170 Peter Macnair — Altered Clastic Rocks 



In the last edition of his textbook, 1 published 1893, Sir A. Geikie 

 gives a short account of the work of the Geological Survey amongst 

 the rocks of the Central and Southern Highlands. He there says it is 

 deserving of remark that the rocks along the southern margin of the 

 Highlands are for the most part so little aifected as closely to resemble 

 portions of the unaltered Silurian series of the South of Scotland, and 

 that they dip towards the mountains, being more highly foliated 

 and crystalline as they recede from the Lowlands. This may be 

 practically said to be all that is at present known of the strati- 

 graphical affinities of these rocks, a fact which was pointed out 

 nearly fifty-one years ago by James Nicol ; and from this fact, 

 with his description of the Southern Highlands and his theory of 

 the north-west, taken together, it will be found that, generally 

 speaking, our knowledge of the geological structure of the Scottish 

 Highlands has not much advanced beyond where Nicol left 

 it. It is quite evident from his writings that he saw clearly 

 the fallacy of Sir E. Murchison's observations and deductions as 

 to the supposed north-west succession, and this being so it left 

 him untrammelled in his descriptions of the other parts of the 

 Highlands, so that we find them, as I have already said, perfect 

 transcriptions of what he saw in nature. He states his position 

 on this point very clearly in the conclusion of his famous paper. 2 

 There is no evidence, he says, to connect the great mass of 

 crystalline schists, stretching from the north coast of Sutherland 

 to the south of Inverness-shire, more closely with the mica-slates of 

 Ben Hope than with the gneiss of Scourie Loch, Inver, and 

 the Gairloch, or to justify us in throwing aside mineral characters 

 for some assumed synchronism in the age of the original but now 

 wholly altered deposits. 



III. Lowee Argillaceous Zone. 



At the end of the paper I have bracketed together into zones 

 and given in the form of a table the chief rock groups occurring 

 in the Southern Highlands. Perhaps a word explaining the table 

 would be of advantage here. The table is a descending one, the 

 highest rocks being placed at the top, the lowest at the bottom. 

 They have been grouped together into zones under their larger 

 lithological features, which will be found to be more or less 

 characteristic of each rock-bed. As I mentioned before, these zones 

 have principally been determined from sections in Perthshire, but 

 they undoubtedly range across the country from shore to shore in 

 a succession similar to that occurring in Perthshire. The lowest 

 member of the series (the lower argillaceous zone) given in 

 my table is one whose lithological features are so well known 

 as scarcely to need description. Outcropping at almost the base of 

 the exposed rocks of the Southern Highlands, it strikes across their 

 flanks from north-east to south-west. It does not, however, mark 



1 Textbook of Geology, 3rd edition, pp. 627, 708. 



2 " North-west Highlands " : Quart. Journ. Geol. Soc, vol. xvii, 1860. 



