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Full text of "Twentieth century atlas of microscopical petrography .."

LIBRARY 

OF THE 

UNIVERSITY OF CALIFORNIA 




MICROSCOPICAL PETROGRAPHY. 



Cbe u)cntictl) Centurp Htlas 



OF 



IHicroscopical RctroarapDp 



BY 



. Reward fldpc 



of THE 

UNIVERSITY 

OF 




ftr bn 
THOMAS MURBY, 3, LUDGATE CIRCUS BUILDINGS, E.G. 

THROUGH THE LABORATORY OF 

JAMES K. GREGORY & CO., 139, FULHAM ROAD. SOUTH KENSINGTON, S.W. 

LONDON, 1906. 

All Rig Jits Reserved. 



OE434 



Printed by 
li. PULLE\N & Co., 
1 1, Princeton Street, 
Red Lion Square, . 
London, W.C. . . 



PREFACE. 



ATLAS was projected for the purpose of placing within 
* the easy reach of students of petrology, a choice selection 
of interesting slides of rock-textures, accompanied by reproductions 
from accurate highly finished drawings and photographs of the actual 
objects, with explanatory text. 

I have to acknowledge, with many thanks, my indebtedness for 
useful suggestions, notes and practical help afforded from time to 
time by many distinguished workers, notably Messrs. DOBBIE, 
GREENLY, HARKER, HILL, HILTON, HINDE, HUE, HUNT, JUKES- 
BROWNE, SIR JOHN MURRAY, PRIEST, TEALL, and WETHERED, as 

recorded in their proper places in these pages. 

E.H.A. 



MICROSCOPICAL LABORATORY & STUDIO, 

141, Fulham Palace Road, London, W. 
April, 1906. 



204668 



CONTENTS. 



PAGES. 

MODERN METHODS OF RESEARCH ... ... ... .... ... ... i to xxviii 

QUARTZ-GABBRO, Carrock Fell, Cumberland ... ... ... ... ... ... 1-3 

GLOBIGERINA-OOZE, Challenger Expedition ... ... ... ... ... ... 4-6 

AuciTE-PicRiTE, Inchcolm, Firth of Forth 7-10 

SPHENIFEROUS HORNBLENDE-GNEISS, Pitlochry 11-12 

//PORPHYRITIC BASALT, Lion's Haunch, Arthur's Seat 13-14 

LEUCOXENIC DOLERITE, Salisbury Crags, Edinburgh 15-16 

OOLITIC LIMESTONE, Clifton ... ... ... ... ... ... ... ... 17-18 

GREEN QUARTZITE, Igtham ... ... ... ... ... ... ... ... 19-20 

GLOBIGERINAL LIMESTONE, Goshi, Cyprus ... ... ... ... ... ... 21-22 

PICRITIC-SERPENTINE, Menheniot ... ... ... ... ... ... ... 23-24 

PHONOLITE, The Wolf Rock 25-^6 

LEUCITOPHYRE, Rieden, The Eifel t 27-28 

HORNBLENDE-SCHIST, Lizard ... ... ... ... ... ... ... ... 29 

OLIVINE-HVPERITE, Old Meldrum ... ... ... ... ... ... ... 30 

SPHERULITIC OBSIDIAN, Iceland 31 

' BARGATE-STONE,' Godalming ... ... ... ... ... ... ... ... 32 

SPHERULITIC FELSITE, Corriegills, Arran ... ... ... ... -;. ... 33 

MICA-LAMPROPHYRE, Sale Fell, Cumberland ... ... ... ... .. 34 

BINARY-GRANITE, Wendron, Cornwall... ... ... ... ... 35- 

RED-SANDSTONE, Penrith, Cumberland ... ... ... 36 

^^GRANOPHYRIC DIORITE, Muckraw ... ... 37 " 

RED GRANITE, Mount Sorrel ... ... ' ... 3$ ' 

GLAUCOPHANE- SCHIST, Anglesey ......... 39 

WENLOCK LIMESTONE, Wren's Nest, Dudley ... ... 4 

/^ LIMBURGITE, Haddington 4 1 



PAGES. 

KENTALLENITE, Kentallen, Argyll ... 42 

SERPENTINE, Lizard 43 

CHIASTOLITE-SLATE, Gefrees, Bavaria .. ... ... 44 

OLIVINE-DOLERITE, Rowley-Regis ... 45 

TRACHYTE, Peppercraig, Haddington , 46 

RiEBECKiTE-MiCROGRANiTE, Mynydd Mawr 47 

LEITHAKALK, Vienna ... ... ... ... ... ... ... ... 48 

LAURVIGITE, Laurvig, Norway ... ... ... ... ... ... ... ... 49 

PORPHYRITIC RHYOLITE, Tardree 50 

BOROLANITE, Loch Borolan ... ... ^. ... ... ... ... ... 51 

CLAY-SLATE, Bethesda ... ... ... ... ... ... ... ... ,.. 52 

QUARTZ-MICA DIORITE, Rubislaw 53 

GARNETIFEROUS MICA-SCHIST, Perth 54 

RED FELSITE, Mount Sorrel 55 

BARBADOS EARTH, Barbados ... ... ... ... ... ... ... ... 56 

ECLOGITE, Eppenreuth, Bavaria ... ... ... ... ... ... ... 57-58 

SCHORL GRANITE, Penzance ... ... ... ... ... ... ... ... 59-60 

PITCHSTONE, Corriegills, Arran ... ... ... ... ... ... ... ... 61-62 

DOLOMITISED LIMESTONE, Birchwood Park 63-64 

GRANOPHYRIC SYENITE, Groby 65-66 

HORNBLENDE-ANDESITE, Stenzelberg 67-68 

PORPHYRITIC PERLITE, Glashutte, Hungary ... ... ... ... ... ... 69-70 

ENDOTHYRA LIMESTONE, Waterhouses 71-72 



INDEX 73-8i 




Cbc CuKiiticiD Ccnturp fltlas 



OF 



microscopical 




BY 



. toward nape. 




Microscopical petroarapDp. 

<^^> 
modern IPctboUs of Research 



EOGNOSY, now-a-days, entails so many methods of research of comparatively 
recent invention, a register of which has not hitherto been collected, that it 
appears advisable in a work of this kind to give a fairly comprehensive sketch 
to enable the student to successfully overcome sundry technical difficulties, which, not 
so very long ago, were accounted to be insurmountable, in exhaustively pursing 
inquiries concerning the materials of the earth's crust. 

The majority of our modern text-books afford ample information about the physical 
and chemical properties of rocks, but few of them essay to give any directions as to 
exactly how such variable specimens as blown sand, soft clays, friable chalks, natural 
glasses, and indurated examples of the nature of granites and " greenstones," not to 
mention such stubborn and complicated things as gritty marls, calciferous sandstones, 
and other admixtures of intensely hard and superlatively soft materials, ought to be 
dealt with, in order to yield " subjects " capable of microscopic examination. It is 
therefore the intention, in the sequel, to deal practically with the methods of preparation 
of typical examples, to enable students of modern lithology to become, in case of need, 
experts in the technics of petrography. 

The study of soils, belongs more properly to the domain of agriculture, and may 
therefore be pardonably neglected in this connection ; but, immediately and after the 
subsoil is reached the petrologist can claim his ground and act accordingly. In doing 
his duty, the petrologist who seeks the assistance of the microscope, finds ultimately, 
that he has to deal with a vast variety of textures, which require special modes of 
treatment to enable him to investigate their true significance. In examining a sand or 
sample of clay or a marl or an oceanic ooze, he may be at his wit's-end how to proceed ; 
for, by simply mounting the specimen in a transparent fluid or balsam, many of 
the particles will inevitably be much too large for minute scrutiny. It will then become 
advisable to bind the particles together in situ, and to take a thin section therefrom, 
before their optical and mineralogical characters can be determined. Nevertheless, the 
old-fashioned method of levigation, washing, sorting and mounting of particles may 
frequently be found useful jn preliminary determinations, and ought not therefore to 
be entirely discarded. 

TYPE 1. THE PREPARATION OF HARD-ROCK SECTIONS.* 

The precise methods of preparation, may best be considered seriatim, under the 
foliowing heads: (i.) Procuring chips or slices; (ii.) Grinding the first surface; 
(iii.) Cementing and reducing the slice ; (iv.) Mounting the section. 

PROCURING CHIPS OR SLICES. Suitable chips can best be obtained in the field by 
plying the square end of the geological hammer against advantageous edges of the rock. 
It will very soon be found that while some specimens can be procured with the utmost 

* The methods here recorded, were first Ccmmunicated by the writer to Technics, February, 1905, and 
pnb!ishei therein at pp. 129-135. 



11 



Cbe Cuxmktb Centurp fltlas of 



ease, others offer almost insurmountable difficulties; for example, the basalt of the Giant's 
Causeway will readily yield flake after flake of over one inch square, tolerably flat, and 
even less than one-sixteenth of an inch in thickness ; while the utmost dexterity may be 
exercised in vain to procure a much smaller chip, one-eighth of an inch thick, from the 
granite of Shap Fells, or almost any other holocrystalline rock. In the laboratory, Mr. 
RUTLEY'S directions* may be followed with advantage. 

Apart from coarsely crystalline (granites, etc.), and heterogeneous fragmentary rocks 
(breccias, etc.), there are numerous minerals and cleavable rocks which require to be cut in 
certain definite directions, or in planes other than those of their bedding or cleavage. In 
such cases it is imperative that the material shall be sliced. Some fairly compact rocks, such 
as hard chalks, limestones, serpentines, and even picrites and other volcanic rocks, can 
be easily sliced by hand, with the aid of a finely tempered tenon-saw lubricated with 
water; others, such as the harder dolerites, basalts, marbles, slates and the like, are also 
amenable to hand-slicing by means of a toothless soft-iron tenon-saw plentifully supplied 
with water and an abrasive powder, such as No. 90 hole! emery, or, better still, the 
wonderful new material known as " carborundum " powder, of about the same calibre. 

Carborundum is the name given by Mr. E. G. ACHESON, of Niagara Falls, N.Y., 
U.S. A., to an artificial silicide of carbon, having the formula C. Si, which has not hitherto 
been found in Nature. It is formed by fusing a mixture of coke and sand, with salt as 
a flux, in an electric furnace. It crystallises 
in the hexagonal system, and possesses abrasive 
properties superior even to those of sapphire. 
While not quite so tough as diamond bort, it 
approaches the hardness of this substance. 

Hand-slicing is, at best, a laborious process, 
so that the assistance of a working lapidary 
is often requisitioned. Every modern well- 
equipped laboratory, however, ought to be 
provided with a good lapidar)' bench for both 
slicing and grinding minerals and rocks for 
microscopic observation. Many such machines 
are made by German laboratory fitters ; but 
there is to-day only one thoroughly trustworthy 
appliance of the kind in the English market. 
This excellent " Amateur's Lapidary Machine " 
was devised by Mr. JAMES B. JORDAN, of the 
Mining Record office, and is made and supplied 
by Messrs. COTTON & JOHNSON, of 14, Gerrard 
Street, Soho, London, W. 

The working parts of Jordan's Lapidary 
Machine are clearly shown in the engravings 
here given. From these it will be seen that 
the appliance consists of a strong framework JORDAN'S LAPIDARY BENCH. 

* " The Study of Rocks." By Frank Rutley, F.G.S.. etc. Long nan's Text-books of Science, London, 
1879, pp. 39-44, 59 et sequentes. 




t The term " No. 90 hole " means, that the grains of the powdered material have been passed through 
a sieve having ninety threads to an inch, but will not pass through one with 100 threads to the inch. 



Microscopical Pctroarapbp. 




of seasoned mahogany, the pillars of which (a, a) support a heavy crank-axle worked by 
the treadle, and a driving wheel two feet in diameter, much as in an ordinary turning- 
lathe. The upper portion of the frame is bound by two cross-pieces a', fixed about an 

inch apart, to form the bed of the lathe. 
Between the cross-pieces is placed an iron 
casting B, which is bored to receive the 
spindle D, the upper free end of which 
is fitted with gun-metal clamps between 
which the slitting disc or grinding wheel 
(lap), is firmly secured by the nut above. 
The lower end of the spindle carries 
a small horizontal driving-wheel ; a 
bracket C supports a pair of angle- 
pulleys, so that the thong from the treadle 
fly-wheel is made to traverse the angle- 
pulleys and thence around the small 
driving-wheel of the spindle D. By this 
means, when the treadle is worked, motion 
is communicated to the driving-wheel 
attached to the spindle D, which rotates 
the slitting-disc or lap at the rate of from 
400 to 500 revolutions per minute. The 
casting B is also bored to receive a 
second spindle at E, to the top of 
which is fixed a circular gun-metal 

plate F, for carrying a small saucer H, into which the specimen to be sliced is cemented. 
The plate F can be raised or lowered by means of the thumb-screw K ; this adjustment 
provides for the regulation of the thickness of the slice to be cut. To the brass plate F, 
a piece of strong string or catgut is attached; this hangs over a pulley, fixed outside 
the framework of the machine, and supports the weight G, which consists of a brass 
cylindrical box containing lead shot, so that it can be lightened at pleasure to accommo- 
date itself to the requirements of the specimen operated upon. This simple contrivance 
ensures a constant pressure of the specimen against the cutting-edge of the slitting-disc ; 
and, when the latter is put into motion, the specimen is kept firmly held in position, 
until it has been sliced through. The plate F may then be revolved backward, and the 
specimen elevated to any desired degree by means of the thumb-screw K, and a second 
cut taken. It may occasionally, however, be advisable to steady the work by hand, and 
not to depend entirely upon the automatic pressure afforded by the counterpoise G. 

Slices of minerals and rocks for microscopical slides ought to range from about 
one-sixteenth to one-eighth of an inch in thickness. The slitting-disc at the upper end 
of the spindle D is made of soft iron or softened steel, and is slightly convex on its upper 
surface. It is about eight inches in diameter and about one-fiftieth of an inch in 
thickness. The edge of the disc should be turned true, at right angles to its surfaces, and 
then carefully primed with a powder of exceptional hardness. For this purpose, 
carborundum " flour " of the water-grades " FF," or " FFF," may be used for slicing 
specimens which reach up to about No. 6 of Mohs's scale of hardness, such as bones and 
teeth, most fossils, calc-spar, fluor-spar, apatite, felspar, and limestones, marbles, 



PLAN AND SECTION OF JORDAN'S 
LAPIDARY BENCH. 



iv Cbc Cioentietb Cciiturp flrlas of 

indurated chalks, serpentines, picrites, dolerites, basalts and even diabases. Minerals 
ranging from No. 7 and more, such as quartz, garnet, topaz, corundum and the majority 
of the acid rocks (i.e., pitchstones, granites, sandstones, quartzites, eclogites, and so forth) 
although capable of being cut by carborundum, are apt to prove too troublesome, and 
therefore call for something harder, such as diamond dust, for charging the edge of the 
slitting disc. As diamond dust is expensive, and when purchased ready-made is often 
adulterated, it is safer to buy diamond bort from some well-known mineralogist, and to 
subsequently pulverise it.* To do -this successfully without fear of waste, a diamond- 
crushing mortar will be found indispensable. This useful accessory may be procured 
from Messrs. COTTON & JOHNSON, 14, Gerrard Street, Soho, London, W. By its means, 
the diamond bort or chips, should be pulverised so finely, that no sparkling is revealed 
by diffused daylight. 

To prime the disc-edge, either flour-carborundum or diamond dust may be used, 
according to circumstances. A few grains of the abrasive should be placed in a 
watchglass or artist's saucer-palette, and made into a paste with a drop or two of 
olive oil. A particle of this paste should then be taken up with a quill and applied, inch 
by inch, to the edge of the slowly revolving disc, until the whole circumference has been 
charged. Care is then required in order to gently rub the particles well into the 
substance of the disc-edge by means of the glass or steel roller provided for that purpose. 
Particular care must be taken to avoid smearing the surfaces of the disc with the abrasive 
paste, as this would not only be wasteful, but destructive alike to disc and specimen. 
The work must be kept constantly lubricated, preferably with oil-of-brick (spirits of 
turpentine or soap and water will do), by means of a drip-tap from a lubricating bottle 
or can, as shown in thre perspective drawing, which also shows a tray surrounding the 
working parts, to collect the waste which flies off the disc or lap during its revolution, 
and serves at the same time to protect the spindle bearings from dirt and grit. This 
contrivance can, with care, maintain the work in such cleanliness as to permit the 
operator to give demonstrations on the machine, if need be, even in a library or drawing- 
room. 

GRINDING THE FIRST SURFACE. Whether with chip or slice, the operations of 
grinding the first surface by hand or machine power remain essentially the same. The 
specimen, held firmly between the tips of the forefinger and thumb, should be rubbed 
perfectly flat and smooth on one side. To do this by hand, either emery or carborundum 
powder may be used, with plenty of water, on a well-flatted zinc plate, measuring about 
eight by ten inches superficially, and one-quarter of an inch thick. These dimensions 
have been found by experience to be the most generally useful. A coarse abrasive (No. 
70 or No. 90 hole emery or carborundum, according to the nature and texture of the 
rock) should be employed for the initial grinding, which must be continued until aji 
absolutely flat face of the desired dimensions say one inch in diameter has been 
obtained. The chip or slice should be rubbed (with moderate pressure, and care to 
keep the tips of the fingers clear of the grinding surfaces), round and round in circles, 
commencing at the top left-hand quarter of the plate, and then proceeding gradually to 
the lower left, the lower right, and the upper right-hand quarter, to finish off with a 
series of concentric movements around the centre of the plate. This procedure will 
prevent the zinc-plate surface from wearing into hollows and ridges, ^and so producing 
irregularities in the ground surface of the specimen. The operator will, of course, learn 

* Diarond chips are valued at about half-a-guinea per carat. 



Microscopical petroarapbp. v 

by practice the necessity for a sufficient supply of water and replenishment of the 
abrasive powder, from time to time. The specimen must next be washed thoroughly 
free from adhering particles, and the process of grinding repeated on a second zinc 
plate of like dimensions, with flour-emery or " FF " carborundum powder. Its surface 
ought then to be quite flat and fairly smooth. If examined with an ordinary reading 
glass, or botanist's lens, however, sundry scratches will be revealed, and these should, 
after careful re-washing of the specimen, be eliminated by rubbing its surface briskly 
forwards and backwards on the prepared top of a very fine-grained hone, well moistened 
with water. Ordinary grades of commercial Water-of-Ayr stone are suitable for this 
purpose, but the picked varieties known as the " Tam O'Shanter," or better still, the 
" Montgomerie," are to be preferred by reason of their superlatively fine texture and 
cutting qualities.* As an alternative, the artificial " Hand- Washed Powder " 
carborundum hone f may, as effectively, be used. 

In preparing the surface of the Water-of-Ayr stone, or other hone, precautions should 
be taken, to ensure the maintenance of a slight convexity ; otherwise the surface of the 
slice or chip will inevitably depart from the desired " dead-level," and the section, when 
completed, will be thicker towards the centre than at its edges. Scientifically 
considered, such a section is of more value to the student of lithology than one which has 
been uniformly cut ; unless, of course, several preparations from the same specimen are 
available. For example, if a section of rock containing clastic grains of quartz is evenly 
cut down to -ooi inch, such grains would show faint lemon and varying shades of 
neutral tints between crossed Nicols, and might therefore be confounded by the 
beginner, with grains of clear felspar, particularly if the grains happen to exhibit 
" strain-shadows " and are fairly free from inclusions ; whereas, in an unevenly cut section, 
the thicker portions will reveal the quartz in brilliant blues and yellows, while the felspars 
unless inordinately thick would remain neutral and faint yellow. The striae of triclinic 
felspars, moreover, remain stationary, while strain-shadows in quartz, shift from one 
end of the crystal to the other, upon its revolution. Commercially, however, unevenly 
cut sections are accounted to be inferior by dealers, as betraying bad workmanship. 

An absolutely flat or " dead-level" surface, free from scratches, when examined with 
an ordinary hand lens or magnifying glass, ought to be secured after sufficient rubbing on 
the Water-of-Ayr stone ; for which, of course, similar stones, such as Turkey, Washita, 
Arkansas and others on the market, or the exquisitely fine-grained varieties of compressed 
carborundum tool or razor sharpeners may be substituted. It is unnecessary to develop 
a polish upon the surface with putty-powder or jeweller's rouge, as recommended by many 
text-books, for microscopic scratches are practically obliterated by the method of 
mounting the finished section in Canada balsam. 

When the Water-of-Ayr stone, or other hone, becomes worn by usage, a slightly 
convex surface should be freshly developed as required from time to time. To secure such 
a surface rapidly, the hone, measuring about 7 inches long, 2 inches wide, and i inch deep, 
should be briskly rubbed with a plentiful supply of water from end to end upon a perfectly 
flat large slab of good fine-grained sandstone. It will be found that the longitudinal motion 
thus communicated by hand, will precisely suffice to develop the slight convexity, about 
equal to that of an engineers' so-called flat file. As an alternative, the hone may be 

* These stones are placed on the market by Mr. JOHN C. MONTGOMERIE, Dalmore, Stair, 
Tarbolton Station, R.S.O., Ayrshire, Scotland. 

t Procurable from THE POLISHERS SUPPLY Co., 27, Chancery Lane, London, W.C. 



VI 



ClK Ctocmictb Ccnturp Atlas of 



firmly fixed upon a stout wooden board by means of strips of wood screwed tightly 
around its edges, and the surface can then be readily dressed to the desired degree 
of convexity, by rubbing it sharply lengthwise with a flat piece of consolidated 
carborundum.* It is essential that the slight convexity of the hone-surface thus 
developed, should be constantly maintained, so as to make sure of securing the 
indispensable dead-level on the first surface of the chip or slice of rock. 

CEMENTING AND REDUCING THE SLICE. The requisites for cementing the slices 
suitably for their subsequent reduction to sections sufficiently thin for microscopic 
scrutiny are : (i.) A suitable cement. (ii.) Pieces of quarter-inch-thick plate-glass, 
cut into squares of about 1-5 inches, (iii.) A Bunsen burner, spirit or paraffin lamp, 
(iv.) A tripod or other iron stand with copper or zinc hot-plate, (v.) A pair of level- 
jawed pliers. 

Suitable cements may be purchased ready-made from oilmen, drysalters and dealers in 
watchmakers' requisites under the name of Waller's wax, red cement, and the like ; but 
as many students prefer to make their own materials, the following reliable recipes, all of 
which have been thoroughly tested, are here given : (a) Crude Canada balsam, 
evaporated down to dryness on a sand or water-bath, or in a hot-air chamber. A small 
portion of the balsam should be removed from time to time, and tested between the 
fingers and thumb. As soon as the pellet becomes hard, but not so brittle as to be 
capable of being rubbed to a powder, the balsam ought to be removed from the bath 
or hot chamber, poured in small quantities upon a cold slab of marble or plate of glass and 
rolled out quickly into sticks like sealing-wax. (/3) Although old-fashioned, the following 
recipe will also be found useful, especially for cementing pieces of rock into the saucer 
of Jordan's lapidary machine for being sliced. Take Burgundy pitch, resin and bees- 
wax. The precise proportions are not important, because samples vary very consider- 
ably, and the mixture is liable to be affected by seasonal or climatic changes. In 
summer more resin, and in winter more pitch and beeswax should be used. The 
ingredients are to be melted together, with frequent stirring, on a sand or water-bath, 
and then formed into lumps or sticks for future use. (y) Shellac sticks however, are 
incomparably the best for fastening the smooth surfaces of slices of hard rocks on to the 
small plate-glass holders specified above. The author's own formula for making this 
cement is : Melt some Venetian turpentine (about three or four ounces), and strain it 
through fine muslin, placed filter-paper-wise in a hot-water funnel, into an evaporating 
basin, to be afterwards placed with great care over a Bunsen flame or other source of 
heat. When quite mobile, add flakes of commercial orange-shellac, freed from dust and 
vdirt, a little at a time. Stir about with a glass rod until thoroughly incorporated, and 
test occasionally, by removing a small portion at the end of the rod. As soon as the 
sample solidifies into a hard and glassy bead, it may be considered ready for use. As 
a confirmatory test, a chip of rock previously heated, should be fastened with some of 
the cement on to a piece of warmed glass and allowed to get cold. If the cement does 
not set hard, tough and glassy in about a couple of minutes or so, but exhibits a greasy 
lustre, and feeble tenacity, more shellac must be added to the mixture until the desired 
result has been attained. When ready, pour a small quantity of the hot cement upon a 
cold (damped) slab of zinc or a dinner plate, and roll or squeeze it with the fingers into 
a lump of about the size of a filbert or small walnut. Continue this process until all 

* A fragment of about three square inches, from a broken No. 60 or No. 90 carborundum 

wheel serves admirably. 



Microscopical Pctrocjrapbp. vii 

the material has been collected from the evaporating basin. The latter should 
immediately be re-heated, wiped out, and finally cleansed with a rag dipped in 
methylated spirit. For the sake of convenience in use, the lumps of cement, like those 
of the hardened Canada balsam, may be rolled or moulded into sticks by being immersed 
in hot water, and, when sufficiently softened, rolled out by hand upon a plate of glass or 
china, or pressed into moulds of any desired shape and size. Professor ROSE-NBUSCH' 
recommends a mixture, made by heating together sixteen parts by weight of Canada 
balsam with fifty parts of shellac ; but this cement, although equally tough and service- 
able for grinding very thin sections, does not amalgamate quite so readily, nor dissolve 
quite so freely in methylated spirit ; besides which Venetian turpentine is cheaper than 
Canada balsam. 

Having prepared the cement as directed above, in the form of suitable sticks, either ' 
of hardened Canada balsam or of shellac-compound, a portion of the chosen stick should 
next be melted on to the surface of one or more of the small one-and-a-half-inch squares 
of plate-glass, by heating the latter on a hot plate of metal, or simply by holding the 
square of glass over the Bunsen or other flame, with the level-jawed pliers or a pair of 
cork-tipped tongs. The smooth surface of the slice or chip, ought to be warmed over 
the flame to dispel any moisture, and placed face downwards in the melted cement. The 
heated plate of glass supporting the specimen should then be transferred on to a smooth 
pad of paper or cardboard, or a cork mat on the adjoining work-table, and the specimen, 
now cooling, should be pressed down firmly with a slight screwing movement on to the 
plate of glass, preferably by means of the thumb and fore-finger guarded by a glove, so 
as to squeeze out all superfluous cement to the borders of the slice or chip. By this 
time the small plate-glass holder will have cooled down sufficiently to be handed and 
examined from below. If any air-bubbles are present between the specimen and the 
glass, they will become plainly apparent as glistening specks when viewed by reflected 
light, especially through a magnifying glass ; and, in that event the plate of glass must 
be reheated, the specimen detached, a little more cement added, and the whole operation 
of pressing repeated, until all air-bubbles are excluded. Specimens, as a rule, require to 
be removed and re-cemented in this way only once, and, after but a little practice, can be 
satisfactorily done in quick succession, without fear of failure; while the operation of 
repeating the process to eliminate air-bubbles, is not by any means so troublesome and 
annoying in practice as it appears to be in print. Cork-tipped tongs, if preferred, may 
be easily improvised from a pair of common compasses, into the pointed ends of which 
the separate halves of a sound bottle-cork cut lengthwise into two, may be inserted. 

When quite cold, the chip or slice of rock, securely cemented upon the small square 
of plate-glass, ought to be surrounded by a fair margin of the squeezed-out cement. This 
serves to prevent the edges of the specimen from being prematurely worn away during 
the subsequent process of reduction ; and, as the plate-glass square is intended to serve 
as a holder or handle to the specimen, it is obvious that it ought to be at least a-quarter- 
of-an-inch thick, so as to afford sufficient grip for the fingers while the slice is being 
reduced by grinding. It may be mentioned, en passant, for the benefit of those who do 
not possess a lapidary bench, that a few broken carborundum wheels, flat on both 
surfaces, may occasionally be purchased at a nominal price, and will be found more 
serviceable than metal or glass plates plus abrasive powders, in rapidly reducing the 
cemented chips or slices into sections fit for the microscope. Pieces of broken vitrified 
carborundum wheels, roughly about 3^ by 4 inches, and i inch thick, approximating to 



Vlll 



Cbe CuKiitietb Cctiturp Htlas of 



No. 60 and No. 90 hole may be used in place of No. 90 and flour carborundum 
powders respectively ; and it will be found, that apart from enhanced cleanliness, the 
work of reduction can be effected far more rapidly by simply rubbing the specimen on 
the flat surface of the solidified carborundum, with a liberal supply of water. These 
vitrified wheels can, of course, be adapted to lapidary machines in place of lead, zinc and 
other laps. The essentials at this stage of the work are, that specimens should be 
washed or even scoured with a nail-brush, so as to thoroughly free them from even traces 
of one grade of abrasive powder, before they are subjected to grinding with a finer 
grade; and that the chip or slice, now assuming the character of a section, should be 
frequently examined under a low power of the microscope, fitted occasionally with a 
polariscope. 

No hard-and-fast rules can be given as to the exact method of procedure in 
reducing the chip or slice ; but the following hints will certainly be found useful : Press 
the cemented specimen, face downwards, on to the grinding surface, firmly between the 
thumb and fore-finger, with care to keep the tips of the fingers free of the grinding slab 
or wheel, the quarter-inch thickness of the plate glass square can now be duly 
appreciated. Proceed with the first or rough grinding, with No. 90 hole carborundum 
powder or its equivalent grade of vitrified wheel, until the section, as it must henceforth be 
called, becomes translucent. These directions, needless to say, must be observed with 
due discretion ; e.g., a quarter-inch thick slice of clear quartz would be transparent, but 
a slice of any dark volcanic rock, such as a basalt, dolerite or diabase, would need to be 
cut to about one-hundredth of an inch before even a glimmer of light is admitted through 
its substance. Then again, such rocks as shales, slates and limestones are not generally 
capable of withstanding the coarse grinding till they become translucent ; while, on the 
other hand, vitreous rocks such as rhyolites and pitchstones, admit of being reduced to 
great tenuity even with coarse abrasives. Judgment gained by experience will, how- 
ever, speedily serve to indicate precisely when the coarse grinding of any particular 
specimen should cease. 

After very careful washing, the section must be still further reduced by the 
process of fine grinding on a separate plate or lap charged with flour emery or flour 
" FF " carborundum. The Powder ought to be mixed with water into a very soft 
paste, and applied to the grinding surface with a brush in order to prevent undue 
waste. As a rule, the grinding may be safely continued, until the bars of a window or 
other bold external object can be just barely detected through the section when it is 
held up to the light. A second thorough washing in water now becomes imperative, and 
the section should be examined under a low power of the microscope (about twenty 
diameters), to determine whether it is thin enough ; for, if not, it must be still further 
reduced, washed, and re-examined. 

When reduced to the desired degree of tenuity, the section should be cautiously 
rubbed down, with frequent washes and peeps through the microscope, until it is 
deemed to be sufficiently thin. A rough and ready test of tenuity for most hard 
rock sections consists in placing the preparation over a printed page of fairly small 
type, " minion," " nonpareil " or "ruby." If the type can be deciphered through a 
dark basalt, dolerite, picrite or limestone, the section may be considered adequately 
thin ; but with rocks such as granites, gabbros, diorites and the like, where much clear 
quartz and colourless felspar are present, this test obviously cannot be relied upon. The 
only positive criteria, as to the tenuity of rock sections must necessarily be elicited through 



microscopical Pctrograpbp* ix 

the agency of varying powers of the microscope under both ordinary and polarised light. 
For example, sections are regarded as sufficiently thin for general study when they are 
reduced to about o-ooi inch. If such sections contain quartz, that mineral will, 
under a lower power (20 to 30 diameters) exhibit weak interference tints between 
crossed Nicols, from neutrals to white of the first order on Newton's scale. If, under 
like conditions, the quartz shows a yellow of the first order, the section may be regarded 
as somewhat thicker than the standard of o-ooi inch ; but, if vivid chromatic polarisation 
ensues, with orange or red hues the section may unhesitatingly be termed " too thick." 
So also with certain sedimentary rocks of the nature of shales and slates : The 
excessively minute particles of which they are composed, secondary mica, rutile 
needles and so forth, cannot be discerned unless the sections are cut correspondingly 
thin. On the other hand, there are many rocks, notably the chalks, in which the 
typical structure and general characteristics become obliterated when the sections are 
cut too thin ; while, in yet others, certain of the mineral constituents are apt to 
disintegrate and drop out altogether long before the section is reduced to the one- 
thousandth part of an inch. 

To remove the section from the plate-glass square, and free it from even the 
slightest trace of the shellac-cement, it should be turned face-downwards in a shallow 
saucerful of good methylated spirit and covered with a pane of glass ; or, better still, 
the nests of porcelain saucer palettes, about four inches in diameter, as sold by most 
artists' colourmen, may be profitably utilised. The usual cabinet of saucers will hold 
five sections at a time, one in each saucer. Just sufficient methylated spirit should be 
poured into each saucer to come above the lower edges of the plate-glass square, and 
the whole nest may then be covered and set aside. It will be found that the sections 
become detached of their own accord by the solvent reaction of the spirit, and sink to 
the bottoms of the saucers. Some of them more porous or loosely textured than others 
may become detached in less than a couple of hours, while others may take as many 
days ; but, on no account must they be tampered with, by heating over a Bunsen or other 
flame, and subsequent shifting off with a pen-knife, needle or other instrument ; neither 
should the sections, at this stage, be washed with a camel's-hair or any other kind of 
brush. These precautions ought to be observed in order to prevent disintegration of 
the majority of sections, which, it is scarcely necessary to add, are very fragile. 

As soon as the sections become detached through the solvent reaction of the spirit, 
the squares of glass should be removed, and immediately wiped clean for future use. 
The discoloured spirit can then be gently poured away from the section, and its place 
supplied by clean spirit. This operation should, if need be, be repeated, and the 
sections washed very gentry with a soft sable brush until every trace of impurity has 
been removed. The spirit should finally be drained away, leaving the sections intact at 
the bottoms of their respective saucers. In the course of a few minutes, especially in a 
moderately warm room, the small quantity of spirit left adhering to the sections will 
evaporate. Rectified benzol should then be added to just cover each section for a few 
minutes and the sections will be ready for mounting. 

When hardened Canada balsam is used for cementing the slices of rock on to the 
handles of plate-glass squares, the sections, when ready, may be successfully removed 
by heating the plates of glass over a flame or on a hot-plate, holding the plate of glass 
by means of level-jawed pliers, and very cautiously pushing the section off with the 
point of a small penknife into a saucer-palette containing clean benzol. A soft camel's- 



x CIK Cioentietl) Centurp fltlas of 

hair or sable brush gently used will suffice, in a minute or two, to cleanse the sections 
from impurities, and the brush, with a very little practice, will also be found an 
admirable lifter, to transfer the sections to another saucerful of clean benzol, in which 
they should remain until wanted for mounting. 

MOUNTING THE SECTION. Canada balsam is still recognised as the most generally 
useful medium for the preparation of objects capable of being permanently preserved 
for microscopical examination, mainly by reason of its physical properties, colour, 
refractive index and so forth. Crude Canada balsam, however is not suitable for 
microscopical manipulation. It requires to be prepared, to fit it for duty in the 
laboratory. Its impurities must be removed and its viscosity modified in order to 
adapt it for use to the best advantage. Properly prepared balsam may be purchased 
from a few dealers in microscopical requisites, but the writer has found it far more 
satisfactory to make his own medium, and strongly advises others to do likewise. The 
following is his method of preparation, which may be relied upon to yield uniformly 
good results : 

Procure, from some trustworthy source*, a sufficient quantity of the best and freshest 
commercial Canada balsam, say, about a-quarter of a pound or so. Immerse about 
two-thirds of the unstoppered bottle containing the balsam in a bath of water, and bring 
the latter to a boil. By this time the balsam will have become sufficiently mobile to be 
poured without waste into an evaporating basin. Place the evaporating basin upon a 
water- or sand-bath, and continue to boil the water or heat the sand, so as to permit 
the uncovered balsam to harden very gradually, by loss of its volatile constituents. This 
process may take a few or many hours to accomplish, according to the quantity and 
condition of the balsam. Some workers may be tempted to expedite matters by heating 
the basin of balsam' directly over a Bunsen burner or other kind of flame ; but, apart 
from the liability to an explosion, through ignition of the highly inflammable vapours 
given off by the balsam, the latter can scarcely fail to become discoloured and otherwise 
altered by dissociation of its particles. By the gentler methods noted above, or by the 
judicious employment of a hot-air chamber, however, the balsam will finally grow ice- 
hard, when allowed to get quite cold, without appreciable alteration of its pale honey- 
yellow colour. 

During the process of hardening, samples should be tested from time to time, by 
removing a drop or two of the balsam at the end of a glass dipping-rod. The drop may 
be allowed to fall on to a piece of paper and immediately thereafter touched by the 
finger-tip. If sufficiently hardened, the particle ought not to remain sticky, but be 
capable of being rolled between the fingers into a hard yet tough pellet, without being 
exactly brittle. In the latter event, it may be concluded that the hardening has been 
overdone, and to remedy this, more fresh balsam should be added to the basin-contents. 
It is essential that the balsam should be hardened to a nicety neither under nor 
overdone ; and, when this has been satisfactorily accomplished by the " rule-of-thumb " 
method above-noted, the balsam may be poured out upon a cold glass or other plate, 
deftly rolled out into sticks for future use, and stored in a wide-mouthed stoppered 
bottle. As such, these sticks of hardened balsam may be used for cementing the first 
surfaces of chips or slices of minerals and rocks, and other compact materials, such as 

* MESSRS. TOWNSON AND MERCER, 34. Camomile Street. London, E.G.; or MESSRS. BAIRD AND 
TATLOCK, 14, Cross Street, Hatton Garden, London, E.G. 



Microscopical Pctro<jrapl)p. xi 

metals, wood, shells, teeth, bones and artificial substances to the small squares of plate- 
glass, in place of the shellac cement, as directed above. 

To prepare fluid balsam for rock-section mounting, a few sticks of the hardened 
balsam should be broken to pieces, placed in a glass-stoppered bottle, and lightly shaken 
down. Sufficient rectified benzol must then be poured upon the balsam chips to just 
about submerge them, and the bottle set aside for a few hours, or until all the hardened 
balsam has been dissolved to yield a fairly mobile solution of about the consistency of 
B.P. Simple Syrup. This solution must be very carefully filtered to render it suitable 
for slide mounting ; and to that end, a hot-water funnel, which can be purchased from 
any laboratory outfitter, will be required. The most generally useful pattern of hot- 
water filter consists of a glass funnel encased in a metal jacket arranged to hold water. 
The water can thus be heated to any desired degree by means of a Bunsen ring 
clamped to an ordinary retort stand. To filter the balsam, a circular sheet of 
MESSRS. SCHLEICHER AND SCHULL'S well-known Rhenish filtering paper, folded to form 
a cone, should be wetted with a little benzol and placed in the funnel. The desired 
quantity of balsam solution may then be poured in, and the gas circle very gently turned 
on, cautiously ignited and kept low so as to avoid an explosion for it must be constantly 
kept in mind that both benzol and balsam are highly inflammable. The filtrate should 
be collected and kept in a special unstoppered, but glass-capped-bottle provided with a 
dipping-rod. 

The best pattern of Canada balsam bottle extant is that provided by the celebrated 
firm of laboratory fitters, MESSRS. BAIRD AND TATLOCK, of 14, Cross Street, Hatton 
Garden, London, E.C. It is possessed of a very substantial, solid base, calculated to 
prevent the bottle from toppling over, when smartly unstoppered and re-stoppered 
during use. The glass cap, moreover, is ground to fit very accurately over the neck of 
the bottle a sine qua non for holding solutions of balsam in benzol or other volatile 
liquids ; while the dipping-rod within is not drawn out to too fine a point, but is of 
exactly the shape and size best adapted to take up and deposit just one good-sized drop 
of balsam at a time. When in use, the bottle ought to be kept a trifle over one-third 
full of the mounting medium. 

Canada balsam thus prepared and kept in readiness for use will be found the best 
and most generally useful of media for mounting, not only rock and kindred sections, 
but a vast variety of permanent microscopic objects. The solvent used may, of course, 
be varied by a substitution of ether, chloroform or xylol for the benzol ; but the benzol, 
by the general consensus of opinion and practice has been found not only the cheapest, 
but the best and most trustworthy solvent for all ordinary technical operations. Then 
again, the Canada balsam itself may be replaced by other gums such as dammar and 
storax for the mounting of hardened and stained biological specimens and diatom 
frustule-valves respectively ; but Canada balsam, nevertheless, holds its premier position 
on the score of all-round utility, by reason of its low refractive index, which averages 
approximately 1-540. Students who are ambitious enough to excel in the art of 
preparing microscopic objects should endeavour to determine for themselves the 
refractive indices of the various media and reagents recommended for microscopical 
work. They will find most of these accurately given in the text-books on Physics, and 
in some of the best treatises on Microscopy; and, by consulting such works and 
experimenting for themselves, they will come to the conclusion that Canada balsam 
duly dried and dissolved in benzol is, without doubt, the best known medium for 



xii chc ciDcmkii) Cetiturp flilas of 

rock-section mounting, as well as for the mounting of the majority of stained and 
hardened animal and vegetable cells and tissues. 

It is, indeed, important to fully appreciate the optical phenomena due to the 
refractive indices of objects under observation. For instance, when a thin slice of 
quartz, which has a mean refractive index approximating to that of Canada balsam, is 
mounted in that medium, its contours and surfaces are almost if not quite invisible in 
ordinary light ; but the inclusions, with which it usually abounds, are mapped out with 
great clearness, and can therefore be studied to advantage. On the other hand, when a 
slice of olivine is similarly^mounted, its moderately high index of refraction, averaging 
about i '68, reveals fairly strong outlines, and a characteristically shagreened surface; 
while a minute crystal of zircon, which is possessed of a very high mean index of 
refraction, is rendered conspicuous by its bold boundaries. 

The technical methods to be followed in mounting objects in Canada balsam, have 
not hitherto been recorded in any of the current text-books ; and are, indeed, only known 
to and practised by a few of the leading expert slide-mounters of to-day. They 
necessitate the strict observance of a general routine to insure invariably successful 
results, and may here be considered seriatim as follows : (i) Cleaning the slips and 
cover-glasses, (ii) Placing the prepared sections, or other objects, in benzoled balsam 
in a special dust-proof cabinet, (iii) Mounting the preparations in their proper positions 
on the slips, (iv) Cleansing, sealing, finishing and labelling the completed slides. 

For the present it will suffice to explain the methods indicated above by reverting 
to the hard rock sections left lying thoroughly freed from cement and other impurities 
in an artist's saucer-palette containing clean methylated spirit. These sections should 
be carefully removed (by means of a mounted needle, fine camel's-hair or sable brush, 
or one of the many forms of section-lifters sold by dealers in microscopes and their 
accessories), and transferred to another saucer-palette or watchglass filled with pure 
benzol, and kept covered therein, until wanted for mounting. It is necessary to drain 
off all methylated spirit from the section before transferring it to the benzol, as these 
two liquids are not easily miscible, and any adherent drops of methylated spirit, how- 
ever minute, may at first mar the beauty of the mounted preparation, although they 
will gradually become absorbed and disappear. To drain off superfluous spirit from 
the section on the lifter, or when poised on the edge of a mounted needle or brush, the 
tip of a finger, or edge of a snip of blotting paper, may be cautiously applied. 

Microscope slips are supplied in the standard sizes of 3 inches by i inch, and 
3 inches by i inch for extra large objects. The former are most generally used. The 
edges should be smooth and the surfaces flat and free from blemishes. It is most 
satisfactory to use the best quality of extra thin white glass slips, guaranteed to be of 
uniform thickness. These are sold at five shillings per gross, and do not usually require 
to be specially cleaned. With regard to cover-glasses, it is most satisfactory to use the 
best quality of extra thin No. i circles, which are usually supplied in standard sizes of 
tb f $ l an d ij inches in diameter, at about five shillings per ounce. The best cover- 
glasses, like the best microscope glass-slips, seldom need to be specially cleaned ; but, 
as instances sometimes arise where any amount of ordinary rubbing with damp and 
afterwards with dry silk cannot remove a faint-blue cloudy film from the surfaces of 
slips and covers, it is advisable to get into the way of cleaning them. Then again, slips 
and cover-glasses that have been used and discarded, on account of a spoiled or faulty 
preparation can be used over again, after being thoroughly cleaned by the following 



Microscopical Petrograpbp. 

simple plan. Plac the slips into one, and the cover-glasses into another covered 
glass basin (four inches in diameter by two inches deep is a convenient size), and pour 
in sufficient strong sulphuric acid to just immerse the glass slips, etc., within ; cover the 
basins and allow them to stand aside for at least a couple of hours. Drain away the 
acid and wash the contents of the basins under a running water-tap for about a couple 
of minutes. Drain away the water, and pour in enough methylated spirit to cover the 
glass-slips and cover-glasses respectively. After remaining in the spirit for two hours 
or so the glass-slips may be removed, wiped dry with a linen rag and stored for 
immediate or future use. They ought to be brilliantly clean. The cover-glasses require 
tenderer handling by reason of their fragile character. They should be removed from 
the spirit and placed separately one by one, on a special india-rubber draining mat. 
This mat consists of a sheet of india-rubber with numerous small short cylindrical rods 
standing up from its surface perpendicularly a mat such as is used by retail tradesmen 
(especially tobacconists) for placing payment coins upon. When drained and quite dry, 
the cover-glasses should be gathered together in the palm of one's hand, wrapped in 
tissue paper and replaced within their original card-board boxes. 

Immediately before use the cover-glasses should be finally recleaned. The old 
fashioned clumsy way of doing this is, unfortunately, still taught in our colleges and 
science schools, and recorded in current text books. By it students are advised to 
take up the delicate cover-glass with an old silk pocket-handkerchief and rub both 
surfaces simultaneously between the thumb and fingers. There can belittle doubt, that, 
by prolonged practice, the operator may acquire dexterity enough to save the majority 
of the fragile glasses from an untimely fate ; but, in the face of other far simpler and 
surer methods, the plan cannot be recommended. A better method for minimising 
waste through breakages is to use the buff-leather pads sold for that purpose by dealers ; 
but even these would be unhesitatingly discarded when it is known that, for a trifling 
outlay, one can provide oneself with the main requisite for cleaning cover-glasses to 
perfection, without fear of breaking any save defective glasses. All that is needed is 
to purchase a small cutting, say six or twelve inches square, .of fine American oil-cloth 
from any draper, and to lay this down, face upwards, by one's side upon the operating 
table. A better plan would be to buy a small cutting of the very finest oil-cloth, 
preferably of a dark colour, from some upholsterer, and to fix this flat, in place of paper, 
upon one of Messrs. GEORGE ROWNEY'S framed drawing boards provided with a 
shifting panel. 

