CHAP. II.] CAMBRIAN PERIOD. 21 



In Scotland we once more find rocks resting on un- . 

 doubted Archaean and covered by Ordovician, but they 

 are very different lithologically from the Cambrian of 

 Wales. They consist entirely of red felspathic sandstones, 

 with beds of breccia and conglomerate at the base, and 

 are in places from 3,000 to 4,000 feet thick. This Torridon 

 Sandstone is probably of early Cambrian age, and has 

 evidently been formed against an Archaean coast-line. Its 

 composition is thus described by Professor Bonney l : "Its 

 coarser basement beds are crowded with fragments of the 

 underlying gneisses and schists, and since the epoch of 

 their formation no important change has taken place in 

 either the one or the other. The finer beds, though other 

 materials occasionally occur, are largely, sometimes almost 

 exclusively, composed of grains of quartz and of felspar 

 identical in every respect with those of the underlying 

 series. It may be a fact of some significance, for it agrees 

 with what I have elsewhere noticed in very old f ragmental 

 rocks, that the felspar appears to have been broken off 

 from the parent rock while still undecomposed, and in 

 many cases is even now remarkably well preserved. It 

 would seem, therefore, as if the denudation of the granitoid 

 rock had been accomplished without material decomposition 

 of its felspar ; but I must not allow myself to digress into 

 speculations on this interesting and suggestive fact." 



The Torridon Sandstone has not, however, been entirely 

 derived from the Archaean gneiss. Mr. B. N. Peach states 

 that in some parts of Sutherland the basal conglomerate 

 consists chiefly of stones derived from older sedimentary 

 rocks, such as greywacke, quartzite, hardened shales, and 

 cherty limestones, together with a few pebbles of slaggy 

 diabase lava. 



Professor Judd remarks 2 : " These rocks in their cha- 



1 Pres. Address to Geol. Sec. of Brit. Assoc., 1886. 



2 Pres. Address to Geol. Sec. of Brit. Assoc., 1885. 



