, and these beyond the limits of the Denver Basin; hence they HORIZONS OF VERTEBRATE FOSSILS. 477 need not be further mentioned here. The next higher horizon of special importance, which the writer has named the Pteranodon beds, from the gigantic pterodactyls found in them, is very rich in vertebrate fossils, and among these remains of marine swimming reptiles are most abundant. Here, too, were found the well-known birds with teeth. In the Denver Basin vertebrate fossils from this horizon have not been seen in place, although isolated specimens have been found. In the foothills to the north interesting discoveries have been made, one locality being on Lodge Pole Creek, Wyoming. On the plains farther east the Pteranodon beds have a great development, especially in Kansas along the Solomon, Saline, and Smoky Hill rivers, where the variegated chalk bluffs are rich in vertebrate fossils. To the south they may be seen at various points nearer the moun- tains, particularly in the vicinity of Pueblo and Canyon. The Pteranodon beds are in part the equivalent of No. 3, Meek and Hayden. The next horizon given in the section, fig. 23, is that of the Montana group, which is well represented in the Denver Basin, but its vertebrate fossils do not call for special mention here. Farther north in Wyoming the Fox Hills section of this group lies directly beneath the Ceratops beds, the horizon next to be considered. CERATOPS BEDS. The Ceratops beds placed next in the section are of special importance, from the remarkable fossils they contain and their great development in the north. They form one of the most distinct vertebrate horizons in this country. In the number and variety of their vertebrate fossils they are surpassed only by the Atlantosaurus beds, and as both horizons are so well developed in the Denver Basin special attention will be given in this chap- ter to the animals entombed in each of them. The exact relation of the Ceratops beds to the adjoining deposits in the vicinity of Denver will not here be discussed, but it is worthy of notice that, to the north, beds con- taining the same fauna have a remarkable extension, mainly as lacustrine shales and sandstones. The most characteristic development of the Ceratops beds known to the writer from personal observation is in the northeastern part of Converse County, Wyoming, in the region represented in the accompanying map, 478 GEOLOGY OF THE DENVER BASIN. fig. 24. The great, importance of the discoveries made here, both from a biological point of view and as accurately defining the geological horizon of the Ceratops beds, made it desirable to fix definitely the position of every important specimen found, and these were carefully noted at the time each discovery was made. As the skulls of the gigantie Ceratopsidee are most characteristic, the precise localities of 30 of these skulls are given in this map, so that the horizon and its age may thus be verified by future explorers. ¥ {i fy sin, a hie, S GAY N\A); TA SN ANITA WA prville ay om lush rz Saas as Fia. 24.—Map of Converse County, Wyo., showing localities where skulls of Ceratopsid# have been discovered. The position of each skull is indicated by a cross, and more than 30 of these specimens were found within the area bounded by the Cheyenne River and the dotted line. The localities given are based upon field notes made, at the request of the writer, by his assistant, Mr. J. B. Hatcher. Still farther north, in the Judith Basin, in Montana, the same lacus- trine strata alternate with beds in which brackish-water invertebrates and marine reptiles are found associated with the gigantic Ceratopside. Along this Ceratops horizon for hundreds of miles the experienced collector can find vertebrate fossils at almost every exposure. HORIZONS OF VERTEBRATE FOSSILS. 479 Fragments of vertebrate remains, evidently of this same characteristic fauna, have been found by the writer in the well-known rock columns of Monument Park, south of Denver, and careful search there would doubt- less bring to light well-preserved specimens. Farther to the south, and also west of the mountains in ‘the Wyoming Basin, typical fossils of this horizon have been observed in place at various points. The horizon thus marked by the Ceratops fauna is indicated by a series of outcrops at various points along the eastern flank of the Rocky Mountains from Canada to New Mexico. From these exposures fossils of this fauna have already been obtained at 15 different localities, although systematic explorations have been made at only a few of the outcrops along the line thus indicated. TERTIARY. The Eocene formation, which has such a great development west of the Rocky Mountains and is so rich in mammalian life, is apparently not represented in the Denver Basin, nor indeed along the eastern side of the mountains to the north. It is, however, well developed to the south in Huerfano Park, and southwest, in New Mexico. BRONTOTHERIUM BEDS. The next higher horizon of the Tertiary, the Miocene, and the succeeding Pliocene have an extensive development along the eastern flank of the Rocky Mountains, and especially in the plains region. The series of Lower Miocene fresh-water clays and sandstones which the writer has called the Brontotherium beds form a well-marked horizon. They have been recognized at various points from southeast of Denver, in Elbert County, far to the north through Nebraska and Dakota, and have recently been found in Canada. On the plains not far from Denver they have been found, with their characteristic fossils, in depressions or pockets in the underlying Ceratops beds, and this is the case, also, at various points in the north, especially in Colorado and Wyoming, where the Brontotherium beds have suffered great denudation. Farther east, in both Nebraska and Dakota, these beds are overlain by other Miocene deposits of great thickness, known as the Oreodon beds: Both series are well developed in 480 . GEOLOGY OF THE DENVER BASIN. northeastern Colorado, at Chalk Bluffs and various other localities, and here interesting discoveries of vertebrate fossils have been made. Southeast of Denver, the Brontotherium beds appear to be covered by later Tertiary deposits, which may include equivalents of the Oreodon series, although these have not been observed in place much south of the South Platte River. The greater part of these overlying strata are of Pliocene age. PLIOHIPPUS BEDS. The most important horizon in the Pliocene is that of the Pliohippus beds, which have a great extension both north and south of Denver, often remaining as table-lands far out on the plains, or as isolated buttes where they have not been entirely removed by erosion. The great Arkansas divide, especially south of the Smoky Hill River, is mainly composed of these strata, as ascertained by the writer in 1870 and 1871, by personal exploration at various points in eastern Colorado and western Kansas. Northeast of Denver these beds form high bluffs above the Miocene, and in Nebraska and Dakota this is their usual position. Pliocene vertebrate fossils have been found in the vicinity of Denver and are quite abundant at various points in the adjacent regions. The Quaternary and Recent deposits above the Pliocene also contain interesting vertebrate remains, which need not be discussed here. These various horizons, marked by characteristic vertebrate fossils, have been determined and named by the writer mainly upon evidence secured during his own explorations in the Rocky Mountain region, begin- ning in 1868. In many of the strata examined no other characteristic fossils except vertebrates were to be found, and this fact should give the present determinations additional value. PART If. JURASSIC VERTEBRATE FOSSILS. After this brief review of the series of geological horizons, Mesozoic and Cenozoic, represented in the region of Denver, and their chief charac- teristics, it remains to discuss the vertebrate life successively entombed as the various strata were deposited. _ The extinct animals thus preserved are of great interest in themselves, as they give conclusive evidence of many JURASSIC VERTEBRATE FOSSILS. 481 phases of ancient life as it developed from age to age in the progress of the world. They thus afford interesting data as to the higher forms of life in ages still more remote, and, more important yet, indicate the lines of descent which have continued on to the present. During Mesozoic time the vertebrate life consisted mainly of fishes, amphibians, and reptiles, although birds and mammals were also represented. Of all these the reptiles were the dominant forms, and the title ‘“‘Reptilian Age” appropriately defines the whole period. In this class the dinosaurs form in many respects the most important group, and the representatives that left their remains in the neighborhood of Denver are not surpassed in interest by those found in any other part of the world. To make clear the descriptions that follow, it may be well to state here that dinosaurs are now divided into several distinct orders: The Theropoda, or carnivorous, bipedal forms; the Sauropoda, including huge herbivorous, quadrupedal forms; and the Predentata, consisting of herbivorous reptiles of several very different suborders, among them the Stegosauria, or plated lizards, the Ceratopsia, or horned forms, and the Ornithopoda, or bird-footed reptiles, which in many respects resemble birds. A full account of these remarkable reptiles will be found in an illustrated memoir by the writer, entitled ‘The Dinosaurs of North America,” and published in Part I of the Sixteenth Annual Report of the United States Geological Survey. With these various dinosaurs lived other reptiles, especially crocodiles and turtles, and numerous fishes. A few birds and some diminutive mammals likewise then existed in the same region. REPTILIA. HALLOPUS. Near the base of the Jurassic strata not far from Canyon, in the hori- zon already defined as the Hallopus beds, remains of a small carnivorous dinosaur have been found, which are worthy of special description. This diminutive reptile, which has been named by the writer Hallopus victor, was but little larger than a domestic fowl, although of much more slender pro- portions. The greater part of the skeleton is preserved, and this shows that the bones were bird-like and hollow, possibly pneumatic, and in their MON XXxVII——31l. 482 GEOLOGY OF THE DENVER BASIN. anatomical features of great interest. In figs. 25 and 26 below, restora- tions of the fore and hind legs, one-half natural size, are given, which show well the delicate proportions of the animal. The scapula is of moderate length, and its upper portion broad and thin. The humerus is slender, with a strong radial crest. The shaft is very hollow, with thin walls, and the cavity extends almost to the distal 26 25 FiG. 25.—Outline restoration of left fore leg of Hallopus victor Marsh. Fie. 26.—Outline restoration of left hind leg of same individual. Both figures are one-half natural size. end. The latter is but little expanded transversely. The radius and ulna are short, and were closely applied to each other. There were but four digits in the manus, the first being short and stout and the others slender. All three pelvic bones aided in forming the acetabulum, as in typical dinosaurs. The ilia are of the carnivorous type, and resemble in form those of Megalosaurus. The pubes are rod-like, and projected downward JURASSIC VERTEBRATE FOSSILS. 