The method of finally cleaning the cover-glasses, without risk of breakage, is 
" simplicity itself ": Place as many of the cover-glasses as are required for immediate 
use say. half-a-dozen at a time on the surface of the oil-cloth. Breathe gently upon 
them, and, with the forefinger, guarded by a piece of old silk pocket-handkerchief, 
press upon and rub the glasses thoroughly clean. The glasses will adhere evenly to 
the fairly flat oil-cloth, and considerable pressure may be exerted without the slighest 
fear of breakage. Indeed, the silk rag may be rubbed swiftly to and fro over the 
surfaces of the whole six glasses with impunity. Now, insert the point of a blunted 
penknife between the cover-glass and oil-cloth, and turn the former over. The oil-cloth 
will give way readily to the gentle pressure thus exerted, so that the six cover-glasses 
can be rapidly turned, and their reverse faces similarly rubbed till quite clean. These 
last clean faces are to be used for mounting the sections or other objects upon ; and the 
cover-glasses, when removed from the cleaning-board, should be placed slantwise 



XIV 



Che Ciocntictl) Centurp fltlas of 



against a rest the edge of a saucer-palette so that any particles of dust from the air 
may not touch nor cling to the surface upon which the object is to be mounted. By 
cleaning cover-glasses in this way, not one out of a hundred need be broken, and the 
process can moreover be carried out with almost lightning rapidity. It has hitherto 
been employed by only a few expert slide-mounters, and, although so simple and 
effective, appears to be unknown in university and other scientific laboratories. 

Both slips and cover-glasses having now been perfectly cleaned for mounting 
operations, the following routine ought to be rigidly observed in order to secure 
uniformly satisfactory results : Breathe upon the surface of a slip and then rapidly 
press one, two, three or more cleaned cover-glasses upon it. The condensed moisture 
from the breath will be amply sufficient to hold the cover-glasses in situ. Now rub the 
surfaces of the cover-glasses briskly with the silk rag (cotton or linen will not do, 
because they leave adherent hairs or fibres behind), warm gently over a Bunsen or other 
clean flame to dissipate any moisture, and place the slip upon the mounting tile or 
piece of cardboard limned with outlines to show central and other suitable positions. 

Next, remove the glass-cap from the balsam-bottle, and, with the blunt dipping- 
rod, transfer a drop of balsam on to the surface of each cover-glass, with care to spread 
it evenly over the whole of its field with the exception of its margins which for 
obvious reasons would make future operations difficult. Re-cover the balsam bottle at 
once. With a sable brush, mounted needle, or any other kind of " lifter," take up the 
object (e.g., a rock or other section), and gently lower it into the balsam on one of the 
cover-glasses, repeating the operation for any more cover-glasses that may happen to be 
placed upon the slip in question. Add another small drop of balsam to the object, so 
as to entirely submerge the specimen, and 
transfer the slip bearing the specimen or 
specimens to one of the compartments 
of the dust-proof cabinet here depicted, 
taking care to shut the lid of the cabinet 
down. This process may be repeated 
until the cabinet is filled, after which 
the latter ought to be laid aside for 
fully twelve hours during wintry weather, 
or for at least six hours in summer time. 
To expedite matters, the dust-proof 
cabinet with its contents may be placed 
in a hot-air chamber or expiring oven, at 
a temperature of 85 F. for at least three 
hours. This process permits the benzol 
in the balsam to evaporate just sufficiently to render the next operation successful. 

Take up a thoroughly cleaned slip between the forefinger and thumb, and move it 
to and fro over the top of (not within) the Bunsen or spirit flame for a few moments. 
Place the slip accurately upon one of the patterns, having a central circle, of the 
mounting tile, and spread a small drop of benzoled balsam evenly over the part indicated 
by the circle. Remove one of the cover-glasses, bearing the balsamed object, from one of 
the slips in the dust-proof cabinet by gently pushing it off the slip with the point of a 
mounted needle, and place it on another clean slip under the microscope with i or 2 
inch objective accurately adjusted. If any particles of dust, hairs or other dirt happen 




DUST-PROOF MOUNTING CABINET. 



microscopical Petrograpbp. 



XV 







to be in the field, they should now be removed with the point of a mounted needle, after 
the slip has been duly warmed. There will be no necessity for this operation, however, 
if it is carried out in the first instance before the objects in balsam are laid aside for the 
twelve hours in the dust-proof cabinet. 

Now take up the cover-glass bearing the balsamed object between the forefinger 
and thumb of the right hand and hold it slantwise over the drop of balsam on the slip 
which lies centered on the mounting tile. With the forefinger of the left hand lower the 
cover-glass with its object gradually into the balsam below ; gently warm the new slip 
(now practically a slide), over the Bunsen flame, and, with the end of a tapered pen- 
holder or bone knitting or crochet needle, squeeze out all superfluous balsam, and at the 
same time contrive to get the object centrally or symmetrically placed by repeated 

warmings (not heatings) and 
a free use of the mounting tile. 
As soon as the now 
mounted slide gets cold (in 
about three minutes), the 
superfluous balsam may be 
washed off with a small piece 
of soft rag dipped in methy- 
lated spirit, and the slide can 
instantly be polished with 
silk and remain ready for the 
finishing touches. It may be 
noted enpassant that the single 
circles on the patterns of the 
mounting tile are merely pro- 
jected as guides to centering 
the object when it is being 
mounted. When the object 
is white, the black circle may 
be used to best advantage ; while for dark objects the white circle would, of course, 
have preference. The pattern with two circles is intended as a guide for double mounts 
on one and the same slip. For example, a transverse section under one small cover and 
a longitudinal section under the other ; or a sample of coarse washings of, say, a 
Globigerina-ooze, under one, and fine washings from the same material under the other. 

SEALING AND FINISHING SLIDES. Many persons are inclined to regard the finishing 
of slides mounted in Canada balsam as superfluous, and savouring of the work of the 
dilettante rather than that of the serious scientific man. As a consequence many 
exquisitely prepared and valuable slides become utterly ruined after a few years by the 
discoloration of the medium or the springing off of the cover-glass ; whereas, when 
properly sealed, the preparation ought to be practically everlasting. Again, when 
sealed, it requires only a trifle of time and trouble to finish or decorate the slide by 
means of a suitable ring of cement, which not only strengthens the seal, but makes the slide 
" a thing of beauty " (viewed from without) and accordingly, commercially valuable. 
Scientific men, moreover, need not be slovenly. 

On the other hand, in conducting researches, slides are seldom preserved, especially 
in biological work, although it is desirable to keep everything that can possibly be 



MOUNTING-TILE. 

To SHOW MOST USEFUL PATTERNS. 



Cbc CUKtitietl) Cetiturp fltlas of 

secured, as evidence of the bona fides of the investigator ; and, again, the use of Canada 
balsam distinctly points to the permanent preservation of prepared objects. Then, too, 
there are exceptional instances, where anything beyond a simple filmy seal becomes 
inadmissible. For example, when a large section of granite containing quartz with 
inclusions carrying cubes of sodium chloride is examined, high powers (-J and ^ inch 
objectives), and careful substage illumination are imperative, in order to demonstrate 
the minute cubes of salt to perfection; and, when a fancy ring encircles the object, it 
may very often prevent the proper examination of the edges of the section, which might 
be richest in such inclusions. 

Nevertheless, it is well that the student should know exactly how to seal and finish 
slides that are mounted in Canada balsam, in a thoroughly workmanlike way ; and as so 
much mystery has been made about the preparation of suitable sealing and finishing 
cements mainly by dealers who know " next to nothing " about the matter it is hoped 
that the following tried and tested recipes, which are so simple that " a child could 
carry them out," may be found useful by many of our readers. 

The requisites for sealing and 'finishing slides duly mounted and cleansed are : 
(i.) A good turntable; (ii.) Suitable brushes for applying the cements to be used; 
{iii.) Methylated spirit and benzol ; (iv.) Shellac sealing cement ; ;v.) Black, white, or 
variously coloured finishing cements; (vi.) A soft silk rag; (vii.) Patience and perseverance. 

German slides, especially of rock sections, for the English market are almost 
invariably mounted with square cover-glasses, and left unsealed and unfinished, 
presumably because, after a very few years, the sections, many of which cannot be 
remounted on account of their frailty, have to be replaced by fresh purchases. In the 
United Kingdom, however, circular cover-glasses are most generally used, as these 
admit of being more easily and neatly sealed and finished. Sometimes, oval cover- 
glasses are also used, and are advantageous for the mounting of elongated sections such 
as not infrequently happen to be cut for both biological and petrographical purposes. 

By far the most efficient turntable extant, is the old-fashioned Shadbolt pattern. 
Some makers usually fall into the error of placing the circular brass disc above (instead 
of flush or a little below) the level of the mahogany hand-rest. In consequence of this 
mistake, the student is often driven to despair. He might as well try to write legibly 
and rapidly upon a surface elevated above the desk upon which his palm rests. In 
choosing the turntable, it should be tested to see that the brass disc revolves accurately 
and freely upon its pivot for fully sixty seconds before it commences to slow down. 
The motion should be initiated by applying the index finger of the left hand to the edge 
of the brass disc, and swiftly moving the latter from left to right. The best brushes for 
applying the cements are those supplied by artists' colourmen under the name of " sable 
stripers " of " crow " size, the free hairs of which are fairly stiff and measure close upon, 
an inch in length. Common methylated spirit and used-benzol are quite good enough 
for this work, as they are employed merely to clean the brushes after use, and sometimes 
to thin off the cements when they commence to clog. The shellac sealing cement should 
be made by loosely filling a small bottle about half full with orange shellac flakes, and, 
before shaking them down, to just barely cover them with good commercial methylated 
spirit. As soon as dissolved, in the course of a few hours, the stoppered bottle should be 
well shaken, and the solution strained through fine muslin, when it will be ready for 
use. The cement should be kept in an ordinary Canada balsam bottle with glass cap 
and dipping rod. 



XV11 

Black, white or variously coloured finishing cements, may all be prepared in 
practically the same way ; the difference, of course, being in the colour of the pigments 
employed. The method of preparation is as follows: For a foundation, make a 
saturated solution of gum dammar in rectified benzol and strain it through fine well- 
washed muslin. A modicum of this cement, thinned down with a little benzol may be 
used, without any previously applied shellac seal, to give slides that have been mounted 
in Canada balsam a pellucid ring, which, when dry, ought to look like a rim of 
burnished glass. 

Black cement, so much in vogue for ringing geological slides, should be made 
thus: Pour some of the "foundation" cement into a small, say, four-ounce bottle, so 
as to fill about one-half of its volume. Purchase, from any good oilman's store, about 
one pennyworth of Brunswick black, and strain it through fine muslin. Now, pour the 
purified Brunswick black into the foundation cement gradually, until the black (which 
will sink to the bottom at first), rises to the top level of the clear cement. The cement 
will immediately, after stirring with a glass rod, be ready for use, and should be kept 
preferably in a glass-capped Canada balsam bottle duly provided with a dipping, or 
rather in this case a stirring, rod. 

White cement should be made in precisely the same way, but, in place of the 
Brunswick black, substitute strained " Paris- White Enamel" or " Aspinall's White 
Enamel," both of which can be procured ready prepared in convenient sized tins from 
oilmen's or drysalter's stores. If desired, the white cement may be tinted with a trifle 
<3f oil-colour (red for animal and green for vegetable preparations), such as may be had 
from any artists' colourman. 

The advantages of cements, prepared by the processes directed above, over those 
usually sold by the majority of dealers is, that they require little or no stirring previous 
to use, because the pigments are uniformly held in suspension, and will not subside even 
after many weeks of rest ; whereas, in the ordinary zinc-white, asphaltum and other 
cements now on the market, a single day will suffice to, as it were, precipitate the 
pigment. It is, however, just as well to be on the safe side by stirring the cements 
immediately before using them ; and, when they become too thick, by evaporation, to 
thin them down with a trifle of benzol. 

It will doubtless be remembered that the Canada balsam, specially prepared by the 
writer's method,* is only moderately hard. If evaporated to dryness, so as to become 
brittle (crumble away to powder when rubbed between the fingers), the slides prepared 
therefrom will be but ephemeral, i.e., they cannot last for more than a year or two 
without deterioration, and it is for this reason that slides prepared by the writer's 
method require to be sealed with the shellac cement before the finishing cement is applied. 

Presuming that a circular cover glass is used, and the slide made thoroughly clean 
by rubbing with methylated spirit, it (the slide) should be briskly wiped over with the 
silken rag to remove adventitious specks of dirt or dust, and the cover-glass, with the 
object in the middle, ought to be very accurately centered on the turntable. The shellac 
cement, previously well stirred, should be placed on the work-table alongside a similar 
bottle containing methylated spirit, and the sable crow-quill-sized striper ought first 
to be dipped into the methylated spirit before being inserted into the bottle of shellac 
cement. The brush, charged with the cement, must be applied slantwise to the edge of 
the rapidly revolving cover-glass, so as to just barely seal its margin to the glass- slip. 

* Ut supra, p.x., et sequcntes. 



cfcc CiDtmieti) Centurp fltias or 

This ring ought to be about one sixteenth of an inch in breadth, and the slide, as soon 
as sealed, should be removed and laid aside in a cabinet to dry for at least three hours. 
To clean the brush, which ought to be done without delay, wash it well in methylated 
spirit, and then with soap and water, wipe dry, and set aside. 

When the sealed slide has become quite dry, after three hours, but preferably after 
a full day, the finishing cement may be applied. For this purpose a separate crow-quill 
sable striper should be reserved ; while, by the side of the cement bottle, another similar 
bottle containing benzol should be placed, in which to wash the brush occasionally so 
as to remove any undesirable clogging cement. Ring the already shellac-sealed slide 
with the chosen cement, white, black, pink or green, as the case may be, and set it 
aside in a cabinet to dry for a day or two. A brilliant clean-edged ring ought thus to be 
secured, and the narrower it can be made, provided it completely covers the shellac 
seal beneath, the neater will the slide be. It may be noted, en passant, that the shellac 
seal effectually prevents any subsequent benzol-dissolved cement from penetrating below 
the cover-glass ; but, if the shellac seal is not very carefully applied, but happens to be 
weak at any point, then the other cement will surely penetrate through the defect and 
mar the beauty, if not altogether spoil the slide. 

TYPE II. THE PREPARATION OF SOFT-ROCK SECTIONS, ETC. 

There are many rocks and kindred materials, such as fossils, bones, teeth, etc., 
which, by reason of their physical properties, effectually resist every effort to slice, shift 
and mount them by the methods of preparation recorded above ; and there are yet 
others of the nature of blown sands, clays and marls, lacustrine deposits and oceanic 
oozes, friable sandstones and grits, coal, fossils generally, and the softer series of 
sedimentary rocks, limestones, chalks, some kinds of slate and the like, not to speak 
of weathered igneous and certain metamorphosed rocks, which require special modes of 
treatment before they are rendered suitable for microscopical examination. 

A few types will therefore be considered in this place, which, with sundry slight 
modifications, to meet individual instances, may be taken to cover the whole range of 
objects likely to come within the scope of the student as follows : (a) Blown sands, 
clays, marls, lacustrine deposits, oceanic oozes and kindred incoherent rocks. 
(3) Coherent soft rocks, such as chalks, some limestones and marbles, hard marls and 
iron-stones, shales and coal, many slates et id genus omne. (y) Bones, teeth, wood and 
fossil remains. 

Pulverulent materials may be prepared for examination in three ways: (i.) By 
Dr. SORBY'S time-honoured, but still very useful plan of washing arid separation of 
particles of approximately the same size, and the mounting of these in a series of slides 
for permanent preservation and future reference. By way of an example, a sample of 
sand may be chosen. Place the specimen in a large test-tube about one-third full of 
water, boil and agitate. Set aside for about one minute, and decant all save the 
sediment into another test-tube. Cover both test-tubes carefully (corks will do), and 
place them upright in their stand, until all suspended particles are deposited. Pour off 
the supernatant water from each without disturbing the deposits, and replace the water 
in each test-tube with sufficient strong methylated spirit to immerse the samples. Now 
empty each tube into a clean saucer and drain off the spirit after it has reacted for about 
an hour or so. The coarser sample when quite dry, may by shaking be separated into two 



microscopical Pctrograpftp. 

or more grades of sizes, while the finer sample may be left intact. Moisten the samples 
with good redistilled turpentine and allow the latter to react for about half-an-hour, after 
which the samples ought to be ready for mounting in Canada balsam. It may be noted 
that many sands contain fine examples of rare mineral crystals, notably tourmaline and 
kaolinite in the Bagshot sands at Hampstead. In such cases the diminutive crystals 
should be picked out with the point of a mounted needle, under a low power (two inch 
objective) of the microscope, and thereafter mounted separately in benzoled balsam. Then 
too there are many sea-side sands that are full of fine grains of magnetite, and these can, 
of course, be very readily isolated by means of a magnet for separate slides. 

To mount samples of such objects successfully, the usual routine of balsaming 
cover-glasses, adherent to slips, by breathing upon the latter, as detailed above at page 
xiv., ought to be put into operation. Next, take up enough of the turpentined sample 
upon the point of a small penknife, and place it in the balsam on the cover-glass. Stir 
about with a mounted needle so as to spread the approximately even-sized particles 
uniformly over the whole surface of the cover-glasses, and place the slips bearing the 
latter into the dust-proof cabinet. After from six to twelve hours, according to the 
season or temperature of the room, shift the cover- glasses bearing the balsamed objects 
with the point of a mounted needle. If they resist immediate removal, a slight heating 
over a Bunsen or other flame will suffice to overcome the difficulty. Mount the objects 
on clean slips, previously annointed at their centres with a trifle of benzoled balsam, 
and, by a slight circular movement applied to the upper surface of the cover-glass with 
the end of a conical penholder, contrive to spread the objects evenly over the whole 
field. In performing this operation, it may be found needful to occasionally re-heat the 
slide over the Bunsen flame. Scrape superfluous balsam away from the edges of the 
cover-glass with the point of a blunt penknife, clean off with methylated spirit and a dry 
silk rag, as already directed, and seal and finish the slides if desired. It may be noted 
that if any stray air-bubbles persist, the slides containing them should be set aside for a 
day or two, before being sealed, in a moderately warm room or chamber, maintained at 
a temperature of about 80 F. All such air-bubbles, by evaporation of the benzol in 
the balsam, will then be found to disappear of their own accord, and an absolutely perfect 
mount thereby secured. 

Useful modifications of the process just described may, at the discretion of the 
student, be applied to the study of many coherent soft rocks, which cannot be 
easily sliced and reduced after the manner of hard rocks. Most marls, many 
grits, and some sandstones, require to be levigated sufficiently, and then washed, 
separated into samples and mounted as a series of slides, while some chalks may also be 
partially pulverised, boiled in water, macerated, and then dried into a series of samples 
to display the organisms and other particles they contain, to advantage, when mounted 
both dry as opaque objects and in benzoled balsam for examination by transmitted 
light. 

(ii.) Incoherent rocks such as oceanic oozes, diatomaceous and radiolarian deposits 
and very soft chalks, may be cleansed by chemical reagents previous to being washed, 
separated into samples and mounted, either for reflected or transmitted light. As an 
excellent typical example, the famous Globigerina-ooze, dredged by the Challenger 
naturalists from a depth of 1990 fathoms, at Station 338, South Atlantic Ocean, may 
be referred to in this connection. Its mode of preparation, by boiling in a solution of 
bicarbonate of soda, in order to cleanse the diminutive tests and fragments without 



xx Cbe cuwitictb Ccnturc fltlas of 

destroying them, is sufficiently explained at page 5 of this Atlas. Many of the larger 
Globigerina, Pulvinulina and Orbulinte, when cleansed and duly dried, may be readily 
picked out with the aid of a hand lens, and then mounted separately or in groups, 
either as opaque objects or in benzoled balsam, and often afford slides of great beauty, 
value and utility to students of taxonomy. 

Other types of incoherent rocks, such as siliceous deposits composed mainly of the 
frustule-valves of diatoms, skeletons of radiolarians, sponge spicules and so forth, ought 
first of all to be mounted intact as a series of slides prepared after the manner indicated 
for pulverulent materials generally, but they should thereafter be cleansed thoroughly 
in order to reveal the siliceous components in all their pristine beauty. To accomplish 
this successfully the samples should be subjected to prolonged boiling in strong 
sulphuric acid, fortified, if need be, with nitric acid, in a large test-tube immersed in a 
water-bath. When cold, the contents of the test-tube ought to be repeatedly washed 
with tap-water, and, finally, with distilled water, until all trace of acid has been 
removed. Now drain off the water and replace it with good methylated spirit for an 
hour or two, or until samples are required for mounting. The mounting may then be 
carried out in the same way as directed above for pulverulent specimens. 

(iii.) Oozes, clays, marls, sands, chalks and soft or friable rocks admit of being cut 
into thin sections after having been artificially indurated in such a way as to preserve 
the natural relationship of their component particles. Some chalks and many lime- 
stones admit of being sliced, reduced, shifted and mounted with the utmost ease, but the 
majority of argillaceous and calcareous rocks, even though they may allow for being 
sliced, fastened and reduced, cannot possibly be removed from the slips or glass-squares 
to which they are originally fixed. On the Continent, therefore, such rocks are usually 
ground thin in situ upon the slips on which they are to be permanently mounted, and the 
surfaces of such slips are usually much disfigured (fortunately outside the field of view), 
by sundry scratches. The writer has, however, overcome this difficulty by devising the 
new method of reducing rock sections, which will not hold together otherwise, on 
detachable cover-glasses, which, upon their removal, can be mounted without fear of 
failure upon clean glass slips. This process has not hitherto been published, and may 
be recorded for the first time as follows : 

Select as compact a piece as possible of the ooze, clay, chalk, coal or other soft 
rock, and trim it roughly with a knife or file into a slice about an inch square 
and ^ or even inch thick. Indurate this slice with hardened Canada balsam or by 
PEARCEY'S process * which consists essentially in induing the rock with gum copal 
dissolved in ether. The method of indurating the rock with Canada balsam or a 
mixture of Canada balsam and gum dammar, is simply to immerse the specimens for an 
hour or two in the medium used, which should be kept at barely boiling point over a 
water-bath, after which the specimens may be removed and allowed to get cold and hard. 

When hard, a flat surface should be developed on the specimen by rubbing it 
briskly on the upper face of a flat or engineers' fairly coarse-cut file, held horizontally 
between the jaws of a vice. The most useful size of file is ten inches long by one inch 
wide, and three such files ought always to be kept in readiness for use, coarse, 
medium and fine-cut. Notice moreover that the surfaces of the files are not absolutely 
flat but slightly convex. This convexity enables the operator to develop dead-level 
surfaces on small specimens when held by the hand. 

* For details of which see Proc. Roy. Physical Society, Edinburgh, Vol. viii., 1884. 



microscopical Petrograpbp* 

Neither carborundum nor emery-powder must be used for soft or artificially 
indurated rocks, because the particles will inevitably become firmly imbedded in the 
specimens ; so that after obtaining a dead-level face with the coarse, and, if desired, also 
with the medium and fine files, the surface so obtained should be rubbed perfectly smooth 
and free from scratches (when viewed through a hand-lens or magnifying glass), on a 
block of prepared Water of- Ayr stone, as indicated on page v. above. 

Grinding on the cover-glass by the writer's new method may now be proceeded 
with as follows: Heat one of the handles off inch thick plate-glass squares, sufficiently 
to melt a very small fragment of hardened Canada balsam placed thereon, and let Sj. 
warmed and thoroughly cleaned No. i cover-glass of suitable size gently slantwise down 
upon the balsam. Move the cover-glass circularly with the points of a small pair of 
forceps upon the glass plate so as to place it centrally, and at the same time exclude 
air-bubbles. When thus cemented, place another fair-sized piece (about as big as a 
small pea), of hardened balsam upon the surface of the cover-glass and reheat from 
below until this is melted. Now warm the smooth surface of the specimen and fasten 
it as centrally as possible over the cover-glass, squeezing out superfluous balsam to the 
exclusion of air-bubbles, and set it aside till quite cold and firm. 

Reduction of the specimen may now be safely effected, upon the files and Water- 
of-Ayr stone, until it is judged by examination under the microscope to be thin enough. 
The square of plate-glass should next be heated from below by holding it with a pair of 
level -jawed pliers over a Bunsen or other flame until the cementing balsam is barely 
soft enough to permit the cover-glass bearing the thin slice of rock to be shifted off the 
square of plate-glass. The cover-glass with its specimen in situ must now be placed 
(specimen upwards) upon a slip of glass, and the thin slice can then be covered with a 
drop of benzoled balsam and placed for a few hours in the dust-proof cabinet, to be 
afterwards mounted on a clean slip,* washed and wiped, sealed, ringed, labelled and 
placed in the slide cabinet. 

With regard to the second section of coherent soft-rocks, such as chalks, some 
limestones and marbles, hard marls and iron-stones, shales and coal, and many varieties 
of slate, as well as for weathered rocks generally, a little judgment must be exercised 
as to the precise modes of treatment to be adopted. For example there are a few 
indurated chalks, such as those in County Antrim, Ireland, near contact with the 
basalt, and the hard chalk with concretionary brown fluo-phosphate from the lower rock 
bed of Taplow Court, which admit of being ground, shifted and mounted like ordinary 
compact igneous rocks ; whereas the crystalline white statuary marble of Carrara, Italy, 
which apparently appears to be more compact, cannot be shifted after grinding without 
disintegration, and must therefore be ground on the glass slip upon which it is to remain 
permanently, or by the method of reduction upon a cover-glass detailed above. 

Most chalks, however, such as the ordinary white chalks of the South Coast and 
the East of England, the remarkable red chalks of Speeton, near Flamborough Head, 
and of Hunstanton, Norfolk, together with the majority of marls, shales, coal and some 
slates, such as the chiastolite slate of Skiddaw, require special treatment in order to 
secure successful results. 

By way of an example, the hard white chalk known as Melbourne Rock in 
Hertfordshire, may be operated upon as follows: Select a fairly compact specimen, and 
with a hardened steel tenon-saw, detach slices, each about one-eighth of an inch thick. 

* Ut supra, pp. xiv. et sequentes. 



Cbe twentieth Centurp fltlas or 

Boil, or rather simmer, these slices in hardened Canada balsam for an hour or so, over a 
water-bath, and, immediately upon their removal, scrape off all superfluous balsam. 
Set the slices aside to harden. When cold and quite hard, rub the first surface quite 
flat on a medium and then on a fine-cut engineer's file, and then upon the Water-of-Ayr 
or carborundum razor-hone. Fasten the slice upon a cover-glass fixed to the square 
plate-glass holder, and proceed to reduce the slice, first on files and afterwards on hones, 
with frequent examinations under the microscope. 

Most specimens of chalk, and indeed of the majority of calcareous rocks containing 
organic remains, are all the better for being ground only fairly and not too thin, so as to 
display the organisms to the best advantage ; but it is also advisable in many instances 
to prepare sections as thin as they can possibly be cut in order to bring them within the 
range of high powers of the microscope. For instance, in examining a chalk-rock 
of the nature of one derived from a globigerinal-ooze, like that dredged at Station 338, 
South Atlantic Ocean, by the naturalists on board H.M. Challenger, from a depth 
of 1990 fathoms, on the 2ist March, 1876,* a thick section would only reveal the 
presence of the larger Foraminifera, while the interstitial matter or ground-mass would 
appear opaque. In a very thin section, on the other hand, most of the tests of the 
larger Foraminifera would drop out, leaving the ground-mass full of empty spaces ; but, 
when resolved under high powers, the true nature of the ground-mass at once becomes 
apparent, and it can be observed to consist of comminuted particles of large tests 
binding together a vast assemblage of other forms coccoliths, rhabdoliths, spicules of 
alcyonarians, tunicates, valves of diatoms and fragmentary tests of radiolarians, thereby 
throwing light on the true nature of the rock mass. 

Common chalk, then, is worthy of being prepared in slices of varying thickness. 
Some to show complete sections through the foraminiferal tests, etc., and green 
glauconite casts, while others are intended for examination under higher powers to 
reveal the intimate structure of the tests, " spheres " and matrix generally. If desirable 
two or more sections, say, one thick and the other or others thinner, may be mounted 
under separate small cover-glasses on one slip of glass. These suggestions can, of 
course, be applied to a variety of compact limestones with organic remains in good 
preservation, such as that from the Silurian series at the Wren's Nest, Dudley,f the 
Miocene Leithakalk of the Vienna Basin.J and many others, most of which, however, 
are so hard and compact as to admit of being sliced, ground, shifted and mounted after 
the manner already directed for fresh igneous rocks. 

Similar observations may be recorded with respect to slates, some of which 
notably the peculiarly altered Chiastolite Slate of Skiddaw, Cumberland rarely admit 
of being removed from the glass upon which they are ground ; while the ordinary clay- 
slate from the Penrhyn Quarries, Bethesda, in North Wales, can be ground excessively 
thin and yet shifted with impunity. Not so, however, with very soft rocks and other 
substances, such as shales and coal. These invariably demand special previous 
treatment, such as induration with hardened Canada balsam, und grinding upon the 
cover-glass by the writer's method. Some specimens of coal, moreover, cannot even 
then be made to adhere to glass except by the use of marine glue ; and there are also 
many oily and bituminous shales which require similar treatment and very careful 
handling while being sliced across the plane of bedding, on account of their extremely 
fissile character. 

* See p. 4 of Text in this Atlas ; f See page 40 ; J See page 48 of this Atlas. 



microscopical Pctrograpbp, 

Fossil remains of bones, teeth, wood, etc., when not completely silicified or calcified, 
require to be specially treated to reveal their intimate structure under high powers of 
the microscope. Slices of bones, teeth and wood (lignite) may be easily cut in any 
direction by means of a tenon-saw ; but it is undoubtedly the best plan to cut true 
longitudinal and transverse sections, instead of tangential or oblique ones. In a few 
instances, especially when dealing with teeth, the tenon-saw may prove unavailing when 
it comes into contact with the intensely hard calcified enamel. Recourse may then be 
had to a saw charged with carborundum powder ; or, better still, to the diamond- 
charged disc of the lapidary machine. Slices necessarily, must vary in thickness from 
fa to of an inch according to the nature and condition of the material in hand. 

Slices thus procured should be washed thoroughly ; and, even in some cases, soaked 
in solvents of grease, such as benzol, followed by a washing in strong methylated spirit. 
After being duly dried one surface of each slice should be rubbed quite flat on a medium 
and then on a fine-cut engineer's file, and finally smoothened on a Water-of-Ayr stone 
or extra fine Carborundum hone. On no account must the slices be reduced by means 
of powdered abrasives of any kind. 

If required the slices may be indurated with hardened Canada balsam ; but, as 
a general thing, this precaution, except for cancellated bony tissues, will not be found 
necessary. The smooth surfaces of the slices may now be fastened with shellac cement 
or hardened Canada balsam, on to squares of plate-glass as usual, and the slice reduced 
to the requisite degree of tenuity, which can be ascertained by intermittent peeps 
through the microscope, preferably with a moderately high power (about 150 to 300 
diameters. 

To remove the sections, they must not be even slightly heated. Otherwise, they 
will curl up, and resist any attempt that may be made to flatten them. Place the 
square plate-glass holders, with the specimens downward in saucerfuls of strong 
methylated spirit, until they float off of their own accord. Remove the sections with a 
soft brush or special metal lifter into another saucerful of clean methylated spirit, and 
after allowing them to soak for an hour or two, to get rid of all dirt and adventitious 
particles, remove the sections, one by one, on to a sheet of fine clean blotting paper 
under a bell-glass, so as to dry them thoroughly, and, at the same time, prevent the 
incursion of dust from the air. 

When quite dry, the sections of bone and teeth should be dipped, one by one, with 
the aid of a finely-pointed pair of dissecting forceps, into a solution of white-shellac, for 
a second or two, so as to completely coat the sections with a thin transparent film of 
shellac. To make the solution of shellac take of broken white-shellac sticks of the best 
quality a sufficient quantity to about half-fill a three- or four-ounce wide-mouthed 
bottle, carefully stoppered, and lightly shake the small fragments down. Cover the 
shellac with strong methylated spirit, and set the bottle aside until the shellac has been 
dissolved. A turbid liquid will result, which, when filtered through fine Rhenish paper, 
ought to become as clear, but not quite so mobile, as pale sherry. When the film of 
shellac covering, but not penetrating, the substance of the section has become quite 
dry and hard, which will probably take an hour or two, according to the temperature 
of the room, the sections may be mounted in benzoled balsam in the ordinary way 
indicated above* with the use of the dust-proof cabinet, etc. 

* Cf. Ut supra, page xiv. 



ClK Cwntietl) Centurp fltlas of 

Examination under the microscope will then demonstrate the intimate structure 
of both bones and teeth to perfection by occluding the spaces left in those tissues of 
organic origin. The difference between the refractive indices of the imprisoned air, 
the surrounding matter, and mounting medium (Canada balsam), is so marked as to 
map out, not merely the Haversian canals, but the lacuna and canaliculi of the bone, and 
not only the pulp-cavity, but the dentinal tubuli of the dentine and the meshwork of 
lacuna and canaliculi of the crusta peivosa in teeth, with the utmost clearness ; while the 
lamella of the bone and the enamel fibres of the teeth are also rendered more distinct by 
the use of the polariscope. This is the writer's " method of occlusion," which he 
communicated over twenty years ago, for the first time, to the QUEKETT Club, at 
a meeting held in University College, London. 

Sections of wood, and of fossil plants of the nature of lignite, do not require any 
special mode of treatment. They should be mounted in benzoled Canada balsam or 
gum dammar or in a mixture of the two, in the same way as hard rock sections. 



megascopic Slides. 



GENTLEMEN who are desirous of making complete collections of minerals and rocks 
are, more often than not, nonplussed where and how to " stow " their hand-specimens. 

The writer has, therefore, devised a plan whereby many hundreds of specimens 
may be kept for ready reference within reasonable space, and yet be more useful for 
consulting or teaching purposes than the so-called bulky collections issued by tradesmen. 
It is this : 

Procure as thin a flake of rock as it is possible to get, by plying the hammer in the 
field. Take the chip into the laboratory and grind one surface flat on the coarse wheel 
of the lapidary bench or otherwise, and then, with flour emery or fine " Carborundum," 
till an absolutely even surface is produced. This face must be fastened with clear 
hardened Canada balsam on to the centre of a 3x1 glass slip; but it must be 
remembered that the upper faces must be cleanly fractured so as to show the full 
characters of the rock-texture for view by the naked eye or field-lens. When turned 
up-side-down the slide will, of course, present a polished face, and its appearances 
thereupon noted. These rough specimens are useful, inasmuch as they teach workers 
in the field how to discriminate, and ergo garner for or eliminate specimens from their 
geological bags. 

Then again, specimens so prepared, say in. square, on 3 x i inch glass slips, can 
be labelled and put by in book-boxes on an ordinary bookshelf, where they will take up 
but very little room, and thus a complete and handy museum of geological specimens 
may be established in his own home by every enthusiast or student at small expense 
within a minimum of space. An ordinary bookcase, preferably on the " GLOBE- 
WERNJCKE " type ought to amply suffice for any collector, and need not take up any 
more room than can easily be afforded by nooks and corners in even a cottage residence. 

Needless to say the microscopic slides, as finishing counterparts to the megascopic 
specimens, ought also to be stored in book-boxes and placed side-by-side with the 
megascopic slides. 



microscopical pctrograpbp. 



PetroarapDical Delineations. 




NDER the microscope the study of rocks cannot better be pursued by the student 
than by an endeavour to depict the forms presented and to colour them 
according to artistic ability. 

Practically, the first thing to do is to get a good camera lucida, that is to say, a good 
working instrument ; not a toy like BEALE'S Neutral Tint Reflector, nor such elaborate 
and costly articles as the ABBE or NACHET Apparatus, but the good old-fashioned 
"WOLLASTON" here depicted. The "shades" in practical work are riot absolutely 
necessary. 

By fixing this simple appliance to the ocular of an ordinary microscope two very 
important factors in micrographic petrology can be immediately accomplished. First, 

with the help of a stage-micrometer, the exact sizes of objects 
can be ascertained ; and, secondly, their outlines can, with 
practice, be faithfully limned. So faithfully indeed, after a 
very little practice, that goniometrical measurements can be 
absolutely relied upon. 

In using the WOLLASTON instrument care should be 
taken to see that the distance from the object to the camera 
is exactly equal to the distance from the camera to the plane 

of projection on the table: 

WOLLASTON s J 

CAMERA LUCIDA. WITH Light has also " something to say " in this connection. 

SHADES. The field must be illuminated to correspond with the illumi- 

nation on the paper or other plane of projection, and then the pencil point can also be 
clearly seen. 

The pencil itself is of some minor importance. It should be of the consistence and 
purity of GEO. ROWNEY & Co.'s Compressed Graphite, H.H.H., usually sold at 4^. per 
pencil. Commoner pencils can be used, but are decidedly unsatisfactory. 

The pencil sketch made with the assistance of the camera lucida ought to be as 
detailed as possible, giving even shades as well as lines, to result in as perfect a picture 
as can be produced in subdued " black and white." The paper used ought to be 
superfine, smooth-faced and preferably hand-made. 

To proceed, project a circle in Indian ink, (ROWNEY'S or REEVES'S Indelible Fluid), 
and place the circle under the field of view, as seen through the camera lucida. Get the 
objects to be portrayed within the field of view, and then take a sketch, dot for dot and 
line for line of the projected picture. That once fairly accomplished "half the battle is 
fought." Take off the camera lucida from the ocular, turn the microscope down to 
a slanting position, adjust the mirror, and bring out the box of paints. 

Any ordinary box of paints will do ; but it is just as well to notify the best 
colours for use in petrographical delineations. ROWNEY'S, REEVES'S, WINSOR & 
NEWTON'S or LECHERTIER FILS colours hard water-colour cakes are the best. The 



XXVI 



Cbc Cioemiefl) Ccnturp fltlas or 



colours of most frequent importance are : sepia, sap-green, burnt-sienna, gamboge, 
prussian blue, ultramarine, light-red, vandyke-brown, crimson-lake, and either ivory- 
black or Indian-ink. From these almost any tint can be improvised by mixing, as SIR 
DAVID WILKIE once tersely put it, " with brains." 

Presuming that a perfect sketch from the camera lucida has been effected, the 
microscope must be turned down to enable the artist to fill in details of both form and 
colour. Peeping through the microscope, with or without polarised light, he must 
endeavour, by dint of perseverance, to put the " right colours into the right places," and 
then, and then only, will a true and faithful picture be produced. 

The best brushes for use in petrographical delineations are those made of the finest 
red or brown sable-hair. ROWNEY'S sizes, Nos. o, i, 2, and 3, will be found amply 
sufficient, and can be procured from any artists' colourman. The sizes made by other 
firms vary slightly from the above, but are nevertheless equally good. ROWNEY'S, 
however, has been chosen in this place merely to indicate the standard sizes required. 
With the No. 3 sable, the entire sketch should be damped by brushing the surface 
lightly with a liberal supply of water, and the superfluous moisture removed by means of 
a clean sheet of white blotting-paper. No. 2 sable will be found adapted for laying 
on broad tints over those crystals, etc., that require them. To do this rapidly and 
effectively the colour should be diluted until tolerably thin and quickly spread over the 
parts, after which the blotting-paper may be pressed lightly thereon. As a result 
a perfectly flat and even-hued tint will be secured. If not quite dark enough the 
operation ought to be repeated and the blotting-paper again brought into requisition 
until the desired depth of colour has been effected. This method of producing large 
areas of perfectly uniform tints either over the whole field or in patches is far preferable 
to the plan sometimes used, to paint in the colour to the correct depth at one operation 
and allowing it to dry thus without the use of the blotting-paper. Some colours 
notably burnt-sienna would, if applied once and for all to produce a large area of 
uniform rich brown, be found to dry in unsightly blotches ; but, when laid on in 
successive coats, duly blotted as directed, can be absolutely relied upon to yield perfect 
results. Nos. i and o sables are very diminutive brushes, such as are used by miniature 
painters. They should be used, along with a hand-magnifying glass of about 3-inch 
focus, for painting in the details, either in fine lines or by stippling. Sepia will be found 
the most useful colour for this kind of work ; but, of course, other colours are also to be 
employed according to circumstances. 

While finishing drawings from the microscope in water-colours it is desirable, 
though not essential, to fix the sketch flatly upon a good drawing-board. As few 
drawings are made exceeding the size of four inches in diameter, the most convenient 
size of drawing-board to use ought to be about 5x8 inches in surface. Such a board 
admits of being turned at a moment's notice into any desired position, to enable the 
artist's hand to limn lines with greater facility and exactitude. For example, a right- 
handed person can usually draw parallel lines (such as occur in plagioclase felspar 
under the polariscope) with the greatest facility, obliquely from left to right ; but, in the 
event of the crystal lying from right to left, considerable difficulty will be experienced in 
brushing in the bands of parallel twin lamellas if the drawing-board and sketch were to 
be immovably fixed ; whereas, with a small board, the sketch can instantly be revolved 
to bring the particular crystal into a suitable position, i.e., to enable the artist to put in 
the parallel lines obliquely from left to right and thus to secure true unwavering lines. 



microscopical Pctrograpbp. 



XXVll 




THE ABBE MODEL CAMERA LUCIDA. 



As many workers would like to learn something definite concerning the use of the 
more elaborate forms of delineating apparatus, the following specification of " The 

ABBE Model Camera Lucida " may not come 
amiss in this connection. This unique instru- 
ment may be used with the utmost satisfaction 
while the microscope-tube is in its ordinary 
position at an angle to the surface of the 
drawing-table, and there is thus no necessity 
for tilting the microscope-tube into a horizontal 
position parallel with the plane of projection. 
This advantage, however, necessitates the 
employment of a Special Drawing Table, 
the best pattern of which is depicted below. 
Mechanical arrangements are here provided to 
enable the surface of the table to be tilted to any 
desired angle in order to correspond with the 
inclination of the microscope, and to thereby 
obviate a distorted image being projected. 

The "ABBE Model Camera Lucida" is manufactured in Aluminium, and is a very fine 
piece of mechanical workmanship. The image 
of the field of view, in this instance, is not pro- 
jected on the paper, as with the WOLLASTON 
instrument ; but the very reverse happens, both 
paper and pencil-point being visible within the 
field of view. The mirror, shown in the figure,* 
reflects the paper and pencil-point on to the 
silvered surface of a prism placed over the eye- 
lens of the ocular of the microscope, and it is 
thereby transmitted to the eye of the observer. 
There is provided a central aperture in the 
silvering through which the microscopic field 
of view can be fully seen, so that when the 
instrument is properly adjusted both pencil- 
point, plus paper and the field-of-view, come into perfect sight simultaneously, and the 
sketch, in its minutest details, can be readily made to one's entire satisfaction after but 
a few trials. MESSRS. WATSON & SONS mount this appliance in a novel way, by means 
of a " Cloth-lined Adapter," which is made to fit accurately over the outside of the 
microscope-tube, thereby saving all the trouble of centering, while centrality is ensured. 
In the event of an instrument being employed which has capped eye-pieces the camera 
lucida must, of course, be adapted to the eye-piece after the cap has been removed. 

Other advantages of "The ABBE M6del Camera Lucida" are, that the appliance 
can be readily disconnected from the fitting adapter by means of a sliding pin, and as 
easily readjusted, while it can also be instantly lifted over out of the way, as shown by 
the figure. Being made of untarnishable aluminium the instrument always remains 
brilliant and is so light that vibrations are practically abolished by its use. Then, again, 
it can be put into perfect operation when the microscope-tube is placed at any desired 

"This and the other figures of "Drawing Apparatus" are inserted here through the courtesy of 
Messrs. WATSON & SONS, of 313, High Holborn, London, W.C. 




ADJUSTABLE DRAWING TABLE. 



XXV111 



Che CiiKiitktb Centurp fltlas or 



angle, the only precaution to be observed being that the paper on which the sketch is 
to be made must be placed, by means of the " Drawing Table " already mentioned, 
at the same angle as the microscope-stage. 

Some students who desire to make sketches with rapidity, 
combined with comfort and precision, can have their wants 
supplied by procuring MESSRS. WATSON & SONS' " Ej-epiece 
Camera Lucida," which is made to order to fit the tube of any 
microscope. This handy little instrument, as its name implies, 
is a combination of eye-piece with a camera lucida fixed in situ. 
All that has to be done is to place the microscope and drawing- 
table at an angle of 45 upon the operating table, when it 
will be found that both paper and pencil-point are reflected into 

the field-of- view with the utmost clearness, so that tracings can be WATSON'S 

. EYE -PIECE 

taken by even the tyro, with fidelity, convenience and celerity. CAMERA LUCIDA 




On tbe Storage of Slides. 

MANY years ago the writer contributed a series of articles on "The Naturalist's 
Laboratory " to Knowledge, a journal which had then just ceased to be 
conducted by the late Mr. RICHARD PROCTOR, the popular astronomer. In 
those papers there appeared, for the first time, a figure and descriptive text on an 
improved plan for the storage of microscopic slides in book-boxes. 

None of the makers of microscopical accessories, however, appear to have since 
adopted the suggestion, which, nevertheless, is worthy of careful consideration by all 
who wish to study economy not only in expense and space, but convenience in the 
handling of slides. The following notes are therefore here recorded to enable collectors of 
both microscopic and megascopic slides to store their specimens in such a way as to be able 
to instantly lay hands on any particular slide that may be wanted for examination. 

Book-boxes are intended to supersede the old-fashioned, expensive and cumber- 
some cabinets with drawers or trays. If they cannot be purchased, anyone can, with 
but a trifle of care, improvise them at a moment's notice, by procuring a few rack- 
boxes, each to hold say two-dozen slides, and place them end-wise, like books upon an 
ordinary bookshelf. For a petrological collection, the backs of the boxes may be 
labelled "Granites," "Syenites," " Diorites," "Basalts," " Sandstones," "Limestones" 
and so forth ; while the inside of the lid may be utilised for a numbered list to indicate 
the contents in detail. 

The advantages of this system of storage ought to be obvious : The slides can be 
referred to without loss of time ; they are maintained in a horizontal position, a matter 
of some importance, where incoherent particles liable to shift are mounted and have 
not " set " properly, such as oozes, sands, etc. ; they occupy a minimum of space, for, 
a few thousand specimens which would otherwise take up a roomful of cabinets, can be 
stored with absolute safety on an easily accessible yet out-of-the-way bookshelf ; while 
in removing, the book-boxes can be quickly packed and shifted without fear of any 
damage to the slides. To conclude, a very large collection of minerals and rocks in 
both microscopic and megascopic slides, which would ordinarily demand the space of a 
museum or warehouse, may by this means be both securely and conveniently held 
within the limits of an ordinary bookcase. 



Microscopical Petroarapbp. 



XXIX 




WATSON & SONS' ADVANCED PETROLOQICAL 
MICROSCOPE. 



OF THE 

UNIVERSITY 

OF 




Carrock Fell, Cumberland, England. 



Microscopical Petrography. 



Quartz=6abbro. 



