﻿240 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 20. 



character in their lateral extension, as well as in their succession 

 from quartz-rocks into mica-slates and vice versa, and seeing that in 

 some of their members they contain Silurian fossils, Ave have natu- 

 rally come to the conclusion, that on the whole the crystalline strati- 

 fied rocks of the Highlands are simply the metamorphosed equivalents 

 of the Silurian grauwacke of the Southern Scottish counties, in which 

 there is quite a large enough area of varied mineral matter to account 

 for the greater part of such stratified crystalline rocks, particularly 

 when we reflect on the enormous masses of granite, porphyry, and 

 syenite which prevail in the North. 



In conclusion, we express our belief that the so-called " foliation " 

 of the Highland rocks is, on the whole, nothing more than such an 

 alteration of the original deposits as caused the siliceous, felspathic, 

 and micaceous ingredients to form separate layers, as seen in some 

 instances, though in others they are intermixed in the same layer, 

 and in some parts are simply altered sandstones and limestones with 

 organic remains. In certain cases, like those pointed out by Mr. Sorby, 

 there may doubtless be cleavage-foliations in the mica-schist, but 

 these are clearly exceptions to the general rule. 



Although we cannot here enter upon the great problem of how 

 such changes in the character of the strata have been brought about, 

 yet, looking to those researches in this line which have already been 

 made (by M. Bischoff, M. Daubree, M. Delesse, M. Deville, Mr. Sterry 

 Hunt, and others), we confidently anticipate that the experiments of 

 the chemical geologist will ere long solve the mystery, and in doing 

 so we trust that they will confirm the conclusions at which we have 

 arrived by a survey of the phenomena in the field. 



