92 



NATURE 



\_N0V. 22, 1888 



could for a moment confuse a sheared granite from the High- 

 lands with a Laurentian gneiss from Canada or with an unin- 

 jured Hebrid-an gneiss. For the former to attain to the condition 

 of the latter, the mass must have been brought to a condition 

 which admitted of great freedom of motion amongst the par- 

 ticles, almost as much, in short, as among those of a molten 

 rock. Clearly, the dynamic metamorphism of Palaeozoic or 

 later ages appears to require some supplementary agency. Can 

 we obtain any clue to it ? 



An explanation of broadly-banded structures was long since 

 suggested, and has recently been urged with additional force, 

 which avoids some of our difficulties. We know that the pro- 

 cess of consolidation in a coarsely crystalline rock has often been 

 a slow one ; the constituent minerals separate gradually from 

 the magma, of which sometimes so little may remain, that a 

 rock with a true glassy base has been mistaken for one holo- 

 crystalline. The residual and still unsolidihed magma would 

 admit of a slow flowing of the mass, but there would be so little 

 of it that the crystals already individualized, though altered in 

 position by differential movements, would be affected by strains, 

 and liable to fracture. Such a rock, when finally consolidated, 

 would exhibit many phenomena in common with a rock modi- 

 fied by dynamic metamorphism, but would differ in the greater 

 coarseness of its structure. This may prove to be the correct 

 explanation of the curious foliated and banded gabbros in the 

 Lizard district. That some crystalline rocks must have passed 

 through this stage I am now in a position to affirm, from evidence 

 not yet published. 



Let us, however, see whether another line of investigation 

 may not throw some light on our difficulty. I have already 

 mentioned the effect produced by the intrusion of large masses 

 of igneous rocks upon other rocks. These maybe either igneous 

 rocks already solidified, or sedimentary rocks. The former may 

 be passed over, as they will not materially help us. In regard 

 to the latter, the results of c mtact-metamorphism, as it is called, 

 are, as we might expect, very various. Speaking only of the 

 more extreme, we find that sandstones are converted into 

 quartzites ; limestones become coarsely crystalline, all traces of 

 organisms disappearing, and crystalline silicates being formed. 

 In clayey rocks all signs of the original sediments disappear, 

 crystalline silicates are formed, such as mica (especially brown), 

 garnet, andalasite, and sometimes tourmaline ; felspar, how- 

 ever, is very rare. Fair-sized grains of quartz appear, either by 

 enlargement of original granules or by independent crystalliza- 

 tion of the residual silica. It is further important to notice that, 

 as we approach the surface of the intrusive mass — that is, as we 

 enter upon the region where the highest temperature has been 

 longest maintained— the secondary minerals attain a larger size 

 and are more free fiom adventitious substances — that is, they 

 have not been obliged as they formed to incorporate pre-existing 

 constituents. The rock, indeed, has not been melted down, 

 but it has attained a condition where a rather free molecular 

 movement became possible, and a new mineral in crystallizing 

 could, as it were, elbow out of the way the more refractory 

 particles. I can, perhaps, best bring home to you the result of 

 contact-metamorphism by showing you what its effects are on a 

 rock similar to that which I exhibited in illustration of the effect 

 of pressure-metamorphism on a distinctly stratified rock. These 

 are, in brief, to consolidate the rock, and while causing some 

 constituents to vanish, to increase greatly the size of all the 

 others. It follows, then, that mineral segregation is promoted 

 by the maintenance for some time of a high temperature, which 

 is almost a truism. I may add to this that, though rocks modi- 

 fied by contact-metamorphism differ from the Archaean schists, 

 we find in them the best imitations of stratification-foliation, 

 and of other structures characteristic of the latter. 