Carrock Fell, Cumberland 



Key- Plate. Upper semicircle. Magnified 26 diameters. L, L, L. Labradorite, 
showing albite lamellation under crossed Nicols. The sub-central L also shows Carlsbad 
twinning and zonary banding with polarised light, o. Orthoclase, showing alteration 
(indefinite kaolinisation). Q. Interstitial quartz, bounded below by idiomorphic 
labradorite. A, A. Allotriomorphic wedges of augite with basal striation, combined with 
twinning parallel to the orthopinacoid ; thereby giving rise to the so-called " herring- 
bone " structure. A. Augite intergrown with a core of rhombic pyroxene (enstatite), 
which has undergone the bastite modification at B ; while the margin of the augite at H, 
on the extreme right, shows the uralitic change. H, H, H. Marginal changes of the 
herring-bone augites into pale yellowish-green hornblende. T. Opaque titanomagnetite. 
Lower left-hand quadrant. Magnified 120 diameters. E. Cross-section of partially 
idiomorphic enstatite, fairly fresh, but undergoing bastite modification at the margin 
B. h. Small patch of brown hornblende, intergrown within the fresh rhombic pyroxene. 
a. Sporadic prism of apatite. L, L. Labradorite. Q. Quartz. Lower right-hand 
quadrant. Magnified 120 diameters. Micropegmatite, showing characteristic transverse 
sections of quartz, Q, and orthoclase, o. L. Labradorite. 

Occurrence. On the South side of Carrock Fell, well exposed in the crags to the 
North of the village of Mosedale, the intrusive mass of this very variable gabbro extends 
Westward to Brandy Gill and Arm o'Grain. The main body of the gabbro is faulted, 
and exposures show it alternating with the granophyre of the summit of the Fell. In 
his masterly paper on the subject, contributed to the Quarterly Journal of the Geological 
Society, HARKER notes that : " The gabbro obeys the law of becoming more basic from 
the centre to the margin." The specimen here under consideration, was gathered from 
the most acid area at the centre of the mass, and shows the felspar intergrown with the 
quartz, in surpassingly clear micrographic patterns, witness the lower right-hand quadrant 
in the plate. This confirms the coeval consolidation of the quartz as an essential 
constituent of the rock ; for, otherwise, the interstitial presence of that mineral, here 
in tolerable abundance, but almost, if not quite absent in the more basic parietes of the 
mass, might possibly pass for accessory quartz. 



2 Cbc CuKiitietb Centurp fltlas or 

Definition. The mineralogical characters of the rock indicate a predominance of 
plagioclastic felspar ; which, however, varies in its proportions from the centre to the 
margin of the mass. The bulk of the felspar, mainly in idiomorphic crystals, and quite 
clear and colourless when fresh, shows markedly fine albite lamellation, frequent 
periclinic cross-hatchings, a few Carlsbad twinnings, and occasional zonary banding. 
These circumstances, coupled with the fact that TEALL, (Cf. British Petrogyaphy], 
observed: " Two cases of symmetrical extinction on opposite sides of the trace of the 
face of composition, in which each angle was as nearly as possible 26 P , making for the 
double angle 52," confirms the conclusion that the dominant felspar is a basic 
labradorite. The presence of a monoclinic felspar is based more upon chemical 
analysis than microscopic observation, by reason of the presence of potash ; but 
careful scrutiny also reveals the fact that some of the Carlsbad twins polarise in flat 
colours, while the graphitic portions also behave in perfect consonance with the 
orthodox orthoclase of other rocks in homologous situations. TEALL observes, that 
what is very remarkable in this rock is : " The existence of colourless well-striated 
crystals of plagioclase in a matrix of quartz"; while the felspar at places, presumably 
of the orthoclase, becomes ill-defined and pulverulent (kaolinised). 

The pyroxene is both monoclinic, (augite), and rhombic (enstatite), occasionally 
intergrown as original constituents (See Plate, at extreme right-hand side of upper 
semicircle). Both of them are invariably modified ; the alteration creeping inwards 
from the margins, sometimes to the entire transformation of the minerals, the former 
into greenish-yellow chloritic matter, referable to secondary hornblende, frayed out 
perchance into actinolitic needles, and the latter, into a bastite modification, likewise of 
a pale greenish-yellow hue. Both pyroxenes are of a faint brownish-pink tint in their 
fresh moieties. The augite is almost imperceptibly pleochroic ; but the enstatite 
remains unchangeable under revolution of a single Nicol. The augite is usually 
presented in allotriomorphic wedges, with delicate striation parallel to the basal plane, 
and when twinned as usual on the orthopinacoid, shows thoroughly typical " herring- 
bone " structure. Much of the chloritic alteration products infiltrate into interstices in 
the adjacent felspar, and limonitic stains are not infrequent. While intergrown with 
the enstatite, the augite often exhibits " Schillerisation " ; and this doubtless led 
CLIFTON WARD to think of the mineral as hypersthene. The crystals however, are 
much too minute for levigation and after analysis, to reveal the 14 per cent., or over, of 
Fe O. The enstatite appears to have taken shape slightly before the augite ; for the 
former generally exhibits itself, when associated, as a core ; interpenetrated, it is true, by 
incursions of augite ; while now and again, there are ill-defined patches of clear brown 
pleochroic blebs in the pyroxene, said to be original hornblende, and certain marginal 
crystals of exactly the same colour, but more idiomorphic, obtain at the edges of the 
enstatite ; showing, with a power of about 120 diameters, the angles of hornblende. 
The enstatite tends to assume idiomorphic outlines, as shown in the lower left-hand 
quadrant of the plate, and undergoes bastite modification generally ; while the augite 
occasionally becomes paramorphic (uralite). Although the augite does not build 
ophitic plates with interpenetrating felspar, HARKER astutely surmises that the rock 
" approaches a diabase." 

Iron occurs in a somewhat ambiguous form ; usually in large plates which rarely 
show grey leucoxenic degeneration, but yet give a bluish-black lustre with reflected 
light. The crystals are therefore presumed to be titanomagnetite. MARKER says 



microscopical Pctrocjrapbp, 3 

that : " Brown Biotite, apparently highly ferruginous (comparable with Haughtonite), 
is frequent as an accessory "; but this does not appear to obtain in the specimens under 
consideration. Most likely, the Biotite happens largely in the basic portion of the 
rock-mass, where also HARKER has estimated that the iron, (sparingly represented in the 
central acid gabbro), is present to the extent of some twenty-five per cent., at the margin 
of the rock, where the quartz is " conspicuous by its absence." He also notes the 
rare circumstance, that the magnetite and ilmenite are sometimes well intergrown, 
and among the first to form, twinned parallel to a crystal boundary. Sulphide of iron, 
(pyrite), occurs only in the very basic part of the rock-mass; while apatite is 
disseminated throughout in the form of sporadic prisms ; and grains of sphene, other 
than leucoxene are said to occur. HARKER concludes, that both acid and basic varieties 
of the gabbro of Carrock Fell were consolidated, presumably under slightly different 
conditions, from one and the same magma. 

Literature. GEOLOGICAL SURVEY MAP, England and Wales. 101 N.E. (New 
Series 23), 1890. Issued to public 1893. GROOM, Quarterly Journal of tlie Geological 
Society, Vol. XLV, 1889, pp. 298-304. HARKER, Q.J.G.S., Vol. L, 1894, anc ^ Vol. LI, p. 125. 
TRALL, Q.J.G.S., Vol. XL, 1884, p. 647, PI. xxix, Fig. 2, and British Petrography, 1888, 
p. 178. TRECHMANN, Geological Magazine, 1882, pp. 210-212. CLIFTON WARD V 
Q.J.G.S., Vol. xxxii, 1876, pp. 16-27. ZIRKEL, Lehrbuch der Petrographie, 2nd Ed., 
1893, p. 781. 




Che Ciociiiictb ccuiurp Atlas or 



Globiacrina=0ozc. 

<92> 

* Challenger " Expedition, Station 338, South Atlantic Ocean, 



Key Plate. Outer zone. A to K, magnified 25 diameters ; N to p, x 120 ; Q to 
T, x 210. All mounted in Canada balsam, after careful cleansing with bicarbonate of 
soda and clearing with turpentine. Viewed by ordinary transmitted light. A. 
Pulvinnlina Menardii; B. Pulvinulina Micheliniana; c, D, E. Globigerina bulloides ; F. 
Globigerina vubva ; G. Globigenna Canariensis ; H. Young Pulvinulina Menardii; i. Young 
Pulvinnlina Canariensis; j. Orbulina universa ; K. Fragment of Orbulina universa; L. 
Young Globigerina sacculifera ; M. Under side of Pulvinulina Micheliniana ; N. Young 
Globigerina cequilateralis ; o. Young Globigerina bulloides; p. Young Pulvinulina, species 
incertce sedis ; Q. Possibly the test of Cymbalopora bulloides; R. Fragment of 'Orbulina 
universa; s. Portion of M highly magnified to show nature of foramina ; T. Fragment of 
Globigerina hirsnta. Inner Circle. Magnified 210 diameters. a, a. Young Globigenna ; 
b. Fragment of Radiolarian ; c. Horse-shoe shaped spicule of Alcyonanan ; d. 
Diatom, Coscinodiscus ; e. Vase-shaped test of probably, Tintinnus ; /. Star-shaped 
spicule of Tunicate ; g, g, g. Rhabdoliths ; h, h, h. Primordial cells of Globigerina. 
The spherical body depicted immediately below the diatom d, is probably a Coccolith. 

Occurrence. Dredged from a depth of 1990 fathoms, at Station 338, of H.M.S. 
Challenger Expedition on March 2ist, 1876. Latitude 21 15' S., longitude 14 2 ' W. 
The temperature of the sea was 76 5 at the surface, and 36 3 at the bottom. 

Definition. This "modern chalk," as the late SIR C. WYVILLE THOMSON happily 
termed it, affords an excellent example of the geological dictum, that, in a wide sense : 
"The present is the key to the past." The naturalists on board H.M.S. Challenger 
discovered, that, at Station 338, in the South Atlantic Ocean, they had obtained 
possession of one of the purest Globigerina-oozes brought to light during their eventful 
expedition ; the deposit at the depth of 1990 fathoms, being almost wholly composed of 
the dead shells of pelagic organisms : the tests of many species of exquisitely beautiful 
Foraminifera, (whose life histories could be studied from hauls made by the surface tow- 
nets,) associated with the remains of Radiolaria, the armatures of delicate calcareous 
Alga, siliceous frustule-valves of Diatomacea, sponge and kindred spicules, the shells of 
sundry molluscs, and more or less mineral matter of purely inorganic origin. Most of 
these matters were shown to have been showered down from the surface as dead bodies, 
and to have been augmented by the remains of organisms from intermediate depths of the 
sea ; while a few probably lived and died in their abyssal tomb at the bottom of the 
ocean. In thus subsiding by gravitation, the minute carcasses, of course, underwent 

* By kind permission of Sir JOHN MURRAY, K.C.B.. D.Sc.. LL.D., Ph.D., F.R.S., etc., Edinburgh. 




o.ex-0 < 




6lobiaerina=0oze. 

"Challenger" Expedition, Station 338, South Atlantic Ocean. 



microscopical Petrograpbp. 5 

decomposition, while, their skeletons, so to speak, were subjected to chemical as well as 
dynamic metamorphism, due to the re-actions of the salt water and its dissolved gases, 
as well as to the continually increasing pressure of the superincumbent water. At 1990 
fathoms, the freshly gathered samples of the ooze exhibited a whitish colour tinged with 
rose ; and, when dry, a granular homogeneous texture, closely resembling chalk, with a 
reddish-brown residue. 

In their exhaustive report of this, among numerous other samples, all of which are 
systematically tabulated for ready reference, the Challenger naturalists record, tinder 
the heading of carbonate of calcium, that the tests of Foraminifera revealed 80 per cent, 
of Globigerinida , including Pulvinulina, and only one per cent, of Miliolida, Textularida, 
Lagenida, Rotalida, and Nummulinidee, while other calcareous matter included remains 
of fish-otoliths, gasteropods, lamellibranchs, pteropods, heteropods, Lepas valves, 
ostracodes, echinoderm fragments, polyzoa, coccoliths and rhabdoliths. Of siliceous 
organisms one per cent, consisted of sponge spicules, radiolarian tests, imperfect casts 
of Foraminifera, Astrorhizidtz, Lituolida, and a few diatoms. Mineral matters were 
evidenced by but one per cent, of particles, m.di. 0-06 mm., consisting of angular 
felspar, hornblende and magnetite ; magnetic spherules, pumice, a few manganese 
grains and bronzite spherules ; while the fine washings amounted to 5.46 per cent, of 
amorphous matter, with small mineral particles and fragments of scoria and siliceous 
organisms. In comparing this particularly pure and typical sample of Globigenna-ooze 
with similar soundings taken nearer the equator, the Challenger report points out that 
the majority of shells are much smaller and thinner than those of the latter, while the 
younger specimens are much more numerous, and the species which predominate are 
different. These remarks are also registered to hold good for the specimens taken 
from the surface of the ocean. 

In mounting such a heterogeneous assemblage of forms for microscopical 
examination, it would obviously be quite impossible to place them side by side, in sitit 
upon a single slide ; nor would the plan of presenting sections, taken through the 
mass, previously consolidated in Canada balsam or gum copal, serve the purposes of 
study nearly so well as the method adopted, namely, to thoroughly cleanse and dissociate 
the diminutive tests and their fragments by boiling for a couple of hours in a fairly 
strong solution of bicarbonate of soda ; to remove all traces of the soda by repeated 
washing in water, and to separate the sample into two parts by agitation in a test-tube 
and decantation of all particles which remain in suspension at the end of one minute. 
The two samples, after being allowed to settle for some hours are collected by pouring 
off the clear supernatant water from the sediments, and the latter are then very gradually 
permitted to become quite dry in a hot-air chamber. The coarser sample is next 
subdivided, by sifting, into shells of uniform sizes ; and these, upon examination, will 
be found to contain fairly typical proportions of GlobtgeritUf, Pulvinulina , Orbulinc?, etc., 
and can therefore be cleared with turpentine, and mounted together in Canada balsam, 
under a single cover-glass, or displayed as an opaque mount. The excessively fine, 
one-minute washing, can then be similarly treated and mounted in Canada balsam 
under a separate cover-glass, upon the same glass slip, and, when viewed with powers 
of over 200 diameters ( to ^ and ^ in. objectives), will be seen to contain, in addition 
to very minute Globigerwa and their primordial cells, the assemblage of spicules, 
rhabdoliths, coccoliths and other infinitesimal particles, which for obvious technical 
reasons, cannot be mounted to advantage by the side of the larger specimens under 



6 Che Cwentierb Cenrurp fltlas of 

the same cover-glass. It is for these reasons that a double mount on one slide has 
been presented to harmonise with the typical plate and its key-plate. 

Literature. MURRAY and RENARD, Deep Sea Deposits. Report on the Scientific 
Results of the Voyage of H.M.S. Challenger, during the years 1873-1876. Prepared under 
the Superintendence of the late SIR C. WYVILLE THOMSON, Knt., F.R.S., etc., and now 
of SIR JOHN MURRAY, K.C.B., F.R.S., etc., London, 1891, pp. 144-144. C. WYVILLE 
THOMSON, The Voyage of the "Challenger," Vol. i, London, 1877. 




fluaite=Picrite. 

Inchcolm. Firth of Forth, Scotland 




microscopical Pcirograpbp. 



Inchcolm, Firth of Forth, Scotland. 



Key -Plate. Upper Semicircle. Magnified 24 diameters. A, A. Large sub- 
idiomorphic plates of purplish-pink augite, tinged with yellow-brown at places. The 
crystals are more deeply coloured at the margins than at the centres. Both are 
appreciably pleochroic. and show optic axes with convergent polarised light, while 
portions of their boundaries are invaded by more or less broad borders of black opaque 
matter, resolvable under high powers, at their margins, into granules and spicules ; 
some of the former tending to assume rhombohedral outlines. These few are therefore 
referable to titaniferous iron. The upper crystal A, shows pcecilitic structure, the 
inclusions being crystals of somewhat rounded serpentinised olivine, ol, ol. The lower 
crystal A, while also exhibiting poecilitic olivine, ol, has a marginal ingrowth of brown- 
red pleochroic hornblende at h, tongues and patches of which penetrate into the 
substance of the augite below, and show characteristic traces of cleavage lines 
intersecting at an angle of 124, immediately above. Augite that has undergone a 
uralitoid change into a deep red-brown hornblende is shown at u, u, u. The two upper 
contiguous it's belong to a single large crystal, insterstitial between olivines ; while the 
lower u at the extreme right hand, above the equator, represents the brown apex of a 
similar wedge of augite, which has undergone paramorphic change, the remainder of the 
crystal, however, retaining its original optical characters. The predominant mineral 
olivine, is shown at 0,0,0,0. Part of an idiomorphic crystal with fresh kernels of 
rough-surfaced, colourless olivine, separated by many anastomosing threads of honey- 
yellow serpentine, shot at places with aquamarine, is seen at o, on the extreme left 
above the diametric line ; while at the extreme right, the letter o shows another 
idiomorphic crystal of olivine, with fresh kernels, but borders more markedly 
metamorphosed into yellow and greenish serpentine. The strings and patches of 
serpentine here, as elsewhere throughout the rock, carry agregates and stray specks of 
both ilmenite and magnetite, some of the forms being rhombohedral and others 
octahedral. The poecilitic structure in grains and rounded crystals of olivine and 
serpentine pseudomorphs after that mineral, already referred to as occurring amid the 
large augite plates, are curiously repeated under similar circumstances, in a matrix of 
Exceptionally fresh felspar, F, F, at the left-hand side of the figure. At the lower F, the 
felspar shows lamellation under crossed Nicols. The two o's, towards the centre 
of the semicircle, indicate cases where complete metamorphism of the olivine into honey- 
yellow serpentine, diffused with green, has taken place. B shows a small indefinite 
patch of dark reddish-brown Biotite, cut parallel with the vertical axis ; while s, s, s, 
show interstices filled with decomposition products, ranging from grey to pale and 



Che Cioemictb Cemurp fltlas of 

bright emerald rings and patches of viridite. T. Compound corroded crystal of ilmenite. 
L. Decomposed remains of ilmenite, showing irregular granules of the iron enshrouded 
in grey leucoxene. Lower left-hand sextant. Magnified 24 diameters. To show at o, a 
particularly symmetrical crystal of olivine, with fresh kernels and anastomosing bands 
and meshes of serpentine developed along the irregular routes of fracture. A greyish 
patch of fibrous zeolite with mammillated structure, occupies the apex of this sextant. 
B. Lath-shaped Biotite, pale-brown. Other crystals of this mineral, varying in 
intensity of hue, occur here and there in the intermediate green decomposition matters 
at s, s, s, except at the topmost patch s, where a small plate of pink augite A obtains. 
H. Fragment of original hornblende, partially enveloping chloritic and zeolitic 
mamillse. o'. Pseudomorph of yellow serpentine after olivine. Lower Central Sextant. 
Magnified 24 diameters. Shows a specially fine crystal of original hornblende, with 
idiomorphic outlines, H, H, which, however, incloses a border of vivid-green viridite 
around a triangular nucleus of grey decomposed matter, (amorphous calcite) ; while the 
body of the prism is invaded on either side by metamorphosed olivines, and harbours, 
at its base, a compound rhombohedral section of black opaque ilmenite. This 
cinnamon-coloured crystal is typical of basaltic hornblende. It is intense!)' pleochroic, 
changing under revolution of a single Nicol to a deep clove-brown colour. If completed 
by projection, the outline would afford an excellent example of a section perpendicular 
to the vertical axis, showing two clinopinacoidal and four prismatic planes. As it is, the 
upper angle, in the figure, formed by a clinopinacoidal and a prismatic plane, and the 
cleavage lines intersecting at an angle of 124, are shown to perfection. The lower 
end of the left-hand prism plane comes into contact with upper margin of the purplish- 
pink plate of augite, A, below. This red-brown border is particularly interesting, 
because it exhibits the paramorphic change of augite into a dark brown strongly 
pleochroic hornblende gradually, and is analogous to the uralitic change seen in acid 
gabbros. It will be noticed that the sub-rectangular imperfect cleavages give the 87 of 
augite at the extreme left-hand of this marginal band ; while towards the right, where 
the border comes into contact with the crystal of original hornblende, the cleavages 
gradually merge into those of the latter. The augite, A, is distinctly pleochroic ; the 
upper and lower edges of the section showing a much deeper purplish-pink hue than 
the central zone under a single Nicol. The section is taken at right angles to the 
clinopacoid ; otherwise there would be a tendency to show the hour-glass structure 
which is not infrequently revealed in this rock, when the augite crystals are cut 
approximately parallel to a vertical axis. The purplish tinge, of course, denotes the 
presence of titanium. The sub-central crystal of olivine o, exhibits kernels of the fresh 
mineral at its sinistral apex ; the rest of the section being changed to yellow serpentine 
with separations of iron-ore grains. The olivine, o, at the lower left-hand side is fairly 
fresh ; while that at the right hand corner, o, has been completely converted into honey- 
yellow serpentine. Ilmenite and viridite are located by the letters T and s respectively. 
Lower right-hand sextant. Magnified 120 diameters, to display the essentials of the 
apatite and Biotite. AP. Colourless glassy acicular prisms of apatite in longitudinal 
sections, seen penetrating the green decomposition viridite s, and the light cinnamon- 
coloured Biotite B. AP'. Cross section of hexagonal prism of apatite, o. Serpentinised 
olivine, with a fresh kernel of the mineral jutting downward, and portion of a 
rhombohedral grain of ilmenite at the dextral edge. o'. Green serpentine pseudomorph 
after olivine. T. Hexagonal grain of ilmenite. The cross section of apatite remains 



Microscopical Petrograpbp. 9 

dark when rotated between crossed Nicols ; while the longitudinal sections with their 
transverse cracks and joints, extinguish straight. The Biotite plates here show their 
frayed-out edges, encroached upon by the green decomposition products. The sections 
depicted are all cut parallel to the vertical axis, and are intensely pleochroic, 
changing from light cinnamon to deep clove-brown during rotation under a 
single Nicol. 

Occurrence. From a knoll named Swallow Craig, forming a mural cliff on the 
east side of the landing place adjacent to St. John's Abbey on the Island of Inchcolm, 
Firth of Forth. The barren exposure of rock weathers to a rich brown colour, and has 
been identified as intrusive through the contemporaneous Carboniferous strata of the 
Basin of the Forth. The freshly fractured hand specimens exhibit a moderately 
coarse-grained texture and deep green colour. With a lens the golden grains of olivine 
and glistening scales of mica may be seen to contrast clearly amidst the darker augite ; 
while the polished face of a chip intensifies the mottled appearance due to whitish and 
greenish patches of decomposition products, felspar, calcite, and zeolites, which are by 
no means uniform throughout the rock-mass. 

Definition. Olivine, never quite fresh, shows every stage of change into serpentine, 
and constitutes the bulk of the rock. Next in abundance, comes augite, frequently 
intergrown with marginal hornblende. The other original constituents, in order of 
importance, are basaltic hornblende, Biotite, plagioclastic felspar (very variable), 
ilmenite, magnetite and apatite. The secondary substances are serpentine in 
pseudomorphs after olivine, interstitial viridite and stellate chloritic spots ; grey 
granular patches indicative of decomposed felspar ; amorphous separated and infiltrated 
calcite, fibrous zeolites at places, leucoxene, and separated specks of magnetite in the 
olivine meshwork, with (probably) secondary ilmenite at the margins of some of the 
augite plates. 

The olivine in its various aspects, from fairly fresh crystals to rounded grains (the 
latter giving rise to pcecilitic structure, mainly in the plates of augite, but also rarely 
within the fresh felspar, when it occurs), has already been sufficiently noted. With 
regard to the augite, TEALL observes that : "The coloured variety is markedly pleochroic. 
It is difficult to define the pleochroism with precision, but sections approximately at 
right angles to the vertical axis of the prism change from a pale yellow when the vibration 
plane of the polariser lies parallel to the optic axial plane of the crystal, to brownish 
violet when the stage is rotated through 90. This renders it highly probable that the 
pleochroism is the same as that of certain augites rich in titanium." 

The hornblende is sometimes almost as abundant as the augite in some sections, 
while in others it is only sparingly present, or may be absent. It is of the variety 
known as basaltic hornblende, and is intensely pleochroic, except, of course, when the 
sections are cut parallel to the orthopinacoid, as shown by u and u, at the upper right- 
hand of the semicircle in the Plate. The hornblende may be considered from a dual 
viewpoint, in the paragenesis of the components of this rock. First, as an independent 
original mineral, which took permanent form along with the augite and felspar, 
subsequent to the olivine, as shown by H, in the lower left-hand sextant, and H, H, in 
the lower central sextant of the Plate ; and next as a derivative through paramorphism 



io Cbc Cuxntieti) Ccnturp fltlas or 

at the margins of many of the augite plates. MACPHERSON has ingeniously conceived 
the idea that the pleochroism of the augite may arise from ultra-microscopic lamellae of 
hornblende intercalated within ; and the view that the augite and hornblende became 
more frequently intergrown than separately crystallised during the consolidation of the 
rock appears to be confirmed by ROHRBACH in his separation and analyses of the two 
minerals from a specimen of Teschenite. It is doubtful whether the Biotite is entirely 
original from the fact that it invariably occurs in the neighbourhood, if not entirely 
within the decomposition patches ; but the apatite, in needles, and, at most, in minute 
prisms, has probably been one of the earliest to take form as it penetrates the other 
minerals, mostly the Biotite, but is not found in constant quantity, and may be 
absent. In the sections from which the Plate was taken, the felspar is particularly 
fresh and abundant ; but this is exceptional, as the mineral may occur but very 
sparingly, or be absent or only represented by grey pseudomorphs, showing faint traces 
of their origin under polarised light. TEALL is of opinion that the felspar is a species 
approaching anorthite, on the grounds that when twin lamellation is preserved the 
extinctions on both sides of the twinning plane are often very high, and that they are 
prone to decay and become replaced by zeolitic pseudomorphs. The titan iferous iron 
plates when large are often corroded and show traces of alteration to leucoxene ; but it 
is probable that most of the magnetite is secondary. 

Literature. ADYE, Cole's Studies in Microscopical Science, June loth and 17th, 1882. 
BONNEY, Q.J.G.S, Vol. xxxvn, p. 137. A. GEIKIE, Trans. R.S.E., Vol. xxix, pp. 504-507, 
1880. GUMBEL, Die paldolithischen Eniptivgesteine des Fichtelgebirges, Miinchen, 1874. 
MARKER, Petrology for Students, p. 89, 1897. JUDD,- Q.J.G.S., Vol. XLI, p. 400. 
MACPHERSON, Bulletin de la Societe Geologique de France, p. 289, 1882. PHILLIPS, Q.J.G.S., 
Vol. xxxiv, p. 478 and Plate xxn, Fig. 5. ROHRBACH, Tschermak's Miner do gische und 
petrograpfnsche Mittheilungen, Neue Folge, Wien, Vol. vn., p. 24. ROSENBUSCH, 
Mikyoskopische Physiographic dcr massigen Gesteine, Band I, Plate xxi, Fig. i, 1885. TEALL, 
British Petrogiaphy, pp. 80, 81, 82, 94, 97, 124 ; Plate iv, Fig. 2, and Plate vn, Figs, i, 2, 
3 and 4. TSCHERMAK, Die Porphyrgesteine Oesterreichs, p. 239, Wien, 1869. 




A 



^ 




Sphcmfcrous horiiNcmlc OIKIS*. 

Pitlochry, Perthshire, Scotland. 



E.H.A .Pinxit. 



microscopical pctrograpDp. 



Spbenifcrous l>ornblciulc 6neiss. 

-^^z~- 

Pitlochry, Perthshire, Scotland. 



Key-Plate. Upper Semicircle. Magnified 25 diameters. H. Hornblende; B. 
Biotite; s. Sphene ; Q. Quartz; p. Plagioclastic felspar; z. Zoned plagioclase; 
A. Faint indication of apatite patch, portrayed in detail at lower left-hand sextant of 
this circle ; o. Interstitial orthoclase. Lower left-hand sextant. Magnified 210 
diameters. A. Longitudinal section of apatite needle ; A'. Transverse section of 
apatite prism ; M. Magnetite ; H. Hornblende ; z. Zoned plagioclase. Lower central 
sextant. Magnified 120 diameters. B. Biotite, much altered, inclosing lenticles, L, 
probably of included felspar; o. Indefinite felspar, kaolinised ; H. Hornblende; 
Q. Quartz. Lower right-hand sextant. Magnified 210 diameters. z. Zircon ; 
Q. Quartz ; F. Probably orthoclase ; H. Hornblende. 

Occurrence. This remarkable rock, now under consideration for the first time, was 
discovered, quite lately, by Mr. GREGORY, who found it in sundry boulders in Aldour 
Quarry, Pitlochry, Perthshire, Scotland ; and recognising its unusual richness in 
idiomorphic sphene, took specimens for laboratory examination, which amply verified 
his observations in the field, and has now added a worthy member to our list of rare 
British rocks. The boulders do not display " slickensides " nor glacial striae ; but, like 
most erratic blocks of their " ilk," are more or less weathered. 

Definition. Petrographical knowledge has of late progressed by such leaps and 
bounds that it has become somewhat embarrassing to credit any newly discovered rock 
with an inalienable name. The writer therefore thinks that the time is at hand when 
lithologists throughout the world would do well to adopt the time-honoured methods of 
their brother biologists, by endowing each specimen with a (Latin or Greek) generic 
plus a specific name. However that may be, this Pitlochry rock may, at all events, be 
adjectivally termed, a gneiss, by reason of its distinctly foliated character, clearly 
induced by dynamic metamorphism. HARKER* is of opinion, however, that, on 
mineralogical considerations, the specimen, minus augite, approaches the Monzonite of 
DE LAPPARENT, rather than a true foliated granite ; but then Monzonite itself, according 
to the new nomenclature here proposed, ought to be of specific rather than generic 
significance. 

The predominant constituent is colourless felspar, accentuated by sepia-grey 
fractures and mottlings due to weathering (kaolinisation,) which sometimes emphasises 

* In a letter to the writer. 



12 cbc Ciocntieib Ccmurp fltlas of 

zonary bandings and albite lamellations. Most of the felspar is plagioclastic, 
apparently oligoclase-andesine ; but there are also numerous interstitial patches 
of orthoclase. The zoned-plagioclase is worthy of attention under crossed Nicols. 
Next in abundance comes the brownish-green hornblende in idiomorphic plates, cut in 
various directions ; and the Biotite, which is much altered, occurs in characteristic lath- 
shaped forms, frayed at the edges, of an olive-greenish-brown hue, bleached towards the 
centres, and exhibiting lenticular inclosures, which simulate the secondary calcite 
lenticles of many minettes. The felspar is literally invaded by minute needles 
and prisms of clear colourless apatite, and subordinate magnetite granules 
occur disseminated throughout the rock mass, but are absent from the 
quartz patches which fill up interspaces between the felspar and hornblende 
and, with a moderately high power (^ in. objective), exhibit characteristic strings 
of sub-spherical inclusions, with occasional colourless prisms of zircon, which may be 
identified by their strongly marked contours and brilliant pinks and greens between 
crossed Nicols. It is the sphene, however, which gives distinctive character to this 
rock, by reason of its unusually deep colour (brown-pink) and pleochroism, abundance, 
and exceptionally regular lozenge-shaped forms. 

Literature. This particular rock has not hitherto been described ; although there 
is reason to believe that it occurs pretty frequently throughout the Southern Highlands 
of Scotland. 



* 












PorpDpritic Basalt. 

Lion's Haunch, Arthur's Seat, Edinburgh. 



E.H.A.Pinxit. 



microscopical peiroarapbp. 13 

Porpbpritic Basalt 

<s&=* 
Lion's Haunch, Arthur's Seat, Edinburgh. 



Key - Plate. Upper Semicircle. Magnified 18 diameters. The porphyritic 
constituents are : p, p, p. Plagioclase felspar in sub-longitudinal sections. P'. 
Plagioclase crystals cut obliquely. A. Cross-section of augite with inclosure of 
ground-mass. A'. Zoned augite cut somewhat obliquely to the vertical axis, o, o. 
Olivine, partially serpentinised along fissures of metamorphism. o'. Olivine almost 
wholly converted into greenish-yellow serpentine. The black specks are aggregates and 
grains of magnetite. The intervening matter consists of a ground-mass mainly of small 
plagioclase crystals ; minute crystals and grains of augite ; crystals, spicules and grains 
of magnetite, and a small yet appreciable proportion of a devitrified glassy base. Lower 
left-hand sextant. Magnified 18 diameters. To show porphyritic olivines, o, o, with clear 
crystallographic contours. I Both crystals, juxtaposed, are fairly fresh, and show 
unaltered kernels of the mineral, with characteristic roughened surfaces, like ground- 
glass, and irregular greenish-yellow fissures of metamorphism. At P, a small corner of 
the longitudinal section of a skeleton crystal of porphyritic plagioclase is shown. The 
rest of the sextant is occupied by the ground-mass already noted in the upper semicircle. 
Lower central sextant. Polarised light ; fully-crossed Nicols. Magnified 18 diameters ; 
but, by reason of the analyser having been screwed above the objective, the tube of the 
microscope was thereby lengthened, and the image consequently slightly amplified when 
compared with the corresponding part depicted towards the median left-hand moiety 
of the upper semicircle. A. Augite. A'. Part of crystal of zoned-augite, cut 
obliquely to the principal axis, emphasising the border of accretion, p. Plagioclase 
crystal exhibiting albite lamellation. o. Olivine showing three large fresh kernels in 
vivid reds and yellows, the remainder giving the sub-aggregate polarisation effects 
characteristic of serpentine. The bulk of the ground-mass remains dark, save for being 
illuminated by the small laths of felspar and glimmerings from the crop of minute 
augites. Lower right-hand sextant. Magnified 210 diameters, to resolve the nature of the 
ground-mass, p, p. Plates and laths of plagioclase, some of the smallest of which are 
microlitic and often exhibit bifid or ragged terminations, a, a, a. Minute prisms, 
variously cut, and allotriomorphic grains of augite. These often occur isolated and in 
groups, invading the margins of the larger porphyritic crystals of felspar, and also as 
inclusions in the bodies of the latter, m, m, m. Crystals and grains of magnetite, which 
also occurs in form of spicules, and skeletal groups of spicules and other microlites 
throughout the interstitial matter of the ground mass. g. A small patch of brownish 
to greenish isotropic glass, devitrified by microlites. 

Occurrence. Overlooking the city of Edinburgh from its Eastern Boundary, the 
hill known as Arthur's Seat towers aloft in form of a recumbent lion. The summit 
outlines the head, while the nether hill at its East end is familiarly called the " Lion's 
Haunch." The rock rises in rude columnar structure, from which large fragments 
become detached, to accumulate in a confused heap at the base of the small escarpment, 
by reason of its exposed position. Many of these blocks are much weathered, but fresh 



H C1K CuKiitictl) Centurp fltlas or 

specimens ma)' always be secured with a little perseverance. When fresh, the rock is 
black to the naked eye, and very compact. With a lens, its porphyritic nature becomes 
plainly apparent and the porphyritic elements can be identified as large glassy crystals of 
felspar, frequent dark glancing augites, and patches of yellowish-green olivines with a 
greasy lustre. SIR ARCHIBALD GEIKIE is of opinion that the rock, which consists of the 
remains of a volcanic neck or dyke, a Southerly offshoot from the main plug of the 
summit, is of Permian age ; but PROFESSOR JUDD assigns to it a more ancient origin, 
in common with the other volcanic rocks of the district, which are undoubtedly 
contemporaneous with the associated stratified rocks of Lower Carboniferous age 
(Calciferous Sandstone Series). 

Definition. A typical porphyritic basalt, consisting of a ground-mass, in which a 
very minute amount of the magma persists as brownish and greenish-grey isotropic 
glass, mostly devitrified by microlites. The glassy base is interstitial, and may be 
studied to advantage in the inclusions which are most frequent in the felspar crystals. 
TEALL says that the " felspar appears to play the vole of matrix in the ground-mass," 
where it obtains in form of small plates and laths, often microlitic in dimensions, with 
bifid or ragged extremities. These minute second crop of felspar crystals also exhibit a 
tendency to flow-structure around the larger porphyritic crystals of felspar, augite and 
olivine. The augites of the ground-mass occur as a secondary crop of small stout prisms 
and allotriomorphic grains, wedged in between the smaller felspars, and often invading 
the substance, especially at their margins, of the porphyritic felspars, where they are 
frequently isolated and consequently conspicuous. Cubes, octahedra, grains and 
spicules of magnetite are plentifully disseminated throughout the ground-mass. 

The porphyritic crystals which diversify the ground-mass and give character to the 
rock, consist, in order of genesis, of olivine, augite and felspar. The two last-named 
probably took shape together, and invariably included portions of the magma. The 
olivine is often fairly fresh, displaying colourless kernels, with characteristic shagreened 
surfaces in the sections. The boundaries of the crystals and their irregular network of 
fissures, exhibit metamorphic changes into serpentine ; metallic yellow, like the 
hyalosiderite found in the Limburgite of the Kaiserstuhl near Freiburg, in some, and 
quite green at other parts. There are also separations in spots and strings of minute 
specks of magnetite ; while many of the smaller crystals are almost completely converted 
into fibrous serpentine giving feeble bluish-grey and neutral tints under crossed Nicols. 
The larger of the augite crystals often measure as much as | in. They are of a pale 
reddish-brown colour and faintly pleochroic, in well-formed idiomorphic crystals, showing 
both eight-sided cross sections and longitudinal rhombs. Some of them are very clearly 
zoned. The large crystals of clear colourless felspar, often \ in. long, are well striated 
on the basal plane, and repeatedly twinned on the albite type. Their high extinction 
angles show that they are allied to anorthite, probably Bytownite. Many of the 
crystals are zoned, having a colourless periphery inclosing a core crowded with 
inclusions ; while skeleton crystals are not infrequent. 

Literature. ADYE, Cole's Studies in Microscopical Science, Vol. i, p. 293, London, 1883. 
ARNOLD-BEMROSE, Q.J.G.S., Vol. 1, p. 624, London, 1894; Pwc. Geol. Assn., Vol. xvi., 
p. 213, 1899. A. GEIKIE, Descriptive Catalogue, etc., p. 19, Edinburgh, 1870; Geol. Survey 
Memoir and Map 32, Scotland. HARKER, Petrology for Students, 3rd Ed., p. 204, 
Cambridge, 1902. JuoD,)/7.G.S., Vol. xxxi., p. 131, London. MACLAREN, Geology of Fife 
and the Lothians, 1839. TEALL, British Petrography, Plate 23, and p. 188, London, 1888. 



OF THE 

UNIVERSITY 

OF 







Dokritc. 

The Hawse, Salisbury Crags, Edinburgh. 



E.H.A.Pinxit. 



microscopical Petrograpbp. 15 



Ccucoxcnic Dokrite, 



The Hawse, Salisbury Crags, Edinburgh, 



Key-Plate. Upper Semicircle. Magnified 25 diameters. P. Transverse section of a 
large, almost porphyritic crystal of plagioclastic felspar, with clear colourless zone, 
enveloping a dark core of altered ground-mass. P', P', P'. Lath-shaped plagioclases, 
disposed in ophitic fashion. T, T, T. Tabular and elongated crystals of Titaniferous 
iron, which have undergone considerable leucoxenic change. A, A. Plates of augite, 
completely metamorphosed into serpentine and a trifle of chlorite, m, m. Magnetite. 
a, a. Interstitial ground-mass showing grains of augite. Lower central sextant. Magnified 
25 diameters. A portion of the upper semicircle under fully crossed Nicols. The 
crystals, etc., appear to be larger than the corresponding components figured above, by 
reason of a slight lengthening of the tube of the microscope with analysing prism 
screwed above the objective, p. Transverse section of large crystal of plagioclase, 
with indications of polysynthetic twinning on the albite type. P', P', P'. Lath-shaped 
plagioclase twins. a, a. Allotriomorphic augite in interstitial ground mass. 
T. Titaniferous iron. m. Magnetite. Lower left-hand sextant. Magnified 210 diameters. 
L. Extremity of an elongated crystal of titaniferous iron, almost entirely altered into 
brown highly refringent grains of leucoxene. a. Transverse section of minute prism 
of apatite, a. Longitudinal aspect of needles of apatite, m, m. Grains of Magnitite. 
p. Plagioclase. Lower right-hand sextant. Magnified 120 diameters. c. Secondary 
calcite. h, h. The variety of vermicular chlorite called helminth, m. Octahedron of 
magnetite. Py. Iron pyrite. 

Occurrence. The bold escarpment facing the Eastern boundary of the " Old 
Town " of Edinburgh, and popularly known as Salisbury Crags, is due to the exposure 
of a thick sheet of dolerite, which was intruded through the contemporaneous strata 
of Lower Carboniferous age, and now forms the picturesque upper slope over which 
Arthur's Seat rears its lofty head. The face of the rock is generally weathered to a 
rusty brown, which extends inwardly for from an eighth to as much as half-an-inch. 
The mass varies somewhat in texture in different situations ; but fragments, gathered 
at " The Hawse,"* at the entrance to the " Hunter's Bog," beloved of volunteer riflemen, 
may be regarded as fairly typical. When freshly fractured, the rock exhibits a medium 
texture of a deep greenish-grey colour mottled with white crystals of felspar, grains and 
streaks of calcite, some analcime and diversified by a few brassy crystals and specks 
of iron pyrites and veinlets of Prehnite and pectolite. 

* Presented by PROFESSOR JAMES J. DOBBIE, M.A., D.Sc., Museum of Science and Art, Edinburgh. 



i6 CDc Ciucmietl) Ccniurp fltlas of 

Definition. The mineralogical characters are : Dominant plagioclase in sub- 
porphyritic crystals ; with lath-shaped clear plagioclase in ophitic arrangement, 
intersecting green patches and tabular, sometimes distinctly eight-sided forms of 
altered augite. Numerous elongated crystals of titaniferous iron which have undergone 
leucoxenic change are scattered throughout, and there are also a fair proportion of 
crystals and grains of magnetite. All these crystals are manifestly held together by a 
ground-mass of felspathic material and obscure grains of augite. No isotropic glass, as 
such, can be detected, but high powers (200 diameters and over) reveal the presence of 
abundant minute prisms and needles of clear colourless apatite, and enables the 
leucoxene to be studied to advantage. Alteration products, are serpentine derived 
from augite, indued with a small proportion of chlorite. Pyrite and calcite are constant 
accessories ; while analcime, Prehnite, and pectolite may be regarded as accidentals. 
The calcite is more or less invaded by the vermicular chlorite called helminth. 

Fairly high extinction angles point to the conclusion that the sub-porphyritic 
crystals are basic labradorite. These usually exhibit clear borders enclosing cores of 
dusty and granular inclusions, evidently caught up from the original magma, and are 
usually twinned on the albite type, occasionally grouped into sheaves. The lath-shaped 
crystals are also labradorite twinned on the albite type, the polysynthetic character of 
which appears in some of the border examples. Occasionally, interpenetration 
cruciform twins may be detected. The greenish octagonal and irregular patches, which 
are generally penetrated by the lath-shaped plagioclase and elongated ilmenites, are 
almost entirely composed of serpentine which has arisen through alteration of the 
original plates of augite. They give the aggregate polarisation effects of serpentine 
which is not pleochroic, with occasional changes here and there under revolution of a 
single Nicol, which serves to determine the presence of chlorite. The chlorite appears 
to have been the result of weathering, and will be found to penetrate and fill up 
interspaces, especially in association with infiltered calcite, where it segregates into 
cinctures and twisted bands of helminth. The calcite may be identified by its cleavage 
lines and bands of twin lamellae which are emphasised by polarised light. The fractures 
in the original augite become the seat of rows of specks and granules with crenated 
contours, opaque in thick sections, but grey, highly refringent and translucent in very 
thin slices. These are probably calcite. 

Leucoxene however, gives character to this rock-section. It mottles and often 
entirely replaces the commonly elongated crystals of titaniferous iron, of which it is an 
alteration product, not white and opaque, but in brownish-red, glassy crystals and 
granules, with strongly marked contours closely resembling ordinary sphene, but giving 
iridescent colours under crossed Nicols. The magnetite occurs in tolerably large 
compound crystals and scattered minute cubes, octahedra and grains throughout the 
rock mass ; while the magma filling, interspaces and as inclusions in the larger felspars, 
is a dusty and ill-defined felspathic base containing grains of altered augite, fibres of 
serpentine and suffused chlorite at places. 

Literature. ALLPORT, On the Microscopic Structure and Composition of British 
Carboniferous Dolerites, Q.J.G.S., Vol. xxx., p. 554, London, 1874. A. GEIKIE, On the 
Carboniferous Volcanic Rocks of the Basin of the -Firth of Forth, Trans. R. S. Edin., Vol. xxix., 
p. 437, et. seq., Edinburgh, 1880; Descriptive Catalogue, etc., p. 19, Edinburgh, 1870. 
TEALL, British Petrography, p. 189, London, 1888. 



OF THE 

UNIVERSITY 

OF *- 





Carboniferous Oolitic timcsionc. 

Clifton, Bristol, England. 



E.B.W. Photo-micro, 



microscopical Pcirographp. 17 



Oolitic Cimcstonc. 



Clifton, Bristol, England. 



Key- Plate. Upper Photomicrograph.* Magnified 20 diameters. A. Well developed 
oolitic grain showing concentric zones of accretion, and appreciable radial arrangement 
of particles, surrounding a central nucleus of granular calcite, rudely hexagonal in 
outline, and probably the transverse section of a Crinoid stem. B. Elongated oolitic 
grain, enveloping a small spine, probably the branch of a Bryozoon. c. Fragment of 
Crinoid which has escaped being coated by oolitic layers. D. Ovoid oolitic grain, 
inclosing what appears to be a vertical section through the test of a Foraminifer. 
E. Apparently the transverse section of an Echinoid spine. F. Hexagonal plate of 
calcite of Crinoid origin. G. Tangential section of spherical oolitic grain, cut external 
to the nuclear region. H. Fragment of shell of Lamellibranch, sparingly encrusted 
with oolitic accretion. K, K. Sections through Crinoid ossicles revealing the axial canal, 
and forming nuclei for sub-quadrangular oolitic grains. M. Amygdaloidal oolitic grain 
formed around a lenticular nucleus. It shows both the concentric layers and radial 
disposition of their particles admirably. N. Irregularly shaped oolitic grain showing 
that the zones of accretion follow the shape of the nucleus, in this instance, a fragment 
of shell. P. Probably the test of a Foraminifer. Q. Fragments of Echinoderm plates 
or ossicles. R. Tessellated interstitial ground-mass of secondary calcite. Lower left-hand 
photomicrograph. Magnified 60 diameters, from a slide in the possession of Mr. WETHERED 
Transverse section through the stem of a Crinoid, with oolitic zones developed to 
perfection in concentric series which have not undergone molecular changes. There 
is also a distinct radial disposition of particles, which, in some of the thinnest circular 
sections at the edges of the slice, exhibit a faint black cross under completely polarised 
light, analogous to the phenomena of dark crosses seen in the spherulites of igneous 
rocks, when examined under crossed Nicols. The surrounding interstitial ground-mass 
is built up of a mosaic of secondary saccharoid calcite, each tessera of which shows 
the banding due to twin lamellae, especially under polarised light. Lower right-hand 
photomicrograph.- -Magnified 60 diameters, from the same slide as above, in the possession 
of Mr. WETHERED. Section through an eccentric oolitic grain formed round a particle 
of shell (Mollusc or Brachiopod), showing with the utmost clearness that the bulk of the 
oolitic accretion is of organic origin, and produced by the interlacing calcareous 
tubuli, g, g. of Girvanella, cut both lengthwise and in cross sections. 