483 and forward. The distal ends are closely applied to each other, but not materially expanded, and in the present specimen are not coossified with each other. The ischia projected downward and backward, and their distal extremities are expanded, somewhat as in the Crocodilia. The femur is comparatively short, with the shaft curved and very hollow. The tibia is nearly straight, much longer than the femur, and its shaft equally hollow. The fibula was slender and complete, but tapered much from above downward. Its position below was not in front of the tibia,-as in all known dinosaurs, but its lower extremity was outside, and apparently somewhat behind, the tibia. The astragalus is large, and covered the entire end of the tibia, but was not coossified with it. The caleaneum is compressed transversely, and much produced backward. It was closely applied to the outside of the astragalus, and although agreeing in general form with that of a crocodile, strongly resembles the corresponding bone in some mammals. The tarsal joit was below the astragalus and caleaneum. There appears to be but a single bone in the second tarsal row, although this may be composed of two or more elements. i There were but three functional digits in the hind foot, and their metatarsals are greatly elongated. The first digit seems to be wanting, and the fifth is represented only by a remnant of the metatarsal. The posterior limbs, as a whole, were especially adapted for leaping, and are more slender than in almost any other known reptile. There are but two vertebre in the sacrum. The other vertebree preserved have their articular faces biconcave. The chevrons are slender and very elongate. This interesting reptile was found near Canyon, in the horizon that now bears its generic name. No other specimen is known. NANOSAURUS. In the same geological horizon in which the type of Hallopus was found, but at a different locality in the same vicinity, the skeleton of another dinosaur was discovered, still more diminutive, and indeed the smallest known member of the group. The remains are well preserved, 484 GEOLOGY OF THE DENVER BASIN, and indicate clearly that this reptile was herbivorous in habit, avian in form, and in many respects the most bird-like dinosaur yet discovered. It was described by the writer in 1877, under the name Nanosaurus agilis. The type specimen consists of the greater portion of the skull and skeleton of one individual, with ‘the bones more or less displaced, and all entombed in a slab of very hard quartzite. The whole skeleton was probably thus preserved in place, but, before its discovery, a part of the slab had been split off and lost. The remaining portion shows on the split surface many important parts of the skeleton, so that the maim characters of the animal can be determined with considerable certainty. Fic. 27.—Dentary bone of Nanosaurus agilis Marsh; seen from the left. Fic. 28.—Ilium of same individual; left side. Both figures are natural size. Fic. 29.—Left femur of Nanosaurus rea Marsh. One-half natural size. a, frout view; b, side view; c, back view; ¢@, proximal end; e, distal end. A study of these remains shows that the reptile they represent was one of the typical Ornithopoda, and one of the most avian yet discoy- ered. A dentary bone in fair preservation, shown in fig. 27, indicates that the animal was herbivorous, and the single row of pointed and compressed teeth, thirteen in number and small in size, forms a more regular and uniform series than in any other member of the group. The ilium, also, shown in fig. 28, is characteristic of the Ornithopoda, having a slender, JURASSIC VERTEBRATE FOSSILS. 485 pointed process in front, but one much shorter than in any of the larger forms. The posterior end is also of moderate size. All the bones of the limbs and feet are extremely hollow, strongly resembling in this respect those of birds. The femur was shorter than the tibia. The metatarsals are greatly elongated and very slender, and there were probably but three functional toes in the hind foot. The remains now known indicate that the animal when alive was about half the size of a domestic fowl. A second form referred by the writer to this genus, under the name of Nanosaurus rex, may, perhaps, belong to the genus Laosaurus. The femur is shown in fig. 29. The animal thus represented was considerably larger than the present type species, and was found in a somewhat higher horizon in the same region. BAPTANODON. In the horizon marked in the section as the Baptanodon beds the most important known vertebrate is the large swimming reptile that has given the name to these deposits, and figures of its characteristic remains are given below as an aid in identifying the strata. This reptile, Baptanodon, was most nearly allied to Ichthyosaurus, but was without teeth, and the paddles had six digits, as shown in fig. 30 (p. 486). The vertebrae, one of which is represented in fig. 31, are very similar to those of Ichthyosaurus. Another interesting marine reptile from this horizon appears to be a true Plesiosaur, with teeth, and has been described by the writer as Pantosaurus striatus. A vertebra of the type specimen is shown in fig. 32. A small crocodile, Diplosaurus nanus, which the writer found in 1870 in the same beds, was the first vertebrate discovered in this horizon. ATLANTOSAURUS, In the succeeding Atlantosaurus beds a new and remarkable reptilian fauna makes its appearance, and the gigantic Sauropoda are the prevailing forms. In the Hallopus horizon the dinosaurs were, as already stated, the most diminutive known, but in the present beds the representatives of this group are the most gigantic land animals yet brought to light. The first genus discovered, which has been named Atlantosaurus by the writer, includes several species, all of huge dimensions. The first discovery was 486 GEOLOGY OF THE DENVER BASIN. made in 1877, by Capt. H. C. Beckwith, U.S. N., and Prof. Arthur Lakes, and the sacrum of the type specimen is represented in fig. 33, below. This fossil, with other remains, was found in place in the upper part of the Atlantosaurus beds, on the western slope of the foothills, south of Golden. Fic. 31.—Cervical vertebra of Baptanodon natans Marsh. One-third natural size. . A, side view; B, front view; C, section; D, top view; a, anterior articular face; n, neural canal; r, r’, faces or rib. Fig. 32.—Posterior cervical vertebra of Pantosaurus striatus Marsh, One-half natural size. A, side view; B, front view; C, bottom view; a, anterior face; n, neural canal; p, posterior face; 7, face for rib. Subsequent researches resulted in the discovery of other specimens near the same locality. Still later, systematic explorations were carried on by the writer at various points along the same horizon farther south, especially in the vicinity of Morrison, and at every locality interesting discoveries were made. JURASSIC VERTEBRATE FOSSILS. 487 In the present specimen the most characteristic bones preserved are portions of the sacrum and posterior limbs. The former is represented by the last three vertebrae with their transverse processes, nearly complete, and by other fragments. The last sacral vertebra has its centrum moderately ' Fig. 33.—Sacrum of Atlantosaurus montanus Marsh; seen from below. One-eighth natural size. a, first sacral vertebra; b, transverse process, or sacral rib, of first vertebra; c, same process of second vertebra; d, Same process of third vertebra; e,same process of fourth vertebra; /, foramen between first and second processes; f’ foramen between second and third processes; f’, foramen between third and fourth processes; g, surface for union with ilium ; p, last sacral vertebra. concave below on each side of the median line, but only near its anterior end can indications of a keel be observed. The next sacral vertebra has its inferior lateral surface so deeply concave as to materially lessen its bulk. This is also true of the next anterior centrum, and may be considered a 488 GEOLOGY OF THE DENVER BASIN. distinctive character of these vertebrae. A more important character of the same centra is a very large cavity in each side, connected with the outer surface by an elongated foramen below the base of the neural arch. The inner surface of this cavity indicates that it was not filled by cartilage, and Fic. 34.—Left femur of Atlantosaurus immanis Marsh; inner view. Fia. 34a.—Proximal end of same. Fig. 35.—The same bone; front view. Fic. 35a.—Distal end of same. All the figures are one-sixteenth natural size. c, innercondyle; c’, outer condyle; f, groove for fibula; h, head; t, trochanter; ¢’/, lower trochanter. it was, perhaps, a pneumatic opening, designed to lessen the weight of the enormous sacral mass. The transverse processes, or more strictly speaking, the sacral ribs, of these vertebrae are very stout and of moderate length. JURASSIC VERTEBRATE FOSSILS. 489 Their distal ends are firmly coossified, forming a powerful support for the ilium. Between these processes are large, oval openings, as shown in fig. 33. The present remains indicate that this reptile when alive was about 50 feet in length. Its general form and proportions were very similar to those of Brontosaurus, an allied genus, the skeleton of which is represented in the restoration on Pl. X XI. In the same series of strata, a few miles to the south, and just above the village of Morrison, various remains of a much larger species of the same genus were found in the following year, and described by the writer as Atlantosaurus immanis. A femur belonging to the type specimen is represented, one-sixteenth natural size, in figs. 34 and 35. The other remains of this enormous reptile recovered show many points of interest to anatomists, which will be discussed fully by the writer elsewhere. It may, however, be stated that the head of this animal was quite small, and the neck very long and lightly built, insuring great flexibility. The vertebree of the trunk were also lightened by large cayi- ties in the sides, similar to those in the sacrum. The tail was powerful and much elongated. All the limb bones were massive and solid. The animal was herbivorous in habit, and its food was probably soft, succulent vegeta- tion, which it obtained along the borders of the great fresh-water lake in which it was finally entombed. When alive, it was about 70 feet or more in length and 20 feet in height. APATOSAURUS, In the same stratum with the Atlantosaurus fossils the bones of an allied genus, Apatosaurus, were also found by Professor Lakes, and described by the writer in 1877. This reptile, although somewhat smaller than the one last described, was of gigantic dimensions, and similar in habit. A neck vertebra of one species is shown in fig. 36. The sacrum of the type specimen, represented one-tenth natural size in fig. 37, has but three coossified vertebree, thus differing from that of Atlantosaurus. The pelvis of the same individual is shown in fig. 38, and this is typical of the group. The cervical vertebrzee of Apatosaurus are strongly opisthoccelian, and of moderate length. The dorsals have their centra similar, and both have 490 GEOLOGY OF THE DENVER BASIN. deep cavities in the sides and in the neural arch resembling those in the corresponding vertebrae of Morosaurus. The lumbar vertebrz have their articular faces more nearly plane, and the last lumbar is much expanded transversely. Fic. 36.—Cervical vertebra of Apatosaurus luticollis Marsh; back view. One-sixteenth natural size. e, cup; d, diapophysis; J”, lateral foramen; h, rib; p, parapophysis; z', posterior zygapophysis. Fic. 37.Sacrum of Apatosaurus ajax Marsh; seen from below. One-tenth natural size. eral vertebra; ), seeral rib, or transverse process, of first vertebra; c, same process of second vertebra; d, same process of third vertebra; e, e/, surfaces for union with ilia; f,/’, foramina between vertebra ; p, last sacral vertebra. The sacrum is characteristic of the genus, and quite unlike any hitherto known. The type specimen on which the genus was established is well shown in fig. 37. It is short and massive, and the three vertebrae which JURASSIC VERTEBRATE FOSSILS. 491 form it are nearly equal in size and general proportions. They are firmly coossified, and their transverse processes are ankylosed to the centra. Those on each side are united distally into a solid mass, which rests on the short ilium. The articular faces of the sacral vertebrae are nearly plane. That of the anterior centrum is a transverse oval in outline, and the posterior face is more nearly round. The centra and their processes are somewhat lightened by cavities, as in the sacrum of Atlantosaurus. Fig. 38.—Pelvis of Apatosaurus ajax; seen from the left. One-sixteenth natural size. a, acetabulum; /, foramen im pubis; tl, ilium; is, ischium; p, pubis. The type species of the present genus is Apatosaurus ajax Marsh, and the known remains indicate a reptile at least 50 feet in length. A much larger species is indicated by various remains from the same locality in Colorado, among which is the huge cervical vertebra represented in fig. 36. 492 GEOLOGY OF THE DENVER BASIN. This species had a short, massive neck, and hence was called Apatosauwrus laticollis. The size of the entire animal may be judged from this vertebra, which measures over 34 feet (1.07 meters) in width. This would imply a neck not less than 5 or 6 feet wide at this point. BRONTOSAURUS. In the Atlantosaurus beds of the Wyoming Basin the genus Bronto- saurus is well represented by two or more gigantic species, one of which Fic. 39.—Tooth of Brontosaurus excelsus Marsh. Natural size. a, outer view; b, posterior view; c, inner view; d, front view. Fic. 40.—Sixth cervical vertebra of Brontosaurus excelsus; front view. Fic. 41.—The same vertebra; bottom view. Both figures are one-twelfth natural size. b, ball; d, diapophysis; f, lateral foramen; n, neural canal; p, parapophysis; r, cervical rib; z, anterior zyga- pophysis; 2’, posterior zygapophysis. is shown in the restoration on Pl. XXI. Various remains apparently identical with this species have been found in the Denver region, and JURASSIC VERTEBRATE FOSSILS. 