﻿1861.] MUKCHISON AND GEIKIE STRATA OP THE HIGHLANDS. 233 



rate communication. At the same time we must specially refer to 

 our preceding memoirs on the structure of the Highland crystalline 

 stratified rocks, in which we have shown that the different mineral 

 formations which succeed to each other offer distinct proofs of 

 original deposit under water, some of them even containing organic 

 remains. Our conviction of the truth of this view has been arrived 

 at, not by any theoretical hypothesis, but by repeated and long- 

 continued observations of the succession and nature of the stratified 

 Highland rocks ; and hence we are under the necessity of recording 

 our dissent from the opinions of a distinguished geologist now no 

 more, who, in a memoir published in the ' Philosophical Trans- 

 actions,' and illustrated by a map and sections, has endeavoured to 

 prove that the very lines which we refer to deposit are lines of a 

 foliation which is the ultimate stage of the action of cleavage. We, 

 on the contrary, referring to our former memoirs for our proofs of 

 sedimentary deposit, will now point out that wherever cleavage 

 does exist in these Highland rocks it is transverse to those laminae 

 which have been referred to foliation by Mr. Sharpe, and which 

 we consider to be simply crystallized stratification. 



In the early days of Scottish geology, it was shown by the illus- 

 trious Hutton and his contemporaries, that the gneiss and schists 

 of the Highlands were truly sedimentary formations, and that their 

 present crystalline structure arose from the action of heat at a 

 period posterior to their deposition. Subsequently the same views 

 were entertained and promulgated on all occasions by Macculloch, 

 during and after his numerous explorations of the Highlands. 

 Those eminent geologists and their successors, during many years, 

 never doubted that the planes or lamina? of the schistose rocks 

 represented former lines of stratification, and they wrote of the dip 

 and " stretch " or strike of these altered rocks precisely as they 

 would have done had they been treating of unaltered sandstones, 

 shales, and limestones. But, after the publication, in 1846, of Mr. 

 Darwin's observations on the metamorphic rocks of South America, 

 attention was drawn more pointedly to the structure and origin of this 

 class of rocks. That distinguished naturalist believed that, in the re- 

 gions of Chili and Tierra del Fuego, the planes along which the sepa- 

 ration of the crystalhne particles had proceeded did not coincide with 

 the original planes of bedding, but with the planes of cleavage. He 

 proposed to apply the term " foliation " to the laminar arrangement 

 of gneiss and schist — a term to which, if used merely to express 

 this arrangement, without reference to its origin, we have no objec- 

 tion. But Mr. Darwin considered that his observations bore out 

 the general inference, that foliation and cleavage are parts of the 

 same process, — " in cleavage, there being only an incipient sepa- 

 ration of the constituent minerals ; in foliation, a much more com- 

 plete separation and crystallization*:" and he applied this theory 

 to the elucidation of large tracts of South America. 



Kow, however this doctrine may be borne out in regions which 

 we have not seen, we hold that it is utterly inapplicable to the 

 * Darwin, Q-eol. Obs. South America, pp. 156-16u. 