* The photographs for this plate were kindly taken by Mr. E. B. WETHERED, J.P., F.G.S., F.I.C., etc., 

of Cheltenham. 



1 8 chc cwentietb Ccnturp fltlas of 

Occurrence. From the Carboniferous limestone formations near Clifton, Bristol. 
The rock is very compact, of a brick-red colour, and takes on a fine polish ; when the 
closely packed oolitic grains can be clearly seen with the naked eye, and give the rock 
its typical character as " roe-stone." Mr. WETHERED'S preparation was made from an 
exceptionally well-developed example, " New Road Oolite," taken from an exposure 
of the Carboniferous Middle Limestone, in the gorge of the River Avon at Clifton. 

Definition. The brick-red rock now under consideration, consists essentially of 
a heterogeneous assemblage of forms of animal life the hard parts of which, some 
entire, (Foraminifera), and others fragmentary (Ccelenterata, Echinodennata, Crustacea, 
Mollusca, Boyozoa, Brachiopoda, etc.], have, as a rule, furnished the nuclei, upon which the 
oolitic zones have been deposited. The oolitic grains, with uncoated organic skeletons, 
and sporadic grains of sand of detrital origin, are imbedded in a matrix of recrystallised 
calcite, probably formed from communicated matter derived from the organic remains 
themselves. The entire mass, after its formation, appears to have undergone molecular 
changes, whereby the intimate structure of the oolitic grains has become somewhat 
obscured ; the calcite matrix granulated, but still showing clear traces of the mosaic ; 
and the occasional grains of sand show a secondary outgrowth, producing minute, and 
sometimes very perfect crystules of quartz. Mr. WETHERED'S specimen, on the other 
hand, appears to be singularly free from alteration. The oolitic grains and uncoated 
organisms are sharply defined in a mosaic of saccharoid, calcite. The only change that 
might have taken place in the rock after its formation, being the alteration of the 
probable aragonite of some of the organic remains, into the stabler form of calcite, which 
now exclusively obtains. The biogenetic formation of some of the oolitic grains, through 
the agency of Girvanella, is here also of special interest. 

Literature. COHEN, Sammlung von Mikrophotographien .... von Minevalien und 
Gesteinen, Plate Ixiii., Figs. 2, 3, Stuttgart, 1899. A. GEIKIE, Text-Book of Geology, 
4th Ed., p. 191, London, 1903. HARKER, Petrology foy Students, 3rd Ed., p. 255, 
Cambridge, 1902. HARRIS, Proc. Geol. Association, Vol, xiv., Plates 3 and 4, 
pp. 59-79, 1895. SORBY, Presidential Address, Q.J.G.S., Vol. xxxv., p. 56, et seq., 1879. 
TEALL, Mem. Geol. Survey, Jurassic Rocks of Britain, Vol. iv.,pp. 8-12, Plates i and 2, 1894. 
WETHERED, Proc. Cotteswold F. N. Club, Vol. x., pp. 101, 102, 1891, also in 1895-6; 
Q.J.G.S., Vol. li., pp. 196-206, Plate vii., 1895. 




Green 

Ightham, Kent, England. 



E.H.A.Pinxjt. 



microscopical Pttrograpbp. 19 

Green Quartzite. 

Ightham, Kent, England. 



Key - Plate. Upper left-hand quadrant. Magnified 25 diameters. From a fairly 
thick section, showing many well-rounded, more or less clear, colourless, allothigenous 
grains of quartz, Q, Q, Q, which vary in size in the rock from 0-015 to 0-04 of an inch, 
but here average about 0*02 in. The authigenous constituents, which bind these grains 
into a closely compacted texture, consist mainly of colourless quartz, q, impregnated at 
places with limonite, /, and stained at other or the same parts with greenish silicate 
of iron. Under higher powers, this thick section shows that the grains of rounded 
quartz are coated with a pellicle of green or brown silicate of iron, and that this 
colouring matter is often diffused through the adjacent secondary quartz ; while the 
limonite frequently insinuates itself through cracks in the detrital quartz grains, and is 
there permanently deposited, as at d. Upper right-hand quadrant. Magnified 25 diameters, 
represents the same part of the section as the above, under fully-crossed Nicols.* The 
slice, being rather thicker than usual, shows the main bulk of the rock-mass, which is 
quartz, in very vivid interference-tints, and these, in their turn, exhibit the optical 
continuity of contiguous parts of the secondary outgrowths of interstitial quartz with 
striking effect. Lower central sextant. Magnified 25 diameters, under fully-crossed 
Nicols. This being taken from an extra-thin section, (about O'ooi in.), the interference- 
tints, are in neutral or blue-grey, and faint lemon, and serve to illuminate structuiai 
details, hitherto obscure. L. Section through the major axis of an ellipsoidal grain of 
quartz densely charged with limonite, and clearly fringed at d, d, with minute odontoid 
outgrowths showing a radial disposition. Q, Q. Allothigenous quartz grains. 
i, i. Interstitial authigenous silica, oriented conformably with the contiguous grain of 
quartz. c. Cryptocrystalline chalcedony, probably indentical with the odontoid 
outgrowths already noted. Lower left-hand sextant. Magnified 210 diameters, under fully 
crossed Nicols. This depicts the angular region marked x, in the lower central sextant, 
resolved by a higher power to show : L. Portion of ellipsoidal grain of quartz, densely 
impregnated with limonite. Q. Colourless quartz grain exhibiting hypo-odontoid 
outgrowths 11, and secondary silica s, in optical continuity with Q. In like manner, the 
odontoid radiating processes r, r, emanating from the grain L, protrude into the 
interstitial secondary clear quartz s, which is in opticial continuity with L, but free from 
its limonitic adjunct. Lower right-hand sextant. Magnified 120 diameters. This picture 
was taken under ordinary transmitted light. The section was rather thick, and, on that 
account the crystal x, appears opaque, although its edges indicate that it might be 
brown if somewhat thinner, and probably tourmaline. Q. Quartz. T. Tablet of 
tourmaline. The dark-brown fenestrated region at the periphery of the sextant is 
without doubt a vestige of organic origin. It forms part of a rounded fragment, about 

* Whenever hereafter polarised light is used in displaying structures already limned under ordinary 
light with the same magnifying power, it must be understood that the details become slightly amplified, but 
not more highly resolved, above the latter, by reason of the elongation of the microscope-tube, caused by 
the addition of the analysing prism screwed immediately above the objective of the instrument. 



20 Cbe Cuxntiet!) Centurp fltlas or 

O'oi by O'O2 of an inch in dimension ; and, very likely, is a silicified remnant of the 
originally calcified areolar tissue of an Echinoderm, probably of the genus Pseudodiadema. 

Occurrence. TOPLEY notes* : " At Ightham, near the Roman Camp, is a hard 
white sandstone, five feet thick, a good deal like the ' Greyweather ' sandstone of the 
Tertiary, and there is too another kind of stone, not observed elsewhere, a hard and 
tough dark-green sandstone, or rather grit. This was not seen in place ; but, on 
Ightham Common it is found in large masses, and is there called ' Ightham Stone,' and 
sometimes ' Firestone ', from its being sufficiently hard to strike fire well." BoNNEvf 
additionally observes that the Ightham Stone is of concretionary origin, like 
the " Sarsen Stones" of the Eocene, ("Grey Wethers"), and the " Cornstones " of 
the Palaeozoic Red Sandstone formations. He records that he has found the rock 
in situ, in a local sandpit at Ightham, but that it is not often so to be seen, and he 
opines, on account of their inclusions, that the allothigenous grains of quartz were 
derived from a granitoid rock. 

Definition. En masse, the rock exhibits a granulated fracture and sub-vitreous 
lustre. It is unique among British quartzites in being markedly green (lighter or 
darker in different specimens), and mottled or streaked with brown. The polished faces, 
in most specimens, show more or less frequent spaces between the grains. 

Under the microscope, the rock is shown to be almost entirely composed of quartz. 
The detrital grains are closely set, sometimes so closely, as to show barely an 
appreciable quantity of micro-crystalline silica between adjacent grains. Between other 
grains, however, the interstitial siliceous cement is well developed, and peculiar. The 
peculiarity, seldom noticed elsewhere, proves that the allothigenous elements were first 
of all subjected to considerable sub-marine attrition, the grains being well-rounded, and 
including remains of Echinoderms and possibly, Annelid-tubes, now silicified. These 
grains were next coated with a deposit of iron, which persists as a pellicle, usually of 
green silicate but sometimes brown, which subsequently spread through the secondary 
outgrowth plus interstitial deposit of chalcedonic silica. The outgrowth is almost unique, 
in that it assumes the form of an odontoid fringe around each grain, probably an 
incipient stage in the endeavour to produce secondary crystal-faces. When this attempt 
is more or less abortive, the fringe develops, at places, into crypto-crystalline chalcedony. 
Any further interspaces, are then occupied by clear infiltered silica sometimes densely 
impregnated with limonite. The quartz grains, per se, are full of inclusions, frequently 
negative crystals, occasional pellucid needles, minute colourless crystules of zircon, and 
sporadic plates of brownish and greenish tourmaline, and, very likely, of mica. Similar 
instances of the transformation of sand into quartzite by the deposition of interstitial 
chalcedonic silica are rare. They occur in the aforesaid Sarsen Stones, and in the 
matrix of the " Puddingstone " of Herefordshire, and have also been noticed among the 
older American quartzites, and during the course of recent explorations in " Darkest 
Africa." 

Literature. BONNEY, Geological Magazine, 1888, p. 297, et seq. IRVINE, Cherty 
Potsdam Sandstone from Wisconsin, Fifth Annual Report of the U. S. Geol. Survey, 
Plate xxxi. TOPLEY, Geological Survey Memoir of the Weald, p. 140. 

*Geol. Survey Memoir of the Weald, p. 140. 
fGeol. Mag., 1888, p. 297. 




limestone. 

Goshi, Larnaca, Cyprus. 



c- u w Phato-micrn. 



microscopical Petrography. 21 

Giobiacrinal Cimcstonc. 

Goshi, Larnaca, Cyprus. 



Key to Plate. Upper figure, x 20. This photograph effectively displays the general 
structural nature of the rock-mass, as essentially constituted by the remains of tolerably 
large and very slightly altered tests of Globigerinida. The portion of the section 
portrayed is too limited, however, to display the stratified character of the rock, which 
nevertheless becomes plainly manifested by examination of the entire slide under a lower 
magnifying power from ten to fifteen diameters. Lower figure, x 60. A portion of the 
upper figure (x) more highly magnified, to show that the tests of the Feraminifera are not 
appreciably altered, nor are there marked evidences of " calcification," such as occur in 
kindred formations. The beautifully preserved skeletons of the Globigerina remain quite 
as hyaline and distinct in contours as their congeners of Globigerina-oozes now being 
formed in the depths of the sea ; nor is there more than a mere trifle of amorphous 
calcareous binding material observable. This picture moreover shows that the inter- 
spaces are in part occupied by smaller shells of Globigerinida, but there do not appear to 
be any extraneous forms, such as the remains of coccoliths, et id genus omne. The rock 
is therefore rare, as a typical example of a pure globigerinal limestone in a perfect state 
of preservation. 

Occurence. Samples of the rock were obtained through the courtesy of E. H. D. 
NICOLLS, Esq., Director of Public Works in Cyprus, from quarries near Goshi, a place 
about eight miles N.W. of Larnaca, the principal port of the Island. It occurs as beds 
in the Idalian Series of Oligocene-Miocene age, and is associated with chalky limestones 
which bear the characters of oceanic deposits. Mr. JUKES-BROWNE, is of opinion f that 
the rock must have been very rapidly formed in comparatively tranquil and shallow 
water ; his reasons being, that the tests of the Foraminifera are not disintegrated nor 
otherwise altered, but practically remain as fresh and perfect as when they were 
deposited upon the sea-floor. 

Definition. Chemically, the rock must rank as a fairly pure limestone carbonate 
of lime with sundry impurities. It is of a pale yellowish-buff colour, especially when 
exposed ; and, by reason of the sizes and closely crowded tests of Globigerinida , bears 
some resemblance (to the naked eye) to a fine and even-grained oolitic limestone. 
Under the microscope, the bulk of the rock will be seen to consist of the tests of large 
and well developed Globigerintz, very like those in point of size and texture, which were 
dredged in medium depths from intertropical waters during the memorable " Challenger" 
Expedition. These remains, which appear to be fairly hyaline, and quite as well 

* The material for the slides accompanying this study was generously presented by A. J . JUKES-BROWNE, 
Esq., F.G.S., etc., upon whose copious notes the text has been mainly based. The Foraminifera were 
determined by Dr. GEORGE J. HINDE, Ph.D., F.R.S., F.G.S., etc., and the photo-micrographs were taken 
by E. B. WETHERED, Esq., J.P., F.G.S. 

t In a letter to the writer. 



22 chc CiDcmkt!) Ccnturp fltlas or 

preserved as the tests in the typical oceanic oozes of to-day, have apparently undergone 
but slight changes, in the way of calcification since they were deposited ; and, in being 
compacted together so densely, have produced barely an appreciable amount of 
comminuted cement. This cement moreover, contains many of the smaller tests 
apparently intact, as confirmatory evidence of the quiescent conditions which obtained 
during their deposition. Some of the cells of the Globigerina, and many of the 
interstitial areas bear grains of limonite, which usually diffuse a yellowish-brown stain in 
their immediate neighbourhoods, and thereby give to the rock-mass, especially when 
exposed to the atmosphere, its buff hue. The surface of the Cypriote seas in Oligocene 
times must literally have been monopolised by these diminutive denizens, even to the 
exclusion of the coccospheres and rhabdospheres which are commonly associated with 
the latter-day fauna of equatorial and sub-temperate ocean-waters. 

DR. GEORGE J. HINDE, F.R.S., having examined the rock microscopically, has 
determined that it is almost entirely made up of the remains of the Globigerina b^tlloides of 
D'ORBIGNY, with stray specimens which probably belong to the genera Pulvinulina and 
Truncatulina, and possibly an occasional erratic Nodosaria and an Entomostracan or two. 
MR. JUKES-BROWNE* observes that the rock closely resembles the somewhat more 
calcined, but equally crowded Foraminiferal Marls of Bissex Hill, Barbados, and the 
well-known Globigerinal Limestones of the Maltese Islands. 

In determining the forms of Foraminifera which compose this rock the following 
notes, taken from BRADY'S elaborate Report on the Foraminifera of the " Challenger " 
Expedition, may be found useful in the examination of sections under the microscope : 
" The Globigerinida have test free, calcareous, perforate ; chambers few, inflated, 
arranged spirally. Aperture single or multiple, conspicuous. No supplementary 
skeleton, nor canal system. In Globigerina bulloides the test is rotaliform, chambers 
opening into an inferior umbilical vestibule. The segments are globular, about seven in 
number four in the outer whorl. If there are only three relatively large segments in 
the outer whorl, the form is G. bulloides, var. triloba, of REUSS. Pulvinulina (PARKER and 
JONES) is rotaliform. Superior side usually thickest. Shell, with rare exceptions, very 
finely porous. Segments fewer than in other Rotalina. Shell thickening usually taking 
the form of external sutural limbation. Aperture, typically a large slit at base of 
umbilical margin of last segment often irregular, both as to form and position. Face of 
terminal segment sometimes inflated and studded with orbuline perforations. 
Truncatulina (D'ORBIGNY). Test free or adherent, rotaliform ; inferior face generally 
more convex than the superior. Shell-wall coarsely porous. Surface sometimes 
tuberculated, especially in old shells. Aperture, a curved slit at or near superior margin 
of inner edge of the final segment, sometimes with phialine neck and lip. 

Literature. The actual specimen here under consideration, has not hitherto been 
described, although kindred rocks from other localities have been exhaustively noticed, 
notably by : BRADY, Report on the Foraminifera. Scientific Results of the Voyage of 
H.M.S. Challenger, during the years 1873-1876, London, 1884. J. H. COOKE, Globigerina 
Limestones of Malta, Geol. Mag., Vol. iii., p. 502. FRANKS AND HARRISON, Q-J.G.S., 
Vol. liv., p. 540, London, 1898. JUKES-BROWNE Arto HARRISON, Q.J.G.S., Vol. xlviii., 
p. 212, London, 1892. SIR JOHN MURRAY, The Maltese Islands, Scot. Geog. Mag., Vol. vi., 
p. 449. 

* Loco fitato, ut supra. 



OF THE 

UNIVERSITY 

OF 




nHM 






-V 











Picrilic=$erpcntinc. 

Clicker Tor, Menheniot, Cornwall. 



E.H.A.Pinxit. 



microscopical Pttrograpbp. 23 

Picritic=$crpcntiiK. - 

<s&> 
Menheniot, Cornwall, England. 



Key=Plate. Upper semicircle, x 25. o, o. Olivine almost completely converted 
into serpentine, and separations, M, of magnetite, o'. Decomposed poecilitic 
olivine inclosed in augite. m. Shows fissures of metamorphism in the olivine. 
A. Augite tolerably fresh and pinkish, much fractured and exhibiting fissures, 
/, filled with serpentine. A' Augite decomposing into greenish, serpentine 
with separations of greyish amorphous calcite. s. Serpentine, derived from both 
olivine and augite. K. Ruddy-brown patch, marginal to augite probably altered 
from basaltic hornblende into picotite. B. Scales of red-brown, fairly fresh biotite. 
T. Titaniferous iron. F. Pseudomorph of serpentine after felspar. F'. Decomposing 
ophitic felspar inclosed in the large plate of augite. x, Marks the part more highly 
magnified in the lower central sextant. Lower left sextant, x 25. o, o. Olivine- 
serpentine showing clear traces of the original crystals. These are, in reality, studded 
in poecilitic fashion throughout a very large plate of augite, A, A, A, which at first sight 
appears to be a series of interstitial allotriomorphic aggregates ; but with polarised light 
all these detached pieces are seen to behave optically as parts of a single large crystal. 
s. Serpentine derived from olivine and augite conjointly. F. Pseudomorphs in 
serpentine after ophitic felspars. Lower central sextant, x 120. Shows" the portion 
marked x, in the upper semicircle. The apical part of this figure was taken with the 
polarising prism only in situ, to show, that upon revolution, the outer border c, of the 
olivine crystal there situated, changes from green to faint brown, and is therefore 
distinctly pleochroic. T. Titaniferous iron. A. Augite, fresh at places, but exhibiting 
alterations into greenish serpentine, chlorite, and amorphous grey calcite. Under 
crossed Nicols the parts converted into serpentine give the aggregate feeble bluish 
polarisation effects so characteristic of that mineral ; which moreover, unlike chlorite 
is not pleochroic. Lower right sextant, x 120. A. Plate of fresh pinkish augite. 
F. Lath-shaped, much decomposed ophitic crystal of plagioclase felspar, originally 
simply twinned. F'. Pseudomorph in serpentine after ophitic crystal of felspar. 
e, e. Empty spaces. 

Occurrence. At Clicker Tor, near the Menheniot Railway Station, about three 
miles S.E. of Liskeard, where it was quarried, this rock was localised by ALLPORT* as 
an intrusive mass, which was well exposed during the construction of the Cornish 
Railway. 

Definition. A deep-green stone with scutiform spots, mapped out by streaks and 
patches of grey and white, not unlike the skin of a lizard or small serpent. A thin slice 
under a low power of the microscope, shows a ground of pale green serpentine, merging 
at places into an almost colourless matrix, imbedded in which are numerous pseudomorphs 
in brighter green, or when weathered, yellow serpentine after olivine ; irregular pink 
patches of augite, decaying at their edges into amorphous grey matter ; flecks of 
interstitial ruddy-brown biotite and probably, when in marginal patches, of basaltic 
hornblende, and numerous specks of magnetite, with occasional elongated crystals and 
compound skeletons of titaniferous iron. Felspar much decomposed and in pseudomorphs 
is also seen to play a subordinate part in piercing and sometimes being enveloped by 
the augite in ophitic fashion. With higher powers, varying from 100 to 500 diameters, 
longitudinal and transverse sections of apatite may be seen in the neighbourhood of the 
frayed-out brownish flecks of biotite, while the augite appears to be diallagic, and a 
very peculiar " schiller " structure is revealed within the pseudomorphs after olivine which 
also exhibit a tendency to the development of celyphitic borders. The evolution of the 
rock may therefore be indicated by assuming it to have been originally an intrusive 
deep-seated diabase, with olivine as the dominant and first formed constituent ; augite 

*Q. J. G. S., Vol. xxxii, p. 422, London, 1876. * 



24 OK CiDcntictl) Centurp fltlas of 

next in abundance, in large plates with pcecilitic olivines and ophitic felspars, the 
latter being subordinate ; while the other original constituents were probably hornblende, 
biotite, apatite, magnetite and ilmenite. The second phase in the history of the rock 
points to its alteration into a peridotite, closely akin to the picrite of Inchcolm* , while 
its final metamorphosis, has resulted in the production of a fairly mature serpentine, 
contributed to, in order of importance, by the olivine, augite and felspar, with the 
peculiarity, that of these three, the augite alone persists at places, in its primary 
condition. 

The changes which the olivine has undergone are profound and interesting. Polarised light shows but 
faint vestiges of the original mineral, the kernels being almost entirely converted into yellowish-green 
serpentine. With 200 diameters and more, the cross-hatched black patches are seen to be made up of 
grains of magnetite, often in perfect octahedra, associated with a schiller structure developed by clear 
colourless acicular and lath-shaped microlites disposed in three directions at right angles to each other. 
The cross sections of these microlites when obliquely cut appear fusiform ; but now and again true 
transverse sections may be detected, giving lozenge-shaped outlines, which correspond to cross-sections of 
the hornblende prism ; while the long-sections give maximum extinction angles of 17. These considerations 
led TEALL to conclude! that ROSENBUSCH! was right in referring the microlites to grammatite-like amphibole. 
They have been described under the name of pilite by F. Becke in a pseudomorph after olivine, and 
doubtless arise in connection with the alteration of that mineral into serpentine. TEALL further notes that 
the olivines are bordered by radially disposed needles of secondary hornblende, also the outcome of 
metamorphic changes kindred to those which occur in the olivines of many gabbros, norites and troctolites 
both at home and abroad. It is manifestly these elements in the serpentine which endow it here and there 
with its anomalous pleochroism. The expansion of the olivine, moreover, in its conversion, into serpentine 
is frequently manifested by radiating fissures through the surrounding textures. 

The augite which is singular in remaining fairly fresh, exhibits fissures filled with 
infiltrated serpentine, and a diallagic structure, which becomes modified into something 
like bastite on the one hand, while on the other, the fractured augite is seen to gradually 
break down into allotriomorphic grains, which finally become grey and amorphous to 
pass insensibly into the surrounding serpentine. The margins of the augite sometimes 
show brown adnate patches. These may be all that is left of a former basaltic or a 
uralitised intergrown hornblende, which now however, is indistinguishable from ruddy- 
brown spinel (picotite). As already stated, the augite is pierced by pseudomorphs and 
contains vestiges of ophitic laths of felspar and precilitic pseudomorphs of olivine ; 
while its fresh portions although sometimes widely scattered, may be seen with 
polarised light to extinguish in the same position, thus proving that they belong to 
large ophitic masses, as in true diabases. Occasionally, as in the lower right sextant of 
the plate, the felspars show their plagioclastic character. 

TEALL || notes, that it is interesting to witness the evidence afforded in this rock, of the alteration 
of felspar into a substance which blends intimately with the serpentine derived from the olivine ; and observes 
that this is probably what has been frequently called pseudophite, i.e., a substance which retains the original 
alumina of the felspar in combination with magnesia and water, the lime and alkalies being removed. 

The biotite is still pleochroic and of brown hue sometimes bleached ; but although 
much altered, the laminated scales show that the principal absorption takes place when 
the long axis of the cleavages lies parallel to the short diagonal of the polarising prism. 
The apatite, generally close to the biotite, is typically developed in short prisms and 
needles, transversely cracked, highly refringent, but no longer pellucid. It has become 
slightly granular. The iron ores are represented by occasional elongated crystals and 
skeletal combinations of ilmenite, and by abundant grains and octahedra of magnetite, 
disseminated throughout the rock mass, but seemingly absent from the substance of the 
remains of unaltered augite. 

Literature. ADYE, Cole's Studies in Microscopical Science, Vol. i, June loth, London, 
1882. ALLPORT, Q. J. G. S., Vol. xxxii, p. 422, London, 1876. BECKE, Tschermak's 
Min. und petrog. Mittheilungen, Neue Folge, Vol. v, p. 163, Wien. PHILLIPS, Phil. Mag., 
Vol. xli, p. 100. ROSENBUSCH, Mik. Phys. dev massigen Gesteine, Band i, pp. 413, 530, 
1885. TEALL, British Petrography, pp. 86, 97 and 124, PI. II, fig. 2. 

* ADYE in Cole's Studies in Microscopical Science, June loth, 1882. 

t British Petrography, p. 97, London, 1888. 

Mik. Phys. der massigen Gesteine, Band i., pp. 413, 530, 1885. 

Tschermak's Min. undpetr. Mittheilungen, Neue Folge, Vol. v., p. 163, Wien. 

II Loco citato. 



V THE 

UNIVERSITY 

O 



ASSi 








\ 





Pbonolite. 

The Wolf Rock, Off Cornwall. 



E.H.A.l'inxit. 



microscopical Pctrograpbp. 25 

Pbonolite. 



The Wolf Rock, off Cornwall, England. 



Key = Plate. Upper left quadrant, x 25. N. Hexagonal phenocryst of nosean with 
clear border and dark core. s. Lath-shaped, much fissured pellucid and colourless 
phenocryst of sanidine felspar. s'. Tranverse section of sanidine phenocryst with 
inclusions of minute crystals of hauyne and aegirine. Upper right quadrant, X 55. To 
show the structure of the parts surrounding the crystal of nosean, in the above, with 
greater detail. N. Nosean phenocryst. s. Tabular phenocryst of sanidine. A. Minute 
crystals of aegirine. Lower left sextant, x 210. To show small crystals of aegirine, A, highly 
magnified, in a ground-mass composed mainly of minute crystals of nepheline and lath- 
shaped crystules of sanidine. Lower central sextant, x 55. Under fully-crossed Nicols. 
T. Tabular phenocryst of sanidine, twinned on the Carlsbad type. P. Plane of 
composition of the dual twins, united on the clinopinacoid, and shewing the basal 
planes inclined in opposite directions, by means of the difference in extinction shown by 
the two halves. N. Included small crystals of hauyne in the sanidine. s. Small lath- 
shaped twin of sanidine in longitudinal aspect, s'. Cross sections of small laths of 
sanidine. A. ^Egirine. Lower right sextant, x 210. Under half-crossed Nicols, to show 
s' and its surroundings (figured in the lower central sextant) in greater detail, s, s. 
Cross sections of sanidine. N, N. Rectangular and hexagonal sections of minute 
crystals of nepheline ; the former cut parallel to the vertical axis, and the latter parallel 
to the basal plane. A. ^Egirine. 

Occurrence. Some fifteen years ago the notorious Wolf Rock, which occupies a 
site about nine miles S.E. of Land's End, Cornwall, lay submerged two feet below the 
level of the sea at high water mark ; while at low water spring-tides it stood only 
seventeen feet above the sea surface and had an area of 175 by 150 feet. Since then the 
rock has been partially demolished ; but, by reason of its unique character among 
British formations, specimens may be frequently found and unerringly identified among 
the debris on the Cornish coast. The original mass took solid form from a lava of 
peculiar constitution ; and being richer in alkalies than the kindred trachytes, 
felspathoids as well as alkali-felspars were developed, and are now considered of specific 
value in the classification of the rock as a true phonolite. The term phonolite, per se, does 
not, however, convey any scientific significance. It was given by KLAPROTH to a 
heterogenous collection of stones (clinkstones) by reason of the ringing sound emitted 
when thin fragments are struck with a hammer. 

Definition. When fresh, fragments present a rough aspect and brownish-grey 
compact ground-mass, diversified by many glassy porphyritic crystals of felspar, and 
black specks of nosean, which can be readily identified with a hand-lens. Like 
many other phonolites, it is very fissile in one particular direction, but breaks irregularly 
and with difficulty in any other way. In thin slices it will be seen to consist essentially 
of porphyritic crystals of sanidine felspar and nosean thickly disseminated in a 
holocrystalline ground-mass of small idiomorphs of sanidine, nepheline, nosean, hauyne, 
aegirine, and a very minute quantity of iron ore. 



26 ClK Cuxmictl) Centurp flrias of 

Some of the larger sanidine crystals are of tabular habit and occasionally exhibit 
binary twinning, on the Carlsbad-type ; but most of them are columnar and give water- 
clear, much fissured lath-shaped sections, which extinguish straight or at the very low 
maximum angle of 4 or 5 ; and rectangular pellucid sections which extinguish parallel 
to their edges. Optical confirmation of this constituent as sanidine may be found in 
sections parallel to the clinopinacoid, which give extinction angles of approximately 5. 
when referred to traces of the basal plane. The smaller laths of the ground-mass 
usually extinguish straight, or deviate therefrom to 5 at the utmost. Nearly all the 
phenocrysts, and many of the smaller sanidines carry inclusions of haiiyne and well 
formed microlites and needles of aegirine. 

The nosean phenocrysts most frequently afford hexagonal outlines ; less often they 
are square. Their boundaries are commonly formed of a clear zone, while the cores 
are black with inclusions, arranged irregularly or in lines. The crop of smaller hauynes 
are also readily distinguishable by their hexagonal contours, and are invariably cloudy. 
All the sections, of course, remain dark when rotated between fully crossed Nicols. 

Under powers of over 100 diameters, the holocrystalline nature of the ground-mass 
becomes plainly apparent. The small laths and cross-sections of sanidine are conformable 
under high powers to the tests already noted ; while many of them exhibit microlitic 
inclusions. The nepheline is more difficult to detect, seeing that its sections are also 
water-clear and of contours similar to those of the sanidines. Transverse sections of 
the nephelines often exhibit rounded angles and remain dark when rotated in fully 
polarised light ; but rectangular, (sometimes square), sections, which are of course 
taken parallel to the morphological axis, give neutral interference tints and extinguish 
parallel to their boundaries. The nepheline sections are, as a rule, smaller than those 
of the ground-mass sanidines, and can only be localised with precision, by their 
gelatinisation with hydrochloric acid, and staining with fuchsin or nigrosin. With 
regard to the small dirty-green crystals of segirine ; they are most frequently prismatic, 
sometimes tabular, and often irregular and in patches. In the latter instance they are 
often associated with iron ores, and iron peroxide grains and stains, presumably derived 
from the hauynes, are not infrequent. In confirmation of the statement that the soda- 
pyroxene crystules are in reality aegirine, TEALL* says : 

" As they are sometimes idiomorphic in the prismatic zone, and when in these cases the cross-sections are 
bounded by the traces of four prismatic and two pinacoidal faces, they are agirine. The angles of the 
prismatic faces are 87 and 93 and thepinacoids truncate the acute angles, corresponding to the form (100) 
which is the characteristic pinacoid of aegirine. Under crossed Nicols the microlites extinguish parallel or 
nearly parallel to their length (maximum angle only 2" or 3), and the major axis of depolarisation is always 
the one which most nearly corresponds with the direction of elongation. In both these respects the 
microlites differ from hornblende and angite, and agree with aegirine. ROSENBUSCH states that the axis of 
elasticity in aegirine makes an angle of only 4 or 5, with the vertical axis. The pleochroism is not very 
marked owing to the small size ; but, such as it is, it agrees with aegirine. The colours for rays vibrating 
parallel to the length of the crystal, i.e., parallel to a, is green, and that for rays vibrating at right angles to 
the length is often yellowish. 

Literature. BORICKY, Petroqraphische Studien an den Phonolithgesteinen Bohmens, Archiv 
d. Naturw. Landesdurchforschung v. Bohmen, Band iii., Geol. Abth., Prag, 1873. 
HARKER, Petrology for Students, p. 178, Cambridge, 1902. HATCH, Text-Book of Petrology, 
p. 166, London, 1903. ROSENBUSCH, Mik. Phys. d. Massigen. Gesteine, p. 218, 1877. 
RUTLEY, Study of Rocks, p. 228, London. 18 79 ; Granites and Greenstones, p. 18, London, 1894. 
TEALL, British Petrography, p. 367, PI. xli., Fig. i. 

"British Petrography, p, 367, London, 1888. 




eucitopi)pre. 

Near Rieden, The Eifel. 



P H A Pinvit 



microscopical Ptrograpbp. 27 



Ccucitopbpre. x 

Near Rieden, The Eifel, 



Key- Plate. Upper Semicircle, x 25. L. Sections of icositetrahedra of leucite, some 
of which are very irregular, much fissured, and contain many extremely well-formed 
microlites of hauyne, aegirine and apatite. A. ^Egirine. AC. Acmite. N. Hauyne. 
s. Sphene. Only the small haiiynes are shown in this figure ; while the ground-mass, 
which is composed of a minutely crystalline base, with little, if any isotropic glass, 
requires a higher magnifying power for its resolution. Lower left sextant, x 25. AC. 
Section in direction of vertical axis of idiomorphic, zoned, phenocryst of acmite. 
N. Portion of large phenocryst of nosean with dark border and lighter centre. It will 
be observed that the smaller crystals of haiiyne, N, in this, and the foregoing figure, are 
characterised by dark red cores and lighter brown-grey borders. A. ^Egirine, in minute 
prisms in the ground-mass, which, in this figure, shows clearly a lath-shaped small 
crystal, F, of sanidine felspar, abutting into an empty space. I. Limonitic ruddy- 
brown stain liberated from alteration at top edge of crystal of acmite. Lower central 
sextant, x 55. Under fully-crossed Nicols. Part of a large crystal of leucite to show 
polysynthetic twinning of lamellae, the planes of which lie in four directions. Where 
the lamella? are of uniform calibre throughout their lengths, they give rise to parallel 
bands of a neutral tint, varying in breadth and intensity of coloration ; by inter- 
penetration a cross-hatched effect, as at c, is produced ; while, when the lamellae 
(probably as the result of strain) vary in tenuity, they become spindle-shaped, like those 
of microcline, and occasion the peculiar " peg-structure," p, so noticeable in the allied 
mineral melilite. A. Microlites of clear idiomorphic forms of aegirine contained as 
inclusions in the leucite. Ap. Two minute longitudinal prisms of apatite adnate to one 
of the microlites of aegirine. Lower right sextant, x 120, to show the hemicrystalline 
nature of the ground mass. Microlites of aegirine, which occur freely throughout the 
ground-mass, are not represented in this particular figure ; most of the greenish-brown 
ragged, elongated, minute crystals AC, being of the variety of soda-pyroxene called 
acmite. F. One longitudinal lath and two cross-sections of small crystals of sanidine, 
the latter exhibiting diminutive prisms of clear, highly-refringent prisms of apatite. 
Ne. Rectangular and hexagonal sections of minute crystals of nepheline. 

Definition. Phenocrysts of leucite, nosean, and acmite in the hemicrystalline 
ground-mass of sanidine, aegirine, acmite, nosean, hauyne, nepheline, and apatite, with 
a few crystals of sphene, and a very little melanite. 

The leucite obtains in a crop of large, clear, colourless icositetrahedra, with jagged 
outlines, often very irregular and much fissured bodies which usually contain inclusions 
of idiomorphic green microlitic prisms of aegirine ; square and six-sided ruddy-brown, 
clear-bordered crystules of hauyne ; and many 'clear prisms and needles of colourless 
apatite. Under fully crossed Nicols, the aegirine is brilliantly illuminated in greens and 
yellows ; the nosean and hauyne remain dark, even when rotated ; the longitudinal 
prisms of apatite with their transverse fractures, extinguish straight, while the cross 
sections remain opaque, and the substance of the leucite is resolved into sets of 
uniform twin lamellae giving rise to parallel lines and cross hatchings, in darker and 
lighter unequal bands of neutral tint, or of spindle shaped lamellae, which produce the 
peculiar " peg-structure " shown in the plate. A second generation of smaller more 



28 Cbc Croemictl) Ccmurp fltlas of 

regularly contoured leucites may also be observed, and it may here be noted, en passant, 
that this mineral, as such, has not hitherto been detected in any British rock ; although 
a very remarkable rock has been noticed by DAKYNS and TEALL * from Loch Borolan 
in Sutherland, in which the rounded patches of orthoclase are said to be pseudomorphs 
after leucite. 

The soda-pyroxine is also manifested in dual characters (i) as phenocrysts of acmite 
often of a greyish-brown zoned with dirty-green and smaller somewhat similar ragged 
and often wholly sap-green crystals, with (ii) microlitic, often perfect crystals, of 
greenish a5girine, which frequently form irregular patches, as shown towards the top of 
the upper semicircle in the plate. In commenting upon the soda-pyroxene, TfiALLf says : 
"The optical properties of aeginne and acmite are markedly different from those of 
ordinary augite. The mineral is usually elongated in the direction of the vertical axis. 
The forms in the prismatic zone are (no) and (100). The negative bisectrix makes an 
angle of only 4 or 5 with the vertical axis. The extinctions therefore, in the 
prismatic zone, are always very low even lower than in hornblendes. Both acmite and 
segirine are pleochroic." The nosean was formerly mistaken for HauynejJ and indeed, 
as DRESSEL demonstrated very long ago, the blue colour of haiiyne may be developed 
in nosean by heating, and the consequent change thereby induced in the contained 
sodium sulphide ; so that VON LASAULX was led to opine that haiiyne and nosean are 
only varieties of the same mineral species. Chemical and micro-chemical tests, however, 
have since determined this mineral as nosean. The large phenocrysts are usually 
bounded by narrow opaque borders, and lighter cloudy interiors ; while the smaller 
crop of haiiyne crystals, either in the ground-mass or as inclusions in the leucites, 
have dusty-red interiors due to the presence of scales of iron-peroxide of secondary 
origin, and clear rims surrounding their rectangular or hexagonal contours. Both 
nosean and haiiyne, of course, remain dark when revolved under fully crossed Nicols. 

Sanidine occasionally shows itself in form of patches of some size, but mainly as 
colourless lath-shaped prisms in the ground-mass, giving elongated, square and six-sided 
sections. The rest of the ground-mass is made up of minute, also colourless four- and 
six-sided crystals of nepheline, which are difficult to identify except by gelatinisation 
with hydrochloric acid, and staining with nigrosin-black or fuchsin-red. There are 
also many colourless crystals and needles of apatite, known by their fairly bold outlines 
(high refractive index) ; minute green acmites and aegirine, and a few lozenge-shaped or 
lenticular lemon-coloured rough-looking crystals of sphene. The blackish-blue, strongly- 
contoured garnet, melanite, occurs very sparingly, in a few of the sections which have 
been examined. 

Literature. BERWERTH, Mik. Struct urbilder dev Massengesteine, Lief iv., Stuttgart, 
1895-1900. COHEN, Sammlung von Mikvophotographien .... von Mineralien und 
Gesteinen, (3), PI. ii, fig. 3 ; iv, fig. 3 ; and xvi, figs, i, 2. FOUQUE et MICHEL-LEVY, 
Mineralogie micrographique, Plate xlviii, fig. i, and Ii. fig. i, Paris, 1878. HARKER, Petrology 
for Students, pp. 179, 180; fig. 40, Cambridge, 1902. MERIAN, Neues Jahrbuch fur 
Mineralogie, etc., Band iii, p. 276. TEALL, British Petrography, Plate xli., fig. 2 and xlvii., 
fig. 4. ZIRKEL, Mik. Besch. dev Min. und Gesteine, p. 163, Leipzig, 1873. 

* Trans. Royal Society, Edinburgh, Vol xxxvii, pp. 163-172, with plate, 1892. 

t British Petrography, p. 276, London, 1888. 

J FOUQUE and MICHEL-LEVY, Mineralogie micrographique, Plate XLVIII., Paris, 1878. 

Neues Jahrbuch fur Mineralogie, p. 565, 1870. 




ScDist. 

Lizard, Cornwall, England. 



H A Piuvit 



microscopical Peirograpbp. 29 

bonibleiKlc=Schist. 

Lizard, Cornwall, England. 



Key- Plate x 55. The section from which this figure was taken is cut approxi- 
mately at right-angles to the schistosity, which is consequently clearly apparent. H. 
Green hornblende, showing traces of idiomorphic contours, and cleavage lines. F. Clear 
secondary felspar, c. Cloudy felspar. F'. Clear felspar showing twin lamellation on the 
albite type. M. Magnetite, m. Magnetite changed partially into blood-red martite. Q. 
Small blebs of interstitial quartz. 

Occurrence. Exposed in bands along the cliffs at Church Cove, near Landedwed- 
nack Church, Lizard, Cornwall, in which neighbourhood many notable rocks such as 
gabbro, picrite, diorite, serpentine and gneiss are also typically developed. 

Definition. In its present condition the rock is of distinctly crystalline texture, 
with a marked parallel arrangement of its main constituents, hornblende and felspar ; 
while the magnetite, which is subordinate, also shows signs of having been crushed out 
into more or less elongated forms between the other elements, and the quartz appears to 
be accidental, occurring only as interstitial blebs of minute size, without any definite 
sequence in the rock-mass. It is probable that the bands of this rock were originally of 
intrusive volcanic origin, and that they were subsequently altered, mainly by dynamic, 
but also by thermal metamorphism, from a diabasic rock into a typical schist. 

The crystals of hornblende, of a more or less brownish to bluish-green hue, are 
strongly pleochroic, and, in addition to exhibiting elongated idiomorphic forms, 
frequently show well-marked cleavage traces, which, when in cross sections, give 
good angles of 124. They undoubtedly represent the uralitisation of an originally 
pyroxenic mineral. Minute prisms of hornblende of actinolitic habit are also developed 
in the substance (especially at the contiguous margins) of the clear felspar. 

The felspar, next in abundance, is of a dual character cloudy and clear. The 
cloudy parts probably represent the slightly altered remains of the cloudy felspar of the 
original diabase ; while the water-clear felspar has no doubt resulted by recrystallisation 
and redisposition of ophitic laths of the mineral, which was probably of the nature of 
labradorite. The reasons for this conclusion being, that under crossed Nicols, manifest 
traces of albite and occasionally pericline twinning obtain. 

Dynamic metamorphism has also been influential in elongating the original particles 
of magnetite and of making them take part in the schistose arrangement ; while thermal 
agencies accompanying or accruing from the mechanical forces, doubtless led to the 
transformation of some of the grains of magnetite into blood-red martite (haematite) or 
orange-yellow limonite. The quartz, which is sporadic and interstitial, appears to be 
accidental. 

Literature. Kindred rock-textures, and their evolution, have been described and 
explained by: LEHMANN, Entstehimg der Altkrystallinischen Schiefergesteine, Bonn, 1884. 
Q. J. G. S., Vol. xliv., pp. 429-435. TEALL, in Q. J. G. S., Vol. xli., pp. 133-144, 1885 ; and 
British Petrography, pp. 197-200, PI. xix., xx., 1888. WILLIAMS, The Greenstone Schist Areas 
of the Menominee and Marqnette Regions of Michigan, Bull, 62 U.S. Geol. Survey, Ch. i., vi., 1890. 



30 C!K Ciocnticfb Centurp Atlas or 



Old Meldrum to Inverurie, Aberdeenshire. 



Key- Plate. Upper Semicircle, x 25. o. Olivine intersected by a dense mesh-work 
of fissures, along which granular magnetite is thickly deposited around fresh kernels of 
the original crystal. Towards the boundaries of the olivine, especially those bordering 
on the rhombic pyroxenes (hypersthene), the margins have undergone changes into 
yellowish-green serpentine, s; but where the olivine comes into contact with the felspar, 
F, as at x and y, profound interactions are seen to have taken place, by the development 
of a celyphitic-structure and reaction-rim. On the other hand, the hypersthene H y , of a 
brownish-pink colour, frequently exhibits a modification into greenish-grey fibrous 
bastite. F. Felspar (anorthite). B. Biotite. Lower left-hand sextant, x 120, to show the 
intimate structure of the celyphitic-border and reaction-rim at x in the upper figure. 
o. Olivine, undergoing changes into serpentine. F. Felspar. Hy. Hypersthene. 

A. Celyphitic border of hypersthene, in continuity with Hy, surrounding the olivine, 
v. Reaction -rim of actinolitic-hornblende and anorthite-felspar. Lower right-hand sextant, 
X 210, to show the portion y of the figure above, still more highly magnified. o. Olivine. 
F. Felspar, at this part, full of minute schiller-inclusions of actinolitic-hornblende, 
h. Celyphitic-border of hypersthene. r. Reaction-rim composed of radial fibres of 
actinolite and felspar intricately interwoven. L. Grain of limonite. Lower central 
sextant, x 25. o. Olivine, with celyphitic border, h, of hypersthene. Hy. Hypersthene, 
showing at its inferior moiety the bastite modification alluded to above. F. Felspar. 

B. Biotite. H. Brown hornblende, exhibiting well-marked prismatic cleavages. 

Occurrence. Discovered on the high-road, as a heap of freshly quarried " road- 
metal " by Mr. GREGORY, on a journey between Old Meldrum and Inverurie,. Aberdeen- 
shire, when on the outlook for a somewhat similar so-called gabbro, which is reported to 
occur thereabouts and also at Huntly. 

Definition. A holocrystalline rock, composed essentially of a lime-soda felspar 
(anorthite) and a pyroxene (hypersthene), diversified by well-marked sub-idiomorphic 
crystals of olivine, a little brown biotite and occasional plates of brown hornblende. 
Original iron-ore does not appear to be represented ; but secondary magnetite and 
limonite are abundantly separated along the fissures and boundaries of the olivine 
crystals. 

