June 23, 1892] 



NA TURE 



181 



much diversity of opinion must exist on some theoretical ques- 

 tions. Into the details of controversies it is not my purpose to 

 - nter ; but I shall content myself with indicating the con- 

 clusions to which I have been led in the time which the many 

 inevitable duties of life permitted me to devote to this branch of 

 geology. 



Before doing this it may be well to indicate very briefly the 

 mode in which the microscope is applied to the examination of 

 rocks. Commonly it is as follows : slices, cut by the lapidary's 

 wheel from minerals or rocks, are ground down smoothly till they 

 are about one one-thousandth part of an inch thick, and are then 

 mounted on glass. By this means most minerals, including the 

 great majority of the ordinary constituents of rocks, become 

 translucent, if not transparent. They are then examined under 

 a specially-constructed microscope, fitted with Nicol's prisms and 

 other contrivances for optical tests. Occasionally also certain 

 ■chemical tests can be applied. To what extent an object is 

 magnified depends on the nature of the investigation. A very 

 minute crystal can sometimes be studied, under favourable 

 circumstances, when enlarged to at least 800 diameters ; but in 

 ordinary cases, where the chief constituents of a rock and their 

 mutual relations are the object of research, a magnification of 

 from 50 to 100 diameters is commonly the most advantageous. 

 Sands, clays, and incoherent materials can be readily studied 

 liy mounting them temporarily or permanently on glass ; some- 

 imes, also, good work can be done, and time saved, by crush- 

 ing up fragments of minerals and of rocks, and by treating the 

 powder thus obtained in the same way. Investigations, which 

 promise to throw light on the problem of the development of 

 minerals, have been recently made by examining the insoluble 

 residues of those rocks which are chiefly composed of carbon- 

 ates. Solutions of different specific gravities have proved very 

 useful in the determination of the mineral constituents of a 

 rock, which are sorted out by them, as by a strainer, from a 

 sand, mud, or a powdered mass, so that each kind can be 

 studied separately either by microscopical or by chemical 

 analysis. 



The subject evidently, in process of time, tends to divide 

 itself into two branches : the one concerned mainly with the 

 characters of the individual constituents of a rock, the other 

 with the wide problem of their mutual relations, or, in other 

 words, with the history of the rock-mass : branches properly 

 denoted by the words petrography and petrology, though these 

 terms are often confused. The former is more strictly a depart- 

 ment of mineralogy ; the latter a department of geology. 

 This it is of which I chiefly speak to-day ; this it is in which 

 the most marked advances have been recently made. 



How great these have been may be more readily appreciated 

 if I mention a few matters, concerning which, even a quarter of 

 a century since, great uncertainty prevailed. Though it was 

 then generally admitted that one great group of rocks, such as 

 clays, sandstones, limestones, &c., were sediments, and that 

 another great group, the rocks called igneous, had solidified in 

 cooling from a fused condition ; the origin of a third, and by no 

 means unimportant group, the crystalline schists and gneisses — 

 the metamorphic rocks, as they were commonly called— was 

 considered very doubtful. Many geologists also believed that 

 not a few igneous rocks had been once sediments, like those in 

 he first group, which had been subsequently fused or " digested " 

 )y the combined action of heat, water, and pressure. Thus it 

 was supposed that clays and felspathic sandstones could be 

 traced through various stages till they became granite, and rocks 

 of the most diverse chemical composition could be transmuted 

 one into the other. The province of metamorphism was the 

 fairy-land of science ; it needed but a touch of the magic wand, 

 and, like Bottom the weaver, a rock was at once " translated." 

 It would be easy, were it worth while, to enumerate instance 

 after instance of these alleged transmutations, everyone of which 

 has been proved to be groundless. No doubt, even at the time 

 named, these assertions were questioned by some geologists, but 

 that they could be made so confidently, that they could be 

 inculcated by the official representatives of geology in this 

 country, shows the hopeless confusion into which petrology had 

 fallen. 



By means of the microscope also much light has been 

 thrown upon the nistory even of the better known rocks. The 

 classification of the igneous group has been simplified, and the 

 relations of its several members have been determined. The 

 microscope has dispelled many an illusion, and reduced a chaos 

 to order. In regard to the sedimentary group, it often has 



determined the true nature of their constituents, and has sug- 

 gested the sources from which they have been derived or the 

 agents by which they have been transported. Thus, through 

 its tube, we have been enabled, not only to gaze at the most inti- 

 mate structure and composition of rock-masses, but also to catch 

 glimpses of the earth's physiography in ages long before the 

 coming of mankind. 



