September 21, 1893] 



NA TURE 



489 



which are obviously unlike any of the products of surface 

 agencies. If these rocks are forming now, it must be beneath 

 the surface. This pDint was clearly realised by Hutton. 

 Granite was proved by him to be an igneous rock of subterranean 

 origin. His conclusions as to the formation of the schists are 

 expressed in a passage so remarkable when viewed in connection 

 with what I regard as the tendency of modern research, that I 

 make no apology for quoting it at length. "If, in examining 

 our land, we shall find amassof matter which had been evidently 

 formed originally in the ordinary manner of stratification, but 

 which is now extremely distorted in its structure, and displaced 

 in its position — which is also extremely consolidated in its mass, 

 and variously changed in its composition — which therefore has 

 the marks of its original or marine composition extremely 

 obliterated, and many subsequent veins of melted mineral 

 matter interjected ; we should then have reason to suppose that 

 here were masses of matter which, though not difl"erent in their 

 origin from those that are gradually deposited at the bottom of 

 the ocean, have been more acted upon by subterranean heat and 

 the expanding power, that is to say, have been changed in a 

 greater degree by the operations of tlie mineral region. If this 

 conclusion shall be thought reasonable, then here is an ex- 

 olanation of all the peculiar appearances of the Alpine schistus 

 masses of our land, those parts which have been erroneously con- 

 sidered as primitive in the constitution of the earth ("Theory 

 of the Earth," vol. i. p. 375). Surely it is not claiming too 

 much for our author to say that we have there, sketched in 

 broad outline, the theories of thermal and dynamic meta- 

 morphism which are attracting so much attention at the present 



day. 



The hypogene origin of the normal plutonic rocks and their 

 formation at different periods, even as late as the Tertiary, are 

 fects which are now so generally recognised that we may leave 

 these rocks without further comment and pass on to the con- 

 sideration of the crystalline schists. 



Everyone knows that the statement, " He who runs may 

 read," is untrue when the stratigraphical interpretation of an 

 intensely folded and faulted district is concerned. The com- 

 plexity produced by the earth-movements in such regions can 

 only be unravelled by detailed work after definite palaeonto- 

 logical and lilhological horizons have been established. But if 

 the statement be untrue when applied to districts composed of 

 ordinary stratified rocks, still less can it be true of regions of 

 crystalline schist where the movements have often been much 

 more intense ; where the original characters of the rocks have 

 been profoundly modified ; and where all distinct traces of 

 fossils have in most cases been obliterated. If detailed work 

 like that of Prof. Lapworth at Dobb's Linn was n quired to 

 solve the stratigraphical difficulties of the Southern Uplands, is 

 it not probable that even more detailed work will be required to 

 solve the structural problems of such a district as the Highlands 

 of Scotland, where the earth-stresses, though somewhat similar, 

 have operated «ith greater intensity, and where the injection of 

 molten mineral matter has taken place more than once both on 

 a large and on a small scale ? With these few general remarks 

 by way of introduction, I will now call attention to what appear 

 to me to be the most promising lines of investigation in this 

 department of geology. 



The crystalline schists certainly do not form a natural group. 

 Some are undoubtedly plutonic igneous rocks showing original 

 fluxion ; others are igneous rocks which have been deformed by 

 earth-stresses subsequent to consolidation ; others, again, are 

 sedimentary rocks metamorphosed by dynamic and thermal 

 agencies, and more or less injected with "molten mineral 

 matter" ; and lastly, some cannot he classified with certainty 

 under any of these heads. So much being granted, it is obvious 

 that we must deal with this petrographical complex by separat- 

 ing from it those rocks about the origin of which there can be 

 no reasonable doubt. Until this separation has been effected, 

 it is quite impossible to discuss with profit the question as to 

 whether any portions of the primitive crust remain. In order 

 lo carry out this work it is necessary to establish some criterion 

 by which the rocks of igneous may be separated from those of 

 sedimentary origin. Such a criterion may, I think, be found, 

 al any rate in many cases, by combining chemical with field 

 evidence.' If associated rocks possess the composition of grits, 

 sandstones, shales and limestones, and contain also traces of 

 stratification, it .'eems perfectly justifiable to conclude that they 



^ H. Rosenbusch, " Ziir AuflTassiin? der chemischen Natur dcs Grundge- 

 fairges," Min. und fietro. Mitth., xii. (1891), p. 49. 