Both felspar and hypersthene are massive ; the former displaying (under crossed 
Nicols) marked albite lamellation, the lamellae being frequently cuneate, and occasional 
pericline markings ; while the latter is frequently altered into the serpentinous mineral 
bastite, and by its characteristic development serves to separate the rock from its allied 
gabbros of the North of Scotland, and place it among the norites, or more precisely the 
hyperites of the Norwegian geologists of to-day. The biotite and original hornblende 
are occasionally intergrown, and both of a light-brown colour ; but the feature par 
excellence of this specimen consists in the very peculiar border which surrounds the 
altered crystals of olivine. The olivine having been the first to take crystalline form, 
appears to have been enveloped by a thin layer of hypersthene, and the outer edges of 
the latter, in coming into contact with the felspar, has clearly been changed into 
actinolitic hornblende, the fibres of which have intermingled with fibres of the felspar to 
constitute a reaction-rim. All these phenomena may be verified stage by stage upon 
examination of different parts of the section, under varying powers of the microscope. 

Literature. BAYLEY, Amer. Journ. Sci., xliii., 515-518, 1892 ; Joum. of Geol. i., 702- 
710, 1893. BONNEY, Q. J. G. S., xxxiii., 884-915. CLEMENTS, Bull. No. 5, Geol. Surv. 
Ala., 171-172, 1896. COHEN, Sammlung von Mikrophot. von Min. und Gest., PI. v., fig. 3. 
JUDD, Q. J. G. S., xlii., 49-89, 1886. SMYTH, Amer. Jouvn. Sci., xlviii., 54-65, 1894; Bull. 
Geol. Soc. Amey., vi., 271, 1895. TEALL, Geol. Mag., 483-485, 1886. WILLIAMS, Bull. 
No. 28, U.S. Geol. Surv., 1886. 





Old Meldrum to Inverurie, Aberdeenshire. 



R.H.A.Pinxit. 






X 








SpDcrulitic OtsiUian. 

Iceland. 



E.H.W.Photo&E.H.A.I'inxit. 



microscopical Pttrograpbp. 3* 

SplKrulitic Obsidian. 

<^> 
Iceland. 



Key- Plate. Upper Quadrant x 20.* R. Rhyolitic region, composed of approxi- 
mately parallel bands of varying width, and darker or lighter according to the presence 
of more or less densely crowded crystallites, s. Spherulite, showing clear colourless 
outer zone and perlitic cracks. Lower left-hand segment x 55, to show the spherulitic 
portion in greater detail. R. Rhyolitic region, s. Isolated spherulite with its " court of 
crystallisation," c, and perlitic cracks, p, invaded by the flow-lines from the rhyolitic 
region. Lower right-hand segment x 210, to show a portion of the same spherulite s, 
more highly magnified, c. " Court of crystallisation " free from colouring matter. 
G. Brownish glassy matrix. M. Minute octahedral grains of magnetite, sometimes 
aggregated into groups of skeleton crystals. /. Longulites which are most densely 
crowded in the rhyolitic regions of the rock. 

Occurrence. The exact locality and mode of occurrence of this specimen are not 
available, beyond the record that the rock was gathered in Iceland from an indisputable 
mass of volcanic glass. The specimens selected show marked evidences of flow structure, 
even to the naked eye, and are of a deep-brown glassy texture, with typical conchoidal 
fracture. They are absolutely fresh, i.e., unweathered, and almost entirely vitreous, save 
for the presence of sundry spherulites, many of which can be seen without the aid of 
a lens. 

Definition. BARON VON RICHTHOFEN gave the name of rhyolite to the group of 
acid lavas, of which this obsidian is a typical early example, by reason of the fact that 
flow-structures are commonly characteristic of the rocks. In the present specimen, the 
vitreous type of ground-mass forms the main bulk of the rock, which, in thin sections 
under the microscope, is seen to consist of a pale yellowish-brown structureless glass 
diversified at places by bands of deeper colour, which under high powers may be 
resolved into more or less closely packed crystallites, of the variety named kngulites. 
Occasionally also, there are nebulous patches of longulites, which tend to assume 
circular contours, and these culminate at other parts into well-defined spherulites ; 
while every here and there distinct minute octahedra and skeleton groups of magnetite 
are developed. It may also be noticed that the flow-lines pass more or less uninter- 
ruptedly through the spherulites ; which, moreover, are usually surrounded by perlitic 
cracks occasioned by contraction. The spherulites upon closer scrutiny appear to be 
dense aggregates of longulites, which tend to take a radial arrangement ; this being most 
apparent at the margins of each sphere. The presence of a clear space around each 
spherulite points to the conclusion that the latter are incipient crystals ; although in the 
present instance, the radial structure is not sufficiently advanced to warrant the belief 
that the fibres are composed of silica and felspar respectively as in some more fully 
developed obsidians, which show similar " courts of crystallisation " surrounding radial 
aggregates of felspar fibres and plates of tridymite. Under crossed Nicols the vitreous 
ground-mass remains dark, while the courts of crystallisation show slightly lighter and 
the longulites, according to their relative positions, become luminescent or extinguish 
straight. 

Literature. COHEN, Sammlung von Mikrophoto. von Min. mid Ges., PI. vi. and PL Ixxi., 
figs, i, 2. CROSS, Bull. Phil. Soc. Washington, xi., 411-414, 1891. IDDINGS, Bull. Phil. Soc. 
Washington, xi., 445-464, 1891 ; Obsidian Cliff, Jth Ann. Rep. U. S. Geol. Surv., 265, 266. 
JUDD, Q.J.G.S., xlix., PI. ii., iii., 1893. RUTLEY, Min. Mag.,ix., 261-271, 1891. ZIRKEL, 
Mic. Petr. $oth Parallel, PI. ix., figs. 1-4. 

* From a photo-micrograph by E. B. WETHERED, Esq., J.P., F.G.S. 



32 ClK Cuxntktl) Ccnturp flilas or 



' 



Holloway Hill Quarry, Godalming, Surrey. 



Key=Plate. Upper left-hand quadrant, Photo-micrograph, x 20. E. Oblique section 
through a spine, or possibly through a rounded granule from the test of an Echinus. 
Cr. Transverse section, somewhat mutilated, probably from the stem of a Crinoid. 
T. Obscurely preserved test-plate of an Echinoid. s. Fragment of shell of Mollusc. 
Q. Detrital quartz grain with calcitic-mosaic immediately above it. c. Calcite. Upper 
right-hand quadrant. Photo-micrograph, x 20. E. Transverse section through the spine 
of an Echinoid, showing its characteristic structure with great clearness of detail. 
G. Glauconite. c. Calcite. T. Test-plate of Echinoid indistinctly preserved, s. Frag- 
ment of shell of Mollusc. Q. Quartz grain. Lower Semi-circle, from Water-Colour Draw- 
ing, x 55. E. At upper left-hand side, shows a part of E depicted in the upper left hand 
quadrant more highly magnified. E. At lower right-hand side, shows a very small part 
of E portrayed in the upper right-hand quadrant. G. Glauconite. c. Calcite. Q. Quartz 
grains, the lower centrally situated one of which shows numerous glass inclusions. 
s. Shell-fragment, probably of a Lamellibranch. 

Occurrence. Dr. G. J. HINDE, F.R.S., saysf : " The Bargate Stone occurs in beds 
and lenticular patches, with intermediate beds of greater thickness, of loose sandstone, 
and small pebbles of Ludite, etc. These loose, readily disintegrating beds are of the 
same materials as the firmly consolidated rock ; but the cementing crystalline calcite is 
apparently absent. A prominent feature of the looser beds is, that they are very 
strongly false-bedded. In the same series with the beds of stone and unconsolidated 
sand, there are nodular beds of chert, from four to five inches in thickness having a 
porous exterior ; the minute cavities being casts of silicious sponge spicules, while the 
central portions of the bed are of translucent silica. The Bargate Stone occurs at the 
summit of the Hythe Division of the Lower Greensand, and the rocks are best 
developed at Godalming, Surrey, and the immediate neighbourhood, where many 
quarries have been opened to obtain the stone for building purposes, one of the largest 
quarries is that at Holloway Hill." The name " Bargate " is possibly derived from 
" Burgh-gate," which appears on the Ordnance Map, between Hambleton and 
Hascombe. 

Definition. The Bargate Stone is a gray or greenish gray rock, consisting mainly 
of grains of detrital quartz, fewer grains of green glauconite, and a varying proportion 
of comminuted fragments of organisms. The latter include plates and spines of 
Echiiwdennata, small broken pieces of Bryozoa, chips of molluscan and brachiopod shells, 
and occasional tests of Foraminifera and sponge spicules. These constituents vary in 
relative proportions in different parts of the rock, and are cemented together by calcite 
to form a hard, compact and durable stone eminently suited for building purposes. 

The quartz grains are mostly rounded, or with obtuse edges, and usually contain 
numerous inclusions. The greenish glauconite grains are rounded, but sometimes fill up 
the interspaces of echinoderm structures and the canals of sponge spicules. Some of 
the organic fragments are probably derived from the disintegration of older rocks, 
doubtless of Jurassic age. This seems likely to be the case with the echinoderm remains 
and some of the molluscan and brachiopod fragments ; while the Bryozoa, Spongida~a.nd 
Foraminifera are probably of the same age as the rock. In some localities the Bargate 
Stone contains numerous oolitic grains, much battered about, and these are considered 
to be of secondary origin, i.e., derived from the disintegration of Jurassic oolitic rocks. 

Literature. F. CHAPMAN, The Bargate Beds of Surrey, Q.J.G.S., I., 677-730, 1894. 
FITTON, Strata below the Chalk, Trans. Geol. Soc., Ser. 2, Vol. iv., Pt. 2, p. 146, 1836. 
HINDE, Beds of Sponge -Remains, Phil. Trans. Roy. Soc., Part II., 1885. MEYER, On the Lower 
Greensand of Godalming, 1868. Supp. to Vol. I., Proc. Geol. Assoc., 1870. TOPLEY, Geology 
of the Weald, pp. 121-125, 1875. WHITAKER AND JUKES BROWNE, Borings at Cul ford, etc., 
Q.J.G.S., I., 494-495, 1894. H. B. WOODWARD, Geology of England and Wales, pp. 368-369, 
1887. 

* From a specimen collected by Dr. GEORGE J. HINDE, F.R.S., to whose kindness the Editor is 
indebted for copious notes on which the text of this study is base>i. The material for the slides was 
generously gathered by J. B. HUE, Esq., Sunnyside, Godalming. The photo-micrographs for the plate 
were kindly taken from the original slide by E. B. WETHERED, Esq., F.G.S,, The Uplands, Cheltenham. 

t In notes sent to the writer. 




K, 




Baraaic Stone. 

Holloway Hill, Qodalming, Surrey. 



. W. Photo &E.H.A .Pinxit. 



34 



imca=Canu)ropbprc. 



Sale Fell, Bassenthwaite, Cumberland 



Key-Plate. Upper semicircle, x 25. o. Orthoclase. P. Plagioclase. p'. Plagio- 
clase with faint zonary banding. B. Biotite, showing sundry brown bands of the 
unaltered mineral, surrounded by pale green bands of altered chlorite with lenticles of 
calcite. Q. Quartz, some of which contains well-marked inclusions of apatite. 
M. Magnetite. A. Interstitial yellowish-green augite. Lower left-hand quadrant, x 120. 
o. Orthoclase. Q. Quartz. Ap. Apatite in cross sections. Ap' Apatite in longitudinal 
section. Ch. Pseudomorph of chlorite after Biotite. Lower right-hand quadrant, x 210. 
Part of the crystal of Biotite marked x, in the upper semicircle, o. Orthoclase. 
B. Unaltered Biotite. Ch. Biotite altered into chlorite. Ca. Lenticular separation of 
calcite in the Biotite. 

Occurrence. MARKER* notes that the best-known British examples of mica- 
lamprophyres occur as small dykes and sills in the North of England, and are of an 
age between the Silurian and the Carboniferous. The Sale Fell rock occurs as an 
intrusion in the neighbourhood of Bassenthwaite, and has been shown to bear 
considerable resemblance to the original kersantites of Brittany. 

Definition. A fine-grained holocrystallme rock of hypabyssal origin composed of 
ruddy felspars, Biotite, apatite, quartz, augite and magnetite. The rock has undergone 
considerable alteration, whereby the structure of the felspars has become somewhat 
obscured ; while the Biotite has also passed through profound changes which are 
manifested by the bulk of the crystals being converted into chlorite, with separations of 
secondary products, (calcite), intercalated as lenticles along the original cleavage 
planes. Many of the crystals, however, show patches of rich-brown unaltered Biotite. 

Between crossed Nicols the small columnar crystals of plagioclase, show traces of 
albite lamellation and often zonary banding, while the crystals frequently show a 
tendency to sheaf-like grouping. By removing the carbonates from the slice, with dilute 
acid, the true structure of the telspars becomes more clearly apparent. The orthoclase 
obtains as short rectangular crystals and Carlsbad twins, and, like the plagioclase is 
much clouded with ferruginous staining, giving to the mass of the rock its ruddy hue. 

The clear and colourless quartz, which is fairly full of inclusions, occurs mainly as 
interstitial grains, but also partly in micrographic intergrowth with the felspars, while 
the augite is only sparingly represented, wedged in between the other constituents. 
Every here and there, irregular grains of magnetite may be detected ; and an 
abundant accessory, is apatite, in well developed prisms, which occur freely throughout 
the rock, within the felspar, biotite and quartz, in both longitudinal and transverse 
sections with bold contours. 

Literature. BERWERTH, Mik. Stmcturbilder der Massengesteine, Lief iii., 1895-1900. 

COHEN, SamnUung von Miktophoto von Mm. und Gesteinen, 1899, PI. xxiv., fig. 4. 

HARKER, Petrology for Students, 3rd Ed., 1902, 141-147. HORNE, Min. Mag., 1886, vii., 
p. iv. IRVING, Ann. N.Y. Acad. Sci., 1899, xii., 287. PIRSSON, A merit an Journ. Sci., 1893, 
xlvi., 374 ; 20^ Ann. Rep. U.S. Gaol. Surv., Part iii., 1900, 526-531, PJ. Ixxxvi., A. TEALL, 
Mem. Geol. Stirv. Silurian tfocks, Scotland, 1899, 628, 629. WADSWORTH, Amer. Journ. Sci., 
1884, xxviii., 99, 100. 

* Geological Magazine, 1892, pp. 199-206. 






I 








mica=ampropbpre. 

Sale Fell, Bassenthwaite, Cumberland. 



.H.A.l'inxit. 





- 




Binarp Granite. 

Copper Hill Quarry, Wendron, Cornwall. 



H.H.A.I'tiiMt. 



microscopical Petrography. 35 



Binarp Granite. 







Copper Hill Quarry, Wendron, Cornwall. 



Key- Plate. Upper semicircle, x 25. o. o. Orthoclase felspar. p. p. Zoned 
plagioclase felspar, which sometimes shows decomposition beginning in the interior, 
while the edges exhibit clear lamellae between crossed Nicols. M. Muscovite, which 
frequently shows a parallel intergrowth with the Biotite, B, or may be posterior to the 
latter in part. Q. Quartz full of " strings " of inclusions. In other parts of the slide, 
there are a few well marked crystals of apatite, usually imbedded in the Biotite, which 
also incloses frequent specks of zircon surrounded by " halos " of deep colour and 
intense pleochroism. Lower semicircle, x 25. o. Orthoclase felspar. B. Biotite of a 
deep brown colour, with numerous inclusions of minute zircons near its base, each of 
which is surrounded by a deep greenish halo. M. Muscovite, intergrown with the 
Biotite. Q. Quartz. A. Large plate of pale brown tourmaline. 

Occurrence. It is probable from the occurrence of a few well-developed plates 
of tourmaline in this otherwise typical binary-granite, that the specimens were gathered 
towards the margin of the mass, which finds full development in the Copper Hill Quarry 
at Wendron, Cornwall, where -the rock may be obtained in a perfectly fresh, 
(unweathered), condition. 

Definition. This constitutes a fairly typical " binary " or " two-mica " granite or 
" granite proper." The colourless brilliantly polarising Muscovite, in more or less 
ragged flakes, is seen to be partly posterior to and partly in parallel intergrowth with 
the brown Biotite. The latter is fairly rich in inclusions, mainly of haloed specks of 
zircon, but also of well-developed prisms of apatite. The felspars are represented by 
both Orthoclase and plagioclase, (oligoclase), and the latter frequently exhibit zonary 
banding and internal decomposition ; while the quartz, which does not show crystal 
boundaries, is full of the usual inclusions, (negative crystals, cavities filled with brine, 
and so forth). Occasionally, there are large crystals of light-brown tourmaline in 
which the rude cross-fracture is often apparent. Under a single Nicol, the dichroism 
of these crystals is seen to be strong ; and, it may be noted that unlike Biotite, the 
strongest absorption is for vibrations transverse to the long axis. At intervals, sundry 
patches of pale brownish green decomposition products may be detected, and very often 
these are clearly traceable to the alteration of some of the Biotite into chlorite. A little 
magnetite may also be seen to have separated from the ferro-magnesian mica. 

Literature. COHEN, Sammlung von Mikrophoto ..... von Mineralien und 
Gesteinen, 3, PI. xxiv., fig. 2. CRAIG, Summary of Progress, Geol. Surv., 1898, 28 and 1900, 
22. GEIKIE, Q. J. G. S., 1883, xxxix., 314. MARKER, Q. J, G. S., 1888, xliv., 444; Bala 
Vole. Ser. Caernarvon, 59, 61. HAUGHTON, Q. J. G. S., 1869, xxv., 166, 167. HUNT Geol. 
Mag., 1894, 102-104. JENNINGS AND WILLIAMS, Q. J. G. S., 1891, xlvii., 380. KEYES, 
i$th Ann. Rep. U. S. Geol. Surv., 1895, 696-730. O'REILLY, Sci. Proc. Roy. Dublin Soc., 
1879, ii., 246-248. ROSENBUSCH-IDDINGS, Microscopical Physiography of the Rock-forming 
Minerals, 1888, PI. xxiii., fig. 3. SOLLAS, Trans. Roy. Irish Acad., 1891, xxix., 427-512 ; 
Proc. Geol. Ass'n. 1893, x "i-> !o6. TEALL, British Petrography, 1888, PI. xxxv., fig. i. 



36 Cbe Cuxntietl) Ccniurp Atlas of 

Red Sandstone. 



Penrith, Cumberland, England. 



Key- Plate X 55. Left-hand semicircle, ordinary light. Right-hand semicircle, between 
crossed Nicols. A. Well-rounded ellipsoidal grain of quartz exhibiting secondary out- 
growths, with good crystal- faces. B. Rounded la'rge quartz grain, full of small needles 
of rutile and other inclusions, c. Large quartz grain, which, upon revolution shows 
distinct strain-shadows. D. Quartz grain traversed by parallel lines of inclosures, 
probably due to mechanical stress on the original rock-mass. E. This large and almost 
spherical grain, shown under crossed Nicols as a much chequered body, doubtless also 
owes its complex structure to previous dynamic metamorphism. F. Rounded fragment 
of felspar, somewhat decayed, but still clearly showing the characteristic structure of 
microcline. G. Ellipsoidal grain of quartz, measuring -02 inch in length, shown 
enveloped by a pellicle of ferric oxide. H. Small sub-angular grains of quartz, each of 
which measures approximately -005 inch. K. Composite grain of quartz, each part of 
which shows secondary outgrowths, x. Empty space. 

Occurrence. Overlying the Carboniferous Limestone Series at the East of 
Penrith, this remarkable rock has been identified to be of Permian age. 

Definition. A somewhat course-grained red sandstone of rather loose texture, 
mainly composed of clastic grains of quartz with occasional fragments of felspar. The 
larger grains of quartz frequently measure as much as -02 inch and are fairly well 
rounded ; while the interspaces are occupied for the most part by small sub-angular 
quartz grains which average about -005 inch in diameter. The grains are coated with a 
pellicle of silicide of iron (ferric) which gives to the rock its characteristic brick-red 
colour ; and, being loosely compacted, affords opportunity for secondary outgrowths of 
clear quartz which frequently show good crystal faces. This is admirably shown by the 
grain at the centre of the picture which is projected, one-half under ordinary and the 
other half between crossed Nicols. It may be noted that the secondary enlargement of 
the grains of quartz is verified by the fact that the outgrowths extinguish simultaneously 
with the original grain between crossed Nicols ; while the substance of the outgrowth is 
usually much clearer than the nucleus. Many of the quartz grains show indications of 
having been derived from rocks which have been subjected to dynamic metamorphism. 
These are manifested by strain shadows which shift from one part of the grain to 
another upon revolution of the slide under crossed Nicols, and also by parallell lines of 
inclusions and the chequered aspect of other grains. 

The felspar grains are few and far between, and more or less decayed. Some of 
them show the peculiar spindle-shaped structure of microcline ; while others exhibit the 
twin lamellation ot albite or the ultra- microscopic texture of orthoclase. If the clastic 
grains were derived from a granite rock, it is probable that the micas were entirely 
destroyed, the felspars largely broken down, while the quartz by reason of its practically 
everlasting nature persists in the form of larger rounded grains and smaller angular 
chips, all cemented together by a silicide of iron, a trifle of calcareous and argillaceous 
material, and the secondary outgrowths of clear silica already alluded to. 

Literature. The principal contributions to our knowledge of sands, sandstones 
and kindred rocks are: BONNEY, Rep. Brit. Assocn., 1886, p. 601, and Nature, 1886, 
xxxiv., 412. BUCKLEY, Build, and Ornam. Stones, Wis., Bull, 4 of Wis. Geol. and Nat. Hist. 
Sur., 1898, PI. Ixiv.-lxvii. CARUS-WILSON, Nature, 1889, xxxix., 591. DERBY, Proc. 
Rochester Acad. Sci. 1991, i., 198-206. DICK, Polarising Microscope, 1890, 41-45. DILLER, 
Educ. Series Rock Specimens, 59-64, 74-84. GARDINER (Miss), Q.y.G.S., 1888, xliv., 755- 
760. HARKER, Petrology for Students, 223-236. HERDMAN, Rep. Brit. Assocn., 1894, 3 2 &- 
339. IRVING and VAN HISE, 1884, Bull. No. 8, U.S. Geol. Sur., PI. iii.-vi. JULIEN and 
BOLTON, Proc. Amer. Assoc'n., 1884, 413-416. MACKIE, T. Edin. G. S., 1897, vii., 148-172. 
PHILLIPS, Q.J.G.S., xxxvii., 1881., PI. ii. RETGERS, Min. Mag., xi., 113-114. RUTLEY, 
P. Liverp. G. S., 1885, v -> 3^1. SOLLAS, Sci. Pr. Roy., Dublin Soc. 1892, vii., 174-184, 
Proc. Geol. Asso., 1893, x "i-> 9*'93' SORBY, Presid. Address, Q.J.G.S., 1880, xxxvi., 62-64. 
TEALL, Br. Petrography, PI. xlv., fig. 2 ; PI. xlvi. 




Sandstone. 

Penrith, Cumberland, England. 



E.H.A.l'inxit. 



6ranopl)pric=DioriU. 

Muckraw, Linlithgow, Scotland 




microscopical Ptroarapbp. 37 

Granopbpric Dior He. 

&e> 

Muckraw, Linlithgow, Scotland. 



Key Plate. Upper semicircle x 25. P. Plagioclase (oligoclase) felspar, sometimes 
with zonary indications, o. Orthoclase felspar invariably intergrown with quartz in 
micrographic fashion. Q. Quartz with the usual inclusions ; the crystals are often 
idiomorphic. M. An elongated aggregate of crystals of magnetite. Ap. Apatite. 
Ch. Chlorite probably degenerated from augite. E. Empty space. Lower semicircle x 55. 
p. Plagioclase (oligoclase) felspar, o. Orthoclase felspar intergrown with quartz. 
Q. Quartz. Ap. Apatite in cross-sections. M. Magnetite. H. Brownish marginal 
hornblende. Ch. Chlorite altered from augite. A. Fresh colourless augite. E. Empty 
space. 

Occurrence. From an exposure at Muckraw in Linlithgowshire, the mass of the 
rock appears to be intermediate, rather than either plutonic or hypabyssal. This is 
evidenced by its granophyric structure. 

Definition. A medium -grained rock of ruddy hue speckled with dark green and 
black patches and frequent blebs of white. Under the microscope, thin sections show 
that the light red colour is due mainly to well-developed idiomorphic crystals of 
oligoclase many of which show marginal zones. These are irregularly arranged, and 
appear to be bound together by a deeper red cement, which when resolved by a higher 
power (fifty diameters or so), is found to be composed of a highly ferruginous orthoclase 
intergrown in micrographic fashion with clear quartz. Every here and there the quartz 
grains are of much larger size, and these interstitial bodies exhibit a strong tendency to 
develop idiomorphic contours. The larger grains of quartz are in all essential details 
precisely similar to those occurring in ordinary granites. 

According to some authorities this rock would be called a quartz-gabbro rather 
than a diorite ; but the distinction, which is based upon the former rock containing 
pyroxene and the latter being possessed of hornblende, is rather an artificial one and 
does not by any means hold good in this case ; for upon examination of the greenish 
patches which occur abundantly in this rock, they are found to consist principally of 
chlorite. In many instances the chlorite shows traces of having been derived from 
herring-bone augites ; while in yet other situations, as shown at H, ch., A, in the lower 
semicircle of the key-plate, the crystal is manifestly composed of a core of colourless 
augite and a marginal intergrowth of brown hornblende, while intermediate and at 
other parts of the elongated crystal its substance has been converted into green chlorite. 
It is interesting to note that the marginal hornblende, which is brown, occurs in the 
neighbourhood of grains of iron ore. 

In the upper semicircle, a very remarkable aggregate of crystals of magnetite is 
shown, with secondary elongated forms developed at right angles to the axis of 
aggregation. All the iron-ore crystals show the characteristic deep blue-black of 
magnetite with reflected light. Apatite is abundantly represented throughout the rock- 
mass in the form of clear colourless needles and well-developed stouter prisms, in both 
longitudinal and cross sections which show bold outlines. 

Literature. This rock has not hitherto been described. 



3 ClK Ciueiitkfl) Cnturp Atlas of 

Red Granite. 

Mount Sorrel, Leicestershire. 



Key = Plate. Left-hand semicircle, x 25. Q. Well developed allotriomorphic crystals 
of quartz, o. Interstitial orthoclase felspar, p. Portions of large idiomorphic crystals 
of zoned plagioclase (oligoclase) felspar. B. Biotite, in a fairly fresh condition, with 
haloed inclusions of apatite. H. Much altered hornblende. M. Magnetite. A. Apatite, 
s. Granule of sphene. Right-hand semicircle x 25. Q. Quartz, o. Interstitial orthoclase. 
o'. Orthoclase crystal intergrown in micrographic fashion with quartz. B. Fresh brown 
Biotite. Ch. Biotite changed into chlorite with interlamellar lenticles of separated 
calcite. H. Altered hornblende. M. Magnetite. E. Epidote. e, Vein of epidote through 
Biotite. A. Apatite. 

Occurence. LAPWORTH says '* : " A mass of beautiful hornblendic granite rises 
through the Trias immediately to the East of Charnwood Forest at Mount Sorrel, near 
Barrow-on-Soar," and ALLPORT adds t, in the same useful handbook, that he discovered 
the junction of the granite with the sedimentary rocks and proved that the former was 
intrusive. This would place the Mount Sorrel formation among hypabyssal rocks, and 
accounts for the granophyric structures to be seen in microscopic slides. 

Definition. A fairly typical example of hornblendic granite, the mineral constituents 
of which, in order of abundance are: Quartz, felspars, Biotite, hornblende, iron ores, 
apatite and a little sphene ; with the secondary products of alteration chlorite, epidote, 
calcite and leucoxene. 

The quartz occurs in well developed, clear, allotriomorphic plates liberally provided 
with the usual inclusions. The felspars consist mainly of very large idiomorphic crystals 
of oligoclase often zoned and showing signs of decay in their interiors, smaller clear 
crystals of the same plagioclase sometimes showing numerous lamellae (albite and pericline) 
between crossed Nicols ; and a lesser quantity of interstitial orthoclase. It may also be 
noticed, especially with polarised light, that the quartz and felspars are slightly 
micrographic. The ferro-magnesian mica, Biotite, is sometimes fairly fresh and of a 
brown colour ; at places streaked with pale green altered matter (chlorite), and frequently 
including small prisms of clear apatite, which are usually surrounded by dark halos. 
More often the Biotite has undergone a complete change into pale green chlorite with 
intercalated lenticles of calcite between the cleavage planes, and occasional separations 
of yellowish-green epidote. The crystal B, in the right-hand semicircle of the plate 
shows a vein of epidote at e. The hornblende has been much altered ; but there remain 
a few clear crystals of a green colour, and, occasionally, some of these show good 
prismatic cleavages. The decomposition products of the hornblende are mainly pale- 
green chlorite and yellowish-green highly refractive cr)'stals of epidote with separations 
of magnetite. The iron ores are mainly magnetite, but ilmenite is also present, and 
now and again shows evidences of breaking down into leucoxene. The specimen now 
under consideration contains abundant needles and small prisms of clear glassy apatite, 
disseminated throughout the rock, but chiefly inclosed by the Biotite, and there are every 
here and there reddish-brown granules of sphene (titanite). 

Literature. The most important contributions to our knowledge of this rock are: 
ALLPORT, Geological Magazine, dec. ii., Vol. vi., 1879, p. 181 ; Handbook of Birmingham, 
prepared for the Members of the British Association, 1886, pp. 255, 256. HILL AND 
BONNEY, Quarterly Journal of the Geological Society, Vol. xxxiv., 1878, p. 219. 

* Handbook of Birmingham, British Association, 1886, p. 223. 
f Handbook of Birmingham, 1886, p 256. 











RcU Granite. 

Mount Sorrel, Leicester, England. 



E.H.A.Pinxit. 





61aucorDanc ScDist. 

Llanfairpwllgwyngyll, Anglesey. 



E.H.A.Pinxit. 



Microscopical petrograpftp. 39 

Gltiucopbnnc Schist. 

<aa 

Llanfairpwllgwyngyll, Anglesey, N. Wales. 



Key = Plate. Upper semicircle, x 55 diameters. G. Pale-lilac glaucophane. E. 
Epidote of a pale brownish-yellow hue. The centrally situated large crystal E' shows a 
tendency to elongation in the direction of the orthodiagonal. F. Veinlets of clear 
secondary felspar, crowded with small inclusions of both glaucophane and epidote. 
s. Speck of granular titanite. . T. Titaniferous iron. Lower semicircle x 120 diameters. -- 
G. More or less idiomorphic crystals of glaucophane. The crystal marked G' exhibits a 
tabular hexagonal section, with a core of the original green hornblende. G." Cross 
section of glaucophane, showing prismatic cleavage lines intersecting at 124. E. 
Epidote. Q. Clear colourless quartz, with a few strings and specks of inclusions. 

Occurrence. From field notes specially taken, and kindly supplied by MP. HAROLD 
HILTON, of Bangor, N. Wales, the specimens of this unique British rock, from which 
the slides illustrative of this study were made, are stated to have been gathered from a 
" road-metal" quarry, situated 700 feet E.S.E. of the MARQUESS OF ANGLESEY'S column, 
on the N. side of the road running between Menai Bridge and Llanfairpwllgwyngyll. 
MR. E. GREENLY, who is studying the geology of the district, describes the rock as a 
member of the " Crystalline Schistose Complex " of Anglesey, which in this part 
consists chiefly of mica-schists. Whatever the origin of these schists, and whatever the 
original relations to them of the glaucophane-schist, the latter now lies among them in 
large lenticular masses whose structures conform in every way to the foliation of the 
region. The balance of evidence is in favour of regarding the whole Complex as of 
Pre-Cambrian age. 

Definition. Essentially a glaucophane-epidote schist with veinlets of clear 
secondary felspar, occasional granular aggregates of quartz, some ilmenite, stray specks 
of titanite, and traces of muscovite. 

A study of the evolution of the rock points to its origin as a massive diorite. By 
subsequent changes (dynamic metamorphism), it appears to have undergone gradual 
transitions into the hornblende-schists which occur between Gaerwen and the Menai 
Bridge ; while locally, as above noted, the hornblende has been modified into glauco- 
phane, and, in part, into epidote. The original felspar has been destroyed ; but is 
represented in bulk by epidote and by frequent veinlets of clear secondary felspar which 
traverse the rock in all directions, and are crowded with minute, more or less idiomorphic 
crystals of both glaucophane and epidote, with occasional ilmenite ; the latter usually 
showing signs of separation into blood-red haematite at its edges. The granules of sphene 
which occur here and there are attributed to secondary developments from the 
titaniferous iron. Rarely, minute scales of white mica may be detected. 

The pleochroic glaucophane (sky-blue to pale lilac), exhibits parallel orientation ; 
the crystals being elongated in the direction of their vertical axes, and becoming 
idiomorphic when penetrating through the occasional granular aggregates of colourless 
quartz, which shows sundry strings and specks of inclusions. The grains of epidote 
moreover, tend to elongation in the direction of the orthodiagonal. In some of the 
sections, most of which were taken parallel with the schistosity, prismatic cleavages 
may be observed. These however are best seen in cross sections under high powers. 
It is also interesting to note that isolated portions of the rock show unchanged green 
hornblende passing insensibly into lilac glaucophane at one end and breaking down 
into granular pleochroic (yellowish-green to pale brownish-yellow) epidote at the other. 

Literature. BLAKE, Rep. By. Assoc'n., 1888, 406 ; Geol. Mag., 1888, 125-127. TEALL, 
British Petrography, Plate xlvii., figs, i, 2. WASHINGTON, Analysis of Glaucophane Schist of 
Anglesey. American Jouvn. Sci., Vol. xi., Jan., 1901, p. 42. 



4 ClK Cuxmktb Ccnturp fltlas or 

Wenlock Cimestone. 

The Wren's Nest, Dudley, England. 



Key- Plate. Upper semicircle, x 25. A. Brachiopod shell showing dorsal and ventral 
valves. The interior is filled with crystalline calcite, a large and well-developed form 
of which exhibiting marked lamellar twins is depicted at B. At other places, F, the 
cleavages intersect parallel to the rhombohedron, thereby producing a characteristic 
cross-hatched appearance; while yet elsewhere, as at E, interspaces are filled with a 
mosaic of irregular minute plates of the colourless to brownish calcite. c. Fragment of 
shell (Lamellibranch). D. Portion of echinodermal plate. Lower left-hand sextant, x 25. 
A. Section through part of a coral (Favosites). B. Piece of brachiopod valve showing 
wavy lamellae, c. Bit of crinoid ossicle. D. Lamellibranch shell. E. Plate of calcite 
showing well marked twin lamellae. F. Mosaic of interestitial calcite. Lower central 
sextant, x 55. c. Transverse section through column or arm of crinoid. A, A. Probably 
rolled bits of crinoids, enveloped by clear crystals of calcite ; the one to the left shows 
how the lamellae of the calcite pass right through the fossil remains, the borders of the 
crystal being quite outside the edges of the fragment. Lower right-hand sextant, x 55. 
A. Section of small brachiopod shell. B. Sponge spicule. c. Plate of echinoderm 
calcified ; and, as is commonly the case throughout the rock mass, showing cleavages of 
the calcite clearly. 

Occurrence. In his lucid notes on the Paleozoic rocks of the Birmingham district, 
LAPWORTH says* : " The Silurian strata are all of the well-known Salopian type, 
showing several thick-bedded limestones, occurring on distinct horizons in a great 
thickness of dark blue or grey nodular shales and mudstones. They make their 
appearance in sharp anticlinal arches in the South Staffordshire coalfield, and along its 
faulted margins. . . . The best localities for fossils at present are the shaly slopes 
on the flanks of the Wren's Nest." The specimen from which the slides for this study 
were prepared was kindly gathered by MR. SIMEON PRIEST, of Fenton. The slices were 
cut from trimmings immediately below a fine trilobite, the famous " Dudley locust," 
(Calymene Blumenbachii) which was recently found in the thick measures of Wenlock 
limestone at the Wren's Nest, at the line where the thin measures approach them. 

Definition. The rock consists essentially of a vast variety of organic remains 
embedded in a matrix of crystalline calcite, and is singularly free from calcareous mud 
and detrital sand grains. The bulk of the specimen is made up of crinoid fragments, 
often much worn and penetrated by crystalline calcite. Next in abundance come 
brachiopod shells of all sizes often well preserved in shape ; many corals, a few tests of 
Entomostraca, sundry Bryozoa, trilobites, lamellibranchs, gasteropods and cephalopods ; 
most of the Mollusca being fragmentary and ill-defined in the general matrix of the rock. 

MR. JuKEs-BROWNEf is of opinion that partly by reason of the absence of fine 
sediment in this particular specimen, it probably came from a part of the rock-mass 
which originally must have been a bank of crinoidal debris, comparable to a bank of 
shell and sand in modern seas that is, the deposit was a loose aggregation of organic 
fragments and not a true sediment, and that the bank was subsequently converted into 
a coherent whole by the infiltration of calcite. 

Literature. GEIKIE, Text-Book of Geology, 4th Ed., 1903, p. 957. LAPWORTH, 
Handbook of Birmingham, Brit. Assoc., 1886, p. 227, et seq. SORBY, Q.J.G.S., 1879, xxxv., 
Presidential Address ; Proc. pp. 56-95. WETHERED, Q.J.G.S,, 1893, xlix., p. 236, on the 
microscopic structure of this limestone. 

* Handbook of Birmingham, Brit. Assoc., 1886, pp. 227, 229. 
t In a letter to the writer. 












wcniock limestone. 

The Wren's Nest, Dudley, England. 



E.H.A.Pinxit. 








s 



istJLvp 




lmDtiralte. 

Chester's Quarry, Haddington, Scotland. 



E.H.A.Pinxit. 



microscopical pctrograpDp. 4 1 

(limburgite. 

<5fk> 

Chester's Quarry, Haddington, Scotland. 



Key - Plate. Upper semicircle, x 55, to show a phenocryst of olivine to advantage. 
The parts of the crystal marked o, represent fresh kernels of olivine colourless, 
strongly contoured and with shagreened surfaces. The crystal is pervaded by 
serpentinised " fissures of metamorphism," and about one-half of its substance is wholly 
converted into serpentine, s. The ground-mass, M, can under this power be resolved 
into an isotropic glass varying from colourless to brownish-yellow, liberally provided 
with a crop of minute, more or less idiomorphic crystals of pale brown-pink augite ; 
studded with octahedra and compound forms of black magnetite, and flecked here and 
there with small scales of ruddy- brown, intensely pleochroic biotite. Limonitic stains 
moreover, are common at the edges of the olivines, and in the neighbourhood of the 
magnetites. Lower left-hand quadrant x 55, to exhibit a phenocryst of zoned-augite, the 
purplish-brown tint of which points to the presence of titanium. A, shows the deeper 
coloured outer zone and A', the lighter core of the augite in cross section, o. A small 
po2cilitic crystal of serpentinised olivine. M. The ground-mass. Lower right-hand 
quadrant x 120, to exhibit the intimate structure of the ground-mass. o. Olivine. 
A. Short stout idiomorphic prism of augite. A'. Granular aggregate of allotriomorphic 
augite. M. Magnetite, c. Colourless isotropic glass. B. Brownish isotropic glass 
slightly devitrified. 

Occurrence. A lava-flow exposed by the cutting called Chester's Quarry at 
Haddington, Scotland, of Carboniferous age. 

Definition. So called by ROSENBUSCH on account of its typical development at 
Limburg near the Kaiserstuhl. These are the magma-basalts of BO&ICKY, and are 
defined as lavas of an ultra-basic character, rich in olivine and augite, but lacking 
felspar. The rock-mass evidently became solid both in the " intratelluric " and 
" effusive " periods, previous to the separation of the felspar. The most abundant 
phenocrysts are of olivine, which is fairly fresh, but invariably shows alterations into 
serpentine, with separations of magnetite, and marginal yellowish-brown limonite, and 
occasionally, blood-red haematite. There are also numerous small crystals of olivine 
disseminated throughout the ground-mass, and sundry inclosures of the same mineral 
developed in pcecilitic fashion within the phenocrysts of augite. The phenocrysts of 
augite are not quite so plentiful as those of olivine. They are usually zoned with light 
cores and darker borders of purplish-brown, and appreciably pleochroic ; but the bulk 
of the ground-mass, and, incidentally of the rock itself, is composed of small idiomorphic 
prisms and granular aggregates of a second crop of augites. The magnetite is plentifully 
represented and uniformly distributed over the field ; while the magma varies from a 
clear colourless to a brownish-yellow, more or less devitrified glass. Occasional scales 
of brown, intensely pleochroic biotite also obtain. 

Literature. BONNEY, Geological Magazine, 1901, pp. 411-417. COHEN, Sammlung der 
Mikrophot .... von Mineral-ten und Gesteinen, Stuttgart, 1899, etc., Plate xxiii., Fig. 3 ; 
xxv., Fig. i ; Ixi., Fig. r. FOUQUE ET LEVY, Mineralogie Micro graphique, 1879, Plate lii., 
Fig. 2. HATCH, Trans. Roy. Soc. Edin., 1892, xxxvii., 116, 117, Plate i, Fig. i. WATTS, 
Rep. Brit. Assoc., 1892, 727 ; Guide to Rocks, etc., Geol. Surv. Ireland, Dublin, 1895, 
pp. 38, 9^. 



42 Che CuKntietb Ccmurp Atlas of 



Kcmallcnitc. 



Druro Quarry, Argyllshire, Scotland. 



Key- Plate X 25. o. Olivine, neutral-tinted, with separations of magnetite M, 
and serpentinised fissures of metamorphism. o'. A centrally situated crystal of olivine, 
which, by reason of its expansion, due to alteration, has caused radiating fissures 
through the surrounding clear felspar. A. Pale-green to faint pink-brown idiomorphic 
crystals of augite. B. Rich-brown strongly pleochroic flakes of Biotite. F. Colourless 
to grey felspar, mainly orthoclastic, but occasionally plagioclastic. Ap. Apatite, some- 
times faintly tinged with blue, but usually colourless. 

Occurrence. A peculiar basic syenite, allied to Brogger's olivine-monzonite, which 
occurs typically at Kentallen in Argyllshire, Scotland. The specimens from which the 
slides for this work were prepared, were gathered at Druro Quarry, by MR. MACONOCHIE 
of the Scottish Geological Survey. 

Definition. A holocrystalline rock, the various constituents of which obey 
ROSENBUSCH'S " order of consolidation." The most conspicuous mineral is olivine in 
large and small well-defined crystals of a neutral-tint with bold contours and much 
fissured interiors. These fissures are usually black with separated granules and 
occasional larger plates of magnetite, sometimes surrounded by limonitic stains. They 
are also more or less converted into green serpentine, and the consequent expansion 
due to alteration frequently results in a radiating system of fine fissures through the 
surrounding felspars. The augite, which is not quite so abundant, obtains in fairly 
large idiomorphic crystals which vary from a pale greenish to a faint pinkish-brown 
hue. They often contain potcilitic inclusions of olivine, stray crystals of apatite, 
Biotite and magnetite. The bulk of the felspar which is interstitial is orthoclase ; but 
under crossed Nicols a little plagioclase is revealed by its twin lamellae. The rich-brown 
intensely pleochroic Biotite occurs in large plates and scales, throughout the field, with 
a tendency to mould themselves around the edges of the olivines. Many of the crystals, 
and especially the smaller scales, become altered into a greenish chloritic material ; 
while, some of the small patches which form inclusions in the augites and olivines, 
show excellent separations of magnetite, sometimes in perfect cubes and octahedra. 

Apatite appears to be plentifully disseminated throughout the rock mass, in both 
cross-sections and longitudinally cut prisms, with bold outlines. The prisms frequently 
exhibit a pale blue tint in ordinary light. 

Literature. For descriptions and drawings of this and kindred rocks, see: 
HILL and KYNASTON, Q. J. G. S., 1900, Ivi., 531-540 ; Plate xxix., xxx. HILL, Summary 
of Progress, Geol. Survey, 1899, 48-53. TEALL, British Petrography, Plate xvi., fig. i ; 
Ann. Rep. Geol. Survey, 1896, 22, 23. WEED and PIRSSON, American Jonvn. Sci., 1895, 
Vol. 1., 467-479; 20^ Ann. Rep. U. S. Geol. Survey, 1900, Part III., 475-479; Bull. Geol. 
Soc. America, 1895, Vol. vi., 408-415 ; 2oth Ann. Rep. U.S. Geol. Survey, 1900, Part III., 
479-484, Plate Ixxii. 









Kcntallcnitc. 

Drouro Quarry, Argyllshire, Scotland. 



E.H.A.Pinxit. 



OF THE 

UNIVERSITY 

OF 











x, 



s 
, 




Serpentine. 

The Lizard, Cornwall, England. 



E.H.A.l'inxit. 



microscopical purograpbp. 43 

Serpentine. 

The Lizard, Cornwall, England. 



Key = Plate X 25 diameters. B. A large bastite-pseudomorph after enstatite, of a 
pale yellowish-green colour, surrounded by a zone of colourless serpentine, s, which on 
the left-hand side merges into a meshwork of yellowish-green serpentine derived from 
olivine, o ; while on the right-hand side, the mesh-structure, also after olivine, is much 
stained with hydrated iron-oxide of a more or less deep red hue. 

Occurrence. In the Huronian district of the Lizard, at an exposure by the flag- 
staff. The serpentine rocks of this district are perhaps better developed than in any 
other part of the British Isles ; but they vary considerably from place to place. The 
specimen from which the figure was taken, is fairly rich in large phenocrysts of enstatite 
which have undergone complete modification into bastite. Similar specimens, with 
fresher enstatite or bronzite occur locally, notably at Kynance Cove, Coverack and 
Cadgwith. 

Definition. PROFESSOR BONNEY and GENERAL McMAHON in summarising the 
characteristics of the Lizard serpentines, note that they " can be roughly separated into 
two groups : in the one a foliated mineral of the enstatite group is a conspicuous 
accessory ; in the other a colourless augite or hornblende, usually the latter. A few are 
non-porphyritic, and in some cases exhibit no certain traces of any pyroxenic mineral, 
rhombic or monoclinic, though of course a spinellid or some iron oxide is always to be 
detected, and in one instance, at the Rill, West of Kynance Cove, the presence of a 
fair proportion of felspar has been asserted."* 

In the specimen under consideration, the rock consists essentially of serpentines of 
various kinds, mainly derived from an olivine-rock liberally studded with large crystals 
of enstatite. The latter has been altered almost completely into the serpentinous 
mineral bastite of a pale yellowish-green hue with slight pleochroism. The pseudo- 
morph figured exhibits the characteristic structure, " Baste-type," composed of 
numerous more or less parallel fibrils, which are traversed irregularly by fissures, which 
are not however crossed by the fibrils. The latter give straight extinction ; and with 
polarised light most of the bastite pseudomorphs are seen to contain poecilitic inclusions 
of serpentine after olivine. A clear zone of colourless serpentine usually borders the 
bastite phenocrysts ; while the rest of the rock is practically built up of serpentine after 
olivine, showing typical " mesh-structure." Some of these meshes are indued with 
yellowish -green cores, separated by grey boundaries, while others remain colourless, and 
are separated by deep-red meshes of hydrated iron-oxide. Occasionally, a few kernels 
of fresh olivine may be detected between crossed Nicols, but the bulk of the rock gives 
the steel-blues and neutral tints which denote its complete conversion into pure 
serpentine. 

