June 23, 1892] 



NATURE 



183 



position correspond with granites, and sedimentary rock thus 

 constituted, though not unknown, is rare. The minerals com- 

 monly exhibit a parallel or foliated and not seldom even a banded 

 arrangement ; in the latter case the layers of different mineral 

 composition in their mutual relations and associations imitate 

 with remarkable success the structure of a banded sedimentary 

 rock. Even a dozen years since little doubt was entertained that 

 this group also had a detrital origin. Occasionally, however, its 

 members, when studied in the field, exhibited characteristics 

 which were difficult to explain on any such hypothesis, and pre- 

 sented resemblances in habit to certain crystalline igneous rocks, 

 from which, however, they were proved to differ in their micro- 

 scopic structures. 



In rocks which have crystallized from a state of fusion some of 

 the mineral constituents usually exhibit their proper crystalline 

 outlines ; but in these others the same minerals had no definite 

 shape, and were simply granular. Of this structure two types were 

 observable : in the one the grains were elongated, in the other 

 they were roundish in outline but slightly wavy or lobed. The 

 former type was commonly found in the more distinctly banded 

 varieties ; the latter in the more massive and faintly foliated 

 kinds which in hand-specimens were not readily distinguished 

 from true granite. 



As in rocks, no less than in living beings, diversities of struc- 

 tures are nature's record of a difference in history, it became 

 a question whether these peculiarities were significant of origin 

 or of environment. By prolonged observations the following 

 results were established: — (i) That crystalline rocks, which 

 could be proved, by their relation to others, to be truly igneous, 

 sometimes exhibited banded structures, (2) That these struc- 

 tures, in certain cases, could not be attributed to any subse- 

 quent pressures or crushings, for no sign of them could be 

 found in neighbouring rocks, which, from their composition 

 and nature, ought to have yielded more readily. (3) That a 

 faint foliation or banding, especially in the case of granitic 

 rocks, could be sometimes detected in irruptive veins, in which 

 •cases the aforesaid granular structure was detected on micro- 

 scopic examination. (4) That cases were occasionally found 

 where a light-coloured granite had broken into a dark horn- 

 blendic rock, and the fragments of the latter had been 

 gradually softened, elongated, and even drawn out into bands, 

 together with the intruder, till they perfectly simulated, as already 

 mentioned, a stratified mass. (5) That in certain of these 

 cases, where a rock, exceptionally rich in hornblende, had been 

 partially fused by a pale-coloured granite, a banded black-mica 

 gneiss had been produced, indistinguishable, macroscopically and 

 microscopically, from those which have been already mentioned. 



It follows from these observations that the great group of 

 crystalline rocks, which are connoted comprehensively as schists 

 and gneisses, includes rocks which may have originated in one 

 of three different ways : — (l) Some have once been molten, but 

 have become solid under rather exceptional circumstances, 

 probably having lost heat slowly, and having continued to move 

 very gradually during the process of consolidation. (2) Others 

 have been produced by the thorough alteration of sedimentary 

 materials, in which a high temperature has been maintained for 

 a long time, in the presence of water and under considerable 

 pressure. (3) Others, again, have been the result of great 

 pressure, which has acted on rocks already crystalline, and has 

 produced mineral changes, sometimes to the complete oblitera- 

 tion of the original structure. The second and third of these 

 groups are truly metauaorphic rocks, to the first the term, strictly 

 speaking, is not applicable. 



As a rule, it is not difficult to distinguish between these three 

 groups, and in all probability the ambiguities which still remain 

 will be solved by patient and persevering work. Cases, no doubt, 

 will occur on which no inference can be founded ; cases where, 

 from one cause or another, nature's record has become illegible. 

 But to this disappointment the scholar and the archasologist has 

 to submit equally with the geologist. Negative evidence of this 

 kind has no disturbing power ; any amount of it is outweighed 

 by a single scrap of clear and positive testimony. It is gene- 

 rally a waste of time to puzzle over bad specimens ; they are 

 much more likely to produce a perplexed agnosticism than a 

 rational faith ; for a creed has its place in science no less than 

 in theology. 



I have mentioned one mode in which materials rather 

 markedly different in mineral character may be, in a certain 

 sense, interstratified and to some extent blended, but should 

 add that recent researches render it highly probable that there 



NO. II 82, VOL. 46] 



are other modes in which mineral or chemical constituents may 

 be differentiated in a magma which was once homogeneous. 