493 likewise near Canyon, in the same horizon. The great perfection of the remains of this genus from the western localities has enabled the writer to make a careful study of the whole skeleton, and thus determine many important points in the anatomy of the Sauropoda hitherto unknown. In fig. 39, opposite, a tooth of Brontosaurus is represented, and this may be regarded as typical of the Sauropoda in general. A cervical vertebra of the same species is shown in figs. 40 and 41, and this indicates the delicate, Fic. 42.—Dorsal vertebra of Brontosaurus excelsus; side view. One-twelfth natural size. Fic. 43.—Fourth caudal vertebra of Brontosaurus excelsus; side view. One-eighth natural size. b, ball; c, cup; ec’, face for chevron; d, diapophysis; f, foramen in centrum; p, parapophysis; s, neural spine; z, anterior zygapophysis; 2', posterior zygapophysis. bird-like neck of this huge reptile. In fig. 42 a dorsal vertebra of the same individual is represented, which also shows how the trunk vertebrae were lightened in a corresponding manner. The sacrum in this genus was quite hollow, and was composed of five coalesced vertebrae. The anterior caudal vertebrae, and even the ribs, were more or less lightened by cavities, but the rest of the caudal vertebrze and all of the limb bones were quite solid. 494 GEOLOGY OF THE DENVER BASIN. RESTORATION OF BRONTOSAURUS. In the restoration given on Pl. XXI the diminutive head will first attract attention, as it is smaller in proportion to the body than in any vertebrate hitherto known. The entire skull is less in diameter or actual weight than the fourth or fifth cervical vertebra. A careful estimate of the size of Brontosaurus, as here restored, shows that when living the animal must have weighed more than 20 tons. The very small head and brain, and slender neural cord, indicate a stupid, slow-moving reptile. The beast was wholly without offensive or defensive weapons, or dermal armature. In habits Brontosaurus was more or less amphibious, and its food was probably aquatic plants or other succulent vegetation. The remains are usually found in localities where the animals had evidently become mired. The present genus, Brontosaurus, together with the genera Atlanto- saurus, Apatosaurus, and Barosaurus, form a distinet family of the Sauropoda, which the writer has called the Atlantosauridze. if ’ DIPLODOCUS., In the same horizon in the Denver Basin remains of another peculiar genus, Diplodocus, are quite abundant, especially the teeth. This genus, described by the writer in 1884, represents a distinct family of the Sauropoda, and one of much interest. The skull and nearly every part of the skeleton are now known. The type specimen was found by M. P. Felch, near Canyon, Colo. Other remains of the same genus have been found near Lake Como, in Wyoming. The skull of Diplodocus is of moderate size. The posterior region is elevated and narrow. The facial portion is elongate, and the anterior part expanded transversely. The nasal opening is at the apex of the cranium, which from this point slopes backward to the oeciput. In front of this aperture the elongated face slopes gradually downward to the end of the muzzle, as represented in fig. 44. Seen from the side, the skull of Diplodocus shows five openings: A small oval aperture in front, a large antorbital vacuity, the nasal aperture, JURASSIC VERTEBRATE FOSSILS. 495 Fic. 45.—Maxillary teeth of Diplodocus longus; side view. One-half natural size. e, enamel; 7, root. ¥1G. 46.—Chevron of Diplodocus longus; top and side views. One-tenth natural size. a, anterior end; p, pos- terior end; v, faces for articulation with vertebra. Fic. 47.—Twelfth caudal vertebra of Diplodocus longus; side view. Fic. 48.—The same vertebra; bottom view. Both figures are one-sixth natural size. ec, anterior face for chevron; c', posterior face for chevron; s, neural spine; z, anterior zygapophysis; z',posterior zy gapophysis. 496 GEOLOGY OF THE DENVER BASIN. the orbit, and the lower temporal opening. The first of these has not been seen in any other Sauropoda; the large antorbital vacuity is characteristic of the Theropoda also; while the other three openings are present in all the known Dinosauria. The lower jaws of Diplodocus are more slender than in any of the other Sauropoda. The dentary, especially, lacks the massive character seen in Morosaurus, and is much less robust than the corresponding bone in Brontosaurus. The short dentigerous portion in front is decurved, and its ereatest depth is at the symphysis. The articular, angular, and surangular bones are well developed, but the coronary and splenial appear to be small. The dentition of Diplodocus is the weakest seen in any of the known Dinosauria, and strongly suggests the probability that some of the more specialized members of this great group were edentulous. The teeth are entirely confined to the front of the jaws, and those in use were inserted in such shallow sockets that they were readily detached. Specimens in the Yale University Museum show that the entire series of upper or lower teeth could be separated from the bones supporting them without losing their relative position. In fig. 45 a number of these detached teeth are shown. The vertebral column of Diplodocus, so far as at present known, may be readily distinguished from that of the other Sauropoda by both the centra and chevrons of the caudals. The caudals are deeply excavated below, as shown in fig. 48, while the chevrons have both anterior and posterior branches, as seen in fig. 46. The type specimen of Diplodocus, to which the skull here figured belongs, indicates an animal intermediate in size between Atlantosaurus and Morosaurus, probably 40 or 50 feet in length when alive. The teeth show that it was herbivorous, and the food was probably succulent vegetation. MOROSAURUS. Another genus of the Sauropoda, represented by several species of large size, has also been found in the Denver region, and especially in the same horizon farther south. Like Brontosaurus, however, it is much more abundant in the Wyoming Basin, west of the mountains. This genus, described by the writer in 1878, is the type of a distinet family, the Morosauridee. JURASSIC VERTEBRATE FOSSILS. 497 The head in this genus was very small. The skull resembles some- what that of Brontosaurus, but the jaws are more massive and the teeth larger. The vertebra were similar in general form to those of Brontosaurus, but were less lightened by cavities, especially in the trunk. The sacrum was much less excavated, and the caudals were all solid. Fie. 49.—Fourth cervical vertebra of Morosaurus grandis Marsh; side view. Fig. 50.—The same vertebra; back view. Both figures are one-eighth natural size. b, ball, c, cup; 4, diapophysis; f, foramen in centrum; p, parapophysis; z, anterior zygapophysis; z’, posterior zy zgapophysis. Fic. 51.—Pelvie arch of Morosaurus grandis; seen from in front. One-sixteenth natural size. a, first sacral vertebra; b, sacral rib, or transverse process of first sacral vertebra; c, the same process of second sacral vertebra; ¢, same process of last sacral vertebra; i, ilium; is, ischinum; ne, neural canal; p, fourth, or last, sacral vertebra; pb, pubis. The scapula has a large anterior projection on its shaft, while the ischia are twisted and were directed backward, characters not seen in the Atlantosauridee. A cervical vertebra of this genus is shown in figs. 49 and 50, above, and in fig. 51 the entire pelvic arch, with the sacrum in position, is likewise represented. MON XXVII——32 498 GEOLOGY OF THE DENVER BASIN. The type species of the present genus, Morosaurus grandis, was about 40 feet in length when alive. MJorosaurus agilis, found near Canyon, was much smaller. The genus Camarasaurus Cope, which includes some of the gigantic forms of the Sauropoda, was apparently a form nearly allied to Morosaurus, and perhaps belonged to the same family. STEGOSAURUS., Although the Sauropoda are by far the most abundant dinosaurs in this horizon, there are other large herbivorous forms well represented, and among these the Stegosauria are the most remarkable. The type genus, Stegosaurus, described by the writer in 1877, is now well known, and is represented by several species, two of which, at least, occur in the Denver region, where the type was found. West of the mountains, especially in the Wyoming Basin, remains of this genus are also numerous in the Atlan- tosaurus beds. One species from that region, Stegosaurus ungulatus, is restored on Pl. XXII. This figure will indicate the general form and appearance of all the species of the genus, although they differ much in minor details. The skull of Stegosaurus is long and slender, the facial portion being especially produced. Seen from the side, with the lower jaw in position, it is wedge-shaped, with the point formed by the premaxillary, which projects well beyond the mandible, as shown in fig. 52. The anterior nares are large and situated far in front. The orbit is very large and placed well back. The lower temporal fossa is somewhat smaller. All these openings are oval in outline and are on a line nearly parallel with the top of the skull. In this view the lower jaw covers the teeth entirely. A single tooth is shown below, in fig. 53. The brain of this reptile was much elongated, and its most striking features were the large size of the optic lobes and the small cerebral hemi- spheres. The latter had a transverse diameter only slightly in excess of the medulla. The cerebellum was quite small. The optic nerve corresponded in size with the optic lobes. The olfactory iobes were of large size. A cast of the brain is shown in fig. 54, on the opposite page. In Stegosaurus the brain was one of the smallest known in any land vertebrate, living or extinct. A still more remarkable feature, however, is JURASSIC VERTEBRATE FOSSILS. 499 seen in the sacrum, where there is a very large chamber formed by an enlargement of the spinal canal. This chamber is ovate in form, and strongly resembles the brain-case in the skull, although very much larger, Fic. 52.—Skull of Stegosaurus stenops Marsh; side view. One-fourth natural size. a, anterior narial opening; an, angular bone; ar, articular; b, orbit; c,lower temporal fossa; d,dentary; fp, post- frontal; j, jugal; 1, lachrymal; m, maxillary; n, nasal; oc, occipital condyle; pd, predentary; pf, prefrontal; pm, premax- illary; po, postorbital; g, quadrate; s, splenial; sa, surangular; so, supraorbital; sg, squamosal. a b Fic. 53.—Tooth of Stegosaurus ungulatus Marsh. a, natural size ; b, e, d, twice natural size. b, outer view; ¢, end view; d, top view. 55 Fic. 54.—Brain-cast of Stegosaurus ungulatus ; side view. ¢, cerebral hemispheres; cb, cerebellum; m, medulla; ol, olfactory lobes; on, optic nerve; op, optic lobes. Fig. 55.—Cast of neural cavity in sacrum of Stegosaurus ungulatus ; side view. a, anterior end; f, foramen between first and second vertebrex ; /’, same between second and third vertebra: ; f between third and last vertebra; p, exit of neural canal in last sacral vertebra. Both figures are one-fourth natural size. ‘same being at least twenty times the size of the cavity which contains the brain. This large, vaulted chamber is mainly contained in the first and second sacral vertebrae, although the canal is considerably enlarged behind this 500 GEOLOGY OF THE DENVER BASIN. cavity. Figs. 54 and 55 show the comparative size of the brain cavity and the chamber in the sacrum. The physiological effects of a posterior nervous center so many times larger than the brain itself is a suggestive subject which need not here be discussed. It is evident, however, that in an animal so endowed the posterior part was dominant. The dermal armor of Stegosaurus is one of its most remarkable features, and suggested the generic name. A plate and spine are shown in figs. 56 and 57, and the series are in place in the restoration, Pl. XXII. In the restoration of Stegosaurus given on Pl XXII, the animal is represented as walking, and the position is adapted to that motion. The Fic. 56.—Dermal spine of Stegosaurus ungulatus. a, side view; b, front view; c, section; d, inferior view of base. Via. 57-—Dermal plate of Steyosaurus ungulatus. a, side view; b, end view of base; d, thin margin; e, rugose base; f, surface marked by vaseular grooves. Both figures are one-twelfth natural size. head and neck, the massive fore limbs, and, in fact, the whole skeleton, indieate slow locomotion on all four feet. The longer hind limbs and the powerful tail show, however, that the animal could thus support itself, as on a tripod, and this position could have been very easily assumed in consequence of the massive hind quarters. In the restoration as here presented the dermal armor is the most striking feature, but the skeleton is almost as remarkable, and its high specialization was evidently acquired gradually as the armor itself was developed. Without the latter many points in the skeleton would be inexplicable, and there are still a number that need explanation. JURASSIC VERTEBRATE FOSSILS. 