Literature. ADYE, Cole's Studies in Microscopical Science, 1882, Vol. i., No. 50. 
BONNEY, Q. J, G. S., 1877, xxxiii., 915 923; 1883, xxxix., 21 23; 1891, xlvii., 466. 
McMAHON, Q. J. G. S., 1891, xlvii., 466, et seq. TEALL, British Petrography, 1888, 115, 
et seq., Plate i., fig. 2 ; Geol. Mag., 1887, 137, 138. 

* Quarterly Journal of the Geological Society, 1891, Vol. xlvii., p. 466. 



44 



CMastolitc=Slatc. 



Qefrees, Fichtelgebirge, Bavaria. 



Key Plate X 55 diameters, c. Cross section, somewhat obliquely taken through 
a well-developed crystal of chiastolite. Ch. Core of chiasmal impurities mainly of 
carbonaceous particles aggregated at the centre and stretching therefrom to the lateral 
edges of the rhombic prism, i.e., to the angles of the section. M. Matrix of the slate, full 
of small flakes of Biotite, B, of frequent ferruginous stains, F, and occasional empty 
spaces, E. 

Occurrence. From the famous formation, metamorphosed by the igneous rocks in 
the neighbourhood of Gefrees, Fichtelgebirge, Bavaria. 

Definition. This specimen presents a singularly good typical example of the early 
effects of thermal metamorphism in an argillaceous rock charged with carbonaceous 
matter, and rich in alumina. The most striking results of the changes that have taken 
place are evidenced by the formation of abundant elongated prisms of chiastolite of 
fairly large size, and the production of numerous minute flakes of dark mica, which are 
probably the variety of Biotite called Haughtonite. 

The large crystals of chiastolite are seen to the best advantage in cross sections, 
when their inner cores of carbonaceous impurities can be observed forming characteristic 
chiasmal aggregations, more or less centrally situated and stretching outwards to the 
lateral edges of the rhombic prism. The matrix of minute argillaceous particles 
frequently affords examples of false cleavage, whereby a parallel system of microscopic 
faults is induced, and has the effect of causing a direction of weakness approaching to 
that of the true cleavage. The numerous scales of ferro-magnesian mica which are 
produced by the metamorphism of the matrix can, of course, be instantly identified by 
their strong pleochroism, when examined under revolution of a single Nicol ; while it 
may be noticed that although the ferruginous stains obtain indiscriminately throughout 
the field they are prone to affect the boundaries of the chiastolite crystals. 

Literature. COHEN, Sammlung von Mikrophotograph. von Miner alien und Gesteinen, 
Stuttgart, 1899, Plate xvii., fig. 4. FOUQUE ET MICHEL-LEVY, Mineralogie micrographique, 
Paris, 1879, Plate iii., fig. i. ROSENBUSCH, On the Various Stages of Metamorphism in the 
Skiddaw District. The Naturalist, 1892, pp. 119, 120. TEALL, British Petrography, Plate 
xxxiii., fig. 2. 





Chiasioliic Slate. 

Qefrees, Fichtelgebirge, Bavaria. 



.H.A.Pinxit. 



m 






Oliuinc Doleritc. 

Hailstone Hill, Rowley Regis, England. 



E.H.A.Pinxit. 



microscopical Petrograpbp. 45 

01iv>inc=D0Icritc. 

Hailstone Hill, Rowley- Regis, England. 



Key-Plate. Upper semicircle, x 55 diameters. The phenocryst which occupies the 
major portion of the field-of-view here depicted, consists in reality of a singularly peculiar 
intergrowth of olivine, Ol. and augite A the latter being revealed, between crossed Nicols, 
as an irregularly developed twin crystal. Granular augite, A', also encroaches and is 
apparently partially enveloped by the borders of the olivine. M. The ground-mass, made 
up mainly of small and sometimes of fairly large laths of colourless felspar (labradorite), 
is likewise peculiar inasmuch as it also contains abundant grains and prisms of augite A' 
distributed in pegmatitic fashion, as pointed out by TEALL.* Iron ores occur as compound 
plates of ilmenite and grains of magnetite, while there are also slender needles and minute 
prisms of colourless apatite, barely discernible under this power of the microscope, and a 
small proportion of interstitial clear or devitrified glassy matter. Lower semicircle, x 120 
diameters, to show the intimate structure of the ground-mass. F. Laths of labradorite. 
A. Small prisms and grains of augite. I. Large compound crystal of titaniferous iron. 
Ap. Hexagonal cross sections of apatite. Ap'. Longitudinal needle of apatite with 
transverse fissures. G. Greenish isotropic glass, somewhat devitrified. 

Occurrence. The rock is of an intrusive character, associated with the 
Carboniferous strata of the Rowley Series, and is now being extensively quarried at 
Hailstone Hill, and cut into cubes for paving stones. The specimens were specially 
gathered for this work, through the courtesy of Mr. SIMEON PRIEST of Fenton, Stokc- 
on-Trent. 

Definition. By reason of its intrusive character the rock must be called a dolerite; 
although its microscopic structure approaches to that of a typical porphyritic basalt. It 
is now of historic interest, since ALLPORT t used it as evidence to destroy the barrier 
between the names melaphyre and dolerite, and may be defined to consist of olivine and 
augite phenocrysts with plates and elongated crystals of titaniferous iron in a ground- 
mass of clear plagioclase felspar in pegmatitic combination with a second crop of olivines 
and augites, mingled with magnetite, minute prisms and needles of apatite, and a little 
interstitial glassy matter. 

The intergrowth of olivine with augite, clearly shown by the upper semicircle in the 
plate, has not hitherto been noticed ; but nevertheless obtains frequently throughout the 
rock-mass. The olivine is distinctly tinged with a faint ruddy hue, and, as pointed out 
by TEALL, when " the colour approximates to that of hyalosiderite and fayalite it seems 
reasonable to infer that it is rich in iron." TEALL also remarks, as a peculiar feature of 
this rock, that " the augite and felspar are intergrown exactly like quartz and felspar in 
pegmatite and micro-pegmatite. Each mineral depolarises uniformly over large areas. 
The structure is due to the simultaneous crystallisation of the felspar and augite and 
therefore differs from the ophitic structure which is determined by successive crystallisa- 
tion." Both crops of olivines are more or less serpentinised. 

Literature. ALLPORT, Geol. Mag., 1870 ; Q.J.G.S., 1874. WALLER, Midland 
Naturalist, 1885, viii., p. 261. TEALL, British Petrography, 1888, PI. xi., Fig. 2 ; PL xxiii., 
Fig. 2, p. 211. 

* British Petrography, 1888, PI. xxiii., Fig. 2. 

t Geological Magazine, 1870; Q.J.G.S., 1874, Vol. xxx., p. 548. 



46 Che Cujcniicib Ctnturp Atlas or 

Cracbpte. y 

<s^> 
Peppercraig Quarry, Haddington, Scotland. 



Key- Plate. Upper semicircle, x 25 diameters, s. A large phenocryst of sanidine 
felspar, slightly zoned, and also with internal intergrowths of plagioclase, P., (oligoclase) ; 
which, in this key-plate are shown by their lamellae under crossed Nicols. Under 
ordinary light the entire crystal appears water-clear, with only slight traces of 
structure. A. Phenocryst of dark green soda-augite bordered with rich brown iron- 
oxides. The crystal to the right has been much decayed into a greyish amorphous 
substance and separations of iron-ore. G. Ground-mass. Lower semicircle, x 120 
diameters. To exhibit the intimate structure of the ground-mass, s. Phenocryst of 
sanidine felspar. Within the boundaries of this crystal there are many minute fairly 
idiomorphic inclusions of pale-green augite, A ; altered augite, degenerated into 
pseudomorphs of rich brown iron-oxide B ; octahedra of magnetite, M ; occasional 
prisms of apatite, Ap. ; and possibly minute prisms of zircon, z. The ground-mass 
itself, can, under this power, be readily resolved into small laths and tablets of 
sanidine s' ; a lesser number of laths of plagioclase, P, which, of course, can only be 
detected by their striae between crossed Nicols ; granules of greenish soda-augite A' and 
a small proportion of pale green isotropic interstitial glass G'. 

Occurrence. A lava-flow of Lower Carboniferous age, exposed by quarrying 
operations at Peppercraig, Haddington, Scotland. 

Definition. An augitic-trachyte characterised by phenocrysts of sanidine-felspar 
crystals which are often intergrown with plagioclase, (oligoclase), and sometimes slightly 
zoned. The augite phenocrysts are pleochroic and decidedly green, even in very thin 
sections. Many of them are fairly idiomorphic ; while not a few show signs of alteration. 
These phenocrysts irregularly stud a ground-mass composed chiefly of sanidine felspar 
prisms with a minor proportion of plagioclastic felspar, detectable, of course, by the 
revelation of their stria? between crossed Nicols. Granular augite of a pale greenish 
hue, almost identical with aegirine, are also liberally represented, and, by reason of the 
fact that many of the minute felspars exhibit a short stout shape, the structure of the 
ground-mass may be termed orthophyvic. 

Between crossed Nicols the ground-mass is proved to be almost hotocrystalline ; 
but, every here and there, there are interstitial patches of isotropic glass, which under 
ordinary light are of a pale green hue, and might therefore be mistaken for allotrio- 
morphic patches of augite. In the key-plate, the large phenocryst of sanidine shown 
in the upper semicircle, was drawn under ordinary transmitted light ; but, in order to 
demonstrate the parallel intergrowth of plagioclase, which usually occurs centrally, or 
sub-centrally, polarised light was employed to reveal the presence of the twin lamellae. 

With powers of one hundred diameters and more, sundry needles and prisms 
of apatite, with bold contours can be detected. These, of course, between crossed 
Nicols extinguish straight when in longitudinal section, and remain dark in the case 
of cross-sections when the slide is revolved. A few of them, however, of very minute 
dimensions, give interference tints from green to pink and are therefore diagnosed as 
zircon. Magnetite occurs sparingly in specks and octahedra. 

Literature. BERWERTH, Mik. Structurbilder dev Massengesteine, Stuttgart, 1895-1900, 
Lief. I. COHEN, Sammlung von Mikrophot. , . . von Mineralien und Gesteinen, Stuttgart, 
l8 99> (3) -PI- xlvii., fig. 3 ; PI. xxiv., fig. i ; PI. xxxii., fig. 3. GEIKIE, Trans. Roy. Soc., 
Edin., 1879, Vol. xxix., PI. xii., figs, i, 2. MARKER, Petrology for Students, 4th Ed., p. 175. 
HATCH, Trans. Roy. Soc., Edin., 1892, Vol. xxxvii., pp. 115-126. McMAHON, Q.J.G.S., 
1894, Vol. 1., pp. 345-346. KOSENBUSCH, BevwevtW s Mtk. Stntcturbilder, Lief. II., (Domite, 
Auvergne). 





CracDpte. 

Peppercraig Quarry, Haddington, Scotland. 



E.H.A.Pinxit. 





Riclxckitc IHicroaranitc. 

Mynydd Mawr, North Wales. 



E.H.A.Pinxit. 



Microscopical Petrography. 47 

Ricbcckiic=inicroaranitc. 

<se> 
Mynydd Mawr, North Wales. 



Key- Plate. Upper semicircle, x 25 diameters. Q. Phenocryst of quartz, with fluid 
inclosurcs at a, and inclusion of ground-mass at b. There are also many "strings ".of 
pores. The crystal of quartz is fairly idiomorphic, but some of its angles show a strong 
tendency to become rounded, and this is quite a feature of the larger crystals of quartz 
throughout the rock-mass ; although a goodly number of the phenocrysts and some of 
the much smaller crystals of the ground-mass show well-developed angles. R. Allotrio- 
morphic patches of the dark-blue soda-amphibole, called " Riebeckite," after Dr. 
RIEBECK, who first discovered the mineral in the rocks of Socotra. G. Ground-mass 
composed essentially of quartz and orthoclase felspar much as in the so-called " Felsites." 
p. Sporadic crystal of iron pyrite, which, under reflected light displays the characteristic 
brassy glimmerings. Lower left-hand quadrant, x 25 diameters, o. Greyish phenocrysts 
of orthoclase exhibiting ultramicroscopic markings. The one to the right is twinned, 
and both show intergrowths with quartz when examined between crossed Nicols. 
R. Spongiform blotches of Riebeckite. These appear to be opaque, but, when examined 
under higher powers (from 100 to 500 diameters), especially in very thin sections, many 
of the individual grains are seen to be of a deep indigo-blue colour. G. Ground-mass of 
quartz and orthoclase liberally speckled with grains of Riebeckite. Lower right-hand 
quadrant, x 25 diameters. H. Crystals of dark-green, intensely pleochroic hornblende. 
Towards the extreme right-hand side, some of these crystals show the characteristic 
prismatic cleavages of the mineral. This hornblende is only of limited distribution in 
the rock-mass. It appears to occur in abundance exclusively at those parts that are 
traversed by veinlets of quartz. Q. A small phenocryst of quartz. R. Patches of 
Riebeckite, many of which show brown discoloration due to iron oxides, which also tend 
to stain the ground-mass G, in their neighbourhoods. 

Occurrence. An acid intrusive mass, towards the summit of Mynydd Mawr near 
Snowdon in North Wales. The rock has long been known as a " Felsite " ; but, by 
reason of its not infrequently developed phenocrysts, is now regarded as a true micro- 
granite, probably a diverticulum from a deeper-seated hornblendic granite. 

Definition. Occupying an undoubted place among the hypabyssal formations of 
North Wales, this unique rock differs from the quartz-porphyries and felsites in the presence 
of well-developed phenocrysts of its constituent minerals, and an abundance of grains of 
the comparatively rare blue soda-amphibole, in independent specks and sponge-like 
patches which can be clearly discerned by the naked eye. Some of the specimens, 
which were kindly sent to the writer by Mr. HAROLD HILTON, of Bangor, are very light 
in colour, while others are deeply hued, red to brownish-black. The microscopic 
structure of all the specimens, however, appears to be fairly constant, in accordance 
with the general description given in explanation of the key-plate above. 

Literature. BERWERTH, Mikroskopische Structurbilder der Massengesteine, Stuttgart, 
1895 1900. COHEN, Sammlung von Mikrophot. ... von Mineralien nnd Gesteinen, 1899. 
HARKER, Petrology for Students, 4th ed., p. 105. 



48 Che Ciuciiiicil) Ctnturp jfltlas or 



Celtftakaik* 



The Vienna Basin, Austria. 



Key- Plate. Upper semicircle, x 55 diameters. L. Longitudinal section through 
part of the thallus of Lithothamnion showing the general disposition of the parenchymatous 
cells which form the tissues of this calcareous Alga. T. Transverse section through the 
thallus of Lithothamnion, showing polygonal outlines of the parenchymatous cells, 
s. Terminal shoot of Lithothamnion, showing regular arrangement of the elongated 
parenchymatous cells. F. Part of a clearly defined test of foraminifer (Pulvinulina) 
showing the interior filled in with tesserae of calcite. M. Fragments of shells of Molltisca. 
c. Mosaic of recrystallised calcareous matrix. Lower semicircle, x 120 diameters. To 
exhibit the intimate structure of the parenchymatous tissues of Lithothamnion, L, in 
sub-longitudinal sections, and the nature of the recrystallised calcareous matrix, c, 
which binds the organic remains into a compact rock-mass. 

Occurrence. That calcareous Alga have contributed largely to the formation of 
rocks is nowhere, perhaps, better evidenced than by this example from the Miocene of 
the Vienna basin. Specimens have also been found in the Tertiary and Recent strata 
of various parts of the world, notably in Malta, the Barbados, Trinidad and the Tonga 
Islands. 

Definition. A very compact whitish rock composed for the most part of the 
remains of calcareous Alga in a very fine state of preservation. Numerous other 
organisms contribute to the rock-mass, such as Foraminifer a, chiefly Globiverinidz 
(Pulvinulina, Textularia, etc.) ; fragments of the shells of molluscs, and a few tests and spines 
of echinoderms, corals, etc. This miscellaneous assemblage of the remains of calcareous 
plants and animals appears to have consolidated, and then been finally bound together 
by recrystallised carbonate of lime, which invariably fills all available interspaces with a 
mosaic of clear, colourless calcite, sometimes in large, but more frequently in minute 
tesserae. 

Literature. GREGORY, Q.J.G.S., 1892, on Trinidad, Vol. xlviii., pp. 538-540. 
HILL, Q.J.G.S., 1891, on Barbados, Vol. xlvii., pp. 243-248, PI. ix. HINDE, Q.J.G.S., 
1893, on New Hebrides, Vol. xlix., pp. 230-231. LISTER AND MURRAY, Q.J.G.S., 1891, 
on Tonga Islands, Vol. xlvii., pp. 602-603. MURRAY, Scot. Geol. Mag., 1890., Vol. vi., on 
Malta, Plate i. MURRAY AND RENARD, " Challenger " Expedition Report on Deep-Sea Deposits, 
1891, Plates xiii., xiv. ROTHPLETZ, Zeits. dents, geol. Ges., 1891, xliii., Plates xv.-xvii. 










tcilhakalk. 

The Vienna Basin, Austria. 



E.H.A.Pinxit. 



OF THE 

UNIVERSITY 

OF 





V 







Laurvig, Christiania, Norway. 



E.H.A.PiHxit. 



microscopical petrograpbp. 49 

Cauroigitc. 

<s&=> 
Laurvig, Christiania District, Norway. 



Key-Plate. Upper semicircle, x 25 diameters. F. Cryptoperthite felspar, in large 
almost colourless plates. A. Large hypidiomorphic crystal of augite of a light brown' 
colour, tinged with violet, exhibiting a diallagic habit and " schiller-structure," s, sub- 
centrally situated. B. Rich brown Biotite. M. Magnetite. AP. Apatite. There are 
also many minute prismatic crystals of zircon in the field of view, but they cannot be 
detected with this low power of the microscope. Lower semicircle, x 55 diameters. 
F. Cryptoperthite felspar showing " schiller-structure " at s. B. Biotite with roughly 
idiomorphic outlines. M. Magnetite. Ap. Apatite, mostly in fairly good cross sections. 
The small prisms of zircon can only barely be detected under this medium magnifying 
power, as specks ; but under powers of from 100 to 500 diameters, are revealed as small 
colourless prisms with pyramidal terminations, and high refringence and birefringence. 

Occurrence. As intrusions through the Devonian rocks of Laurvig, in the 
Christiania district of Norway. Much used as an ornamental building stone, this 
peculiar augite-syenite is often to be discovered in the form of boulders on the East 
Coast of England, notably of Yorkshire. 

Definition. Although WERNER first applied the name " Syenite " to the horn- 
blendic-granite of Ancient Syene (Assouan) on the Nile, modern lithologists have long 
since agreed to Confine the term to those granitic rocks that are typically free from 
quartz. In this comparatively new sense, the specimen under consideration comes 
clearly under the group of syenites, but is nevertheless so peculiar as to merit the 
distinction of a variety called the Laurvig type of augite-syenite, after the locality 
whence it was first taken. 

Laurvigite consists essentially of felspar, Biotite, augite, magnetite and apatite; 
with zircon as a constant accessory. The felspar is peculiar, inasmuch as it is an ultra- 
microscopic intergrowth of a potash and a soda-felspar, mainly orthoclastic, with 
bands and threads of plagioclase. It is frequently schillerised. The augite is distinctly 
diallagic, slightly pleochroic, and often markedly schillerised, especially in the central 
regions of the larger hypidiomorphic crystals. Rich-brown intensely pleochroic Biotite 
clusters around the augite and iron-ore (magnetite), and, when free, shows fairly idio- 
morphic contours ; but, when in boundary-intergrowths with the augite, displays 
indefinite edges. Magnetite occurs in allotriomorphic plates, sometimes of compara- 
tively large size, but always glints of steel-blue by reflected light. The apatite, in 
colourless crystals, much fractured, appears to occur in fairly large stout prisms, and 
a few smaller needles, distinguishable by the extinction of cross-sections, in all positions 
between crossed Nicols, the straight extinction of longitudinal sections, and their high 
refractive indices. The apatite is particularly well developed within, and in the 
neighourhood of the biotite : but is also frequent, although in smaller prisms, inside the 
felspars. Zircon can only be detected, with high powers, in form of very highly 
refringent translucent bipyramidal prisms, disseminated mainly within the region of the 
felspars. 

Literature. On allied rocks, the following papers may be profitably consulted : 
DILLER, Igneous Rocks of Arkansas, Vol. ii. Annual Report of the Geological Survey, Arkansas, 
1890, pp. 194 197. DRESSER, American Geologist, 1901, Vol. xxviii., pp. 209 210. 
WEED AND PIRSSON, American Journal of Science, 1896, Vol. ii., pp. 136 137. WILLIAMS, 
Igneous Rocks of Arkansas, Vol. ii. Annual Report Geol. Survey, Arkansas, 1890, pp. 55 69. 
YORKSHIRE, Proceedings Geol. Pol. Society, 1889 90, Vol. xi., pp. 303, 304, 4.10. 



50 ClK CuKiilictb Ceniurp Atlas or 

Porpbpritic Rbpolite. 

^^> 
Tardree, Near Antrim, Ireland. 



Key- Plate. Upper semicircle, x 25 diameters. Q. Idiomorphic crystal of quartz 
with slightly rounded angles and sundry indeterminate cracks. The bulk of the clear 
colourless quartz is singularly free from inclusions, and practically devoid of " strings " 
of pores. G. Ground-mass of a nascent-crystalline character, full of small blebs of 
quartz, indications of felspars, and grains of magnetite. Lower left-hand quadrant, x 25 
diameters. s. Crystal of sanidine felspar, water-clear, but with more or less parallel 
fissures and many minute inclusions. G. Ground-mass, practically the same as above. 
Lower right-hand quadrant, x 120 diameters. Q. Quartz, with rounded angles, the cracks 
of which are innltered with silicified dark-brown oxide of iron (stained glass). 
A. Hexagonal negative crystal of glass of a deep-brown colour, in the path of one of the 
cracks through the quartz. B. Hexagonal negative crystal in the quartz, composed of 
clear glass, full of fractures, and exhibiting a well-defined air-bubble, z. Prism of zircon 
included in the quartz. M. Octahedra of magnetite in the ground-mass, G. At x, 
surrounding the borders of this particular crystal of quartz, there are spongy masses of 
deep-brown (probably iron-oxide) irregular crystals, recalling the somewhat similar 
patches of Riebeckite (blue soda-amphibole) in the Microgranite of Mynydd Mawr 
N. Wales. 

Occurrence. From the fresh Tertiary formations at Tardree, about eight miles 
from the town of Antrim, Ireland, this rock exhibits extreme variations ; being distinctly 
rhyolitic, i.e., showing flow structure at parts, and markedly porphyritic at others. The 
porphyritic varieties, moreover, appear to be quite different from each other to the naked 
eye ; some remaining red en masse, while others are buff or even grey in colour, and 
seemingly devoid of rhyolitic structure, although in reality they belong to the same 
mass. 

Definition. The specimen under consideration was taken from samples of the buff 
to grey porphyritic varieties of the rock, which, in hand specimens, have a stony 
appearance, liberally studded with large glassy crystals. Under the microscope the 
rock would unhesitatingly be classed as a quartz-porphyry or porphyritic felsite ; but 
its method of occurrence at once forbids the use of such old-fashioned and obscure 
names, and places it unequivocally among the Rhyolites of VON RICHTHOFEN, as a true 
acid lava of porphyritic structure, the porphyritic elements of which are mainly crystals 
of quartz and alkali-felspars. 

The ground-mass is cryptocrystalline in character, consisting of an intimate 
admixture of quartz and felspar on so minute a scale that the individual grains 
cannot be resolved even under high powers of the microscope. This part, moreover, 
is fairly full of minute grains and octahedra of magnetite, which also frequently tends 
to segregate around the quartz crystals. Pellucid quartz, often idiomorphic, practically 
free trom strings of pores, but frequently containing glass inclusions with fixed air- 
bubbles, and occasional prisms of zircon, is the most abundant porphyritic element. 
Next come the large clear crystals of sanidine, and finally a clear plagioclase, which 
may or may not be intergrown with the monoclinic felspar. 

Literature. COLE, Set. Trans. Roy. Dublin Soc., 1896, Vol. vi., pp. 77 118, PI. iv. 
GEOLOGICAL SURVEY, Summary of Progress, 1897, pp. 132 134. HARKER, Petrology for 
Studtnts, 3rd Ed., 1902, pp. 154 169. WATTS, Q.J.G.S., 1894, pp. 367375, PI. xviii. 





PorpDpritic RDpolite, 

Tardree, near Antrim, Ireland. 



E.H.A.Pinxit. 



Of THE 

UNIVERSITY 

OF 








Jfl 







Borolanitc. 

Loch Borolan, Sutherland, Scotland. 



E.H.A.Pinxit. 



microscopical Pctrograpbp. 51 



Borolanitc. 



Loch Borolan, Sutherland, Scotland. 



Key = plate. Left-hand semicircle, x 25 diameters. F. Orthoclase felspars in pseudo- 
morphs after leucite, of a greyish-white colour, full of inclusions of greenish biotite 
and greenish pyroxene (aegirine). P, Probably plagioclastic felspar intergrown with 
orthoclase. B. Intermediary border, composed for the most part of green to yellow- 
green Biotite, liberally intermingled with pinkish-brown, slightly pleochroic sphene, 
.magnetite granules, and occasionally small grains and crystals of melanite (black garnet). 
Under the polariscope a small proportion of greenish pyroxene may be detected ; while 
every here and there, glistening colourless specks of apatite, Ap, obtain. Right-hand 
semicircle, x 55 diameters. F. Orthoclase felspar pseudomorph after leucite. B. Greenish 
to yellow green Biotite associated with allotriomorphic sphene of a pinkish-brown hue. 
M. Portion of a fairly large crystal of melanite, which, in sections under the microscope 
are ruddy-yellow. The black spots and minute octahedra are magnetic oxide of iron. 
AP. Apatite. 

Occurrence. Borolanite is associated, says GEIKIE, with a large mass of igneous 
material which has disrupted and altered the Cambrian Limestone of Loch Borolan in 
Sutherlandshire. It occurs also in the adjacent part of Ross, N.B. 

Definition. This specimen has been diagnosed by TEALL as the only known 
vestige of an orignally leucite-bearing rock in the United Kingdom. It is essentially 
a syenite of medium-grained texture, of a dark grey colour, in hand specimens, blotched 
with white patches. The white patches are found to consist mainly of orthoclase, with 
obscured traces of plagioclase, and, by reason of the majority of these patches simulating 
the outlines in section of leucite, are reasonably presumed by TEALL to be pseudomorphs 
after that mineral. They were formerly supposed to be crystals of the felspathoid 
nepheline. The rest of the rock is made up of both large and small crystals of black 
garnet (melanite), as an essential constituent, which in microscopic sections appear rich 
yellow-brown, plentifully cracked and full of magnetite separations. 

Green pyroxene occurs plentifully in some and sparingly in other samples ; but 
green Biotite and brown-pink sphene seem to be constant and intermingled in all 
Apatite as an accessory also occurs in detectable quantity, mostly in stout colourless 
prisms. 

Literature. DAKYNS and TEALL, Trans. Roy. Soc. Edin., 1892, Vol. xxxvii., pp. 
163 172. GEIKIE, Text-Book of Geology, 4th Ed., p. 222. HARKER, Petrology for Students, 
p. 55. PLAYER, Trans. Roy. Soc. Edinr., Vol. xxxvii., p. 178. TEALL, Geol. Magazine, 1900, 
p. 389. 



52 Che Cuxmictl) Ceulurp fltlas or 

Clap=$iatc. 

^5>a^ 

Penrhyn Quarries, Bethesda, N. Wales. 



Key- plate. Left-hand semicircle, x 120 diameters. The section from which this 
drawing was taken is cut at a right angle to the cleavage. B. Biotite deformed into 
lenticular shapes, partially chloritised and partially converted into superposed thin 
tablets of epidote, traces of which can be very clearly seen when only the polarising 
prism is used. G. Ground-mass composed essentially of comminuted mica, mostly lying 
flat in the plane of cleavage, mixed with many rutile (sagenite) needles, commonly called 
" clay-slate needles," which can only be resolved under very much higher powers of the 
microscope, and sundry indications of detrital grains of sand and minute fragments of 
felspar. H. Sub-circular scale of deep red micaceous haematite. Right-hand semicircle, 
X 1 20 diameters. Taken from a section cut parallel with the cleavage. The letters in 
this refer to the counterparts of those given above. It will be seen that the lenticular 
forms of the latter are here represented by more or less disc-shaped outlines, and that 
the pseudomorphs of chlorite and epidote after Biotite, B, also show distinct traces of 
being built up of tablets of epidote. The rutile needles, moreover, would in this section, 
when viewed under powers of from 500 to 1,000 diameters, appear mostly as mere 
specks. 

Occurrence. Gathered and supplied by courtesy of Mr. HAROLD HiLTON 1 of 
Bangor, from the quarries of Lord Penrhyn at Bethesda, Carnarvonshire, North 
Wales, this rock is a typical example of the purple Cambrian roofing-slates which 
are held in such high esteem by builders throughout the United Kingdom. 

Definition. A very fine purple to purplish-red argillaceous rock, which represents 
a fairly advanced stage of secondary alteration, both with regard to its structural and 
mineralogical characters. Hand specimens exhibit a marked cleavage structure which 
passes independently through the original layers of deposition, and by the glossy 
aspect along the cleavage plane, closely approach the so-called phyllites of Continental 
petrologists. 

Under the microscope, thin sections taken parallel with and also perpendicular to 
the plane of cleavage, should be examined under tolerably high powers from 100 to 
1,000 diameters, and to that end a double-mount on each slide has been provided. These 
show that the bulk of the rock is composed of finely crushed mica, with numerous 
patches of a light yellowish -green hue, and frequent specks of deep red. The larger 
yellowish green bodies are pleochroic, and, under crossed Nicols give bands of brilliant 
colours. They are regarded as pseudomorphs after Biotite and are probably built up of 
superposed tablets of epidote intercalated with chlorite. The red spots are referred to 
micaceous haematite. Every here and there may be seen small detrital grains of sand 
and occasional traces of felspar ; while rutile needles may be detected under the higher 
magnifying powers. Sporadic crystals of iron pyrite also occur and often take on the 
so-called "eye-structure," the crystal of pyrite as nucleus being enveloped by a layer 
of chlorite, which gets torn away at its lenticular apices from the former to leave small 
spaces which subsequently become infiltered with clear quartz. 

Literature. CLOUGH, Mem. Geol. Surv. Scot., Geol. of Cowal, 1897, PP- 57, 80. 
DILLER, Bull. Geol. Surv. Amer., 1891, pp. 317 323. HARKER, Petrology for Students, 
3rd Ed., 1902, pp. 237 247. HUTCHINGS, Proc. Liverpool Geol. Soc., 1900, Vol. viii., 
pp. 464 471, PI. i. ; 1901, Vol. ix., pp. 113, 114, PI. vi., figs. E., F. PIRSSON, Amer. 
Journ. Science, 1893, Vol. xlvi., pp. 376, 377. RENARD, Geol. Mag. 1883, pp. 322 324. 
WILLIAMS, Bull. Geol. Surv. Amer., 1891, pp. 305 307. 










Clap-Slate. 

Penrhyn Quarries, Bethesda, N. Wales. 



E.H.A.ftnxit. 










w-n* 

-4 






Qudrtz=mica=Diorite. 

Rubislaw Quarry, Aberdeen, Scotland. 



E.H.A.Pinxit. 



microscopical Petrograpbp. 53 

Quartz mica Diorite. ? 

<s>&, 
Rubislaw Quarries, Aberdeen, Scotland. 



Key- Plate. Upper semicircle x 25 diameters. F. Felspar, mainly plagidclastic 
(oligoclase), but also intergrown with orthoclase. o. Orthoclase felspar in patches 
forming micrographic intergrowths with quartz. H. Hornblende, sometimes decaying 
at edges, and also giving rise to actinolitic bundles at sundry spots. H'. Cross section 
of fairly idiomorphic crystal of hornblende, showing distinct prismatic cleavages. 
B. Biotite, which is very largely intergrown with the hornblende, the hornblende being 
moulded on the Biotite. Ap. Apatite, mostly inclosed by both Biotite and hornblende. 
Q. Quartz of the ordinary granitic variety, containing " strings " of pores, and not 
infrequently good minute refringent and birefringent crystals of clear zircon, with very 
bold contours, which, however, cannot be detected under this low power. Lower 
left-hand quadrant x 55 diameters. The lettering in this figure may be taken from the 
above; but, in addition, fine idiomorphic crystals of sphene are depicted at s, s, s, and 
a small prism of zircon, z, becomes apparent in the adjacent quartz, Q. Lower right-hand 
quadrant x 55. To show finely developed cross-sections of apatite at Ap. F. Oligoclase 
felspar, which, under crossed Nicols, shows fine albite-lamellation, and extinguishes 
at an angle of 5. F'. Oligoclase crystal, with clear zone and cloudy (kaolinised) 
interior, showing scales of colourless mica (payagonite}. H. At the upper portion of the 
field shows the position of a crystal of clear green hornblende, degenerating below into 
a brownish mass, (epidotisation], and inclosing a nucleus of sphene, s, the outer borders 
of which are transformed into black iron oxide (magnetite ?). H'. Here, as in the 
adjacent left-hand quadrant, the hornblende shows a strong tendency to become 
converted into epidote. 

Occurrence. The ordinary Rubislaw grey-granite of Aberdeen, one of the most 
enduring of British building stones, appears to be precisely similar in hand-specimens 
to the rock here described, which was supplied, as coming from their Rubislaw quarries, 
by MESSRS. A. MACDONALD & Co., LTD., the celebrated granite merchants of Aberdeen. 
This rock, however, is distinctly a spheniferous quartz-mica-diorite, and not a binary 
granite. 

Definition. No sharp line of demarcation can be drawn between the granites and 
diorites ; but modern nomenclature ordains that the first shall contain alkali felspars in 
predominance, with fair proportions of quartz and one or two micas as essentials ; 
whereas the diorites ought to be constituted mainly by soda-lime felspars and hornblende, 
with or without quartz, and in the latter instance, an accompaniment of mica (Biotite). 
Under this definition our rock is unquestionably an acid diorite, and not a granite, 
inasmuch as the bulk of the specimen is made up of oligoclase-felspar, a small proportion 
of orthoclase-felspar, micrographic intergrowths of felspar and quartz here and there 
(indicating a tendency to hypabyssal rather than plutonic origin), well-developed 
hornblende which tends occasionally at its margins to epidotisation, a fair representation 
of Biotite, sometimes chloritised, with small separations of magnetite, and constant 
accessories in the form of sphene and zircon. Apatite also is fairly abundant. 

Literature. A rock of this kind has not hitherto been described as coming from 
the Rubislaw Quarries at Aberdeen ; but it is just possible that it does occur there 
in situ. However, this must be taken cum grano salts, as the writer cannot vouch for 
the gatherer. 



Clx CuxHikfb Centurp Xilas or 



6arnetjferous mica=Scl)ist. 



Near Perth, Scotland. 



Key- Plate. Magnified 25 diameters. G. Rudely hexagonal section of crystal of red 
garnet, elongately contorted, and full of inclusions of separated magnetite in black 
specks granules and octahedra ; blebs of clear quartz, Q, here and there and sundry 
cracks which trend tortuously in sub-perpendicular directions to the schistosity of the 
general mass. M. Ground-mass composed mainly of clear colourless quartz, brown 
(Biotitic), and green (chloritised) mica and separated (black) magnetite. o\ Portions 
of adjacent garnet crystals, more or less idiomorphic, which exhibit, in common with all 
the garnets in this specimen, a chloritised greenish border at places. By reason of the 
dynamic forces, many of the garnets, particularly the larger crystals, are drawn out at 
their longitudinal apices by triangular continuations in ciyptocrystalline quartz, forming 
altogether a kind of " augen-structur" A little zircon may be detected here and there 
in the clear quartz, with higher powers of the microscope. 

Occurrence. Unsatisfactory. The specimen was obtained from the collection of 
MESSRS. GREGORY & Co., labelled " Near Perth." It undoubtedly comes from the 
summit or thereabouts of the Great Highland Plateau of Scotland, and is certainly 
a very fine example of Garnetiferous Mica-Schist. 

Definition. The evolution of the mica-schists is still a problem practically 
unsolved. However that may be, whether the rock was derived as a later development 
of an originally argillaceous or an altered igneous rock, the mineralogical characters 
of the mass are indisputable. That profound metamorphism (dynamic) has been exerted 
goes without saying, for the rock is visibly (to the naked eye) contorted. Under the 
microscope this becomes plainly evident, not only by the " augen-strtictiir " of the larger 
garnets, but also by the general trend of the particles of mica ; while the quartz under 
the polariscope shows marked signs of strain structure, not only by lines of por^s 
developed at right angles to the schistosity, but by " strain shadows." The magnetite 
is evidently of secondary origin. Chloritic and limonitic degenerations, of course, are 
to be expected throughout the mass. 

Literature. This particular rock has not hitherto been described; but kindred 
specimens have been analysed and fully commented upon in the papers of almost every 
Geological Society in the World of to-day. 









Garnctifcrous mica=Scl)i$t. 

Near Perth, Scotland. 



E.H.A.Pinxit. 







Red 

Mount Sorrel, Leicestershire. 



E.H.A.Pinxit. 



microscopical petrograpfcp. 55 



Red Fclsitc. 



Mount Sorrel, Leicestershire. 



Key - Plate. Uppey semicircle, x 120 diameters. G. Ground-mass of felspar and 
quartz ultra-microscopically intergrown, giving to the rock-mass a ruddy hue. At 
places this ground-mass becomes stained with brown iron oxides, while every here and 
there lighter patches are due, as at G', to the quartz developing hazy outlines ; and more 
or less idiomorphic contours as at Q. F. Nascent crystal of zoned-felspar, probably due 
to an intergrowth of orthoclase and oligoclase. H. Allotriomorphic crystals of green 
hornblende, showing slight prismatic cleavages at H', and separations of iron oxide 
(magnetite) at H" and limonite at L. M. Well defined minute aggregate grain of 
magnetite. Lower semicircle x 120 diameters. F. Portion of a fairly large nascent 
crystal of felspar, containing inclusions of grains of green hornblende at H, H, H. 
G. Ground-mass composed of ultra-microscopic particles, sometimes exhibiting a 
tendency to radial arrangements, (incipient crystallisation), composed of reddish 
felspar and quartz (felsite) with a few black specks of magnetite and discolourations of 
iron oxides. Q. Hexagonal section through a fairly idiomorphic crystal of clear quartz, 
which is full of dusty inclusions in specks and " strings." Q'. Patch of quartz, with a 
strong tendency to develop crystal outlines. L. Spot of limonite. 

Occurrence. As a dyke forming part of Mount Sorrel at the East of .Charnwood 
Forest, Leicestershire, in the neighbourhood of Barrow-on-Soar. The dyke is evidently 
a diverticulum from the main mass of red hornblendic granite of which a description 
with figure is given at page 38 of this Atlas. 

Definition. This rock furnishes an excellent example of the somewhat equivocal 
"felsite" of British petrologists, inasmuch as the bulk of the mass consists of an 
intimate admixture of felspar and quartz, both of which show a tendency to develop 
into definite crystals. The probability is, that lower down the dyke, in proximity to 
the original red hornblendic granite, the rock would exhibit phenocrysts of felspar and 
especially of quartz, and would then deserve the equally obscure name of " quarts- 
porphyry " ; but in its present condition only incipient crystals of zoned felspar, 
(orthoclase and oligoclase) are apparent, while the clear quartz crystals which do take 
on definite crystal forms are comparatively small and occur at wide intervals. 
In the plate it must be remembered that the crystal of quartz with definite boundaries 
is highly magnified, and therefore of such minute dimensions that it can scarcely be 
called " porphyritic." In hand-specimens, of course, the rock appears to be practically 
of homogeneous texture throughout. Green hornblende in very minute grains and 
larger allotriomorphic patches, which show sundry secondary changes, are likewise only 
capable of careful resolution under higher powers of the microscope (from 100 to 200 
diameters), than those that are ordinarily employed for the elucidation of coarse and 
medium-textured rocks such as the granites, diorites, gabbros, etc. 

Literature. See page 38 of this Atlas in connection with the plate of the Red 
Granite of Mount Sorrel, Leicestershire. PROFESSOR LAPVVORTH'S map, including notes 
on the Charnwood Forest Series of rocks, given in the Handbook of Birmingham, published 
by the British Association in 1886, may also be profitably consulted. 



56 ClK Cuxntietb Ccmurp fltlas or 

Barbados artb. 

Island of Barbados, West Indies. 



Key = Plate. Upper semicircle x 120 diameters. R. Cross section through siliceous 
test of Radiolarian showing outer extracapsular skeleton and inner test of the central 
capsule connected by bars, as in most Polycystina (Podocyrtis, Anthocyrtis, etc.). R. 
Radiolarian tests. R'. Siliceous test of Polycystin, probably Heliospkara. P. Test 
of small Podocyrtis. M. Siliceous matrix of a yellowish-brown colour, composed of 
comminuted and fragmentary tests and spines, s., of the Radiolaria, spicules of sponges, s., 
and tunicates, and the frustule- valves of Diatoms. Lower semicircle x 120 diameters. 
R. Tests of Radiolaria. A. Section through a fairly large test of Astromma, showing 
inner capsular skeleton joined by bars to the test of the outer or external calymma. 
M. Matrix, s. Straight spines of Radiolaria. s. Sponge spicules (curved). 

Occurrence. A purely siliceous earth, which forms the central mass of the Oceanic 
Series, of Miocene age in the Island of Barbados, which lies about 125 miles East of the 
chain of the Lesser Antilles in the West Indies. Mr. JUKES BROWNE* is of opinion 
that the specimen here under consideration was gathered from an exposure at Burnt 
Hill, near Codrington Theological College. 

Description. The rock in situ varies in colour and texture from a whitish to 
yellowish-brown incoherent earth to a fairly compact mass which admits of being 
cut by a saw into thin slices. It consists mainly of the well preserved siliceous 
skeletons of Polycystina, and a lesser number of diatom valves, imbedded thickly in 
a matrix composed of siliceous debris derived from Radiolaria, Diatomacea, Porifera and 
Tunicata. 

By boiling samples in a strong solution of washing soda, and, after thorough 
washing, in nitric acid, the entire skeletons of the Polycystins may be separated intact 
from the matrix and duly classified. The deposit was discovered by ROBERT SCHOMBURGK 
in Barbados ; but it was not till 1847 that GOTTFRIED EHRENBERG described no less 
than 282 species, of which, in 1854, ne figured 33 of the most important in his 
Mikrogeologie, belonging to the genera Spumellaria and Nassellaria. HAECKEL, in 1887, 
observed that the species then known numbered "not less than 400 and probably more 
than 500." 

The most frequent forms of Polycystina found in Barbados Earth are figured in many 
text-books of Zoology and Palaeontology, notably in PROFESSOR VON ZITTEL'S work, 
where diagrammatic drawings are given of Heliodiscus Humboldti, Astromma Aristotelis, 
Podocyrtis Schowburgki, Petalospyris foreolata, and Anthocyrtis ntespilus. 

Literature. BURY (MRS.), Polycystins, Figures of Remarkable Forms in the 
Barbados Chalk Deposit, 2nd Edition by M. C. COOKE, 1868. BUTSCHLI, Radiolaria, in 
BRONN'S Klassen mid Ordnnngen des Thierreichs, Band I, Protozoa, pp. 332-478, Taf. xvii-xxxii. 
EHRENT.ERG, Mikrogeologie, 1873 ; Monatsbey, d. k. preitss. Miki'ogeologischen Studien specieller 
Rucksicht an f den Poly cy stiffen- Mergel von Barbados. HAECKEL, Die Radiolarien, 1880; Report 
on the Radiolaria of the "Challenger" Expedition, Part I., 1887, p. i., p. clxiv., p. clxv. 
JUKES-BROWNE & HARRISON, Q.jf.G.S., The Geology of Barbados, Part II., The Oceanic 
Deposits, Vol. xlviii., pp. 174, 205. NICHOLSON AND LYDEKKER, A Manual of Paleontology, 
Vol. I., pp. 33-34. VON ZITTEL, Text-Book of Paleontology, Vol. I., pp. 39-40, 1900. 

* In a letter to the writer. 




Barbados artb. 

Island of Barbados, West Indies. 



E.H.A.Pinxit. 



f ,v - ---.<{ Y2 

Or THE 

UNIVERSITY 





ecioaitc. 

Eppenreuth, Fichtelgebirge, Germany. 



E.H.A.Pinxit. 



Microscopical Petroflrapbp. 57 






Eppenreuth, Fichtelgebirge, Bavaria. 



Key=PIate. Magnified 25 diameters. G. Portions of sections of large dodecahedra 
of red garnet, full of cracks and inclusions. The large crystal, G', which occupies the 
major portion of the upper left-hand field -of- view, shows many small inclusions of 
yellow, highly refractive crystals of rutile. o. Green pyroxene (omphacite). B. Flake 
of Biotite, clinging to the edge of a large garnet. Q. Clear quartz full of " strings" of 
pores, of the usual granitic type. M. Opaque black magnetite showing bluish glints 
under reflected light. 

Occurrence. The most authentic record of the mode of occurrence of this 
beautiful rock of doubtful affinities, is noted in a paper by MR. DAVID HALE 
NEWLAND, A.B., in the Transactions of the New York Academy of Sciences. He observes : 
" The eclogite exposures form a series of hills and ridges extending nearly North and 
South from the city of Hof to the village of Markt Schorgast, a distance of about fifteen 
miles. They comprise a part of the central gneissic core of the mountain-range which 
is surrounded on either side by Siluro-Cambrian strata and intrusions of various igneous 
rocks. The region has undergone great orographic disturbances, as is instanced in the 
overthrust by which the gneiss has been made to overlie the later sedimentary rocks, 
and in the general folding and crumpling which is to be seen everywhere. Long 
continued weathering and erosion have, however, removed all sharp outlines of the 
topography, and when any notable elevations occur, they are to be accounted for by the 
greater resistance which the rock has offered to these influences, as seen in the eclogite 
hills." 

" The latter form a well-marked topographical feature of this part of the mountain- 
range, extending along the easterly edge of the gneiss, near the contact of the latter with 
granites and diorites, and then curving off to the West, where they are in contact with 
sediments and basic eruptions. The lack of good exposures necessarily renders the 
relations which these rocks bear to one another, difficult to ascertain. Where contacts 
of eclogite and gneiss were found they proved in most cases to be faulted ones, and 
could not be used to draw any conclusions as to interbedding or intrusion." 