One other group of facts requires notice before we proceed to 

 draw our inferences from the preceding. Very commonly, when 

 a stratified mass rests upon considerably older rocks, the lower 

 part of the former is full of fragments of the latter. Let us 

 restrict ourselves to basement beds of the Cambrian and Ordo- 

 vician — the first two chapters in the stone-book of life. What 

 can we learn from the material of their pages? They tell us 

 that granitoid rocks, crystalline schists of various kinds, as well 

 as quartzites and phyllites, then abounded in the world. The 

 Torridon sandstone of Scotland proves that much of the sub- 

 jacent Ilebridean had even then acquired its present charac- 

 teristics. The Cambrian rocks of North and South Wales 

 repeat the story, notably near Llynfaelog in Anglesey, where 

 the adjacent gneissoid rocks from which the pebbles were 



derived, even if once true granites, had assumed their differences 

 before the end of the Cambrian period. By the same time 

 similar changes had affected the crystalline rocks of the Mal- 

 verns and parts of Shropshire. It would be easy to quote other 

 instances, but these may suffice. I will only add that the fre- 

 quent abundance of slightly-altered rocks in these conglomerates 

 and grits appears significant. Such rocks seem to have been 

 more widely distributed— less local — than they have been in 

 later periods. Another curious piece of evidence points the 

 same way. In North America, as is well known, there is an 

 ancient group of rocks to which Sir W. Logan gave the name 

 Huronian, because it was most typically developed in the 

 vicinity of Lake Huron. Gradually great confusion arose as to 

 what this term really designated. But now, thanks to our fellow- 

 workers on the other side of the Atlantic, the fogs, gendered in 

 the laboratory, are being dispelled by the li<iht of microscopic 

 research and the fresh air of the field. We now know that the 

 Huronian group in no case consists of very highly-altered rocks, 

 though some of its members are rather more changed than is 

 usual with the British Cambrians, than which they are supposed 

 10 be slightly older. Conglomerates are not rare in the 

 Huronian. Some of these consist of granitoid fragments in a 

 quartzose matrix. We cannot doubt that the rock was once a 

 pebbly sandstone. Still the matrix, when exainined with the 

 microscope, differs from any Palaeozoic sandstone or quartzite 

 that I have yet seen. Among grains of quartz and felspar 

 are scattered numerous flakes of mica, brown or white. The 

 form of these is so regular that I conclude they have 

 been developed, or at least completed, in sHii. More- 

 over, the quartz and the felspar no longer retain the 

 distinctly fragmental character usual in a Palaeozoic grit, but 

 appear to have received secondary enlargement. A rock of frag- 

 mental origin to some extent has simulated or reverted to a truly 

 crystalline structure. In regard to the larger fragments we can 

 affirm that they were once granitoid rock, but in them also we 

 note incipient changes such as the development of quartz and 

 mica from felspar (without any indication of pressure), and 

 there is reason to think that these changes were anterior to the 

 formation of the pebbles. 



To sum up the evidence. In the oldest gneissoid rocks we 

 find structures different from those of granite, but bearing some 

 resemblance to, though on a larger scale than, the structures of 

 vein-granites or the surfaces of larger masses when intrusive in 

 sedimentary deposits. We find that pressure alone does not 

 produce structures like these in crystalline rocks, and that when 

 it gives rise to mineral banding this is only on a comparatively 

 minute scale. We find that pressures acting upon ordinary 

 sediments in PalaeozDic or later times do not produce more than 

 colourable imitations of crystalline schists. We find that when 

 they act upon the latter the result differs, and is generally dis- 

 tinguishable froiTi stratification-foliation. We see that elevation 

 of temperature obviously facilitates changes, and promotes 

 coarseness of structure. We see also that the rocks in a crystal- 

 line series which appear to occupy the highest position seem to 

 be the least metamorphosed, and present the strongest resem- 

 blance to stratified rocks. Lastly, we see that mineral change 

 appears to have taken place more readily in the later Archaean 

 times than it ever did afterwards. It seems, then, a legitimate 

 induction that in Archaean times conditions favourable to mineral 

 change and molecular movement — in short, to metamorphism — 

 were general, which in later ages have become rare and local, 

 so that, as a rule, these gneisses and schists represent the 

 foundation-stones of the earth's crust. 



On the other side what evidence can be offered ? In the first 

 place, any number of vague or rash assertions. So many of 

 these have already come 10 an untimely end, and I have spent 

 so much time and money in attending their executions, that I 

 do not mean to trouble about any more till its advocates express 

 themselves willing to let the question stand or fall on that issue. 

 Next, the statement of some of the ablest men among the 

 founders of our science, that foliation is more nearly connected 

 with cleavage than with structures suggestive of stratification. In 

 regard to this I have already admitted, in the case of the more 

 coarsely crystalline rocks, what is practically idemical with 

 their claim, for they also assert that when the banding was 

 produced, very free movement of the constituents was possible ; 

 and in regard to the rest I must ask whether they were speaking 

 of cleavage-foliation or stratification-foliarion, which had not 

 then been distinguished, and I know in some instances what the 

 answer will be. The third objection isof a general nature. To 