NO. 1247, VOL. 48] 



must have been originally formed by processes of denudation 

 and deposition. That we have such rocks in the Alps and in 

 the Central Highlands of Scotland, to mention only two 

 localities, will be admitted by all who are familiar with those 

 regions. Again, if the associated rocks possess the composition 

 of igneous products, it seems equally reasonable to conclude that 

 they are of igneous origin. Such a series we find in the North- 

 west of Scotland, in the Malvern Hills, and at the Lizard. 

 In applying the test of chemical composition it is very necessary 

 to remember that it must be based, not on a comparison of 

 individual specimens, but of groups of specimens. A granite 

 and an arkose, a granitic gneiss and a gneiss formed by the 

 metamorphosis of a grit, may agree in chemical and even in 

 mineralogical composition. The chemical test would there- 

 fore utterly fail if employed for the purpose of dis- 

 criminating between these rocks. But when we introduce 

 the principle of paragenesis it enables us in many cases 

 to distinguish between them. The granitic gneiss will be 

 associated with rocks having the composition of diorites, gab- 

 bros, and peridotites ; the sedimentary gneiss with rocks 

 answering to sandstones, shales, and limestones. Apply this 

 test to the gneisses of Scotland, and I believe it will be found 

 in many cases to furnish a solution of the problem. Caution, 

 however, is necessary ; for crystal- building and the formation 

 of segregation veins, and patches in the sedimentary schists 

 clearly prove that a migration of constituents takes place under 

 certain circumstances. 



Recent work on the gneisses and schists of igneous composi- 

 tion has shown that the parallel structure, by no means invari- 

 ably present, is sometimes the result of fluxion during the final 

 stages of consolidation, and sometimes due to the plastic deform- 

 ation of solid rocks. When compared with masses of ordinary 

 plutonic rock, the principal points of difference, apart from 

 those due to secondary dynamic causes, depend on what may 

 be called their extreme petrographical differentiation. Indica- 

 tions of differentiation may, however, be seen in the contempo- 

 raneous veins and basic patches so common in ordinary irrup- 

 tive bosses, but they are never so marked as ingneissic regions, 

 like those of the North-West of Scotland, where specimens 

 answering in composition to granite?, diorites, and even peri- 

 dotites, may be collected repeatedly in very limited areas. 

 The nearest approach to the conditions of gneissose regions is 

 to be found in connected masses of diverse plutonic rocks, such 

 as those which are sometimes found on the borders of great 

 granitic intrusions. 



The tectonic relations of those gneisses which resemble 

 igneous rocks in composition fully bear out the plutonic theory 

 as to their origin. Thus, the intrusive character of granitic 

 gneiss in a portion of the Himalayas has been demonstrated by 

 General McMahon. ' The protogine of Mont Blanc has been 

 investigated by M. Levy- with the same result. Most signifi- 

 cant of all are the discoveries in the vast Archiean region of 

 Canada. Professor Lawson^ has shown that immense areas of 

 the so-called Laurentian gneiss in the district north west of Lake 

 Superior ai-e intrusive in the surrounding rocks, and therefore 

 newer, not older, than these. Professor Adams'* has quite re- 

 cently established a similar fact as regards the anorthosile rocks 

 — the so-called Norian— of the Saguenay River and other dis- 

 tricts lying near the eastern margin of the " Canadian shield." 

 Now that the intrusive character of so many gneisses is being 

 recognised, one wonders where the tide of discovery will stop. 

 How long will it be before the existence of gneisses of Tertiary 

 age will be generally admitted ? At any rate, the discoveries of 

 recent years have compelled the followers of Wernerian methods 

 to evacuate large slices of territory. 



Turning now to the gneisses and schists which re.semble 

 sedimentary rocks in composition, we note that the parallel 

 stiucture may be due to original stratification, to subsequent 

 deformation, or to both of these agencies combined. It must 

 also be remembered that they have often been injected with 

 igneous material, as Hutton pointed out. Where this has fol- 

 lowed parallel planes of weakness, we have a banding due to 

 alternations of igneous and sedimentary material. This injection 



^ ''The Geology of DsAhou^i^," Records ofGcol. Survey of India^ vol. xv 

 part 1 (1882), p 3^. See also vol. xvi. part 3(1883), p. 129. 



^ "Les Rouhcs Crystallines et Erupiives des Isnvirons du Mont-Blanc," 

 Bull, des Serz'ices de la Carte G^olosiqne de la France, No. g (1890). 



^ "On ttie Geulogy of the Kainy Lake Region," AttftJtol Report Geol 

 Survey of Canada for 1887, 



■* " LJeber da-i Norian Oder Obcr-T-aurentian vonCanada," -V^wcf /a^f 

 btichf. Minerah^ict &c., Heilage, Band viii. p. 419. 



