740 EEroET— 1893. 



considered as piimitive in the constitution of tlie earth.' ' 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 metamorphism 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 facts which are now so generally 

 recoo-nised that we may leave these rocks without further comment and pass on to 

 the consideration of the crystalline schists. 



Everyone Imows that the statement, ' lie who runs may read,' is untrue when 

 the stratigraphical interpretation of an intensely folded and faulted district is 

 concerned. The complexity produced by the earth-movements in such regions 

 can only be unravelled by detailed work after definite palajontological and litho- 

 logical 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 Professor Lapworth at Dobb's Linn was required 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 some- 

 what similar, have operated with 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 un- 

 doubtedly 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 be 

 classified with certainty under any of these heads. So much being granted, it is 

 obvious that we must deal with this petrographical complex by separating 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 to 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, at 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 seems perfectly justifiable to conclude 

 that they 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 

 io-neous 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 neces- 

 sary to remember that it must be based, not on a comparison of individual speci- 

 mens, 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 therefore utterly fail if 

 employed for the purpose of discriminating 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, gabbros, and peridotites ; the sedimentary gneiss, with rocks 



' Theory of the Earth, vol. i. p. 375. 



2 H. Rosenbusch, ' Zur Auflassung der chemischen Natur des Grundgebirges,' Min. 

 undpetro. Mitth., sii. (1891), p. 49. 