But in speaking of the services rendered by the microscope, 

 I must not forget a needful caution. If the instrument be 

 employed for petrological rather than petrographical purposes, 

 it must never be divorced from work in the field. No training 

 in the laboratory, however complete, no research in a library, 

 however laborious, can of themselves make a petrologist. No 

 question can be completely mastered, unless it be also studied 

 in the field ; nay, even the specimens for examination under 

 the microscope, as a rule, should be collected by the student 

 himself, and the characters and relationships of the rock- 

 masses from which they are detached should be carefully noted. 

 It was said, on no mean authority, some fifty years since, 

 that, in the education of a geologist, travel was the first, 

 the second, and the third requisite. Perhaps the state- 

 ment, like most epigrams, was somewhat one-sided, but tht 

 truth in it has not been diminished by the increased perfection 

 of our instrumental methods. In petrology, the chimjeras of the 

 home-keeping student of the laboratory have been, and still are, 

 as hurtful to progress, as the dreams of the peripatetic geologist, 

 whose chief appliances are a stout pair of legs and a hammer. 



This, then, was the problem which, some thirty years since, 

 presented itself to geologists who were interested in petrology. 

 Here are two groups of rocks, the sedimentary and the igneous. 

 The origin of these we may be said to know, but as to that third 

 group, which, though not as large, is far from unimportant — what 

 is its history ? what are its relations to the other two ? The 

 records of its rocks at present are illegible. Is there any hope 

 that success will reward the attempt to decipher them ? Time 

 and perseverance have given an answer, and though much is still 

 uncertain, though much remains to be done, some real progress, 

 in my opinion, has been made. As the stones sculptured of old 

 by the hand of man are yielding up their secrets, as the hiero- 

 glyphs of Egypt and the cuneiform characters of Assyria are 

 telling the tale of the conquerors whose bones are dust, as the 

 tongues of the children of Heth, and of the black-headed race of 

 Accad, are being learnt anew, so the records of the rocks, 

 wherein no trace of life is found, are being slowly, painfully, but 

 ever more surely deciphered, and knowledge grows from year to 

 year. 



To obtain success the problem must be attacked in the follow- 

 ing way. As the first step, the two great groups already men- 

 tioned, the origin of which is known, must be thoroughly studied. 

 The examples selected must be nearly or quite unaffected by any 

 agent of change, such as heat, water, and pressure. Among the 

 specimens representative of the sediments, the materials must 

 range from fine to coarse — for the grains in the latter serve also 

 as samples of the rocks from which they have been broken, and 

 suggest their own inferences. Among the igneous rocks, types 

 ranging from the most glassy to the most crystalline forms must 

 be examined, in order to ascertain not only the constituent 

 minerals, but also their associations and mutual relations. 

 Suppose this done — suppose a fairly good idea obtained of the 

 characteristic structures and possit)ie variations in either class 

 — we have then to ascertain how far and in what way each 

 representative can be modified by natural agencies. At the out- 

 set, probably, it will be found convenient to trace the processes 

 of mineral and even of structural change without any immediate 

 reference to the efficient cause. It soon appears that in the case 

 of minerals, which differ in physical properties, but not in chemi- 

 cal composition, the one species replaces the other ; the less stable 

 gradually altering into the more stable form. Thus calcite takes 

 the place of aragonite, hornblende of augite ; one mineral may be 

 broken up into a group, as a colloid into crystalloids, or felspar 

 into quartz and white mica ; new species may be produced by 

 addition or subtraction of constituents from without, or by 

 exchange from within ; the replacement of silicates by carbonates, 

 the conversion of granite into tourmaline-rock, the formation of 

 epidote, chlorites, and serpentine, are a few among the many 

 instances of this kind of change. By tracing the process from 

 one part of a rock to another, numerous facts are collected and 

 relationships ascertained. But during these investigations 

 questions are raised in a student's mind which begin to clamour 

 for an answer. Why does such and such a rock change, now in 



NO. II 82, VOL. 46] 