5O1 The small, elongated head was covered in front by a horny beak. The teeth are confined to the maxillary and dentary bones, and are not visible in the figure here given. They are quite small, with compressed fluted crowns, and indicate that the food of this animal was soft, succulent vegetation. The vertebra are solid, and the articular faces of the centra are biconcave or nearly flat. The ribs of the trunk are massive, and placed high above the centra, the tubercle alone being supported on the elevated diapophysis. The neural spines, especially those of the sacrum and anterior caudals, have their summits expanded to aid in supporting the massive dermal armor above them. The limb bones are solid, and this is true of every other part of the skeleton. The feet were short and massive, and the terminal phalanges of the functional toes were covered by strong hoofs. There were five well-developed digits in the fore foot and only three in the hind foot, the first toe being rudimentary and the fifth entirely wanting. In life, the animal was protected by a powerful dermal armor, which served both for defense and offense. The throat was covered by a thick skin in which were imbedded a large number of rounded ossicles, as shown in the restoration, Pl. XXII. The gular portion represented was found beneath the skull, so that its position in life may be regarded as definitely settled. The series of vertical plates which extended above the neck, along the back, and over two-thirds of the tail, is a most remarkable feature, which could not have been anticipated, and would hardly have been credited had not the plates themselves been found in position. The four pairs of massive spines characteristic of the present species, which were situated above the lower third of the tail, are apparently the only part of this peculiar armor used for offense. In addition to the portions of armor above mentioned, there was a pair of small plates just behind the skull, which served to proteet this part of the neck. There’were also, in the present species, four flat spines, which were probably in place below the tail, but as their position is somewhat in doubt, they are not represented in the present restoration. All these plates and spines, massive and powerful as they now are, were in life protected by a thick, horny covering, which must have greatly 502 GEOLOGY OF THE. DENVER BASIN. increased their size and weight. This covering is clearly indicated by the vascular grooves and impressions which mark the surface of both plates and spines, except their bases, which were evidently implanted in the thick skin. CAMPTOSAURUS. Another group of dinosaurs, the Ornithopoda, is well represented in the Atlantosaurus beds, on both the eastern and western sides of the Rocky Mountains. This suborder includes the smaller herbivorous forms, which have many bird-like features. The genus Nanosaurus, already Fic. 58.—Skull of Camptosaurus medius Marsh; seen from the left side. One-fourth natural size. a, anterior narial opening; an, angular bone; bo, basioccipital; d, dentary; /, frontal; j, jugal; m, maxillary n, nasal; o, orbit; pd, predentary; pm, premaxillary; q, quadrate; s, squamosal; sa, surangular. Fic. 59.—Fifth lower tooth of Camptosaurus medius. Natural size. a, outer view; b, posterior end view; ec, inner view. described, is the smallest member of the group, or, in fact, of all the dino- saurs, while Camptosaurus includes some species of quite large dimensions. All this group are bipedal in locomotion, and thus quite unlike the gigantic forms previously described from this horizon, which were all quadrupedal. The general form and structure of the animals of this order are indicated by the restoration on Pl. XXIII, which represents the skeleton .of one of the typical species of the genus Camptosaurus. In fig. 58, above, is shown JURASSIC VERTEBRATE FOSSILS. 503 the skull of another species, and in fig. 59 a tooth of the same individual is also represented. Other allied genera of this group from the same horizon are Dryosaurus and Laosaurus, the latter containing several species of very small, bird-like forms. A restoration of one of these (Laosawus consors) is shown in Pl. XXIV. The restoration given on Pl. XXIII is based upon the type specimen ot Camptosaurus dispar, one of the most characteristic forms of the great group Ornithopoda, or bird-footed dinosaurs. The reptile is represented on this plate one-thirtieth natural size. The position chosen was determined after a careful study, not only of the type specimen, but of several others in excellent preservation belonging to the same species or to others nearly allied. It is therefore believed to be a position frequently assumed by the animal during life, and thus, in some measure, characteristic of the genus Camptosaurus. The present species, when alive, was about 20 feet in length and 10 feet high in the position here represented. CERATOSAURUS. The Jurassic dinosaurs above described from the Atlantosaurus beds have all been herbivorous forms, but in the same horizon there are abundant remains of carnivorous species that preyed upon them. These are members of the order Theropoda, and the most important genera are Allosaurus and Ceratosaurus, both large of size and ferocious in habit. A small, bird-like form (Ccelurus) of this order lived at the same time, and although of much scientific interest can not be discussed here. All these carnivorous dinosaurs were bipedal in locomotion, and their general form and appearance are suggested by the restoration of Ceratosaurus, shown in Pl. XXV. The genus Ceratosaurus is the best known of this group, and may be taken as a typical form. The skull of the type species is shown in fig. 60, and the pelvis of Allosaurus in fig. 61. The skull of Ceratosaurus nasicornis is very large in proportion to the rest of the skeleton. The posterior region is elevated, and moderately expanded transversely. The facial portion is elongate, and tapers gradually to the muzzle. Seen from above, the skull resembles in general outline that of an alligator. The nasal openings are separate and lateral, and are placed near the end of she snout, as shown in fig. 60. DO04 GEOLOGY OF THE DENVER BASIN. Seen from the side, this skull appears lacertilian in type, the general structure being light and open. From this point of view, one special feature of the skull is the large, elevated, trenchant horn-core situated on the nasals. Other features are the large openings on the side of the skull, four in number. The first of these is the anterior nasal orifice; the second, the very large, triangular, antorbital foramen; the third, the large oval orbit, and the fourth, the still larger lower temporal opening. The parietal bones are of moderate size, and there is no pineal foramen. The median suture between the parietals is obliterated. ‘The frontal bones are rather short, and are closely united on the median line. The nasal bones are more elongate than the frontals, and firmly coossified. ( Fic. 60.—Skull of Ceratosaurus nasicornis Marsh; side view. One-sixth natural size. a, nasal opening; b, horn-core; c, antorbital opening; d, orbit; e, lower temporal fossa; f, foramen in lower jaw; t, transverse bone. ; These bones support the large, compressed, elevated horn-core, on the median line. The lateral surface of this elevation is very rugose, and furrowed with vascular grooves. It evidently supports a high, trenchant horn, which must have formed a most powerful weapon for offense and defense. The premaxillaries are separate, and each contained three functional teeth. The maxillary bones are large and massive, as shown in fig. 60. They are each provided with fifteen functional teeth, which are large, powerful, and trenchant, indicating clearly the ferocious character of the animal when alive. JURASSIC VERTEBRATE FOSSILS. DOS “The cervical vertebra of Ceratosaurus differ in type from those in any other known reptiles. With the exception of the atlas, all are strongly opis- thoccelian, the cup on the posterior end of each centrum being unusually deep. In place of an equally developed ball on the anterior end, there is a perfectly flat surface. The size of the latter is such that it can be inserted only a short distance in the adjoining cup, and this distance is accurately marked on the centrum by a narrow articular border, just back of the flat anterior face. This peculiar articulation leaves more than three-fourths of the cup unoccupied by the succeeding vertebra. Fig. 61.—Pelvis of Allosaurus fragilis Marsh; side view, seen from the left. One-twelfth natural size. a, acetabulum ; i, ilium; is,ischium; p, pubis. The pubes have their distal ends coossified, and expand into an elon- gate, massive foot, which is one of the most characteristic parts of the skele- ton. It is probable that this foot in connection with the distal ends of the ischia served to support the body in sitting down. That some Triassic dinosaurs sat down on their ischia is proved conclusively by the impressions in the Connecticut River sandstone. In such cases the leg was bent so as to bring the heel to the ground. The same action in the present reptile 506 GEOLOGY OF THE DENVER BASIN. would bring the foot of the pubes to the ground, nearly or quite under the center of gravity of the animal. The legs and ischia would then naturally aid in keeping the body balanced. Possibly this position was assumed habitually by these ferocious biped reptiles in lying in wait for their prey. The most interesting feature in the extremities of this dinosaur is in the metatarsal bones, which are completely ankylosed, as are the bones of the pelvis. There are only three metatarsal elements in each foot, the first and fifth having apparently disappeared entirely. The three metatarsals remaining, which are the second, third, and fourth, are proportionally shorter and more robust than in the other known members of the Theropoda, and being firmly united to each other, they furnish the basis for a very strong hind foot. The phalanges of the hind feet are of moderate length, and most of them are quite hollow. The terminal phalanges evidently supported strong and sharp claws. The unique cervical vertebrae, the coossification of the pelvic bones, and the union of the metatarsals, as in modern birds, distinguish Cerato- saurus widely from all other dinosaurs, and make it the type of a well- marked family, the Ceratosauridze. The nearest allied form is apparently Ornithomimus, from the Laramie, recently described by the writer. The type specimen of Ceratosaurus was about 22 feet long when alive, and 12 feet high, as restored on Pl. XXV. It was found by M. P. Felch in the Atlantosaurus beds of the Upper Jurassic, near Canyon, Colo. The associated fossils were mainly other dinosaurs, especially Sauropoda and Ornithopoda, together with various small mammals. OTHER VERTEBRATES. In addition to the dinosaurs here described, many other reptiles lived in this region during Jurassic time, and not a few left their remains in the deposits now known as the Atlantosaurus beds. Among these were various crocodiles of moderate size, and some nearly as large as existing species. The genera appear to be distinct from those now living. One of the most interesting is Diplosaurus, the type specimen of which, found at Morrison in 1887, is represented in the diagram, fig. 62, on the opposite page. All these crocodilians had biconcave vertebrae, and also seem to have been protected by a dermal covering of bony plates, as in existing species. JURASSIC VERTEBRATE FOSSILS. 507 The turtles appear to have been more abundant, and still more distinct from living forms. The most characteristic genus is Glyptops, and a skull and carapace of one species are represented in figs. 63 and 64, below. The 62 64 Fic. 62.—Skull of Diplosaurus feliz Marsh; top view. One-fourth natural size. n, nasal aperture: o, orbit; oc, occipital condyle; s, supratemporal fossa. Fic. 63.—Skull of Glyptops ornatus Marsh; top view. Natural size. Fic. 64.—Carapace of same species; top view. One-fourth natural size. generic name refers to the sculptured surface of the skull, which is not known in any living form of this order, although this character is not unusual 67 Fic. 65.—Jaws of Macelognathus vagans Marsh; seen from above. One-half natural size. Fic. 66.—The same specimen; side view. Fa. 67.—Tooth of Oeratodus giintheri Marsh. Natural size. in the carapace of many species, living and extinct. Numerous remains of the above species have been found in the Denver region, especially near Morrison and Canyon. 