" In general, the eclogite possesses a certain degree of sameness over the whole 
area, but there is too much minerological variation to place any definite limits on the 
term." 

Definition. The name Eclogite was first given by HAUY, in the year 1822, to rocks 
which consist essentially of an aluminous augite (omphacite) and red garnet, with or 
without quartz, some hornblende, actinolite (grass-green smaragdite), cyanite, and 
magnetite or other accessory minerals. 



5 ClK Civcmierb Cemurp fltlas of 

From VON GERICHTEN'S analysis, the Eppenreuth eclogite gives the following 
results : 

Si O 2 57'io 

A1 2 O 8 H-6& 

Fe 2 O 8 2-84 

FeO 3 . 22 

Mn O 0-31 

Cu O 13-80 

MgO 6-37 

K 2 O 0.81 

Na 2 O 2-21 

H 2 O ... ... ... ... ... ... ... 0-54 



Total 98-86 

No detailed account of the affinities of the rock, which had however long before 
been recognised by petrologists, was published until the year 1878, when VON GUMBEL* 
observed that it constituted part of the Miinchberg gneiss-group ; but he did not go so 
far as to discuss its origin. This remained for NEWLAND to attempt in 1895, under the 
instigation of GEHEIMRATH PROF. HEINRICH ROSENBUSCH, of Heidelberg ; and the result 
of his researches on the spot, are, in part, quoted above. 

The predominating mineral in the eclogite of Eppenreuth is a garnet which varies 
in colour from pale pink to deep red. This contrasts boldly with the next abundant 
mineral a pale green to grass green pyroxene so that freshly fractured hand-specimens 
present singularly beautiful surfaces, even to the naked eye. 

The garnets build idiomorphic crystals, bounded by the dodecahedron, and, in 
sections, are invariably seen to be traversed by innumerable cracks, often showing very 
perfect cleavages parallel to the crystal boundaries. The crystals themselves vary in 
size, from microscopic dimensions, to as much as an inch or even more in diameter, and 
are usually crowded with inclusions of quartz, pyroxene and hornblende the quartz 
inclusions often assuming a regular order parallel to the planes of the dodecahedron. 
In addition to the above, the garnet sections may, with higher powers, be frequently 
seen to be more or less plentifully sprinkled with very minute acicular bodies of a 
yellowish colour and high refractive index. These were former!}' referred by RIESS to 
zircon ; but, by reason of their low pyramidal terminations, and brilliant polarisation 
colours, have since been determined as rutile. The pyroxene varies from pale to dark 
green, but does not exhibit distinct pleochroism. It is said to be the form of augite 
known as omphacite. The individuals, in comparison with the garnets, are decidedly 
small and allotriomorphic. Their extinction angles in the prismatic zone, range from o 
to 40, and, although they seldom show cleavage lines, pulverised pieces of the 
omphacite have, with the goniometer, been found to give 87 to 93 for the prism angle. 
These crystals also contain many inclusions, chiefly of garnet, hornblende (actinolitic 
smaragdite), cyanite, quartz, rutile and magnetite. Fibrous actinolite occurs sparingly 
or may be absent from the Eppenreuth specimens, and a few laths of Biotite may now 
and again be detected clinging to the sides of the sections of garnet. The cyanite varies 
in quantity or may be absent. It is usually intergrown with the omphacite to which it 
sometimes imparts a bluish tinge, and is generally in idiomorphic stout crystals with 
twinning on the macropinacoid. Rutile is fairly constant in irregular grains and 
aggregates ; and, in this particular specimen magnetite (although scarce) is fairly well 
represented. In other sections zoisite and zircon have also been identified. 

Literature. COHEN, Sammlung von Mikrophotographien...von Mineralien und Gesteinen, 
Auf. 3, PI. xxi., Fig. i. VON GUMBEL, Gcognostische Beschreibung des Baierischen Fich- 
telgebirges, 1878. HARKER, Petrology for Students, 3rd Ed., 1902, pp. 337-340. NEWLAND, 
Trans. New York Academy of Sciences, 1897, Vol. xvi., pp. 24-29. 



* Gcognostische Beschreibung des Baierischen Fichtelgebirges, 1878, 









Scborl Granite. 

Sheffield Quarry, Penzance, Cornwall. 



E.H.A.Pinxit. 



microscopical Ptfrograpftp. 59 

Schorl Granite. 

Sheffield Quarry, Penzance, Cornwall. 



Key- Plate. Upper Semicircle, x 55 diameters. T. Green plates of tourmaline, 
showing incomplete idiomorphic outlines. s. Greyish-brown to indigo-blue radiating 
bundles of schorl needles, which average about 0-03 inch in length, and exhibit 
transverse fractures, s'. Cross sections through the extremity near to the centres from 
which the schorl needles radiate. The needles themselves are thicker towards these 
centres and gradually taper off at their free ends, usually into the substance of the 
secondary quartz, Q. The clear colourless quartz contains many comparatively large 
inclusions, which are augmented by minute colourless to pale indigo-blue, perfect 
prisms of schorl, several of which when transversely cut, show triangular contours, 
o. Orthoclase felspar intergrown with small blebs of quartz and showing its ultra-- 
microscopic structure, in the rectangular disposition, at places, of its excessively fine 
ruddy-hued particles, p. Portion of a large porphyritic crystal of plagioclastic felspar 
(oligoclase) showing distinct vestiges of lamellation between crossed Nicols. It is also 
of a light-red colour. A. Portions of oblique sections through . colourless highly 
refractive crystals of apatite. Lower semicircle x 55. The above lettering also applies 
here ; the drawing having been made from another portion of the same slide, to show a 
good transverse section of apatite at A, which remains extinguished when revolved under 
crossed Nicols. The quartz, Q, moreover shows good inclusions, and, with polarised 
light, at once proclaims its secondary character. 

Occurrence. Towards the margins of the main mass of normal granite, exposed by 
Sheffield Quarry, near Penzance in Cornwall. It has been frequently observed that the 
proportion of tourmaline found in the granites of Devonshire (Dartmoor) and Cornwall 
increases, and is often limited to the margins of the principal masses, becoming 
converted at times, usually at the extreme edges, into schorl-rock, which consists essentially 
of tourmaline and quartz to the exclusion of felspar. 

Definition. The specimen obtained from Sheffield Quarry near Penzance, is 
composed of ruddy felspar, quartz, green, brown and "black" tourmaline (schorl) with 
accessory apatite. 

Tourmaline gives character to this type of granite, and occurs in brown and brownish- 
green crystals developed mainly in the neighbourhood of the clear quartz, and also 
manifested in the form of diminutive spheroidal masses and nests of radiating needles 
of the variety schorl. These needles, which measure only about -03 inch in length are 
usually developed as inclusions in the clear secondary quartz, but also to a lesser degree 
as intergrowths with quartz within the porphyritic felspars, and vary in colour from 
greyish-brown to indigo-blue respectively, for longitudinal and transverse vibrations. 
The tourmaline is decidedly dichroic, the maximum absorption always taking place, 
in longitudinal sections, when the short axis of the polarising prism is at right angles to 
the length of the crystal section. In this particular, tourmaline can almost always be 



GO CIK CuKiiticth Cciiturp Hrias of 

distinguished from biotite.* Both minerals extinguish straight. The cross-sections of 
tourmaline in rhombohedral prisms are often triangular, and the prisms show a vertical 
striation. The specific gravity of the mineral varies from 2-94 to 3*24 and its hardness, 
on MOHS' scale, from 7 to 7*5. It exhibits strong double-refraction : The cross-sections 
in convergent light give the figure of a uniaxial mineral with negative double- 
refraction. As the long axis of the prism is the direction of vibration for the fastest ray, 
tourmaline can be distinguished from prisms of hornblende ; while the latter is also 
usually recognisable in cross-sections by its well-marked prismatic cleavages, which 
intersect at an angle of 124. 

The name tourmaline is stated to be derived from the Cinghalese tnrtnali, applied to 
zircon and also to precious tourmaline by jewellers in Ceylon. Its precise chemical 
formula is still uncertain, but it may fairly be called a complex silicate of boron and 
aluminium, with pronounced iron, magnesium, or one of the alkali metals. 

The absence of mica in this intrusion from a micaceous main-mass of granite, 
points to the deduction that the original mica must have been replaced by the 
tourmaline and schorl. The quartz, moreover, which envelops the other mineral 
constituents, is of secondary derivation, being replacements in part from the felspars. 
It is water-clear and usually full of fairly large inclusions of the nature of irregular 
pores, felspathic matter in a transitional state and many perfect minute prisms and 
needles of both tourmaline and its variety schorl. Between crossed Nicols the patches 
of clear quartz give evidences of their secondary nature by polarisation colours into 
mosaics, often with sharply defined angles. 

The felspars are of somewhat complicated microscopic structure in both twinned 
porphyritic crystals, sometimes large enough to be seen clearly by the naked eye, arid 
interstitial patches. On careful scrutiny under the microscope the mottled ruddy 
crystals and patches may be observed to show sometimes the ultra-microscopic 
rectangular structure of orthoclase, and yet, under polarised light, to reveal vestiges of 
polysynthetic twinning on the albite type, while at the same time they are full of 
alteration-patches of quartz and schorl needles, with occasional minute plates of 
greenish tourmaline. It is therefore probable, that the bulk of the felspars consist 
of intergrowths of orthoclase and plagioclase (oligoclase). 

Irrespective of position, the colourless, much fractured crystals of apatite are to be 
found in fairly large as well as small prisms, scattered throughout the field-of-view, and 
obey the law that longitudinal sections extinguish straight, while true transverse 
sections remain obscure in all positions between crossed Nicols. 

Literature. - BONNEY, Mineralogical Magazine, Vol. i., pp. 215-222. COHEN, 

Sammlung von Mikvophot von Minevalien und Gesteinen, 3rd Ed., 1899, Coloured 

fig., Plate xxii., fig. 2. COLE, Aid's in Practical Geology, 4th Ed., 1902, pp. 183-184. 
DANA, System of Mineralogy, 6th Ed., 1892, pp. 237, 492, 551. DE LA BECHE, Geology of 
Cornwall, p. 160. HARKER. Petrology for Students, 3rd Ed., 1902, pp. 33, 35, 41-42. HATCH, 
An Introduction to the Study of Petrology, 1891, p. 56. MIERS, Mineralogy, 1902, p. 479. 
PISANI, Comptes Rendus, 1864, p. 913. RUTLEY, Study of Rocks, pp. 138, 265, 298. 
SEMMONS, Proc. Liverpool GeoL Society, Vol. iii., 1878, pp. 357, 358. SENFT, Die 
Krystallinisch Felsengenungtheile, etc., p. 505. TEALL, British Petrography, 1888, pp. 169, 
35i 375- WORTH, Trans. Royal Geological Society of Cornwall, Vol. x., 1884, pp. 177-188. 

*Recantly however, the writer's attention was directed by PROF. ANDREW YOUNG, of Cape Town, to the 
biotite found in the Melilite-Basalt, from the diamondiferous series of Cape Colony, which behaves in 
this respect precisely like tourmaline. 





PitcDstone. 

Corriegills, Arran, Scotland. 



E.H.A.Pinxit. 



Microscopical Pirograpbp. 61 



Pitcbstonc. 



Corriegills, Arran, Scotland. 



Key=Plate. Upper left-hand quadrant. Magnified 31-25 diameters. Shows brownish- 
grey glassy ground-mass devitrified by microlitic dust, and harbouring numerous 
arborescent aggregations (compound-microlites), surrounded by clear and colourless 
"courts of crystallisation." A. Traverse section of large belonitic-microlite with four 
radial plumose groups of trichites attached thereto. B. Longitudinal section of 
compound-microlite, showing central midrib of actinolite. c. Tangential section of 
compound-microlite showing two plumose groups of trichites attached. Lower left-hand 
quadrant. Magnified 131.25 diameters. Shows A, as above, in detail. The trichites are 
seen to radiate from a centre, but in reality they lie in planes above and below the 
central stem or midrib, not here visible, c. " Court of crystallisation." D. Devitrified 
ground-mass full of actinolitic needles in transverse and longitudinal aspects. Uppey 
right-hand quadrant. Magnified 131.25 diameters. E. Transverse section of microlite 
midrib, showing structure of skeleton crystal deficient on one side. F. " Petiolar " 
portion of frondoid microlite, showing longitudinal aspect with transverse fracture, as in 
actinolite, of a clear sap-green colour. There is also a small group of three similar 
stems below, and sub-transverse sections of frondoid- microlites at G, G, G. The 
ground-mass is shown to be devitrified by myriads of acicular microlites, which 
encroach upon the courts of crystallisation surrounding the larger microlites. Lower 
right-hand quadrant. Magnified 131.25 diameters. H, H. Longitudinal sections of 
frondoid microlites, showing devitrified ground-mass and courts of crystallisation. 

Occurrence. Centre of dyke on the Corriegills shore, North of Clauchland Point, 
near the town of Brodick, in the Isle of Arran. The dyke is intruded through beds of 
sandstone of Carboniferous age, and measures from about twelve to thirteen feet on the 
face of the cliff adjacent to the foot of Dun Fion. 

Definition. A dark bottle-green glassy rock, which exhibits a greasy pitch-like 
lustre and typical conchoidal fracture, diversified by sundry specks (spherulites and 
phenocrysts). Chemical analysis shows that the mass is closely allied to granite, while 
microscopical examination further differentiates the texture into an isotropic glassy 
base or magma, devitrified by myriads of microlites, the largest of which are surrounded 
by appreciable clear spaces (" courts of crystallisation "). Occasional specimens show 
phenocrysts of felspar, mainly sanidine and anorthoclase, and blebs of quartz. Both 
the felspar and quartz are generally bristled with subperpendicular acicular microlites; 
while the quartz is often encysted in an envelope of hyalite, sometimes showing 
segregations of tridymite and occasional nuclei of the same ^triclinic modification of 
silica, recognisable only under moderately high powers. Stray specimens are said to 



62 cbc CuKiitictb Centurp fltlas or 

contain well-defined crystals of augite, and most of the sections display the presence of 
magnetite ; but the microlites, which give character to the section under the microscope, 
are now correctly determined as hornblendic probably of the variety actinolite. 
These microlites afford a very interesting subject for a study in the paragenesis 
of minerals. They occur as very minute acicular forms, devitrifying the glassy 
base ; and, by their segregation into crude crystals at irregular intervals, produce the 
exquisitely charming fern-frond combinations which characterise this rock-section. 
Upon careful scrutiny the frondoid aggregates can be resolved into lath -like 
" midribs " or skeleton crystals (acicular and actinolitoid) of hornblende, with pinnae 
of presumably incipient crystallisations (trichites). Cut transversely, the stems of 
the largest of these frondoid arrangements, as shown in the key-plate at E, upper 
right-hand quadrant, give the characteristic angle of hornblende, 124 and there- 
abouts; while the maximum extinction in the prismatic zone is 15. TEALL first 
pointed out that these stems are almost invariably of the nature of skeleton 
crystals, and are frequently deficient .along one part, thereby giving continuity to the 
core with the glassy magma outside. The trichites are generally arranged in tufts, with 
a tendency to radial disposition, in diminishing series from near the base of the so-called 
stem to its extremity (see key-plate H, lower right-hand quadrant) giving rise to the fern- 
frond arrangement already alluded to when longitudinally sliced ; but, when cut 
crosswise, these curved, straight and forked trichites exhibit a whorl-like habit, often in 
opposite fours and twos, reminding one, as ALLPORT says, of the delicate tufts and 
sprays of Batrachospermum moniliforme. Under crossed Nicols, the greenish microlites 
become illuminated, presumably by the diffraction of light from the trichites ; for the 
stems, either in longitudinal or transverse sections, extinguish with the isotropic glassy 
magma and the more diminutive acicular microlites. These latter diminish in size as 
they approach the arborescent microlites, and cease altogether immediately without the 
zone of the clear and colourless " courts of crystallisation " (key-plate c, lower left-hand 
quadrant). It is therefore surmised that the dendritic microlites take origin from the 
acicular crystallites, which in their turn are the nascent derivatives of the magma. 

Literature. ALLPORT, Geological Magazine, 1872, pp. i-io and 536-545 ; also 1881, 
p. 438. BONNEY, Geological Magazine, 1877, pp. 506-508. COHEN, Sammlnng von 

Mikrophotographien von Mineralien und Gesteinen, PJ. i., Fig. xi. HARKER, Pefrology for 

Students, 1897, pp. 107-108. HATCH, Text-Book of Petrology, 1892, pp. 16, 22, 127. JUDD, 
Quarterly Journal Geological Society, 1893, Vol. xlix, pp. 546-551, 559-561. PI. xix. 
ROSENBUSCH, Mikroskopische Physiographic der massigen Gesteine, 1877. RUTLEY, The Study of 
Rocks, 1879, PP- 195-198. TEALL, British Petrography, 1888, pp. 344 et seq., PI. xxxiv. 
Figs. 3 and 4. VOGELSANG, Die Krystalliten, 1875, p. 124. ZIRKEL, Zeitschrift der deutschen 
geologischenGesellschaft, and Lehrbuch der Petrographie, 1866. 




r 




DolomitiscU imc$iotK. 

Birchwood Park, Norbury, Derbyshire. 



E.H.A.Pinxit. 



microscopical Petrography. 63 

Dolomitised Cimestonc. 

Birch wood Park, Derbyshire. 



Key- Plate. D. Sections through rhombohedra of dolomite, a few of which are 
twinned, c. Amorphous granular calcite. c'. Veinlet of crystalline calcite. cr. Tran- 
verse section through joint of a crinoid stem. Q. Sections of quartz grains, many of 
which are idiomorphic. 

Occurrence. From a Carboniferous-Limestone quarry surrounded by Keuper beds, 
in Birchwood Park, near Norbury, Derbyshire, this typical dolomitised limestone inlier, 
is characteristically full of fissures of calcareous spar, which has manifestly been 
deposited from aqueous solutions of the limestone dissolved by the agency of carbon 
dioxide. The mass of rock contains but few fossils, such as the much altered remains 
of crinoids and sundry shells of molluscs. There are also present veins of galena, but 
these are not now of sufficient importance to be worked. In general structure the 
stratification at places exhibits twisting and contortion, and is frequently faulted ; while 
the rents so occasioned are now filled up with sand and small facetted pebbles which 
were clearly washed in while the mass was still under water.* 

Definition. The specimens under considerationf furnish good types of well 
dolomitised limestone ; the rhombohedra of dolomite being clearly mapped out by dark 
brown borders of oxidised ferrous carbonate, when viewed through the microscope. 
These lozenge-shaped forms are surrounded, more or less, by amorphous carbonate of 
lime in greyish ultra-microscopic granules ; while the field is frequently crossed by veins 
of crystalline calcite and re-crystallised patches of calcite therefrom in clear colourless 
mosaics. At sundry intervals a few sections of barely distinguishable crinoid stems and 
fragments of mollusc shells may be identified; and there are also, mainly in the 
amorphous patches of limestone, a fair sprinkling of clear and colourless hexagonal and 
elongated crystals and blebs of quartz, which can best be seen under fairly high powers 
(100 to 300 diameters) of the microscope. 

There are two good reasons why dolomite ought to be interesting to petrologists, 
and these are succintly recorded by MiERsJ in his classic work for students of 
mineralogy, as follows : " Dolomite is interesting because it belongs to the dioptase or 
hexagonal and not to the calcite or di-hexagonal class of alternating symmetry and 
secondly, because it is a double salt, in which calcium carbonate and magnesium 
carbonate are united in the proportion of one molecule each, and not an isomorphous 
mixture of calcite with magnesite." To confirm the statement that dolomite is not an 
isomorphous mixture, it may be pointed out that the specific gravity of the mineral, 
2-850, is less than that of a mixture of Ca CO 8 and MgCO 3 in equal molecular 

* Dr. WHEELTON HIND. Trans. N. Staffs Field Club, 1889, pp. 44-46. 

f Kindly gathered, with notes, for this work, by MR. SIMEON PRIEST, of Fenton, Stoke-on-Trent, Staffordshire. 

I Mineralogy ; An introduction to the Scientific Study of Minerals, 1902, p. 401. 



64 Che Ciucmictl) Ccmurp fltlas of 

proportions, viz., 2-843 > an( ^ this shows that, as usual in such cases, there has been a 
contraction by the union of the two carbonates. The mean refractive index of dolomite 
in Na. -light is given by MIERS as 1*59. 

Carboniferous limestones are very prone to become, like the sample under discussion, 
dolomitised. WETHERED* attributes this, either to the percolation of sea-water through the 
original limestone which serves to partially dolomitise the rock, or to the circumstance 
that the magnesia may have been derived by decomposition from the magnesian 
silicates which were deposited as detritus with the limestone a parallel instance of 
dolomite being replaced by a magnesian silicate in trap-rocks, having previously been 
recorded by BONNEY and HouGHTON.f 

Dolomite was so named, in honour of MONSIEUR D. DOLOMIEU (1750-1801), who 
announced some marked characteristics of the rock in 1791. % Our specimen is partially 
ferriferous the rhombohedra being coated with a pellicle of ferrous carbonate, which 
has become brown by oxidation. As the proportion of ferrous carbonate increases, the 
dolomite graduates into ankerite. Sometimes the rhombohedra develop curved faces ; but 
twinning in the rock-building forms of dolomite is not at all general, as it is in calcite ; 
while another distinction between these two closely allied minerals is, that the lozenge- 
shaped sections of rhombohedra, so frequent in dolomite and dolomitised rocks, are rare 
in calcite. Again, calcite effervesces at once when touched with cold hydrochloric acid, 
whereas dolomite requires hot H Cl. in being tested. This test may be applied to 
sections micro-chemically, and rhombohedra of dolomite can also be dissociated from the 
limestones in which they occur by the reaction of dilute H Cl, and subsequent washing. 

Reliable micro-chemical tests to distinguish dolomite from calcite and from the 
Brucite contained in the rock Predazzite were discovered by LEMBERG|| some nineteen 
years ago, and are well worth while repeating in this place. The tests may be applied 
to both powders and sections, and are as follows : (a). Calcite is known to precipitate 
ferric hydrate from a solution of ferric chloride much more rapidly than does dolomite. 
LEMBERG therefore directs that one part of the crystallised chloride be dissolved in ten 
parts of water and filtered. The nitrate should react on the sample to be tested for a 
few seconds only (always less than one minute), and immediately decanted. Calcite 
becomes coated with a film of ferric hydrate, while on dolomite the solution has but 
slight effect. By treatment thereafter with ammonium sulphide, the film of hydrate is 
converted into black sulphide on calcite, but only shows a pale green on the grains of 
dolomite ; while Brucite takes on a slightly stronger colour than the dolomite. If the 
three minerals are heated slightly after treatment with (N H 4 ). 2 S, and Ag N O solution, 
the Brucite becomes black, the calcite turns brown, but the dolomite remains 
colourless. (3). Prepare a solution of four parts A1 2 C1 6 in sixty parts of distilled water, 
add six parts of logwood, and heat for twenty-five minutes with stirring. Treat samples 
with the solution for from five to ten minutes. Calcite will be stained violet, while 
dolomite and Brucite remain but slightly affected. 

Literature. BONNEY AND HOUGHTON, Q.J.G.S., Vol. xxxv., p. 167. COLE, Aids in 
Pract. Geology, 4th Ed., 1902, pp. 161, 206, 274. DANA, Syst. Min., 6th Ed., 1892, pp. 
271-273. DOLOMIEU, Jour, de Phys., 1791, xxxix., p. i, et seq. HIND, Trans. N. Staffs., 
Field Club, 1889. LEMBERG, Zeits. d. deutsch. geol. Gesellsch., xxxix., 1887, p. 489, and xl., 
1888, p. 375 ; Abstract in Min. Mag., Vol. viii., 1889, p. 166. MIERS, Mineralogy, 1902, 
p. 401. RUTLEY, Q.J.G.S., Vol. L, pp. 377-392 ; PI. xix., figs. 5, 6. SAUSSURE, Dolomie, 
Voyage Alpes, 1796, 1929. SORBY, Q.J.G.S., Vol. xxv., 1879, Proc., p. 89. WETHERED, 
Q.J.G.S., Vol. xlviii., pp. 377-387; fig. at p. 381. 

* Q. J. G. S., Vol. xlviii., page 381. 

t Q. J. G. S., Vol. xxxv., p. 167 " On some Mica-traps from the Kendal and Sedbergh Districts." 

J Journ. de Phys., Vol. xxxix., p. i, 1791. 

Cf. RUTLEV. Q.J.G.S., Vol. 1, p. 391, PI. xix., Figs. 5, 6. 

|| Zeitschr. d. deutsch. geol. Gesellsch., Vol. xxxix., p. 489, 1887, and Vol. xl., p. 357, 1888. 





Gnmopftprlc Spcnitc. 

Qroby, Leicestershire. 



E.H.A.Pinxit. 



microscopical Petrography. 65 

firanopDpric spcnite.^ 

<S*&r 

Qroby, Leicestershire. 



Key- Plate. Upper Semicircle, x 25 diameters. G. Granophyric ground-mass, com- 
posed of a micro-pegmatitic intergrowth of pink orthoclase felspar and colourless quartz, 
p. Zoned idiomorphic crystals of plagioclase (oligoclase) felspar, probably more or less 
intergrown with orthoclase. Q. Larger patches of clear quartz, full of pores and other 
inclusions. A. Portion of an altered crystal of augite showing traces of diallagic habit. 
The unaltered augite remains colourless, while the altered parts have become trans- 
formed into fibrous greenish uralite and pale green chlorite. Lower left-hand Quadrant, 
X 25 diameters. P. Part of a zoned crystal of plagioclase (oligoclase) felspar. Q. A 
large patch of clear colourless quartz, full of strings of pores and other inclusions. 
H. Portion of a large green crystal of hornblende undergoing alteration. At H' the 
hornblende has become fibrous, and at E it has been changed into yellowish epidote. 
Ap. A longitudinal section of colourless prism of apatite. T. Reticulated crystal of 
titaniferous iron, showing degeneration into greyish-white granular leucoxene. Lower 
right-hand Quadrant, x 55 diameters. G. Granophyric ground-mass of orthoclase and 
quartz. H. Portion of a large cross-section of fairly-fresh brown hornblende, partially 
altered into greenish chlorite, but still showing traces of its clinopinacoidal and prismatic 
boundaries, with exceedingly clear prismatic cleavages intersecting at about 124. 
Q. Portion of a patch of quartz. B. Biotite converted into pale green chlorite. T. Plates 
of titaniferous iron. M. Minute octahedron of magnetite. Ap. Transverse (hexagonal), 
and longitudinal sections of colourless apatite, shifted to this part from an adjoining 
place just without the field of view. 

Occurrence. " The Precarboniferous Rocks of Charnwood Forest " formed the 
theme of a still authoritative paper by the REV. E. HILL, M.A., F.G.S., and PROFR. 
T. G. BONNEY, M.A., F.G.S., read before the Geological Society of London, as far back 
as the year 1878. In the Southern portion of the Old- World Forest good exposures of 
the rock occur at Groby, Markfield, Clift Hill, Bradgate Park and Hammer Cliff; while 
closely allied rocks crop up in the Northern portion of the district at Bawden Castle, 
Long Cliff and New Cliff, hard by Garendon. The rocks at Groby, Markfield and Clift 
Hill are closely similar in general characters, and were classified together as " Syenites." 
More correctly, however, they ought to be qualified by the designation " Granophyric " 
to indicate the peculiar nature of their ground-mass, which points unerringly to a 
hypabyssal rather than a plutonic mode of origin. Doubtless the rock represents an 
apophysis of the main mass. 

Definition. The name Syenite was given by PLINY in referring to the famous rock 
of Syene in Upper Egypt, the modern Aswan (Assouan) at the first cataract on the banks 
of the Nile, from whence material was anciently obtained in huge blocks for the con- 
struction of the obelisks and other well-known Egyptian monoliths. This original rock 
was taken as a scientific type by WERNER to specify hornblendic granites free from 
quartz, and he took as his example the Plauenscher Grund rock from the neighbourhood 
of Dresden, which, however, he subsequently classed as a greenstone. Both the 
original rock from Syene, which is a typical hornblende-biotite-granite, and the 



66 ClK CiDcntietl) Ccururp fltlas or 

Plauenscher Grund hornblende-syenite contain more or less quartz ; so that modern 
lithologists, in endeavouring to maintain WERNER'S definition, have been obliged to fall 
back upon chemical considerations in drawing an imaginary line between the granites 
and the syenites by making the abundance or paucity of silica their criterion. This 
artificial distinction gives to granite anything over 66 to nearly 75 per cent, of Si O , and 
anything under 66 to as low as 56 per cent, of Si O 2 , as distinctive of syenite. Under 
these conditions the Groby, Markfield and allied rocks of the Charnwood Forest, come 
well within the limits of so-called syenites ; for BERRY gives their percentages of silica* 
as ranging from 51-54 (Garendon), 5678 (Markfield), and 64-30 (Huncote Quarry, Croft 
Hill). 

Notwithstanding the figures quoted above, the Groby rock is not strictly a syenite, 
by reason of its departure from the true holocrystalline structure. Hand specimens 
examined by the naked eye or with a field-lens show it to be of coarsely crystalline 
texture and of intermediate composition, in that it contains almost evenly balanced 
proportions of monoclinic and triclinic felspars. A fractured surface shows it to be made 
up of pale-green, deep-green to black, and pink patches, with occasional colourless glassy 
spots. Under the microscope the pale green parts may, between crossed Nicols, be 
recognised as fairly idiomorphic and more or less zoned large crystals of plagioclase 
(oligoclase) felspar suffused with greenish alteration products, inclusions and vividly 
polarising granular matter and scales of, probably, paragonite. The crystals also bear 
evidences of intergrown orthoclase. The deep-green to black patches represent the 
ferro magnesian constituents of the rock ; the glistening white specks are blebs of 
quartz ; while the pink parts which play the role of ground-mass, and endow the rock 
with its granophyric character, and ergo, hypabyssal origin, maybe resolved, even under 
comparatively low powers 20 diameters or thereabouts as micropegmatitic inter- 
growths of undoubted orthoclase and quartz. 

Careful scrutiny reveals the fact that the ferro-magnesian minerals, hornblende, 
augite and biotite, have all undergone profound changes. The hornblende, of an 
originally brown hue is largely replaced by chloritic alterations (greenish), with which 
epidote (yellowish and strongly contoured) is frequently associated. Sometimes, as 
shown in the lower right-hand quadrant of the plate, indications of idiomorphic 
boundaries are preserved, and prismatic cleavages clearly shown. The augite is very 
variable in its proportions, being sometimes more abundant than the hornblende, and at 
others difficult to detect. It appears to have been originally colourless, like granitic- 
augite, and of diallagic habit, with striations parallel to the basal plane ; so that 
clinopinacoidal sections have been found to give the high extinction angles of augite 
(37 to 50)> and to exhibit the "herring-bone" lineation.f More often than not the 
augite as invaded by greenish fibrous uralite'and chloritic changes. The biotite has 
been altered to a green colour, showing marked dichroism in sections perpendicular to 
the principal cleavage, and not infrequently being altered into pale-green feebly polarising 
chlorite. 

Apart from its occurrence as an integral part of the micro-pegmatitic ground-mass, 
allotriomorphic patches of quartz full of "strings" of pores, and otherwise identical 
with the quartz of plutonic rocks, completes the essential constituents of the rock-mass ; 
while original accessories are present in the form of iron-ores and apatite. The iron 
occurs mainly in plates of ilmenite, often much reticulated and more or less degenerated 
into greyish white granular leucoxene ; but there are also small crystals of magnetite, 
frequently developed as octahedra, and stray particles of pyrite. Apatite obtains in 
clear colourless boldly-contoured hexagonal and elongated sections of prisms in all parts 
of the field. Lastly, the alteration products and accidental minerals may readily be 
identified as epidote, chlorite, uralite, leucoxene, paragonite, calcite and minute infiltra- 
tions of the last-named mineral. 

Literature. BERRY, Q.J.G.S., Vol. xxxviii., 1882, p. 197. HARKER, Petrologv for 
Students, 3rd Ed., 1902, pp. 49 50. HARRISON, Geology of Leicestershire, p. u. HILL and 
BONNEY, The Precarboniferous Rocks of Charnwood Forest, Part II., Q.J.G.S., Vol. 
xxxiv., 1878, pp. 199, 214, 215. SORBY, GeoL Mag., Vol. ii., p. 448. TEALL, British 
Petrography, 1888, pp. 270271. VALLE-POUSSIN et RENARD. On the allied rock from 
Cjuenast, Belgium, Memoire sur les characteres mineralogiques et stvatigraphiques des roches dites 
pltttoniennes de la Belgique, et de VArdenne Frangaise, L'Acad. Roy., Bruxelles, Tome xl. 

* Q.J.G.S., Vol. xxxviii, 1882, p. 197. 
^Ut supra, p. 2, A. A, in plate. 





hornblende flmlcsitc. 

Stenzelberg, Siebengebirge, Germany. 



E.H.A.Pinxit. 



Microscopical Petrography. 67 

hornblende flndesite. 

<se> 

Stenzelberg, Siebengebirge, Germany. 



Key- Plate. Upper semicircle, x 55 diameters. F. Cross-sections of large colourless 
phenocrysts of andesine felspar. Both of these sections show zonary bands of accretion 
even when viewed by ordinary transmitted light. The one to the left, in addition to 
being very strongly zoned, exhibits good inclusions of apatite and glass-cavities, some of 
which possess nuclei of magnetite. F'. Tabular clear crystal of andesine, which, between 
crossed Nicols, shows polysynthetic twinning on the albite-type. H. Hornblende crystals 
of a greenish-brown pleochroic variety, with well-developed resorption borders. The 
hexagonal section at the right-hand side of the figure incloses a clear cross-section of 
apatite towards its upper parietes. M. Black crystals of magnetite, more or less rounded 
and with corroded edges. M'. Good lozenge-shaped section of octahedron of magnetite. 
Ap. Transverse and longitudinal sections of prisms of clear apatite tinged with blue, 
deported to this from another part of the ground-mass. G. Hyalopilitic or " andesitic " 
ground-mass. Lower left-hand sextant, x 120 diameters. H. Hexagonal section of small 
crystal of hornblende, cut approximately at right angles to the prism zone, showing good 
cleavages intersecting at an angle of 124, bisected by the extinction directions. In 
convergent light a positive obtuse bisectrix, inclined at an angle of about 15 to the 
normal results. F. Aggregate of small crystals of clear colourless andesine, cut in a 
variety of directions with very little interstitial ground-mass between the individuals. 
M. Grains of black magnetite. Ap. Strongly contoured cross-sections of apatite prisms, 
adnate to the resorption border of the hornblende. G. Ground-mass, which, under this 
and higher powers of the microscope, can be readily resolved into a felted texture of 
minute laths of clear andesine, patches of pale-geen allotriomorphic mosaics of augite, 
granules of black magnetite and a residuum of isotropic glass. The ground-mass, more- 
over, is indefinitely subject to brown limonitic stains, e. Empty space. Lower central 
sextant, x 55 diameters. A. Pale-green octagonal phenocryst of augite. The substance 
of the augite phenocrysts, in this particular sample, is so brittle, that most of it cracks 
and falls away when the slices are reduced to the desired degree of tenuity for thorough 
microscopic examination, as exemplified by the present instance, where the crystal has 
been cut at right angles to the prism-zone. The characteristic cleavage cracks conse- 
quently intersect at close upon a right angle, or 87, thereby differing markedly from the 
prismatic cleavage angles of the hornblende; while the extinction angle is comparatively 
high, over 37. B. Flake of intensely pleochroic brown biotite, with strongly developed 
resorption rim at its upper borders. M. Grains of opaque magnetite. F. Andesine. 
G. Ground- mass. e. Empty spaces. Lower right-hand sextant, x 120 diameters. 
B. Part of a large phenocryst of brown cinnamon-coloured, strongly pleochroic biotite 
with inclosures of apatite Ap. ; andesine F ; and a rounded patch M, of what might 
have originally been a large cross-section of apatite, thickly enveloped by magnetite, 
which has subsequently been altered by pseudomorphism into the blood-red variety of 
haematite called martite. The borders of the biotite, moreover, under this power, show in 
detail the resorption phenomena common to that mineral and the hornblende in this 
typical hornblende-andesite. 

Occurrence. As a bed of lava at Stenzelberg, in the Tertiary volcanic district of 

the Siebengebirge near Bonn-on-the-Rhine, which has long been famous among petrolo- 

gists as the home of the Drachenfels type and other typical trachytes, from which it can 

be instantly distinguished by its " intermediate" character, denoted by the presence of 

andesine and other plagioclaslic felspars in place of sanidine. 

Definition. The appropriate name "Andesite" bestowed by VON BUCH upon the 
lavas which form the vast volcanic belt of the Andes, is now restricted by petrologists to 
rocks which occupy an intermediate position, linking the more acid trachytes on the one 



68 cbe Ciocntieifc Centurp fltlas of 

hand with the more basic basalts and dolerites on the other. They consist essentially of 
phenocrysts of soda-lime felspars and one or more ferro-magnesian minerals, studding 
a ground-mass composed of second crops of the same constituents varying from a glassy 
to a minutely crystalline texture. 

Petrologists in France separate these rocks into " andesites " proper and " labra- 
dorites," according as to whether the ground-mass is composed mainly of andesine or of 
labrador-felspar. The distinctions between these two varieties of triclinic felspar depend 
upon their chemical constitution and optical properties, and are as follows : According 
to TSCHERMAK'S theory, which is now generally accepted, all plagioclase felspars are 
isomorphous admixtures of albite, Na Al Si 3 O 8 , which is the only pure soda felspar, and 
anorthite, Ca A1 2 Si 2 O 8 , the only pure lime felspar. Taking Ab as the symbol for 
albite and An for anorthite, it has been found that andesine would be approximately 
represented by Ab 3 An to Ab t An x ; while labradorite, ranges from Ab x An x to Ab x 
An 3 . Again, the maximum extinction angles in sections at right angles to the albite 
lamellae are 16 to 22 for andesine, and 27 to 45 for the more basic labradorite. 

The Stenzelberg rock, now under consideration, comes clearly within the group of 
andesites, inasmuch as its felspars, both of the phenocrysts and the ground-mass, consist 
of andesine. Further, it is a typical hornblende-andesite, because its dominant ferro- 
magnesian mineral is hornblende. The phenocrysts consist of numerous large colourless 
idiomorphic crystals of andesine, many of which display marked zonary bands ; indica- 
tive, according to I DOINGS* of a change from a comparatively acid core to a more basic 
periphery. These zonary bands become emphasised between crossed Nicols, which also 
reveal the albite lamellation in longitudinal sections, associated sometimes with pericline 
cross-hatchings and twinning on the Carlsbad law. Most of the crystals show numerous 
inclusions, chiefly of glass-cavities in rounded forms or as negative-crystals sometimes 
holding specks of magnetite or other bodies ; larger irregular cavities usually filled with 
portions of the ground-mass, and small stray crystals of magnetite, apatite, etc. The 
other phenocrysts consist of largely predominating well-formed crystals of hornblende, 
a lesser number of crystals of augite, and a fair sprinkling of plates of biotite. The 
hornblende is of a greenish-brown colour and moderately pleochroic ; while the biotite, 
which is of a more brownish to cinnamon hue is intensely pleochroic. Both hornblende 
and biotite exhibit resorption phenomena and commonly contain inclusions of apatite, 
felspar and magnetite. The resorption rim occasioned by the corrosive influence of the 
magma penetrates more or less deeply, and leaves a black border of small crystals and 
granules of magnetite, and sometimes diminutive patches of pale-green augite. The 
augite phenocrysts are pale-green in colour, often eight-sided, and with characteristic 
cleavage traces. 

The magnetite, in small black octahedra and larger irregular crystals occur through- 
out the rock-mass, and is sometimes pseudomorphed in blood-red martite, or may 
degenerate at others into the hydrated variety, limonite, which occasionally develops 
into patches of a yellowish -brown colour, but more frequently stains the ground-mass 
every here and there to a rusty hue. The apatite is a constant accessory, in form of 
boldly-contoured glassy prisms sometimes tinged with blue. 

The ground-mass, under powers of over 100 diameters, may readily be resolved into 
the " andesitic " or hyalopilitic type of ROSENBUSCH. This can be clearly made out, by 
reference to the lower-right and left-hand sextants in the plate, to consist of a felted 
texture of a second crop of microlitic laths of andesine, which show a tendency to flow- 
structure around the phenocrysts ; mingled with sundry patches, like mosaics, of 
granular very pale-green augite ; diversified by black specks of magnetite and all 
cemented together by a residuum of isotropic glass. 

Literature. COLE, Sci. Trans. Roy. Dublin Soc.,Vol. vi., 1897, p. 222, et seq. CROSS, 
Bull, i, U.S. Geol. Surv., 1883; Amer. Joitrn. Sci., Vol. xxv., 1883, p. 139. FOUQUE ET 
LEVY, Min. Micrographique, 1879, Pis. xxii., xxviii., xxix., xxxviii. HARKER, Petrology for 
Students, 3rd Ed., 1902, pp. 181 189. IDDINGS, Journ, Geol., Vol. i., 1893, pp. 164 175 ; 
Mon. xx. U.S. Geol. Surv., 1893, PI. v., figs, i, 3, 4; PI. vi., fig. 2; I2ih Ann. Rep. U.S. 
Geol. Surv., 1892, pp. 610 612, PI. li. REYNOLDS AND GARDINER, Q.J.G.S., Vol. Hi., 
1896, p. 602. TEALL, Geol. Mag., 1883, pp. 145 148; British Petrography, 1888, p. 287, 
PI. xxxvii., fig. i. WADSWORTH, Bull. 2, Geol. Surv. Minn,, 1887, PI. x., xi. ZIRKEL, 
Micro. Petr., ^oth Parallel, PI. v., fig. 3, PI. xi., fig. 2. 

* Monograph xx., U.S. Geol. Survey, 1893, PI. v., figs, i 4; PI. vi., fig. 2. 










Porpbpritic Perlitc. 

Glashiitte, Schemnitz, Hungary. 



E.H.A.Pinxit. 



microscopical Petrograpbp. 69 



PorpDpritic Fertile. 

<^&-* 
(ilashiitte, 5chemnitz, Hungary. 



Key- Plate. Upper Semicircle x 25 diameters- G. Glassy ground-mass, crowded 
with microlites and crystallites, with occasional rhyolitic streaks and bands, R, and 
pervaded by numerous perlitic cracks in the form of straight as well as delicately curved 
rifts. In this ground-mass there are scattered numerous phenocrysts of colourless 
felspar, F; brown biotite, B~ and a few crystals of greenish-brown hornblende, H. The 
comparatively large crystal of hornblende here shown contains a core (intergrowth) of 
biotite crystals in both vertical (yellowish-brown and laminated) and basal (deep olive- 
brown) sections. B shows phenocrysts of vertical and B' of basal sections of biotite. 
Lower left-hand Sextant .x 25 diameters. The portion of the field of view here depicted is 
from a section cut parallel with the general minutely columnar structure of the rock, and, 
as a consequence, the delicately curved rifts of the perlitic structure are not so apparent 
as in the upper semicircle, being mostly seen as longitudinally developed cracks. 
F. Colourless felspars, variously cut. The sub-centrally situated crystal of felspar shows 
a fairly large faintly blue prism of inclosed apatite, Ap, with transverse cracks. G. Ground- 
mass. B. Biotite. Lower right-hand Sextant x 120 diameters, to show part of a vertical 
section of yellowish-brown biotite, B, with inclosures of apatite, Ap, and magnetite M. 
The ground-mass, G, shows a few of its crystallites, many of which assume a radiate or 
star-shaped arrangement, stellites. Lower central Sextant x 120 diameters, to show a 
selection of typical forms of the felspar inclusions, and microlites and crystallites of the 
ground-mass. The felspar inclusions, In, exhibit a variety of forms containing glassy- 
matter, with or without smaller inclusions (devitrifications), magnetite granules, etc., 
and are either irregularly shaped or in 'negative crystals.' Ap. Slender needles of 
apatite, faintly tinged with blue, occurring as inclusions in the clear colourless crystals 
of felspar, p. Microlites of ferro-magnesian minerals, mostly of biotite (pale yellowish- 
brown hexagonal plates), and of hornblende (pale greenish-brown, lozenged-shaped or 
hexagonal forms of sections perpendicular to the prism zone. These occur throughout 
the ground-mass, but principally in the neighbourhood of the felspar phenocrysts. 
M. Microlites of magnetite, which, however, are not very numerous in the ground-mass. 
F. Microlites of felspar, in colourless laths, often with bifid (swallow-tailed), ragged or 
brush-like terminations (the scopulites of RUTLEY) mostly developed at the borders of the 
felspar phenocrysts. The crystallites assume many forms. At c, a portion of the 
rhyolitic band R (in the upper semicircle) is shown duly resolved by this higher power 
into cumulites, composed of aggregates of excessively minute spherical forms called 
globulites, g. The globulites show a strong tendency to collect into straight or curvi- 
linear successions of more or less uniform sized individuals to form margarites, m, so-called 
from their fancied resemblance to strings of pearls. These are especially well seen in 
the sinuous rhyolitic band (R in the upper semicircle), but also occur throughout the 
ground-mass, notably in sections taken parallel with the columnar structure of the rock- 
mass, as in the lower left-hand sextant. When the globulites collect together in linear 
series, but in diminishing sizes, they give rise to curved or contorted crystallites, 
named trichites, t, also best seen in sections cut parallel with the columnar structure. 
On the other hand, when globulites of uniform size collect in linear series, the separate 
beads tend to coalesce and give rise to the elongated rod-like bodies called longulites, I. 
By far the most frequently represented form of crystallites in this rock, however, is the 
star-shaped variety, s, or stellite, which consists of several acicular rods, the pointed 
extremities of which radiate outwardly from a common centre. Immediately to the 
left-hand side of the small group of stellites shown in this sextant there is a curved line 
somewhat blurred at its edges by a faint brownish stain. This represents a portion of 



70 Cbe twentieth Century flilas of 

one of the curved rifts which constitute the perlitic cracks of the rock-mass, the 
margins of which are slightly discoloured by the development of limonite due to 
weathering. 

Occurrence. Among the Tertiary volcanic rocks so richly represented in the hilly 
region around Schemnitz in Hungary, at the borderland of the Little Carpathian 
Mountains, this well-known and beautiful example of Perlite occurs exposed at Glashiitte. 
It is usually termed a "Liparite" by German petrographers in accordance with 
ROTH'S* name for VON RiCHTHOFEN'sf " Rhyolite," now in general use among modern 
lithologists. 

