﻿1861.] MTTRCHISON AND GEIKIE— STKATA OF THE HIGHLANDS. 237 



foliation." The latter structure is partially to be seen in the schistose 

 rocks on the coast to the south of Aberdeen, which are highly meta- 

 morphosed, and have never been examined by us ; but we have to 

 thank Mr. Sorby for the pertinent induction respecting them, " that 

 the peculiarities in the rocks having £ cleavage-foliation ' cannot be 

 explained except by supposing that they have been metamorphosed 

 stratified rocks," adding " that the cleavage-foliation is the effect of 

 previously existing cleavage, and not that slaty cleavage is a partially 

 developed foliation*." 



In dissenting from the views of Mr. Sharpe on this one essential 

 point, we must add, that there are many passages in his memoir 

 with which we entirely coincide. Thus, nothing is more true than 

 his declaration, that the geographical separation between gneiss and 

 mica-schist and chloritic schist throughout the north of Scotland, as 

 laid down by Macculloch, has been drawn too arbitrarily. "We fully 

 admit, and have shown, that many of the strata so defined by that 

 author have " the same geological relations." But when Mr. Sharpe 

 applied to their internal divisions the term "foliation," and, separating 

 it entirely from stratification, contended that such foliation is the 

 ultimate term of the same action which produced slaty cleavage, we 

 are completely at issue with him. 



Whether we admit, with Phillips f, Sharpe j, Sorby §, Tyndall||, 

 and others, that the parallel cleavage of rocks Avas produced by 

 mechanical lateral pressure of the strata, or, following the original 

 speculation of Sedgwick, conceive that it was the result of crystalline 

 and polar forces, it is quite obvious throughout the Highlands of 

 Scotland, that this action was not the same as the modus operandi, 

 whatever it was, which changed the original layers of sand, mud, 

 and lime into crystalline laminae. 



In short, as the one set of planes traverses the other, and as the 

 straight lines of parallel slaty cleavage (wherever they occur) are 

 strikingly distinguished from the convoluted and twisted layers of 

 different colour and composition, we are at a loss to imagine how 

 these two results could ever have been referred to different degrees 

 of intensity of one and the same cause. 



As we have already dilated on the true cleavage as affecting the 

 slaty rocks of May, Easdale, and the adjacent tracts of Argyll and 

 Ballyhulish, &c, it will be remembered that in rare instances only 

 does this feature coincide with the layers of deposit, any more than 

 in Wales and other slaty tracts. Even along the Highland border, 

 where Mr. Sharpe says that the foliation of the mica-schist and the 

 cleavage of the slate " are both vertical along lines so closely corre- 

 sponding that they may be considered continuous," we found in 

 the slate-quarries of Dunkeld that there was a clear and manifest 



* Report Brit. Assoc. Adv. of Science, 1850, Trans, of Sections, p. 78. 

 f See Report Brit. Assoc. Adv. of Science, 1843, Trans. Sect. p. 60 ; and 1857, 

 p. 386, &c. 



\ Quart. Joum. Geol. Soc. vol. iii. p. 74; vol. v. p. 111. 



§ Edinb. New Phil. Joum. 1853, vol. lv. p. 137 ; Phil. Mag. 1856, xii. p. 127. 



| Notices Royal Institution, June 6, 1856. 