508 GEOLOGY OF THE DENVER BASIN. In the Wyoming Basin, west of the mountains, the same species are abundant, and with them have been found remains of a very remarkable reptile (Macelognathus), figs. 65 and 66, and also a pterodactyl (Dermo- dactylus montanus). A hatrachian (Eobatrachus agilis) and a peculiar fish (Ceratodus giinther’), fig. 67, have likewise been found in this horizon. A single bird (Laopteryx priscus), discovered near Como, Wyo., has also been described by the writer. All these species are probably represented in the Fic. 68. —Left lower jaw of Stylacodon gracilis Marsh; outer view. ‘Three times natural size. Fic, 69. —Right lower jaw of Diplocynodon victor Marsh; outer view. Twice natural size. Fig. 70.—Left lower jaw of Ctenacodon serratus Marsh; inner view. Three times natural size. a, canine; b, condyle; ¢, Coronoid process; d, angle. Denver region, and may be brought to light at any time. Many other forms of much scientific interest, but known only from fragmentary remains, have been found in this horizon. JURASSIC MAMMALS. The most important of the remaining vertebrate fossils from the Atlantosaurus beds are the diminutive mammals, of which a few only have CRETACEOUS VERTEBRATE FOSSILS. 509 been found on the eastern side of the mountains, but on the western slope, and especially near Lake Como, a large number have been discovered, and described by the writer. In figs. 68, 69, and 70, on the opposite page, are shown the lower jaws of three of these small mammals, all from different localities in this horizon. In the Atlantosaurus beds east and west of the mountains various invertebrate fossils have been found, but they are apparently all of fresh- water species, and hence of little value as evidence of geological age. The most abundant of these are Unios, and a typical locality is on Oil Creek, north of Canyon, just above the well-known strata containing vertebrate fossils. Remains of plants also occur in this horizon, but those yet found throw no light on the problem of age, which the characteristic vertebrate fossils have fully determined. PART Il. CRETACEOUS VERTEBRATE FOSSILS. PTERANODONTIA. PTERANODON. The next horizon in this region that contains important vertebrate remains is known as the Pteranodon beds, and its general position is shown in the section, fig. 23, where the characteristic genera found in these strata are also recorded. The gigantic toothless pterodactyls belonging to the order Pteranodontia are of special interest, and the skull of the typical genus, Pteranodon, is represented below in figs. 71 to 74. These huge flying reptiles, when alive, had a spread of wing of from 15 to 25 feet, and their remains are now quite abundant in the chalk deposits of this horizon, especially in Kansas. With them are found the remarkable birds with teeth, of the genera Hesperornis and Ichthyornis (Pl. XXVI), described by the writer. In the same strata the remains of the mosasaurs are the most numerous of all the vertebrate fossils there entombed, while plesiosaurs and turtles are also represented, and fossil fishes are especially abundant. DINOSAURS. CERATOPSID.E. The next horizon rich in vertebrate fossils is the Ceratops beds, so gigantic horned dinosaurs that are especially tole o) named by the writer from the 510 GEOLOGY OF THE DENVER BASIN. abundant in these deposits, and characteristic of the horizon wherever it - has been found. ‘The entire vertebrate fauna of these strata is of great interest, and is only surpassed in this respect by that of the Atlantosaurus beds above described. The dinosaurs are here still the dominant forms, but are more highly specialized than those of the Jurassic horizon. Other reptiles were also abundant, including plesiosaurs, crocodiles, turtles, and serpents, while amphibians and fishes were much more numerous. Bones b] Vig. 71.—Skall and lower jaw of Pteranodon longiceps Marsh; side view. The same skull; top view. —The same skull; bottom view. Wic. 74.—Lower jaw of Pteranodon longiceps; top view. All the figures are one-eighth natural size. a, antorbital aperture; b, orbit; c, sagitfal crest; d, angle of jaw; e, lower margin of upper jaw; e’, upper margin of lower jaw; f, articulation of lower jaw; oc, occipital condyle; q, quadrate bone; s, symphysis of lower jaw. of several birds have been found, while remains of small mammals of primitive types are abundant in many localities. The gigantic horned dinosaurs, as the most characteristic forms in this horizon, will first claim attention, and the skull of one of the best- known genera of the group is well shown in fig. 75. A front view of the same skull is represented in fig. 76, and the posterior view of another in fig. 77. A tooth of Triceratops is given in fig. 78, showing the double roots, the only case among the Reptilia. Another interesting specimen, CRETACEOUS VERTEBRATE FOSSILS. at although only a fragment of the skull with the horn-cores, is shown in figs. 79 and 80. This specimen was found near Denver by George L. Cannon, jr., who has secured other important specimens from the same horizon. Still others of interest were found by George H. Eldridge, while 75 Fic. 75.—Skull and lower jaw of Triceratops prorsus Marsh; seen from the left side. About one-sixteenth natural size. Fic. 76.—Skull of Triceratops prorsus; seer from the front. Fic. 77.—Skull of Sterrholophus flabellatus Marsh; seen from behind. Both figures are one-twentieth natural size. d, dentary; e, epoccipital; h, horn-core; h’, nasal horn-core; p, parietal; pd, predentary; g, quadrate; r, rostral bone; 8, squamosal. ollz GEOLOGY OF THE DENVER BASIN. working out the geology of this region. The restoration of the skeleton on Pl. XXVIT shows the general form and position of one of these reptiles when alive. Fig, 78.—Maxillary tooth of Triceratops serratus Marsh. Natural size. a, outer view; b, side view; ¢c,inner-view; d, seen from below. Fic. 79.—Fragment of skull, with horn-cores, of Ceratops alticornis Marsh; front view. Fig. 80.—Left horn-core of same specimen; side view. Both figures are one-eighth natural size. CRETACEOUS VERTEBRATE FOSSILS. ilies The skull of Triceratops, the best-known genus of the family, has many remarkable features. First of all, its size, in the largest individuals, exceeds that of any land animal hitherto discovered, living or extinct, and is surpassed only by that of some of the Cetaceans. Another striking feature of the skull is its armature. This consisted of a sharp, cutting beak in front, a strong horn on the nose, a pair of very large, pointed horns on the top of the head, and a row of sharp projections around the margin of the posterior crest. All these had a horny covering of great strength and power. For offense and defense they formed together an armor for the head as complete as any known. This armature domi- nated the skull, and, in a great measure, determined its form and structure. In some species the armature extended over portions of the body. The skull itself is wedge-shaped in form, especially when seen from above” The facial portion is very narrow and much prolonged in front. In the frontal region the skull is massive and greatly strengthened to sup- port the large and lofty horn-cores which formed the central feature of the armature. The huge, expanded, posterior crest, which overshadowed the back of the skull and neck, was evidently of secondary growth, a practical necessity for the attachment of the powerful ligaments and muscles that supported the head. The front part of the skull shows a very high degree of specialization, and the lower jaws have been modified in connection with it. In front of the premaxillaries there is a large massive bone not before seen in any ver- tebrate, which has been called by the writer the rostral bone (os rostrale). It covers the anterior margins of the premaxillaries, and its sharp inferior edge is continuous with their lower border. This bone is much compressed and its surface very rugose, showing that it was covered with a strong, horny beak. It is a cartilage ossification, and corresponds to the pre- dentary bone below. The latter, in this genus, is also sharp and rugose, and likewise was protected by a strong, horny covering. The two together closely resemble the beak of some of the turtles, and as a whole must have formed a most powerful weapon of offense. The frontal bones are quite short, and early unite with each other and with the adjoiming elements, especially those behind them. The frontal or MON XXVII 33 514 GEOLOGY OF THE DENVER BASIN. central region of the skull is thus greatly strengthened to support the enor- mous horn-cores which tower above. These elevations rest mainly on the postfrontal bones, but the supraorbitals and the postorbitals are also absorbed to form a solid foundation for the horn-cores. These horn-cores are hollow at the base, and in general form, position, and external texture agree with the corresponding parts of the Bovidee. They vary much in shape and size in different species. They were evi- dently covered with massive, pointed horns, forming most powerful and effective weapons. The orbit is at the base of the horn-core, and is surrounded, especially above, by a very thick margin. It is oval in outline, and of moderate size. The enormous posterior crest is formed mainly by the parietals, which meet the postfrontals immediately behind the horn-cores. The posterior margin is protected by a series of special ossifications, which in life had a thick horny covering. These peculiar ossicles, which extend around the whole crest, have been called the epoccipital bones. In old animals they are firmly coossified with the bones on which they rest. ‘The lateral portions of the crest are formed by the squamosals, which meet the parietals in an open suture. Anteriorly they join the postfrontal elements which form the base of the horn-cores, and laterally they unite with the jugals. The supratemporal fossee lie between the squamosals and the parietals. The teeth of Triceratops and its near allies are very remarkable in having two distinct roots. This is true of both the upper and lower series These roots are placed transversely in the jaw, and there is a separate cavity, more or less distinct, for each of them. One of these teeth from the upper jaw is represented in fig. 78. The teeth in this family are entirely confined to the maxillary and dentary bones. The rostral bone, the pre- maxillaries, and the predentary are edentulous. The atlas and axis of Triceratops are coossified with each other, and at least one other vertebra is firmly united with them. These form a solid mass, well adapted to support the enormous head. The cup for the occipital condyle is nearly round and very deep. The rib of the second vertebra is coossified with it, but the third is usually free. The centrum CRETACEOUS VERTEBRATE FOSSILS. 515 of the fourth vertebra is free, and the remaining cervicals are of the same general form, all having their articular faces nearly flat. The anterior dorsal vertebrae have very short centra, with flat articular ends, and resemble somewhat those of Stegosaurus, especially in the neural arch. The posterior trunk vertebrae have also short, flat centra. The sacrum was strengthened by the union of several vertebree, ten being coossified in one specimen of Triceratops. The middle or true sacral vertebrze have double transverse processes, diapophyses beg pres- ent, and aiding in supporting the ilium. This character has been seen hith- erto in the Dinosauria only in Ceratosaurus and some other Theropoda. Besides the armature of the skull the body, also, in the Ceratopsidee was protected. The nature and position of the defensive parts in the different forms can not yet be determined with certainty, but various spines, bosses, and plates have been found that clearly pertain to the dermal covering of Triceratops or nearly allied genera. Several of these ossifications were probably placed on the back, behind the crest of the skull, and some of the smaller ones may have defended the throat, as in Stegosaurus. a In the restoration on Pl. XX VII the animal is represented as walking, and the enormous head is in a position adapted to that motion. The mas- sive fore limbs, proportionally the largest in any known dinosaur, corre- spond to the head, and indicate slow locomotion on all four teet. The skull is, of course, without its strong horny covering on the beak, horn-cores, and posterior crest, and hence appears much smaller than in life, The neck seems short, but the first six cervical vertebrae are entirely con- cealed by the crest of the skull, which in its complete armature would extend over one or two vertebra more. No attempt is made in this restoration to represent the dermal armor of the body, although in life the latter was more or less protected. Various ossifications indicating such dermal armature have been found with remains of this group, but the exact position of these specimens can be, at present, only a matter of conjecture. 