2g 6 NATURAL SCIENCE. Oct., 



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 stratifi- 

 cation, 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 igneous products, it seems equally reason- 

 able 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 composi- 

 tion. 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 perido- 

 tites ; 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." 



"... The origin of gneisses and schists, in my opinion, is to 

 be sought for in a combination of the thermal and dynamic agencies 

 which may be reasonably supposed to operate in the deeper zones of 

 the earth's crust. If this view be correct, it is not improbable that 

 we may have crystalline schists and gneisses of post-Silurian age in 

 the North-West of Europe formed during the Caledonian folding, 

 others in Central Europe of post-Devonian age due to the Hercynian 

 folding, and yet others in Southern Europe of post-Cretaceous 

 age produced in connection with the Alpine folding. But if the 

 existence of such schists should ultimately be established, it will still 

 probably remain true that rocks of this character are in most cases 

 of pre-Cambrian age. May not this be due to the fact, suggested by 

 a consideration of the biological evidence, that the time covered by 

 our fossiliferous records is but a small fraction of that during 

 which the present physical conditions have remained practically 

 constant ?" 