 To discuss these would carry us into questions of crystallo- 

 genesis, which have no direct bearing on my present subject, 

 though in these also the microscope has rendered the most 

 valuable services by suggesting inferences and by testing theo- 

 retical conclusions ; questions upon which so much light has 

 been thrown by the researches of Guthrie, Lagorio, Sorby, and 

 others ; but I may refer in passing to the law established by 

 Soret, that by a change of temperature a homogeneous solution 

 may be rendered heterogeneous ; since any compounds by which 

 it is nearly saturated tend to accumulate in the colder parts. 

 Gravitation also, when certain minerals are crystallizine from 

 a magma, may cause them to rise or to sink, and in this way also 

 heterogeneity may be produced. So when the mass of mingled 

 fluids and solids is constrained to move, a streaked or banded 

 structure may be the result produced ; as in a process familiar 

 to the glass-blower. 



But when the geologist has learnt from the microscope to 

 recognize differences of structure in crystalline rocks, and to 

 appreciate their significance, he finds that a wider problem is 

 presented to his mind, provided he has not been led by the 

 fascinations of laboratory studies to despise or neglect work in 

 the field. Granted that one group of rocks, covered by the term 

 metamorphic, has undergone great changes since its members 

 were first deposited or solidified, can these be connected with 

 any phase in the earth's history ? Have they any chronological 

 significance? Even twenty years since few geologists would 

 have hesitated to reply : — "None whatever: a rock may have 

 undergone metamorphism at any epoch in the past. Muds 

 and sands of Eocene, Jurassic, Carboniferous, Silurian, of 

 any geological age, have been converted into crystalline 

 schists. Proofs of some part of this assertion can be found 

 even within the limits of the British Isles ; it can be completely 

 established within those of Europe." But during the last 

 few years this hypothesis has been on its trial ; witness after 

 witness in its favour has teen, so to say, brought into court, 

 and has broken down under cross-examination. I can assert 

 this without hesitation, for I have some personal knowledge of 

 every notable instance in Europe which has been quoted in the 

 debate. Microscopic study, combined with field work, has in- 

 variably discovered that some very important link in the supposed 

 chain of proof is wanting, and has demonstrated without exception 

 that these crystalline schists are very old ; much more ancient 

 always than any neighbouring rock to which a date can be 

 assigned, if not older than the first rocks in which any trace of 

 life has been found. It has been also demonstrated that sedi- 

 mentary masses, after they have been buried deep beneath super- 

 imposed strata and exposed to great pressure, have emerged 

 comparatively unchanged. Such rocks are most valuable as 

 illustrations of the effects of dynamical and other agencies ; but 

 they are sufficiently distinct from the crystalline schists to indi- 

 cate that the environment in the one case must have differed 

 greatly from that in the other. The results of contact meta- 

 morphism prove that heat is an important agent of change ; hut 

 as these also present their own marked differences, they fail to 

 afford a complete solution of the problem. 



Moreover, among ordinary sedimentary rocks, we cannot fail 

 to notice that, as a rule, the older the rock the greater the 

 amount of mineral change in its constituents. A good illustra- 

 tion of this is afforded by the Huronian system of North America, 

 the rocks of which are rather older than the Cambrian of this 

 country. Some of them, while still retaining distinct indica- 

 tions of a sedimentary origin, have become partially crystal- 

 line, and supply examples of a transition from a normal sediment 

 to a true crystalline schist. Even the older Palaeozoic rocks 

 almost invariably exhibit considerable mineral changes, though 

 with them it is only on a microscopic scale. Hence, taking 

 account of all these results, we seem to be forced to the conclu- 

 sion that the environment necessary for changing an ordinary 

 sediment into a crystalline schist existed generally only in the 

 earliest ages, and but very rarely and locally, if ever, since 

 Palaeozoic time began. 



Further, in regard to those peculiar structures which, as 

 already stated once led geologists to consider certain rocks, 

 really of igneous origin, as metamorphosed sediments ; they 

 also appear to have been much more frequently produced in the 

 earliest ages. They are common in association with the ordinary 

 crystalline schists ; they are found, so far as I know, rarely, if 

 ever, with little altered sediments. The microscopic study of 