516 GEOLOGY OF THE DENVER BASIN. This restoration gives a correct idea of the general proportions of the entire skeleton in the genus Triceratops. The size, in life, would be about -25 feet in length and 10 feet in height. This specimen was found by J. B. Hatcher,in the Ceratops beds of Converse County, Wyo, CLAOSAURUS. Another large herbivorous dinosaur, Claosaurus, has left its remains in the same horizon as the gigantic Ceratopsidze, but owing to their smaller size and more delicate proportions they are much less conspicuous, and hence appear to be less abundant. The best-known species of the present genus is Claosaurus amectens, and a restoration of the type specimen will be found on Pl. XXVIII. This animal was bipedal in locomotion and had very small anterior limbs. The head was comparatively large and the tail long and massive. The skull of Claosaurus is long and narrow, with the facial portion especially produced. The anterior part is only moderately expanded trans- versely. Seen from the side, fig. 81 (p. 517), the skull shows a blunt, rugose muzzle, formed above by the premaxillary and below by the predentary, both probably covered in life with a thick, corneous integument. Behind the upper part of this muzzle is an enormous lateral cavity, which includes the narial orifice, but was evidently occupied in life mainly by a nasal gland, somewhat like that in the existing Monitor, and also seen in some birds. This cavity is bounded externally by the nasal bone and the premaxillary. The median septum between the two narial orifices was only in part ossified, the large oval opening now present in the skull probably having been closed in life by cartilage. The orbit is very large and subtriangular in outline. It is formed above by the prefrontal, frontal, and postfrontal, and below mainly by the jugal. There are no supraorbital bones. ’ ] ‘ e / Try { , 7 J F *, ; ; ” ' 1 nl x Mi wee e Pi AC, Xexeloge JURASSIC DINOSAURS.—SAUROPODA. Restoration of Brontosaurus excelsus Marsh..-------------------- Ea panosiaedoanmocdcodowoo She. One-ninetieth natural size. ; ; ; Jurassic, Wyoming. , = 530 \ , ee = b, a en — = E RESTORATION OF BRONTOSAURUS EXCELSUS Marsh. One-ninetieth natural size. Jurassic, Wyoming. Wid bya s bel ve GH a PLATE Run, JURASSIC DINOSAURS.—STEGOSAURIA. Restoration of Stegosaurus ungulatus Marsh......--- +++. -+-+-++-s2eers rete ceee ove eee ae One-thirtieth natural size. : Jurassic, Wyoming. - : 532 — 4 \ ’ ® 7 > U. S. GEOLOGICAL SURVEY MONOGRAPH XXVIII PL, XXII —_——— ————————— — — —_ = = eae ian} RESTORATION OF STEGOSAURUS UNGULATUS Marsh One-thirtieth natural size. Jurassic, Wyoming. Jed opatd hs DCG PLAT Re XoOohLee JURASSIC DINOSAURS.—ORNITHOPODA. Restoration of Camptosaw'us dispar Marsh 2.2.2 Jcsc-ssenanav os ela sen ol ecicln == mentee eee melee en One-thirtieth natural size. : : Jurassic, Wyoming. 5B U. S, GEOLOGICAL SURVEY MONOGRAPH XXVII_ PL. XXIII rc —_ = = = — _ RESTORATION OF CAMPTOSAURUS DISPAR Marsh. One-thirtieth natural size. Jurassic, Wyoming. Dds Bia JE Ich so. TA | Po Ar nes Ke 7 ‘ : JURASSIC DINOSAURS.—ORNITHOPODA. Restoration of Laosaurus consors Marsh ........------++-+-++-++- eh, ee ene i One-tenth natural size. e Jurassic, Wyoming. . . | 536 ; U. S GEOLOGICAL SURVEY MONOGRAPH XXVII PL. XXIV RESTORATION OF LAOSAURUS CONSORS Marsh. One-tenth natural size. Jurassic, Wyoming. 16 Diva 0d er eo aa PLAT xe JURASSIC DINOSAURS.—THEROPODA. Restoration of Ceratosauwrus nasicornis Marsh, ......---- ---------- ---+ --+0 ene eee wane none is One-thirtieth natural size. Jurassic, Colorado. 538 ‘. MONOGRAPH XXVIII PL. XXV U. S. GEOLOGICAL SURVEY RESTORATION OF CERATOSAURUS NASICORNIS Marsh. One-thirtieth natural size, Jurassic, Colorado. es aTeeAvAlE ese Vl, 7 , ee or t ’ P) = i f cup Shi oe d ‘ : ae ‘ ! 7 4 Pa A execu CRETACEOUS BIRDS.—ODONTORNITHES. Fic. 1.—Restoration of Tehthyornis victor Marsh ........---.----- PNB REER peat qceaersecogorsacins 50! One-half natural size. iad cs ¢) Fic. 2.—Restoration or Hesperornis regalis Marsh. .------------------- Saleeine bicadesesionse vseees One-eighth natural size. Cretaceous, Kansas. 540 ae S. GEOLOGICAL SURVEY MONOGRAPH XXVil PL. XXVI | Fig. 1.—RESTORATION OF ICHTHYORNIS VICTOR Marsh. Fig. 2 RESTORATION OF HESPERORNIS REGALIS Marsh. One-half natural size. Cretaceous, Kansas. One-eighth natural size. Cretaceous, Kansas Oe Dp ld thee. oes SEI PUPA Pee sven, ORETACEOUS DINOSAURS.—CERATOPSIA. \ . Restoration of Triceratops prorsus Marsh --.... ---- BS wre oe See een e a naie see eae ee One-fortieth natural size. ’ Cretaceous, Wyoming. e , S ; 542 U. S. GEOLOGICAL SURVEY MONOGRAPH XXVII PL. XXVIE r ID) DP) we, was RESTORATION OF TRICERATOPS PRORSUS Marsh, One-fortieth natural size. Cretaceous, Wyoming deel beac blah ed. < oS i ea oa Bulg ADE, eX Xe Vahl CRETACEOUS DINOSAURS.—ORNITHOPODA. Restoration of Claosaurus annectens Marsh..... 2... -.0. 22.0 s0eeeeveeeee ees dat woke see set 516 One-fortieth natural size. : Cretaceous, Wyoming. . * Sie . o \ ’ ' e oe | * i *, i 7 , PUA Rex QUATERNARY MAMMALS.—PROBOSCIDEA. , _ Page. Restoration of Mastodon Americanus Cuvier ...-..-------0+ --0 nee cneeccccnecscccecenccaccces- 524 One thirty-second natural size. Quaternary, New York, 550 U. 5. GEOLOGICAL SURVEY MONOGRAPH XXViI PL. XXX! 35 \ 3 LS pF! rare creat 8 RESTORATION OF MASTODON AMERICANUS Cuvier. One-thirty-second natural size Quaternary, New York INDEX. A. Page. Absorption of water, capacity of different strata for.. 419-420 Aceratherium, figure of remains of 525 Algonkian formations......-----+-+---+++-+++++++++- 10-13 Alkire artesian well, Denver, description of...-. 442, 459-460 Allacodon, figures of remains of..-......------------ 521 Allen-Bond coal district, features of.....-.---------- 350-351 American House, Denver, description of artesian VGA 3 oe Soo deinod ate ce Oe Re OS ORES EES oS 434, 453 Analyses, rocks.... 55, 66, 67, 301, 306, 308, 314, 375-387, 388-390 SL LOR IRD ALON ote mia oes elle leant eae 461-463 Anderson artesian well, Denver, description of..-.-.. 432, 448 analyses of water from.......-.---.------------- 461, 462 Andesitic pebbles of conglomerates, character of. --.. 315-316 Animas River beds, description of...--.....--------- 217-219 fossils of = 218 Apatosaurus, description of..........-.-.----+---- . 489-492 Arapahoe epoch, orographic movement at close of... 32-33 phenomena of -....--.---.- 5 246 Arapahoe formation, features of.......-... 31-32, 148, 151-155 PORS UA Ulan soctan cose ate 78-79, 154, 208, 215, 217, 218, 221-244 line between Laramie and 146, 212 PONAT CividIOW Oleaice se orie olen sale ele 152-153 APN POLORISION Ole een. =--0 === = tana 153-154 ND, See oa Boe ee eee once eeeerc eens 206-252 stratigraphic break between Laramie and....--. 212 relation between Denver and..-...-.-.-.----- 212-213, 245 HOS? Qe sen Sees ceccce se gor cee eee ee ses 225, WELLeprace LOSSUIS) Of. so = - 306, 308 Bear Creek, Denver beds on and near. --- 190-192 Beckwith, H. C., fossil plants collected by --- 468, 486 Beecher, C. E., fossils identified by.-.-..........---. 234 Bellevue, Colo., stone quarries at.-........-...-..-.- 395 Belt, Thomas, human remains found in loess by-..-- 264 Benton formation, description of....----..-.--.--- 65-66, 107 LUSH) menos SoS oS ook oe eneeas ee nas sees 66, 78-79 outcrops near Golden, Colo...---.-....-.-.------ 87 Berthoud, FE. L., cited................ - 157,332 Bijou Creek, Colorado, fossils from....-- -. 243-244 Black Butte, Wyoming, Ceratopsid from ... Ss 237 Boulder, Colo., structure of the region about........ 105-111 unconformities near..--..-.-.-.---------- : 109 Pen WE Se Sassen sce actors sah BicSesosceroes 114 section near... 142-143 CORT G1U Cheer ee eee ia aaeeiatenta aie oes ne ao 339-371 Boulder peaks, structure of....---.-- ieee anemia ers 6 Boulder Valley region, geology of..--....-----..-..- 120-141 HOVGE iseansa4 Ce atocec toa ESS se eR BB SONS S5 122-128 faultsiofe-=----ce--- - 128-141 Brick clays of the district 27. Brontops, restoration of. ... os 546 Brontosaurus, description of. - 492-494 TESLONAONOL «2 ssenieee ne ene om at 530 Brontotheridx, figures of remains of.........--.---. 546 Brontotherium, beds containing remains of......--- 479-480 figures of remains of ...-...--..-------.--..----- 522 Brown artesian well, description of -.-..-.-.---.- 436, 454-455 Building stones, occurrence and character of. .....-- 392-401 product of. -----------.------~ 02-2 oe owen ---- 400-401 Burnt Knoli, section through..........----.----..-- 123 C. Cache la Poudre River, discharge of .........-.----- 414 Cambrian and pre-Cambrian formations and topog- DAY eee ntede Soa ete cem ec anne ncaa mses 10-15 Camptosaurus, description of....--.---.- 502-503 restoration of ....---.----- 534 Canal fault, description of ...........--..-.--.------ 141 502 Page Canfield-Erie trough, features of.............+.----- 127 Canfield-Erie coal district, description of.......---- 367-368 Cannon, George L., acknowledgments to.-....--..--- xxi, 255 vertebrate remains found by--..------------ 226, 511, 518 Cnt | Mase cinnecoscosngshacoduacoqodsuensntzocie6s 257, 263 Canyon, Colo., strata near..---.--------------------- 216 vertebrate fossils from near..-.--.--.----------- 526 Carboniferous period, orographic movement during- 17-18 Castle Gate, Utah, dinosaur-bearing beds at .. .----- 241-243 invertebrate fossils from near....---.----------- 241-242 Castle Rock, Colo., stone quarries near...----------- 399 Cenozoic age, vertebrate fossils of. . -- . 520-525 Ceratodus, figure of remains of ...--..-------------- 507 Ceratops, figures of remains of..--....--.----.------ 512 Ceratops beds, localities and geologic place of..----- 229- 240, 477-479 Ceratops fauna, localities of.........-..-.----------- 228 significance of ..---------.-----.------- E 229 OYE) O}9 cee sesccoscacsss 26 251 Ceratopsid, description of. ----- -- 509-520 Ceratopsia, figures of remains of- 55 542 Ceratosaurus, description of ---.- -- 503-506 ALT INO NO Poe een ongeescie ssaeoedesnetases 538 Chamberlin, T. C:, cited -..-.....-..--...... 278, 405, 408-410 Chamberlin and Salisbury, cited.....-...-------.--- 275 Gharmrencterin. Chen seme see a ea teat oi sini 76 analyses of artesian water by--.-----.------- 461, 462-463 Cherry Creek, Denver beds near.-.----..----------- 194-195 BULAL AEX OSCCLOM rete ate at oste janie tntel se lete a eletate tate 197-198 evaporation of. 417 Cimolomys, figures of remains of ..-..--..---------- 521 Clear Creek, Denver beds near-...-..-...-----------. 191-193 Claosaurus, description of .-.......--..-.---..--..-. 516-518 fieuresiof remains of--=-------.----- eee <2 517 TESS HOE AU OSU hte are oie anette alee Wie ae eats acters 544 Clays, description of---- 3 69, 75 for brick-making---..-----..-.- 3 274 occurrence and character of - -- 387-392 analyses of..----.---... -- 388-390 valne of product from . at 392 Coal, geologic place of --. ~ 323-325 ABYGAS| Obie mee eee a el =e e20=305) occurrence, development, and character of. - 317-387 BLOOM CHON Of mee een am =n mel --- 317-320 character and chemical composition of...-.-.-.-. 375-387 Coal beds of the Laramie formation, character of -.- 74 Coal Creek, tag) Ones sean seater ee 137-188 Denver UGd Si Nealae == oes = oan ace ete 194-195 coal measures in area east of....--..-.------.--. 368-371 Coal Creek Peak, description of region about-.-.... 112-114 geological development of the area about -.----- 113-114 Coal Creek syncline, description of .--...... 124-128, 353-368 OnE ROADS Ole essen cass a <3 /-— =~ = 353, 354, 357, 358, 359-868 avaiiaple cosliares Ofte. -252 ses. Seca e else 363, 364 Superior district of...-.--...-.....-.....-..-.-.. 364 Louisville district one one ee meeen ian 364-365 Lafayette district of-..............-..-.-...--.-. 365-366 Mitchell district!0fes.. ces. <= 0. -= =) aimee ne 366 Canfield-Erie district of ----...-..-...-.-.-.-.--- 367-368 Coal measures, strikes and dips of. 328 ROCMOM Olea civics samntcielseameioe tl ae 334 Coal mines, list of. -- 320-323 Colorado formation, character of. .--. 26, 64-68 Colorado range, characters of ancient shore-line of. . 2-3 Conglomerates, descriptions of.............--.+-+---+ 63 INDEX. Page. Converse County, Wyo., Ceratops beds of... 231-236, 477478 Cope, E. D., cited 60, 218, 237, 238-239, 243, 244, 250 vertebrate remains examined and described by.. 226, 228, 257, 238-239 Court-house artesian well, Denver, analyses of water from enews mee cee eee ete ee ie near 461, 463 Cragin, F. W., cited .--.-..--.-.--- ac 264 Creamy sandstones, character of.--. - 19, 53-54 Cretaceous formations, descriptions of..-...---- 62-79 building stones of..--.-....-----------------...- 397-398 Cretaceous period, orographic movements during... 23-25, PY 26-27 vertebrate fossils of...........-----.------sse-<0 509-520 Cretaceous vertebrates, beds containing.