464 



SCIENTIFIC NEWS. 



[Nov. 



appear, either by enlargement of original granules or by 

 independent crystallisation of 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 trom adventitious sub- 

 stances — 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 move- 

 ment became possible, and a new mineral in crystallising 

 could, as it were, elbow out of the way the more refrac- 

 tory particles. I can, peihaps, best bring home 

 the result of contact-metamorphism by showing 

 what its effects are on a rock similar to that which I 

 exhibited in illustration of the effect of pressure-meta- 

 morphism 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 modified 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 pro- 

 ceed to draw our inferences from the preceding. Very 

 commonly, when a stratified mass rests upon consider- 

 ably older rocks, the lower part of the former is full of 

 fragments of the latter. Let us restrict ourselves to base- 

 ment beds of the Cambrian and Ordovician — 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 subjacent Hebridean had even then acquired 

 its present characteristics. 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 Malverns 

 and parts of Shropshire. It would be easy to quote 

 other instances, but these may suffice. I will only add 

 that the frequent 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 light 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 

 to 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 

 examined with the microscope, differs from any Palaeo- 

 zoic 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 situ. Moreover, 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 fragmental 

 origin to some extent has simulated or reverted to a 

 truly crystalline structure. In regard to the larger frag- 

 ments, 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 com- 

 paratively minute scale. We find that pressures 

 acting upon ordinary sediments in Palaeozoic 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 distinguishable 

 from stratification-foliation. We see that elevation of 

 temperature obviously facilitates changes, and promotes 

 coarseness of structure. We see also that the rocks in 

 a crystalline series which appear to occupy the highest 

 position seem to be the least metamorphosed, and present 

 the strongest resemblance 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 after- 

 wards. It seems, then, a legitimate induction that in 

 Archaean times conditions favourable to mineral change 

 and molecular movement — in short, to metamorpbism — 

 were genera], which in later ages have become rare and 

 local, so that, as a rule, these gneisses and schists repre- 

 sent the foundation-stones of the earth's crust. 

 (To be continued.) 



= — <-^t^>'^fe u * — 



of papers, 2Lecture#, etc* 



JUNIOR ENGINEERING SOCIETY. 

 At the opening meeting of the eighth session, which 

 took place on October 12th, the presidential address was 

 delivered by Professor W. Cawthorne Unwin, F.RS., 

 M.Inst. C.E., on " Illustrations of the Use of Theory in 

 the Work of the Engineer." After some introductory 

 remarks from the chairman, Professor Unwin said that 

 the name of the society suggested to him that he might 

 venture to speak to them as an older to younger engineers. 

 He began work as an engineer more than thirty years 

 ago, and they could hardly imagine with what scanty 

 means of attacking the problems before them they were 

 provided with at that period in the days before Rankine. 