Definition. Hand-specimens of this typically developed perlite exhibit to the 
naked eye or field-lens a very well developed minutely columnar structure, almost 
schistoid. Of a dark-grey colour liberally mottled with dull white specks and glistening 
black points, the rock is manifestly glassy. Under the microscope thin sections viewed 
through low and medium powers (20 to 50 diameters), are seen to be composed of a 
greyish-brown glassy ground-mass full of fine particles, which occasionally segregate 
into more or less dark sinuous bands (rhyolitic), while the isotropic glass itself has by 
shrinkage during consolidation given rise to innumerable cracks. It is these cracks 
which endow the rock with its columnar character. They - divide the glass into 
numerous polygonal columns which, in their turn, are subdivided by concentric rifts, 
disposed much after the fashion of the leaves of an onion or other bulb, so that when 
cut parallel with the columns they are seen as irregular elongated cracks, divided at 
intervals by transverse and occasionally by curved fissures ; whereas, when cut at about 
right angles to the columnar structure, the cracks are seen in a series of sub-polygonal 
areas, enclosing one or more internal, usually incomplete, rings, which, of course, give 
rise to the microscopic "perlitic" structure. Under higher powers (from 100 to 500 
diameters) the perlitic cracks show thin margins of clear, colourless glass, comparable 
to the " courts of crystallisation " seen in many obsidians and pitchstones-t These 
margins in weathered and even very slightly weathered specimens show yellowish- 
brown limonitic stains. The glassy matrix, moreover, within the perlitic cracks is also 
shown to be closely crowded with incipient crystals (microlites and crystallites, the 
disposition and peculiarities of which have already been described. 

Phenocrysts, as already mentioned, are liberally developed throughout the ground- 
mass. They include idiomorphic crystals, single or in groups, of colourless felspar,, 
biotite and a little hornblende, with apatite as a constant accessory, and magnetite in 
very minute grains or as inclosures, principally in the ferro-magnesian constituents. 
The felspar is mainly monoclinic (sanidine), often in Carlsbad twins and groupings 
thereof, but is also met with in zoned crystals which show intergrowths of plagioclase 
(oligoclase and probably andesine), between crossed Nicols. Inclusions are common. 
The biotite obtains in numerous well-formed yellowish-brown intensely pleochroic 
cleaved plates (vertical sections), and deep greenish-brown more or less hexagonal basal 
sections. The vertical sections usually display numerous inclusions of the ground- 
mass of fairly stout prisms of colourless apatite and sundry developments of black 
magnetite, while the basal sections are frequently so deep in colour that save for 
slightly hyaline coloured edges they might, when small, be mistaken for iron-ores. The 
hornblende is only sparingly represented in tolerably large greenish-brown and 
appreciably pleochroic crystals. It is often intergrown with the biotite as shown in the 
upper semicircle of the plate. 

Literature. ALLPORT, Perlitic Obsidians of the Wrekin, Q.J.G.S., 1877. BONNEY,. 
Q.J.G.S., 1876, p. 151. COLE, Aids in Practical Geology, 4th Ed., 1902, pp. 98, 244, 264, 
265. DILLER, Monograph xx., U.S. Geol. Surv., 1893, pp. 160, 161. FOUQUE ET LEVY, 
Min. Micrographique, 1879, p l- xv *- fig- 2 - GEIKIE (SiR A.), Text-book of Geology, 4th Ed., 
1903, pp. 133, 154, 211, 214, 664. HARKER, Petrology for Students, 3rd Ed., 1902, p. 156. 
I DOINGS, Monog. xx., U.S. Geol. Surv., 1893, pp. 374-380, PI. viii. RUTLEY, Q.J.G.S., 
1881, Vol. xxxviii., pp. 391-396, PI. xx. TEALL, British Petrography, 1888, pp. 60, 185,. 
342. ZIRKEL, Mik. Beschaffenheit der Min. u. Ges., 1873, pp. 365-369. 

* Die Gesteinsanalysen, 1861, p. xxxiv. 

t Jahrbitch d. h. k. Geol. Reichsanstalt, Bd. xi., pp. 156, 165. 

} Ut supra, pp. 31, 61, 62. 




naotfcpra=imestotie. 

Waterhouses, Leek, Staffordshire. 



E.H.A.Pinxit. 



microscopical Pcirographp. 71 

tjiclothpra Cimcstonc. 

<s&=> 
Waterhouses, Leek, Staffordshire. 



Key-Plate. Upper Semicircle. Magnified 55 diameters. E. Sections of the 
foraminifer Endothyra Bowmani (PHILLIPS), which gives character to the rock, cut in 
various directions. As these tests vary considerably in size, they might be supposed to 
belong to more than one species, c. Recrystallised interstitial calcite-mosaic binding 
the organic remains and detrital matter into a coherent compact mass. Lower 
Semicircle. Magnified 55 diameters. E. Sections of tests of Endothyra Bowmani. These 
tests being cut fairly through their centres, display the polythalamous nature of their 
spiral arrangement. The septation of the chambers is quite distinct, although occa- 
sionally imperfect ; while in the lower smaller section the simple aperture is shown to be 
situated at the inner margin of the final chamber. On closer scrutiny with a higher 
power the calcareous tests may be seen to consist of an outer coarsely perforated and an 
inner compact layer of minute grains of limey matter. Cr. Plate of criniod. Cr'. Plate 
of crinoid showing minute structure. M. Patch of unaltered calcareous mud or fine 
sediment, c. Mosaic of interstitial recrystallised calcite. 

Occurrence. Waterhouses village, which is located about eight miles from Leek, 
in North Staffordshire, is the seat of a thriving hydraulic-lime industry ; several quarries 
being now worked by Mr. J. BRIDGWOOD for material wherewith to feed his kilns. Of 
Carboniferous age, the black hydraulic measures are well stratified, but contain only a 
few fossils, EttOMtpkalus, Eskdalia, etc., and the rock, moreover, is peculiar in the circum- 
stance that while being burnt the material expands and rises over the top of the kiln, 
thereby differing from ordinary limestones, which usually sink in the kiln as the burning 
proceeds. The black beds are overlain by light-coloured, less steeply inclined broken 
masses of richly fossiliferous rock, from which Prodttctus giganteus, Syringopora vamulosa, 
and an abundance of crinoid remains may be gathered. Although the bedding of these 
varieties stands strongly contrasted, there appears to be no break between the two, 
but merely a change in chemical composition. The richly fossiliferous "Selfs" or 
"Odd-uns," as the quarrymen term the masses of shattered blocks, behave like other 
limestones by sinking in the kiln during the process of burning. 

The specimens which furnish the theme of the present brief sketch, were specially 
gathered for this work by the courtesy of Mr. SIMEON PRIEST of Fenton. They were 
taken from the " Selfs " beds of the formation, concerning which an interesting report* 
of the joint excursion of the North Staffordshire Field Club and Geological Association 
to the spot, notes that: "Dr. WHEELTON HIND drew attention to the fine series of 
quarries in Waterhouses village. That farthest from the village shows the beds of lime- 
stone (Carboniferous), standing almost vertical, and the series of quarries shows the beds 

* Trans, N . Staffs. Field Club, Vol. xxxviii., 19031904, p. 174. 



72 Cbe CuKiiiict!) Centurp fltlas of 

gradually becoming less and less inclined. In the quarry with vertical beds was once 
a cave, and in it, in bygone days, a mammoth had become stuck. Its bones were found 

in very perfect condition, and are now in Manchester Particular attention was 

called to a bed of limestone at the top of the series, consisting of a mass of more or less 
rolled shells. This bed could be traced in this position in various parts of Derbyshire,. 
N. Staffordshire and Yorkshire, and indicated a shallowing of the Carboniferous sea, the 
rubbed and waterworn shells showing exposure to tidal action, or waves on some shell- 
bank or beach." 

Definition. This specimen, when examined under the microscope, throws light 
upon the evolution of the rock-mass as having been deposited upon a bank of crinoid 
and coral remains in shallow water, and during or after its consolidation to have been 
percolated by water highly charged with dissolved calcium carbonate, which finally 
crystallised to form the interstitial cement of calcite. Where the interspaces were com 
paratively minute a mosaic of calcite was developed ; but when the interspaces (original 
or absorbed) became sufficiently large, the calcite was deposited in the form of clear 
crystalline masses with well marked twin-lamellae and intersecting cleavage lines. 

The rock, moreover, is a true hydraulic limestone by reason of its holding its full 
share of over ten per cent, of silica, in the shape of grains of sand and blebs of quartz, 
and also an appreciable portion of aluminous-calcareous mud, which can be detected 
every here and there in the microscopic field of view, as pointed out in his notes on the 
rock, sent to the writer, by Dr. G. J. HINDE, F.R.S., of Croydon. 

As already noted, the tests of Endothyra (PHILLIPS) endow the rock sections with 
interest on account of their abundance and the excellent state of their preservation. 

The sub-family Endothyrina is placed by BRADY under the Lituolidez and by CARPENTER 
amongst the Rotalidce. Its members are defined by PHILLIPS to be possessed of calcareous 
polythalamous tests, composed of exterior coarsely perforated and interior compact 
layers consisting of minute grains of limey matter. They are irregularly spiral, with a 
simple aperture at the inner margin of the final chamber. 

At parts of the field of view, other than those depicted by the figures in the plate, 
there are a fair number of transverse sections like diminutive thick rings, and longi- 
tudinal moniliform counterparts thereof, which point to the former presence of other 
Foraminifera, allied to Nodosinella (BRADY). Most of the tests of the Foraminifera are filled 
with a fine mosaic of calcite. Sections of joints of crinoid stems and plates are of 
frequent occurrence, but only now and again exhibit traces of minute structure ; as a 
rule the transverse and longitudinal sections of crinoid stem-joints consist of a single 
crystal of calcite, sometimes traversed by cleavage lines, and these, as well as the plates 
of crinoids and of other calcareous remains, appear to have been well rolled. Finally,, 
fragments of the shells of Brachiopoda and Mollusca are to be detected every here and 
there in almost all sections of the rock. The brachiopod shell-fragments are more 
slender than those of the molluscs, especially the lamellibranchs, and can likewise be 
distinguished from the latter by their histological characteristics ; for, while the brachio- 
pods exhibit laminae of flattened fibres arranged in more or less parallel series and lying 
obliquely to the surface of the shell, the much stouter shells of lamellibranchs tend to 
disintegrate into their component prisms or fibres ; and, as most of them were originally 
composed of aragonite, subsequently pseudomorped in stabler calcite, the latter generally 
breaks up into a delicate mosaic to form the substance of the shell-chip. 

Literature. BRADY, Monograph on Carboniferous and Permian Foraminifera, Palaeonto- 
logical Society, 1876, p. 92 ; PI. v. figs, i 4 ; PI. xii., fig. 2. CORNISH AND KENDALL, 
Geol. Mag., 1888, pp. 6673. GEIKIE (Sir A.) Text-Book of Geology, 4th Ed., 1903, Vol. I., 
pp. 190 191. HIND, Trans. N. Staffs. Field Club, 1903-4, Vol. xxxviii., p. 174. KENDALL, 
Rep. Brit. Assoc., 1896, pp. 789 791. SORBY, Presidential Address Geol. Soc., Q.J.G.S., 
Vol. xxxv., 1879, Proc., pp. 56 95. VON ZITTEL, Text-Book of Paleontology, 1900, Vol. i., 
PP- 3 -3i- 



microscopical Petrograpbp. 



73 



INDEX. 



Authors' names are printed in SMALL CAPITALS, localities in italics and subjects 

in ordinary type. 



ABBE, xxv, xxvii 

Aberdeen shire, 30, 53 

Abrasives, ii 

ACHESON, ii 

Acid, intrusives, 47 ; lavas, 31, 50 ; nitric, 

xx ; sulphuric, xx 
Acmite, 27, 28 

Actinolite, 2, 29, 30, 53, 57, 61, 621 
Adjustable drawing-table, xxvii 
ADYE, 10, 14, 24, 43 
jEgirine, 2528, 51 
Air-bubbles, 50 
Albite, 68 ; lamellation, 2, 12, 14, 16, 29, 

34, 36, 38, 53, 60, 67 
Alcyonarian, 4 
Aldous quarry, ii 
Algae, 4, 48 
Allothigenous, 19 
Allotriomorphic, i, 24, 41, 47, 49, 51, 55, 

58,66 

ALLPORT, 16, 23, 24, 33, 38, 45, 62, 70 
Alteration, 52 
Aluminium, 60 

Amateur's lapidary machine, ii 
Amorphous matter, 46 
Amphibole, grammatitic, 24 ; soda, 47 
Andes, 67 

Andesine, 67, 68, 70 
Andesite, hornblende, 67 
Anglesey, 39 
Ankerite, 64 
Annelid-tubes, 20 
Anorthite, 14, 30, 68 
Anorthoclase, 61 
Anthocyrtis, 56 
Anticlinal arches, 40 
Antilles, 56 
Antrim, xxi, 50 
Apatite, i, 8, ii, 12, 15, 16, 23, 24,37,38, 

42, 45, 46, 49, 51, 53, 59, 60, 65, 6770 
Apophysis, 65 
Aragonite, 18, 72 
.Argillaceous, 44, 52 
Argyllshire, 42 



Arm o'Grain, i 

ARNOLD-BEMROSE, 14 

Arran, 33, 61 

Arthur's Seat, 13 

Artificially indurated rocks, xxi 

Artists' saucers, ix 

Assouan, 49, 65 

Astromma, 56 

Astrorliizidae, 5 

Atlantic, xix, 4, 56 

" Augen-structur," 52, 54 

Augite, i, 2, 79, 1316, 23, 24, 37, 

4 1 43 45 4 6 49. 57. 5 8 61, 6568 ; 

picrite, 7 ; syenite, 49 ; trachyte, 46 
Austria, 48 
Authigenous, 19 
Avon, 1 8 



Bagshot-sands, xix 

BAIRD & TATLOCK, x, xi 

Balsam, Canada, vi, vii, ix, x, xii, xx 

Barbados, 48, 56 

Bargate-Stone, 32 

Barrow on-Soar, 38, 55 

Basalt, 13, 45 

Basaltic hornblende, 9, 23, 24 

Basic syenite, 42 

Bassenthwaite, 34 

Bastite, i, 2, 30, 43 

Batrachospermum, 62 

Bavaria, 44, 57 

Bawden Castle, 65 

BAYLEY, 30 

BEALE, xxv 

BECKE 24 

Bees'-wax, vi 

Belonite, 61 

Benzol, ix, xvi, xvii, xxiii 

BERRY, 66 

BERWERTH, 34, 46, 47 

Bethesda, xxii, 52 

Bicarbonate of soda, xix, 5 

Binary granite, 35 



74 



Che CuKiiuetb Centurp fltlas or 



Biotite, 3, 7, 1012, 23, 24, 30, 34, 35, 38, 
41, 42, 44, 49, 5154, 57, 60, 6570 

Birchwood Park, 63 

Bissex Hill, Barbados, 22 

Black cement, xvi, xvii 

Black garnet, 27, 28, 51 

BLAKE, 39 

BOLTON, 36 

Bones, xxiii, xxiv 

Bonn, 67 

BONNEY, 20, 30, 33, 36, 38, 41, 43, 60, 62, 
64, 65, 66, 70 

Book-boxes, xxiv, xxviii 

Bookcase, Globe- Wernicke, xxiv 

BORICKY, 26, 41 

Borolan, Loch, 51 

Borolanite, 51 

Boron, 60 

Bottle for Canada balsam, x, xvi, xvii 

Boundary intergrowths, 49 

Brachiopods, 17, 18, 32, 40, 72 

Bradgate Park, 65 

BRADY, 22, 72 

Brandy Gill, i 

Breccias, ii 

BRIDGWOOD, 71 

Brine-cavities, 35 

Bristol, 17 

Brittany, 34 

Brodick, 33 

BROGGER, 42 

BRONN, 56 

Bronzite, 43 

Brucite, 64 

Brunswick black, xvii 

Brushes, ix, xvi, xxvi 

Bryozoa, 18, 32, 40 

BUCK, VON, 67 

BUCKLEY, 36 

Burgundy pitch, vi 

Burnt Hill, Barbados, 56 

BURY, MRS., 56 

BUTSCHLI, 56 

Bytownite, 14 

Cadgwith, 43 

Calcareous binding, 21, 63 ; mud, 71 

Calciferous sandstone, 14 

Calcite, 16, 18, 23, 32, 34, 38, 40, 48, 63, 

64, 66, 71, 72 ; mosaic, 32, 40 
Calymene Blumenbachii, 40 
Cambrian, 51, 52 
Camera lucida, xxv xxviii 
Canada balsam, vi, vii, ix, x, xii, xx 
Cape-Town, 60 
Carbide of silicon, ii 
Carbon dioxide, 63 
Carbonaceous particles, 44 
Carbonate, ferrous, 63, 64 
Carboniferous, 9, 14, 17, 18, 33, 34, 36,41, 

45, 46, 61, 63, 72 



Carborundum, ii, v, vi, viii, xxiii 

Carlsbad twinning, i, 26, 34, 68, 70 

Carrara, xxi 

Car rock Fell, i 

CARUS WILSON, 36 

Cementing rocks, v 

Cements, v, xvi, xvii 

Celyphytic, 23, 30 

Cephalopods, 40 

Ceylon, 60 

Chalcedony, cryptocrystalline, 19, 20 

Chalks, xxi, xxii 

Chalky Limestones, 21 

Challenger Expedition, xix, xxiii, 4, 5, 
21, 22 

CHAPMAN, 32 

Charnwood Forest, 38, 55, 65, 66 

Chester's Quarry, 41 

Chiasmal impurities, 44 

Chiastolite-Slate, xxi, xxii, 44 

Chloritic matter, 2, 9, 15, 16, 23, 33 35, 
37, 38, 42, 52, 54, 65, 66 

Christiania, 49 

Church Cove, 29 

Clastic grains, 36 

Clauchland Point, 33, 61 

Clay-Slate, 52 ; needles, 52 

Cleaning slips, etc., xii 

Cleavages, calcite, 40 ; false, 44 ; horn- 
blende, 29, 30, 38, 39, 53, 55 ; intersect- 
ing, 29, 30, 38, 39, 40; structure, 52 

CLEMENTS, 30 

Clicker Tor, 23 

Cliff Hill, 65, 66 

Clifton, 17, 1 8 

Clinkstones, 25 

CLOUGH, 52 

Coal, xxii 

Coccolith, 4 

Codrington, Barbados, 56 

Coelentetata, 18 

COHEN, 18, 30, 31, 34, 35, 41, 44, 46, 47, 
58, 60, 62 

COLE, 50, 60, 64, 68 

Conchoidal fracture, 31 

COOKE, 22, 56 

Copper-Hill Quarry, 35 

Corals, 40, 48, 72 

Cork-tipped tongs, vii 

CORNISH, 72 

Cornstones, 20 

Cornwall, 23, 25, 29, 35, 43, 59 

Corriegills, 33, 6 1 

Coscinodiscus, 4 

COTTON AND JOHNSON, ii, iv 

Courts of Crystallisation, 31, 61, 62, 70 

Cover ack, 43 

Cover-glasses, cleaning, xii, xiii, xvi ; 
grinding on, xx, xxi 

Cracks, perlitic, 69, 70 

CRAIG, 35 



microscopical Petrograpbp. 



75 



Crinoids, 17, 32, 40, 63, 71, 72 

CROSS, 31, 68 

Crustacea, 18 

Cryptocrystalline, 19, 50, 54 

Crystalline Schistose Complex, 39 

Crystallisation, courts of, 31, 61, 62, 70 ; 
incipient, 55 ; simultaneous, 45 ; suc- 
cessive, 45 

Crystallites, 31, 62, 69, 70 

Crystals, negative, 35 

Cubes of salt, xvi 

Cumberland, xxi, xxii, I, 25, 34, 36 

Cumulites, 69 

Cyanite, 57, 58 

Cymbalopora, 4 

Cyprus, 21 



DAKYNS, 28, 51 

Dammar, xvii, xx 

DANA, 60, 64 

Dartmoor, 59 

DE LA BECHE, 60 

DE LAPPARENT, ii 

Delineations, xxv xxviii 

DERBY, 36 

Derbyshire, 63, 72 

Devitrified, 33, 41, 61 

Devonian, 49 

Devonshire, 59 

Diabase, 2, 23 

Diallage, 23, 49, 65, 66 

Diamond, iv 

Diatomaceae, xx, 4, 56 

DICK, 36 

DILLER, 36, 49, 52, 70 

Diorite, 29, 37, 39, 53, 57 ; granophyric, 

37 ; quartz-mica, 53 
DOBBIE, 15 

Dolerite, leucoxenic, 15; olivine, 45 
DOLOMIEU, 64 
Dolomite, 63, 64 
Dolomitised limestone, 63 
D'ORBIGNY, 22 
Drachenfels, 67 
Drawing, board, xiii, xxvi ; microscopical, 

xxv xxviii ; table, xxvii 
Dresden, 65 
DRESSEL, 28 
DRESSER, 49 
Druro quarry, 42 
Dudley^, xxii, 40 ; ' locust,' 40 
Dun Fion, 33, 6 1 
Dust-proof cabinet, xii, xiv 
Dyke, 55 
Dynamic metamorphism, 29, 36, 54 



Earth, Barbados, 56 
Echinoderms, 17, 18, 20, 32, 40, 48 



Eclogite, 57 ; analysis, 58 

Edinburgh, 13, 15 

Effusive period, 41 

Egypt, 65 

EHRENBERG, 56 

Eifel, 27 

Emery, ii 

Enamels, xvii 

Endothyra Limestone, 71, 72 

Engineer's files, xx 

Enstatite, i, 2, 43 

Entomostraca, 22, 40 

Eocene, 20 

Epidote, 38, 39, 52, 53, 65, 66 

Eppemeuth, 57, 58 

Eskdalia, 71 

Euomphalus, 71 

Extinction, straight, 43 

Eye-structure, 52, 54 



False, bedding, 32 ; cleavage, 44 

Faults, 40 

Favosites, 40 

Felsites, 33, 47, 55 

Felspar, albite, 68; andesine, 67, 68, 70; 
anorthite, 14, 30, 68 ; anorthoclase,'6i ; 
bytownite, 14 ; cryptoperthic, "49 ; 
labradorite, i, 45, 68; microcline, 27, 
36 ; oligoclase, 35, 37, 46, 53, 55, 59, 
60, 65, 66 ; orthoclase, i, i r, 34 38, 42, 
47, 5i 53> 55 59- 65, 66; plagioclase, 
9, 11 13, J 5> 16, 3438, 42, 45> 4 6 49> 
5 1 ' 53' 59. 6 5> 66 > 68, 70; sanidine, 
2527, 46, 50, 6r, 67, 70 ^secondary, 
29, 39. 

Ferric oxide, 36 

Ferrous carbonate, 63, 64 

Ferruginous stains, 44 

Fichtelgebirge, 44, 57 

File, engineer's, xx 

Filtering-paper, x 

Finishing slides, xv 

P'irestone, 20 

biith of Forth, 7 

Fissures of metamorphism, 41, 42 

FITTON, 32 

Fixed air-bubbles, 50 

Flamborough Head, xxi 

Flow-structure, 31, 50 

Fluid inclosures, 47 

Foraminifera, 4, 5, 17, 18, 21, 22, 32, 48, 
71, 72 

Foraminiferal Marls, 22 

Fossil-remains, xxiii 

Foundation cement, xvii 

FOUQUE, 28, 41, 44, 68, 70 

Fracture, conchoidal, 31 

Fuchsin, 26, 28 



7 6 



Che CiDcmietb Ccnturp Julias of 



Gabbro, i, 24, 29, 37 

Gaerwen, 39 

Galena, 63 

GARDINER, 36, 68 

Garendon, 65, 66 

Garnet, black, 51 ; red, 54, 57 

Garnetiferous mica-schist, 54. 

Gasteropods, 40 

Geftees, 44 

GEIKIE, SIR A., 10, 14, 16, 18, 35, 40. 46, 

5i 70, 72 
Geognosy, i 

Geological Survey, 3, 50 
GERICHTEN, VON, 58 
Giant's Causeway, ii 
Girvanella, 17, 18 
Glashutte, 69 
Glass, cavities, 67, 68; devitrified, 13, 33, 

45, 61, 69; isotropic, 13, 41, 45, 46, 

50; matrix, 31, 41, 68, 69 
Glauconite, xxii, 32 
Glaucophane-Schist, 39 
Globigerinae, xx, 4, 5, 21, 22 
Globigerinal Limestone, 21 
Globigerina-ooze, xix, xxiii, 4, 21 
Globulites, 69 
Gneiss, n, 29, 57 
Goda lining, 32 
Goshi Quarries, 21 
Grammatite-like, 24 
Granites, ii, 35, 37, 47, 53, 57,^5961; 

binary, 3 "5; hornblende-biotite, 65; 

difference from syenites, 66 
Granophyre, i 

Granophyric, diorite, 37 ; syenite, 65 
Granular augite, 45, 46 
Graphite pencils, xxv 
GREENLY, 39 
Greenstone, 65 

GREGORY, MR. A. G. F., n, 30, 54 
GREGORY, PROF., 48 
Greyweather, 20 

Grinding rocks, iv ; on cover-glasses, xx 
Groby, 65, 66 
GROOM, 3 
Ground-mass, 13, 15,26,27,31,41,45 

47. 5 52, 54' 55. 61, 65, 6769 
Gum dammar, xvii, xx 

GUMBEL, 10, 58 



Haddington, 41, 46 
HJECKEL, 56 
Haematite, 39, 41, 52 
Hailstone Hill, 45 
Haloed inclusions, 38 
Hambleton, 32 
Hammer Cliff, 65 
Hampstead, xix 
Hard-rock sections, i 



HARKER, i 3, 10, ii, 14, 18, 26, 33 36, 

4 6 . 47, 5052, 58, 60, 62, 6570 
HARRIS, 18 
HARRISON, 22, 56, 66 
Hascombe, 32 

HATCH, 26, 33, 41, 46, 60, 62 
HAUGHTON, 35 
Haughtonite, 44 
HAUY, 57 
Haiiyne, 25-28 
Hawse, 15 
Heidelberg, 58 

Heliodiscus Humboldti, 56 
Heliosphaera, 56 
Helminth, 15, 16 
Hemicrystalline, 27 
HERDMAN, 36 

Herring-bone augite, I, 37, 66 
Highland Plateau, 54 
HILL, 38, 42, 48, 65, 66 
HILTON, 39, 47, 52 
HIND, 63, 64, 71, 72 

HlNDE, 21, 22, 32, 48, 72 

Hof, 57 

Holloway Hill, 32 

Holocrystalline, 25, 30, 34, 42, 46, 66 

Hornblende, i, 7, 8, ii, 12, 23, 24, 26, 29, 

3739. 43. 47- 53 55. 57. 5 8 . 62, 6570 ; 

andesite, 67; biotite- granite, 65; gneiss, 

ii ; schist, 29; syenite, 66 
Hornblendic granite, 38, 47, 65 
HORNE, 34 
HOUGHTON, 64 
Hour-glass structure, 8 
HUE, 32 
Huncote, 66 
Hungary, 69 
Hunstanton, xxi 
Hunter's Bog, 15 
Huntly, 30 
Huronian, 43 
HUTCHINGS, 52 
Hyalite, 61 
Hyalopilitic, 97, 68 
Hyalosiderite, 45 
Hydrated iron-oxide, 43 
Hydraulic Limestone, 71 
Hypabyssal, 33, 34, 47, 65, 66 
Hypente, 30 
Hypersthene, 2, 30 
Hypidiomorphic, 49 
Hythe-division, 32 



Iceland, 31 

Idalian-series, 21 

IDDINGS, 31, 35, 68, 70 

Idiomorphic, 26, 27, 29, 38, 39, 41, 42, 46, 

47. 50. 53. 54. 55. 58, 63 
Ightham, 19, 20 
Ilmenite, 8, 9, 16, 24, 38, 39, 45 



Microscopical Pctrograpbp. 



77 



Inclusions, 16, 38, 46, 47, 55, 59, 65, 67, 

68 ; haloed, 38 
Inchcolm, 7, 24 
Incoherent rocks, xix 
Index of refraction, xii 
Indian-ink, xxv 
Indies, West, 56 
Indurated rocks, xxi 
Indurating materials, xx, xxii 
Intergrowths, 49, 53, 55, 65 
Intersecting cleavages, 29, 30, 38, 39, 40 
Interstitial, 37, 38, 40, 42, 60 
Intratelluric, 41 
Intrusive, 29, 38, 45, 47, 49, 57 
Inventrie, 30 
Iron, 2, 8, 15, 2426, 37, 43, 4547, 5, 

53; 55 60, 65 ; pyrite, 3, 15, 16, 44, 47 ; 

silicates, 19, 36 
IRVINE, 20 
IRVING, 34, 36 
Isotropic Glass, 13, 41, 45, 46, 61 



ENNINGS, 35 

ONES, 22 

ORDAN'S Lapidary-bench, ii, vi 
UDD, 10, 14, 30, 31, 62 
UKES-BROWNE, 21, 22, 32, 40, 56 
ULIEN, 36 
urassic, 32 



Kaiserstuhl, 14, 41 
Kaolinisation, i, n, 53 
Kaolinite, xix 
KENDALL, 72 
Kentallen, 42 
Kentallenite, 42 
Keuper, 63 
KEYES, 35 
KLAPROTH, 25 
Kvnance Cove, 43 
KYNASTON, 42 

Labradorite, i, 16, 29, 45, 68 

" Labradorite," 68 

Lamellae, twin, 2, 14, 29, 36, 38, 40, 42, 

46, 53. 59 

Lamellibranchs, 17, 32, 40, 72 
Lamprophyre, mica, 34 
Landedwednack, 29 
Land's End, 25 
LAPWORTH, 38, 40, 55 
Lavnaca, 21 
LASAULX, 28 
Laiirvig, 49 
Laurvigite, 49 
Lava, 31, 46, 50 
LECHERTIER FILS, xxv 
Leek, 71 
LEHMANN, 29 



Leicestershire, 38, 55, 65 

Leithakalk, xxii, 48 

LEMBERG, 64 

Lenticles, calcite, 34, 38, 52 

Lepas, 5 

Leucite, 27, 51 

Leucitophyre, 27 

Leucoxene, 2, 8 -10, 15, 16, 38, 65, 66 

Leucoxenic dolerite, 15 

LEVY, 28, 41, 44, 68, 70 

Lignite, xxiii 

Limburg, 41 

Limburgite, 41 

Limestone, chalky, 21 ; dolomitised, 63 ; 
endothyra, 71 ; globigerinal, 21 ; litho 
thamnion, 48 ; oolitic, 17 ; Wenlock, 40 

Limonite, 19, 27, 29, 41, 42, 54, 55, 67, 
68, 70 

Linlithgow, 37 

Lion's Haunch, 13 

Liparite, 70 

Liskeard 23 

LISTER, 48 

Lithophysial, 33 

Lithothamnion, 48 

Little Carpathian Mountains, 70 

Lituolidae, 5, 72 

Lizard, 29, 43 

Llanfairpwllgwyngyll, 39 

Loch Borolan, 51 

' Locust ' 40 

Long Cliff, 65 

Longulites, 31, 69 

Lower Greensand, 32 

Ludite, 32 

LYDEKKER, 56 



MACDONALD & Co., 53 

MACKIE, 36 

MACLAREN, 14 

MCMAHON, 43, 46 

MACONOCHIE, 42 

MACPHERSON, to 

Magnesium, 60 

Magnetite, 9 11, 13 16, 24, 29 31, 34, 

35- 37. 3 8 . 4 J 4 2 > 45. 4 6 > 49 5 1 . 53 

55. 57- 58. 62, 65, 67 
Maltese Islands, 22, 48 
Mammoth, 72 
Marble, xxi 
Margarites, 69 
Marine-glue, xxii 
Markfield, 65, 66 
Markt Schorgast, 57 
Marl, foraminiferal, 22 
Martite, 29, 67, 68 
Matrix, devitrified, 33, 41 
Megascopic slides, xxiv, xxviii 
Melanite, 27, 28, 61 
Melaphyre, 45 



7 8 



Cbc tiocntictl) Centurp jfitlas of 



Melbourn-rock, xxi 

Melilite, 27 ; basalt, 60 

Menai-Bridge, 39 

Menheniot, 23 

MERIEN, 28 

Mesh-structure, 43 

Metamorphism, dynamic, 29, 36, 54 ; 

fissures of, 41, 42 ; thermal, 29, 44 
Methylated spirit, ix, xvi, xxiii 
MEYER, 32 

Mica, Lamprophyre, 34 ; schist, 54 
Micaceous haematite, 52 
Microcline, 27, 36 
Micro-chemical tests, 26, 28, 64 
Microlites, 13, 24, 27, 61, 62, 69, 70 
Microgranite, 47, 50 
Micrographic intergrowths, i, n, 34, 37, 

38,53 
Micro-pegmatite, i, 45, 65, 66 

MIERS, 60, 63, 64 

Miliolidae, 5 

Miocene, xxii, 48, 56 

Modern methods of research, i, et seq. 

MOHS'S scale, iii, 60 

Mollusca, 18, 32, 40, 48, 63, 72 

Monoliths, Egyptian, 65 

MONTGOMERIE Stone, V 

Monzonite, n, 42 

Mosaics, calcite, 32, 40, 48, 63, 71, 72; 

quartz, 60 
Mosedale, i 
Mount Sorrel, 38, 55 
Mounting rock sections, x, xii, xiv 
Mounting-tile, xv 
Muckraw, 37 
Mudstones, 40 
Mnnchberg, 58 

MURRAY, SIK JOHN, 4, 6, 22, 48 
Muscovite, 35, 39 
Muslin, xvii 
Mynydd Mawr, 47, 50 



NACHET, xxv 

Nassellaria, 56 

Needles, clay-slate, 52 

Negative crystals, 35, 50, 68 

Nepheline, 25 28, 51 

Neutral-tint reflector, xxv 

New Cliff, 65 

New -Road oolite, 18 

NEWLAND, 57, 58 

NEWTON'S scale of colours, ix 

NICHOLSON, 56 

NICOLLS, 21 

Nigrosin, 26, 28 

Nile, 49, 65 

Nitric acid, xx, 56 

Nodasaria, 22 

Nodosinella, 72 

Norbury, 63 



Norite, 24 
Norway, 49 
Nosean, 25 28 



Obsidian, 31, 70 

Ocean, Atlantic, xix, 4, 56 

Oceanic-series, 56 

4 Odd-uns,' 71 

Odontoid outgrowths of quartz, 19 

Old Meldrum, 30 

Oligocene-Miocene, 21, 22 

Oligoclase, 12, 35, 37, 46, 53, 55, 59, 60, 

65, 66 
Olivine, 79, 13 14, 23, 24, 30, 4143, 

45 ; dolerite, 45 ; hyperite, 30 
Omphacite, 57, 58 
Oolitic Limestone, 17, 18, 32 
Oozes, xix, xxiii, 4, 21 
Ophitic, 2, 16, 23, 24, 45 
Orbulinae, xx, 3, 5 
O'REILLY, 35 
Orientation, 39 
Orographic, 57 
Orthoclase, i, u, 3438, 42, 47, 51, 53, 

55. 59. 6 5. 66 
Orthophyric, 46 



Paragonite, 53, 66 

Parallel orientation, 39 

Paramorphic, 8, 9 

Parenchymatous cells, 48 

PARKER, 22 

Paving-stones, 45 

PEARCEY, xx 

Pectolite, 15, 16 

Peg-structure, 27 

Pegmatitic, 45 

Pencils, lead, xxv 

Penrith, 36 

Penrhyn quarries, xxii, 52 

Penzance, 59 

Peppercraig, 46 

Pericline, 2, 29, 38 

Peridotite, 24 

Perlite, 69, 70 

Perlitic cracks, 31, 69, 70 

Permian, 14, 36 

Perthshire, u, 54 

Petalospyris, 56 

Petrographical delineations, xxv xxviii 

Phenocrysts, 27, 28, 41, 43, 45, 46, 47, 69, 

70. 

PHILLIPS, 10, 24, 36, 71, 72 
Phonolite, 25 
Phyllites, 52 
Picotite, 23, 24 
Picrite, 7, 24, 29 
Picritic-Serpentine, 23 
Pilite, 24 



Microscopical Petroarapbp. 



79 



PIRSSON, 34, 42, 49, 52 

PlSANI, 60 

Pitchstone, 61, 62, 70 

Pitlochry, n 

Plagioclase, 9, 1113, !5> 16. 34~ 3 8 . 4 2 

45, 46, 49, 51, 53, 59, 65, 66, 68, 70 
Plateau, Highland, 54 

Plauenscher Grund, 65, 66 

PLAYER, 51 

Pleochroism, 9, 23, 26, 28, 29, 39, 41 44, 

46, 47, 49 ; anomalous, 24 
PLINY, 65 

Plutonic, 65 

Podocyrtis, 56 

Poecilitic, 7, 23, 41, 42, 43 

Polycystina, 56 

Polysynthetic twinning, 15, 16, 27, 60 

Pores, strings of, 47, 53, 55, 57, 65, 66 

Porifera, 56 

Porphyritic, 13, 50; basalt, 13, 45; felsite, 

50 ; perlite, 69 ; rhyolite, 50 
Pre-Cambrian, 39 
Predazzite, 64 
Prehnite, 15, 16 

Preparation of rock-sections, i, et seq. 
PRIEST, 40, 45, 63, 71 
Prismatic cleavage, 29, 30, 38, 39, 53 
PROCTOR, xxviii. 
Productus giganteus, 71 
Pseudodiadema, 17 
Pseudomorphs, 23, 34, 43, 51, 52 
Pseudophite, 24 
Puddingstone, 20 
Pulverulent materials, xviii 
Pulvinulinae, xx, 4, 5, 22, 48 
Pyrite, iron, 3, 15, 16, 52 
Pyroxene, i, 2, 30, 51, 57, 68; green, 51, 

57,58 



Quartz, i, n, 19, 29, 32, 3439, 47, 5. 
52 55, 57 61, 63, 65, 66; odontoid, 
19; porphyry, 47, 50, 55; secondary, 
19, 36; strain-shadows, 36 

Quartz-Gabbro, 1,37 

Quartzite, green, 19 

Quartz-mica Diorite, 53 

QUEKETT Club, xxiv 

Quenast, 66 



Radial structure, 17, 18, 31, 55 
Radiating fissures, 42 
Radiolaria, xx, 4, 56 
Reaction-rim, 30 
Recent strata, 48 
Recrystallisation, 29, 48 
Red Chalks, xxi 
Red Felsite, 55 
Red Sandstone, 20, 36 
REEVES, xxv 



Refraction index, xii 
RENARD, 6, 48, 52, 66 
Resorption, 67, 68 
RETGERS, 36 
REUSS, 22 
REYNOLDS, 68 
Rhabdoliths, 4 
Rhombic prism, 44 
Rhombic-pyroxene, i, 30 
Rhombohedron, 40, 63, 64 
Rhyolite, 31, 50, 69, 70 

RlCHTHOFEN, VoN, 31, 50, 70 
RlEBECK, 47 

Riebeckite, 47, 50 

Rieden, 27 

RIESS, 58 

Ringing slides, xvii, xviii 

Rocks, chips, i ; incoherent, xix; prepara- 
tion, i; pulverulent, xviii; section- 
mounting, x ; slicing, i ; soft, xviii 

ROHRBACH, 10 

ROSENBUSCH, 24, 26, 35, 41, 42, 44, 46, 
58, 62, 68 

Ross, 51 

Rotalinse, 22, 72 

ROTH, 70 

ROTHPLETZ, 48 

' Rowley- Rag,' 45 

Rowley-Regis, 45 

ROWNEY, xxv, xxvi 

Rubislaw, 53 

Rutile, 52, 57, 58 

RUTLEY, ii, 26, 31, 36, 60, 62, 64, 69, 70 

Sable brushes, ix, xvi ; stripers, xvi 

Sagenite, 52 

Sale Fell, 34 

Salisbury Crags, 15 

Salopian-type, 40 

Salt cubes, xvi 

Sand-grains, 36, 52 

Sandstone, 36 

Sanidine, 25 27, 46, 50, 61, 67, 70 

' Sarcen-stones,' 20 

Saucers, artists' nest of, ix 

SAUSSURE, 64 

Schemnitz, 69 

Shillerisation, 2, 23, 24, 30, 49 

Schist, garnetiferous mica, 54 ; glauco- 

phane, 39 ; hornblende, 29 
Schistose-Complex, 39 

SCHOMBURGK, 56 

Schorl, 59, 60 ; granite, 59 ; rock, 59 
Scopulites, 69 
Sealing slides, xv 
Seaside sands, xix 
Secondary alteration, 52 
Sections, thinness of, ix 
' Selfs,' 71 
SEMMONS, 60 



8o 



Cbc twentieth Centurp fltlas of 



SENFT, 60 

Serpentine, 7, 8, 13 16, 2224, 29, 30, 

4i4345 

SHADBOLT'S turntable, xvi 
Shagreened, 41 
Shales, xxii, 40 
Shap, ii 

Shellac, vi, xvi ; solution, xxiii 
Sheffield quarry, 59 
Siebenqebirge, 67 
Silica, in granites, 66 ; in limestones, 72 ; 

secondary, 19, 59, 60 ; in syenites, 66 
Siliceous earth, 56 
Silicide of carbon, ii 
Silicide of iron, 36 
Silk rag, xvi 
Silurian, xxii, 34, 40 
Siluro-Cambrian, 57 
Simultaneous crystallisation, 45 
Skeleton crystals, 14, 31 
Skiddaw, xxi, xxii 

Slate, chiastolite, xxi, xxii, 44 ; clay, 52 
Slides, finishing, xv ; megascopic, xxiv 

xxviii ; sealing, xv ; storage, xxviii. 
Slips, cleaning, xii 
Smaragdite, 57 
SMYTH, 30 
Snowdon, 47 
Socotra, 47 

Soda, bicarbonate, xix ; washing, 56 
Soda-amphibole, 47 
Soda-pyroxene, 27, 46 
Soft-rock sections, xviii 
Soils, study of, i 
SOLLAS, 35, 36 

SORBY, xviii, 18, 36, 64, 66, 72 
Sorrel, Mount, 38, 55 
South Atlantic Ocean, 4 
Speeton, xxi 

Sphene, n, 12, 27, 28, 38, 51, 53 
' Spheres ' in Chalk, xxii. 
Spherulites, 31, 33, 61 
Spherulitic Felsite, 33 
Spherulitic Obsidian, 31 
Spinellid, 43 

Spirit, methylated, ix, xvi, xxiii 
Sponge spicules, xx, 4, 32, 40, 56 
Spumellaria, 56 
Staffordshire, 40, 71, 72 
Statuary marble, xxi 
Stellites, 69 
Ste.nzelberg, 67, 68 
Stone, Bargate, 32 ; Ightham, 19 ; Mont- 

gomerie, v, Tarn O'Shanter, v ; Water- 

of-Ayr, v, xxiii 
Storage of slides, xxviii 
Straight extinction, 43 
Strain shadows, 36, 54 
' Strings ' of pores, 47, 53, 55, 57 
Stripers of sable-hair, xvi 
Structure, augen or eye, 52, 54 



Successive crystallisation, 45 

Sulphide of iron, 3 

Sulphuric acid xx 

Sutherland, 51 

Swallow Craig, 9 

Syene, 49, 65 

Syenite, augitic, 49 ; basic, 42 ; grano- 

phyric, 65 ; leucite-melanite, 51 
Symmetrical extinction, 2 
Syringopora ramulosa, 71 

Tarn O'Shanter stone, v 

Taplow, xxi 

Tar dree, 50 

TEALL, 2, 3, 9, 10, 14, 16, 18, 24, 26, 28, 

29. 30, 33 34. 35. 36, 39. 42, 43> 44. 45. 

51, 60, 62, 66, 68, 70 
Teeth, xxiii, xxiv 
Tertiary, 20, 48, 50, 67, 70 
Teschenite, 10 
Tesserae of calcite, 48 
Textularia, 48 

Thallus of Lithothamnion, 48 
Thermal metamorphism, 29, 44 
Thinness of sections, ix 
THOMSON, SIR C. WYVILLE, 6 
Titaniferous iron, 15, 16, 23, 39, 45, 65 
Titanite, 38, 39 
Titanium, 8, 9, 41 
Titanomagnetite, i 
Tonga Islands, 48 
Tongs, cork-tipped, vii 
Toothless saw, ii 
TOPLEY, 20, 32 

Tourmaline, xix, 19, 35, 59, 60 
TOWNSON & MERCER, x 
Trachyte, 46, 67 
TRECHMANN, 3 
Trias, .38 

Trichites, 61, 62, 69 
Tridymite, 31, 61 
Trilobite, 40 
Trinidad, 48 
Troctolite, 24 
Truncatulina, 22 

TSCHERMAK, IO, 68 

Tunicates, 4, 56 
Turmali, 60 
Turntable, xvi 

Twin lamellae, 2, 14, 29, 36, 38, 40 42, 46, 
60 



Ultra-basic, 41 

Ultra-microscopic, 47, 55, 59, 60 
Upper Egypt, 65 
Uralitic, i, 2, 7, 24, 65, 66 



VALLE-POUSSIN, 66 
Valves of diatoms, xx, 4, 56 



microscopical , Petroarapbp. 



81 



VAN HISE, 36 

Venetian turpentine, vi, vii 

Vermicular chlorite, 15, 16 

Vienna Basin, xxii, 48 

Viridite, 8, 9 

Vitreous, 31 

VOGELSANG, 62 

Volcanic glass, 31 

VON BUCK, 67 

VON GERICHTEN, 58 

VON GlJMBEL, 10, 58 
VON RlCHTHOFEN, 31, 50 
VON ZlTTEL, 56, 72 

WADSWORTH, 34, 68 

WALLER, 45 

WARD, 2, 3 

Washing soda, 56 

WASHINGTON, 39 

Water- bath, xx 

Water-colours, xxvi 

Water-of-Ayr stone, v, xxiii 

Waterhouses, 71 

WATSON & SONS, xxvii, xxviii, xxix 

WATTS, 41, 50 

Weathering, 16 

WEED, 42, 49 

Wendron, 35 

Wenlock limestone, 40 

WERNER, 65 



West Indies, 56 

WETHERED, 17, 18, 21, 32, 40, 64 

WHITAKER, 32 

White cement, xvi, xvii 

White-shellac solution, xxiii 

WILKIE, SIR DAVID, xxvi 

WILLIAMS, 29, 30, 35, 49, 52 

WINSOR & NEWTON, xxv 

Wolf Rock, 25 

WOLLASTON, XXV 

Wood, xxiii 
WOODWARD, 32 
WORTH, 60 
Wren's Nest, xxii, 40 



Yorkshire, 72 

Yorkshire Geological Society, 49 

YOUNG, PROF. ANDREW, 60 



Zeolite, 8 

Zinc-white cement, xvi 

Zircon, n, 12, 35, 46, 49, 50, 53, 54, 58 

ZIRKEL, PROFR., 3, 28, 31, 62, 68, 70 

ZITTEL, PROFR. VON, 56, 72 

Zoisite, 58 

Zonary banding, i, n, 12, 34, 35, 37, 38, 

46, 55 6 5> 66, 67, 70 
Zoned augite, 13, 41 





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