-.-..---.--. 476-479 ESCriptiOns | Of esas sateen seen eee een 509-520 Cross, Whitman, cited......---- 11, 34, 36, 203, 204, 214, 238, 257 vertebrate remains found by.-.---.---.--------- 226 Cross, Whitman, and Eakins, L. G., cited.........-- 316 D. Dakota formation, character of......-.---------- 25-26, 62-64 plantsof sess aee eee fe oais cet mend este osote 64, 469-471 outcrops near Golden, Colo.---...-.-..-------... 86-87 FOaUUTOS! Ohi ae eit eee ee ee 106-107 Dakota quartzite, building-stone value of........... 400 Davidson coal district, description of........-....-.. 351-352 Davidson faults, description of.....---- 133-134 Davidson section, description of...--.-.-.---.------. 143-144 Davidson syneline, description of. 122-124, 339-345 coal bedsiofee- pens. - eee eee eee eee sree eee 340-345 Deer Creek fault, description of.-......-..--..------ 119-120 Denver, Colo., exposures of Denver formation at and HEM Aaa SoS aconstosnoceassdiensonesedncnd cence 193-194 artesian wells at and near. 402-403, 422-465 Denyer and suburbs, detailed account of wells of..-. 422-465 Denver artesian basin, form and structure of.....--. 410-412 Denver epoch, orographic movement following..-.--. 36-38 Life ofertas 34-36, 78-79, 208, 215, 217, 218, 222, 244, 248 volcanic eruptions immediately preceding the... 246-247 phenomena, Olieneaaniee eee een eee 247-248 deposits and phenomena of period following.-... 248-249 Denver formation, character of.....---...----------- 33-36 fossils of 34-36, 78-79, 208, 215, 217, 218, 224, 226-252, 471-473 line between Laramie and.....-.--.------..----. 146, 245 relation between Monument Creek and...-.-.-.- 149-150 detailed description of..-..----.:- - 155-206 estimated thickness of... - - 4 172 occurrence and extent of - « 184-199 sources of materials of . - - 199-206 CXEGIA) tome ee ereeemsc . 206-252 volcanic materials of - - SeemotcdsocncenaseS 210-211 relation between Arapahoe and -. 212-213, 245 HES fein soa crondenia gence ate aes Sas Sssos5 224, 471-473 TOCKSiOfere eae ee heen eee ER ee ce eee ee ere 311-316 water-bearing strata in.......-----.---.----. 408, 423-425 Denver Water Company, description of artesian wells SUT Dyes as se eae ee ee eee 436, 454 Dinosaurs of Ceratops beds, age of . 3 251 description of ......-.-.---...-- -- 509-520 Diplocynodon, figure of remains of- cic 508 Diplodocus, description of..-...--- --- 494-496 Diplosaurus, figure of -- - 507 Discharge of streams, table showing. -.-------.----- 413-416 Dolerite, analysis\of---. 2. 0 eee === 301 Drift, deposits of...... eoseewceer Basoodce Sect een ae cGD—s00, INDEX. a Page. Fakins, L. G., analysis by -----------+++--++-- 55, 67, 308, 310 Eckhart artesian well, Denver, description (CS ea 432, 440, 450, 458-459 Eggleston syncline, description Ohits ace nana wen 124 coal seams of ..----.--------------+ +--+ +72 2077-7 352 Emmons, A. B., cited ---..---------+--++------++++- 380 Emmons, S. F., cited 10,45 Eldridge, George H., cited... . 19, 20, 21, 22, 27, 30, 159, 206, 216 vertebrate remains found by ------------ 226, 228, 230, 511 Electric Light Company's artesian well, Denver, description of ...--.----------+-+---+r22-2 72220007" 438, 457 Entelodon, figures of remains of -- 523 | restoration of...------------- 555 548 | Erie fault, description of. ...-.---- 139-140 Ester artesian well, Denver, description of...---.--- 432, 449 Evans artesian well, Denver, description (seer ose 444, 460 Evaporation of the region, amount Of ceees eee ena= = 416417 FE. Fault near Boulder, description of .----------------- 114 Fault of South Boulder Peaks, description and fig- ures Of .----- ------ --- = 22 een ne nnn renee neers = 115-119 Fault south of South Boulder Creek, description of- 119 Faults (overthrust), description and figure of ..-..-- 48,49 Faults near Golden-.--------------++-++-+-+++20+2++- 90 Favlts, analysis and classification Die asa 116-118 Fanlt system of Boulder Valley region.-...--------- 128-141 Felch, M. P., fossils found by --------------++------- 494 Fire clay of the region, character of ....-.------- 64, 387, 391 analyses Of ..--.-----------------220e5eerte> 388-389, 390 Fisher, 0., cited. .....-----------------+2esstesteete+ 117 Fleming artesian well, description of --- 436, 455 Fluvial loess, deposits of ..-.------------------- -. 260-261 Folds (transverse), description of ---.--- 50 Foothill area, topography of. .----------- 5-7 structural geology of----- 45-50 coalin..-.-..----------s----- 2-2 eeen sense nenae- 326-339 mines of. ....------------------2=--- 20ers eter ee 327 Fossils, Laramie and allied formations...----------- 249-250 vertebrate..--.------------ 222202022022 t rte 250-251 Denver Basin... --.------------22---2++--00 2002227 466-550 Fossil plants, geologic age of.-----------------++---- 35 Denver formation -.----.------+-----+-++++--->-- 222-226 Denver and Fork Union floras 249 work of F. H. Knowlton on - 249 section by F. H. Knowlton on-.--...------------- 466473 localities for-..------------------ .- 468-469 horizons indicated by-.---------- _. 469-473 Dakota group ----------------- -- 469-471 Laramie and Denver-.--------- --- 471473 Fox coal mine, description of------ cote 348 Fox Hills formation, description of-.-.------------- 28, 71-72 transitional zone between Pierre and. --.-------- 71 fossils of ..-..--------cee-e-e--- == seen nnn ene 72, 78-79 building stones of. S97--398 Fox Hills sandstone, water-bearing character of. ..--- 406 G. Gas Works, Denver, description of artesian well at.. 436, 456 Gate Creek, strata exposed on 196-197 Gehrmann, C. A., analyses of artesian water by-- 461, 462-463 Geikie, A., cited 59 Gilbert, G. K., cited | 553 Page. | Glacio-natant drift, deposits of..-.....--.--+----++++ 265-266 Glencoe, Colo., stone quarries at..-..---.------------ 397 Glytops, tigure of remains of..---..----------+--+--- 507 Gold deposits of river drift. ...--..-..--..------+---+ 269-272 Golden, Colo., description of the region about.....-- 82-104 structural development of the region about. 91-101 unconformity near...-..-------+---+-+-+++- - 98-100 section of coal measures at.-.--- a 334 Golden arch, period of elevation Olice aces eenetaae 25, 27 Golden coal mine, product of...-...----------------+ 317 Golden district, coal of...- Golden Star coal mine} description of-..-...--.------ 336 Grant Smelting Works, Denver, description of arte- sian well at.....--------------+---222eee erro ee 438, 457-458 Green Mountain, rocks of..-..-.-----------++++--- 171-179, 189 H. Haddon, Geo., fossil plants collected by ...---------- 468 Hallopus, beds containing remains of ...---.- : 475 description of ....------+------+-+++++++-+-- - 481-483 Halodon, figures of remains of 521 Hatcher, J.B., cited .......----- 231, 232, 233, 234, 235, 238, 239 acknowledgments to 237 SBC sols eta nlai le inte aro 478 vertebrate remains found by) 516, 518 Hay, Robert, cited ..--------------+-------+----+---- Hayden, F. V., cited Hayes, C. W.,and Willis, Bailey, cited--.------.---- 44 Hecker artesian well, Denver, description of...-...-- 440, 458 Hesperornis, restoration of------.------------------- 540 Hillebrand W. F., cited. ----------------------------- 292, 390 analyses by -------------------+++-----+--+------ 306, 314 Hills, R. C., cited ..-.----- 37, 60, 204, 213, 216, 217, 219, 220, 340 fossil plants collected by ------------------+----- 468 Hogback ridges, character (Mis essore asec sepcor Boos 5-6, 62 Home artesian well, Denver, description of.-...-..-. 436, 456 Huerfano Basin, formations of 217 Hydrography of the Denver Basin.--.-------------- 413-422 1G Ichthyornis, restoration (S iereise pipe Ae eee cnoseaacae 540 Igneous formations, description of 279 Invertebrate fossils of Laramie and allied formations. 249-250 Tronstones, descriptions of -. . 65-66, 70, 75-76 analyses Of ...-..-+++---+---+----02- 222 creer tees 66, 76 J. Jackson-Star fault, description of. ....-------------- 140-141 Judith Basin, Montana, vertebrate remains in------ 478 Judith River beds, fossils of .....------------- . 240-241 Jurassic period, orographic movement during - 21-22 vertebrate fossils of.-.-------------------- --- 480-509 Jurassic rocks, description of...... 60-62, 86, 106 Jurassic vertebrates, beds containing. .-.----------- 475476 description of ...--------------+-----++-200522 25> 480-509 K. Kinner Run, strata along. ....----------+--+-+------- 161 Knowlton, F. H., acknowledgments to. -------------- xxi OitOd ees en nah nance tes neces an=-n nse mavewen=ns 220, 221 work on fossil plants by. --..-------------------- 77, 208, 215, 221, 224, 225, 235, 237, 238, 249 detailed account of fossil plants by-..----------- 466473 Knox artesian well, description of....--------------- 436, 455 504 Page. L: Lafayette coal district, description of. Lafayette trough, features of................--.----. Lake terraces in the Plains region.......... .....--. Lakes formed in post-Arapahoe time, locations of... 32-33 Lakes, A., acknowledgments to xxi CONG le eR eS ESAS aon Reet nance Reece rpeesegcecacs 12 fossil plants collected by. 226, 467, 468, 486, 489 Laosaurus, restoration of.......-...-..-.------------ 536 Laramie formation, characters of. - - 28-29, 72-77 coal beds of ............-- os 74 local alteration of. 74-75 TOSHIBA ON 2 = 52 Sooo en ora cinenea a= eee ae me ace 77, 78-79 outcrops near Golden, Colo..--.- sons eceasSac=-ee 89 line between Arapahoe and..-.........-...-..---- 146, 212 thickness\ofess- sce once -\-ceas-s sce eaceeascenes 147 line between Denver and.----.............------ 146, 245 stratigraphic break between Arapahoe and...... 212 CHU fel IOs ee oe secoesces male(einle nieieeia eee ces ate 317-465 section of lower division of-........---.- Reeanece 346 water-bearing strata in...............-.. 406-407, 423-425 Laramie period, orographic movement at closeof.... 29-31 plants of -. -- 471-473 Le Conte, J. L., cited . 156, 466 Lesquereux, L., fossil plants determined by-..-..-- 5 59, 222-223, 466, 468 Cite dieses cea cee wap erp passes nce se ae eee 157, 467, 469 Leyden coal mine, description of.........-.-.-...-.. 338-339 Limestones, descriptions of.........-....-.-------.-- 66, 69 CHEN SIGH ooemaie SSrgeccoetarene= bop seesaneee as 67 Livingston formation of Montana, description of. ... 220-222 fossila Of Jose eee o eee cope eeaeakeosecee uses 221-222 Loess, character, divisions, and origin of.....--...-. 41-42, 258-266, 274-278 economic valueiof..----- ene nen ennn-- 212-203 Water CONTSM KO teemena sees nna lo (asin «omens eee 273 BNAlYSOS\0 tee ener nee heen te ee oa ae ee 263 TORRE IS a5 55 sq QSOs “ne Bec pe ggSnesccomeSsSeime 264 BLOSION Of see eer enone eae nae ees 266-269 thickness of...........----- Sasa cseeecsncss TET) Louisville, Colo., cross-section at 125 coal district at... - 364-365 fav Neagle alee else = 139 Loveland coal mine, description of... = 335 Lyons, Colo., stone quarries at..-.....--...-------.-- 396-397 M. Macelognathus, figure of remains of................. 507 Mammals, Jurassie, figures of remains of 508 Manitou building stone, occurrence and character of. 394 Margerie (de) and Heim, cited.....................-. 117 Marquis artesian well, Denver, description of.....--- 440, 458 Marsh ONG cured eanieat eos 23, 26, 31, 34, 39, 60, 77, 243, 250 vertebrate remains identified by........ 226-233, 241, 242 detailed account of vertebrate fossils by-..-...- 473-550 Marshall coal mines, description of.................. 347-350 Marshalldistnichcoaliotecs--nsena-ssssee. .s-oe eee 345-350 section of coal measures in.. 346 Marshall subsystem of faults, deseription of-...-... 129-133 Marshall, Murphy, and Golden coal inines, product of 317 Marvine, A. R., cited.........2. - 12, 157, 158, 214, 281, 285, 332 WManrtodon; restoration \Ol-. +-22esasecsenen= ssc ace 550 McGee, W. J., cited...... 274 Mesohippus, figures of remains of... 524 Mesozoic formations, structure of...........-.------ 79-150 INDEX, Page. Mesozoic geology, description of..................... 51-150 Middle Park beds, plant remains of.................. 215 Ceseription of siamese eee 214-216 Milwaukee Brewery, Denver, description of artesian Well ationic os canst a lease See eee eee eee 434, 453 Mitchell coal district, description of. ale 366 Mitchell trough, features of........ --- 126-127 Montana formation, features of... ee 28 outcrops near Golden, Colo...........----...---- 87-89 Montana group (Pierre+ Fox Hills), characterof.... 68-72 Monument Creek formation, character, age, relations, ANd Tite ofa. o~ tS eneenaesn oes 38-39, 146, 149-150, 252-254 line between older formations and.............-- 146 relation between Denver and....-.....-.---.---. 149-150 Morosaurus, description of - 496-498 Morrison; Colo: sectionjat.s<-s52s5= see eee eee 52 vertebrate fossils from near....-.........------- 526 Morrison formation, age and characters of..-.-.....- 22-23 POscriptionjobaccsewaees eck ae ee eee 60-62 Mountain topography, character of...... .-..--..--. 2-5 Mount Carbon district, coal of...........-..-..-..--- 329-331 Mullen artesian well, description of...........-.---- 434, 453 Murphy coal mine, product of....-..--...--.-------- 317 Murphy Creek, Denver and Laramie beds on...-...- 195-196 N. Nanosaurus, description of.....-...-. --..--.---.--- 483-485 Newberry; JS (cited...0 520-0: 2. seoctacccseee eee 60 fossil plants examined by. 5 223 Newell, F. H., cited........-. . 404 Newlin Gulch placers, description of. --... --- 270-272 New White Ash coal mine, description of. -- -.- 335-336 Niobrara formation, description of----....-. - 66-68, 107 fONSiIS:Of2 2... so cee se eeeee ees aeee ee Oe 68, 78-79 outcrops near Golden, Colo.........---....-..-.. 87 North Boulder faults, description of...............-. 136-137 North Table Mountain, vertical section at........_-- 157 topography and rocks of.........-.......-..- 170, 287-288 0. Oakes artesian well, description of................-- 432, 450 Odontornithes, figures of remains of. 540 Ohio Creek formation, description of. . 219 Old White Ash coal mine, description of. : 335 Oracodon, figures of remains of...-.-.-- a 521 Ornithomimus, description of.............-.....-... 518-520 ALSNTCS Oi TOM BING) O fee — nee eee eee 518 Ornithopoda, figures of remains of....--....---.- 534, 536, 544 Ostrea bed in Laramie formation, character of. -.--- 74 Overthrust faults, description and figure of.-....... 48,49 Pe Paleontology of the Denver Basin, detailed account (S$? oclsancooceccact cost ces fic st - 466-550 Paleozoic sediments. 15-17 Palmer, C.5S., cited ....-.-. E 297 Palmer, Gen. W. J., early exploration for artesian AEC Oh ice eee en-e Socbaseeciecendcrcestamos sae 401 Peale, A. C., discovery of Wyoming fossils by-.------ 57 ORT ere aanar ao Sbocneaesrma snore sSasqacosascs 221 Dakota plants collected by......-..--..--------- 467 Physiography of the Denver Basin. .....-...-..----- 1-10 Pierre formation, description of. -...---.--.---.---- 28, 69-70 transition zone between Fox Hills and.-....-..... 71 HOLE HEROS Ae AnbagocnoacsecOonooge mem csobce! Sasags9 78-79 INDEX. Page Plains area, topography of........-.-..-.0--+------- 7-10 REGLOL Ole mntteeiesm= seee a 43-45, 120-150, 141-150, 179-182 nt HAG! 35.06 sense cnce ad SoS ROSIE ICDS DSRe 182 Plants (fossil), geologic age of..-....---.----.-.----- 35 detailed account of........--------- Beech hiteos 466-473 ROGRIL GION OSes n werent ne eee ae ine 468-469 horizons indicated by.....-.-...----------------- 469-473 Dakota group ..-..---- . 469-471 Laramie and Denver.. - 471-473 Platte River, terraces of. -- - 8-9 Denver beds near. ~~... eo neo we wwe n- se ener n=-- 198-199 character of drift of. 259 Pleistocene formations, character and extent of .... 40-42, 255-278 PORBUS OL ec sa ccd cseeten sae = aslo ar aaa ieee 260, 264 GTO Gsocecceoscoosrbossscencnecetseson bans 266-269 economic features of ......---.......------------ 269-274 Pleistocene geology, description of .......----------- 255-278 Pliohippus, beds containing remains of .....-..----- 480 fipnres.0f LEMANS) Ol. ~~ .---==-=-~=—=-l= => pel 524 Plum Creek section, description of - . 149-150 Price River Canyon, fossils from ---..- - 241-242 Proboscidea, figures of remains of 550 Protohippus, figures of remains of 524 Pteranodon, beds containing remains of.....-.-..-.. 476-477 description of ...--- eileen scan ieee mercer sate 509 figures'of remaing)0fs+...-c-c.-+ccncweccees= cen 510 Q. Quartz-porphyry, occurrences of .....---..--.-.----+ 297 Quaternary period, geology of......---.----.-------- 255-278 LORE @ Re eactcns ssehoco cee sa aao ao sccocenocS CHSE 258-266 river drift of - - 258-260 fossils of .-.- - 260, 264 Gye HG Bessa soe oreo sonconaccsoaoocc ses: 266-269 R. Rainfall of the Denver Basin, table showing..... = 413 amount absorbed by different geologic forma- HILOT IE oS cbrece Seong secs acectb ade CSc ane 417, 418, 419-420 Ralston Creek coal mines, description of --.--......- 337-338 Ralston dike, location of. .....-- 0000.2 ---ccncenn-- 7 MEBOLID ONY Olncmen saan eee sain nase ae eee 281-283 LIES ET Gt peared sesocc nada scaeconos sascsmncSanscc 302 Ralston section, description of..-.-..----.----------- 144-145 Ralston Springs coal mine, description of ...-......- 337 SPOR BUN ed a\S5 ey AICI D Yj coms dineieee anise emrninia wan ttmmereta 205 Red Beds, stratigraphy of......----..-..-----.------ 51-56 correlation of - 56-60 fossils of 57-60 building stones of. - 393-397 Reptiles of Jurassic age, descriptions of .. --- 481-508 Rhinoceros, Cenozoic forms of..........--- mies 525 Richthofen; F.-von, cited......----- 2.02. sncnesweceene 274 River drift, deposit of ........---------------- SOoacics 258-360 POLO DORIS Of - oo nec cee ena a n= eam nine eee 269-272 ALG RI CONTE Y Of a)a ane ania = ete reales cielo 272 River terraces in the Plains region. 2 8-9 Rock Creek fault, description of.........---. 2 139 Rocky Mountain coal mines, description of... 336 Ruby formation, description of ------.--.-- 5 219 Run off of streams, table showing.-...---.---------- 413-416 Ss. Salisbury and Chamberlain, cited ........--.....---- 275 Sand Creek, Denver beds near .....-..-..--+-+-+---- 194 5dDd Page. Sand Gulch and Harper faults, description of ....... 134-134 Sandstones, descriptions of.........-.-..-.-+..-+ 63, 71, 73, 75 Sauropoda, figures of remains of ........----...... = 530 Savage artesian well, Denver, description of..-....-. 438, 457 SCOuu Vets aC LCC aorta saeeaislana i ee eee ere 39 Scranton coal field, description of seveeee-- 313-310 Seudder, S$. H., Wyoming (Red Beds) fossils identi- GOD yates sateen cece See ce ctace soasee eee eee 58,59 Section ravine, strata of.............2...0---------0- 160-161 Sedalia district, coal of... 40 329 Shales, descriptions of..............---.---- . 65,71, 74 Shannon artesian well, Denver, description of -- 440, 458 Shore-line deposits, description of--- - 182-184 Silurian rocks, building stones of-.- 393 Slack, C.S., acknowledgments to.....--.....-.- xxi South Boulder Peaks, description of fault near...... 115-119 South Park area, former connection with theocean of. 4, 14-15 South Table Mountain, strata of-....-........-...+.- 161-170 [RN Te ao pa CUD BOU ANS Ech OC CSUR ASO OD Ro 165-168 DasalolG TOCKAOL-- cna aeamen ses se aeen een sam 288-289 Stagodon, figures of remains of. ..........-.-.....--. 521 Stanton, T. W., fossils collected by....-...------ 193, 226, 241 table of invertebrate fossils prepared by........ 234 (ORME) aoe ee sminick amdgcinss ALOR O ree COR Eon 239-242, 250 Steam Heating Company's artesian well, Denver, description of ......--...- - 436, 456 Stegosaurus, description of. - 498-502 restoration of. .-.------..5..2... 532 Sterrholopus, figures of remains of. 511 Stone for building purposes, occurrence and ¢ liar. HOLST Laer ete te elena te thea ten ni olin --- 392-401 produet of - 400-401 Stout, Colo., stone quarries at....... 396 Streams of the region, discharge of --- 413-416 Strikes and dips, coal measures. --..--..--..--....--- 328 Structural geology of the region, generalcharacterof. 42-50 Stylacodon, figure of remains of............---.--.-- 508 Sullins, A. L., vertebrate remains found by.--.....-.. 518 Superior coal district, features of............-.-..--- 364 Te Table Mountain, Denver strata of-..........-.-...-. 184-186 fossils of Denver beds of. =. 222-223 surface flows of. ......-- -- 285-296 zeolitic minerals of. -- 292-296 character of basalt of. - 304-308 analysis of basalt from.....-........-.------..-- 306, 308 PCH] CAEL] GILLEN Eu Rates ete re tate tea sla nw ee ela eto 312-313 SNALYSi8 (Of (HMMEHEROM pee aemie an =inn em ncn nae 314 Tectonic geology of the region, general ohanictes of. 42-50 Telacodon, figures of remains of .-.....-.--.-------- 521 Terraces in the Plains region...-.-....-..-------..-- 8-9 Tertiary formations, fossils of.......-....-----.----- 479-480 Tertiary geology of the region..-....---..------..-. 151-254 Testudo, figures of remains of .---- Sia de ananassae 523 Theropoda, figures of remains of.--.-..-----.--..--- 538 ‘Litanops, figures of remains of.....-..----.--------- 522 Triassic formations, description of. . 51-60, 84-85, 105-106 building stones of.-..-..--.----- --. 393-397 Triceratops, figures of remains of. . - -- 511-512 description of'................. - 513-516 restoration of. 542 Tuff, deposits of. 311-315 analysis of - 314 Turkey Creek, cade sis sof augite-mica-syenite Faia 310 Twelvetrees, W.H., cited ....----.---.0-s--c00.0--- 59 556 INDEX. Page. | Page. U. Water (artesian), analyses of. .-..-.......-<.00------ 461-463 in Pleistocene deposits. .-...-..-.-.-. 272, 273 Unconformities in the region about Golden. ..-.....- 91-97, | Water-bearing strata, enumeration of..-... .. 406-408 98, 99-100, 111 power of transmission of.....--.---.- =. 420-421 Unconformities in the region about Boulder ..---... 109 capacity of iscses eee See ee ee ee ee 421-422 Union depot, Denver, description of artesian well at- 453 charactenot:. 2-20. ee eee #.. 423-495 Union Pacific hospital, Denver, description of arte- Water content, Pleistocene deposits--.-.. =e) 272/273 Bian gy el baits piase eet aa ener senate entence ns 438, 457 various strata _.. 419-420 220, 221, 240 Vv. ||, Wells; artesian; detailed account of-..-.--..-..----2. 401-465 Bacon ocr sonoscnondseseoseseateetsccsssess 425-427 Valmont, Colo., analyses of rocks from ...--.....-.- 301 . 428-429 Valmont dike, description of........----.-----.----- 280-281 causes of decrease in flow of....-.-...- .. 429-430 DASA Ofee eee mame ee eee ee aetna 298-302 data)concerning.-----22-— 2 —- “ 1210-1220 | 409-419 3, 634 25 | ope Pennessee I ae - Has | Titus ey al ee ee eee Se cag r eee : 83° 30/-84° 35° 30/-36° 925 25 17 | Marysville*. {orth Carolin ad 1219 30/1220 | ceoczaoe 0" | 925 | 25 18 Smartsville* California. - 1219-1219 30/ | 39°-39° 30! | 925 25 ee |( Alabama. == 19) ) Stevenson® - 2.2.0. < enw n ae enennen=-= {Georgi aot < \ 85° 30'-86° 34° 30/-35° | 980 25 aes | Tennessee - = | 20 | Cleveland*. Tennessee - 84° 30/-85° 35°-35° 30! | 975 25 21 | Pikeville* . ---| Tennessee - 859-85° 30! 35° 30/369 | 969 25 CoMeMLGMOUNVALLO® .o concer eneeesree ease ees =e Tennessee - 85° 30/862 | 35° 30/-36° | 969 25 a | ues ener ye (Virginia 3 \ 76° 30'-779 | 380-380 30/ 938 25 24 ree Forks. - ontana. - - a 1119-1129 45°-46° | 3, 854 50 29) || LUST TS is SRR SS ese csoasecmoaconcs Tennessee - 849-819 30/ 35° 30/-36° | 969 25 Boubeacshontas-<_.<.cs2--<2ssiesseeeesee (Wwoet ing \ate-si° 30" | 379-379 30! 951 25 Po PREIS GOT oan oo wale cies eee Tennessee 83°-83° 30’ | 369-369 30’ 963 | 25 * These folios can now be sent only on prepayment of price stated in the last column. Vill ADVERTISEMENT. STATISTICAL PAPERS. Mineral Resources of the United States [1882], by Albert Williams, jr. 1883. 8°. xvii, 813 pp. Price 50 cents. : Mineral Resources of the United States, 1883 and 1884, by Albert Williams, jr. 1885. 8°. xiv, 1016 pp. Price 60 cents. Mineral Resources of the United States, 1885. 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Correspondence relating to the publications of the Survey should be addressed To THE DIRECTOR OF THE UNITED STATES GEOLOGICAL SURVEY, WASHINGTON, D. C., November, 1596. WASHINGTON, D. C. Series. Author. Subject. LIBRARY CATALOGUE SLIPS. United States. Department of the interior. (U.S. geological survey.) Department of the interior | — | Monographs | of the | United States geological survey | Volume XXVII | [Sealof the depart- ment] | Washington | government printing office | 1896 Second title: United States geological survey | Charles D. Walcott, director | — | Geology | of the | Denver basin | in | Colo- rado | by | Samuel Franklin Emmons, Whitman Cross, and George Homans Eldridge | [Vignette] | Washington | government printing office | 1896 4°. 556 pp. 81 pl. Emmons (Samuel Franklin), Cross (Whitman), and Eldridge (George Homans). United States geological survey | Charles D. Walcott, di- rector | — | Geology | of the | Denver basin | in | Colorado | by | Samuel Franklin Emmons, Whitman Cross, and George Homans Eldridge | [Vignette] | ji Washington | government printing office | 1896 4°. 556 pp. 31 pl. < {UNITED STATES. Department of the interior. (U. S. geological survey.) Monograph XXVII.) United States geological survey | Charles D. Waleott, di- rector | — | Geology | of the | Denver basin | in | Colorado | by | Samuel Franklin Emmons, Whitman Cross, and George Homans Eldridge | [Vignette] | Washington | government printing office | 1896 4°.- 556 pp. 31 pl. [UnrreD States. Department of the interior. (U. S. geological survey.) Monograph XXVIL.} ae oe at a a 4 >" =; . =. es a. » e | eo? % ° re bay = > _ _ _ a PP a ia cS * n j cs ‘See as be eee - ans a~. | | 1363 2369 = =] Ee = 0 s ——o se) =——}. © ——___ <>