THE PRE- CAMBRIAN ROCKS, BRITISH ISLES. 3 



same basement members of the Cambrian system are persistent 

 and lie directly upon the pre-Cambrian series. 



Lezvisian Gneiss. Ever since the researches of Murchison and 

 Nicol in the north-west of Scotland, it has been known that two 

 distinct systems of rock underlie the quartzites to which I have 

 just alluded. Murchison regarded the upper of these as of 

 Cambrian age, while he assigned the unconformable quartzites 

 and limestones above it to the Lower Silurian period. But the 

 recent discovery of the Olenelliis zone intercalated conformably 

 between the quartzites and the overlying limestones may be 

 regarded as proving that all the rocks which underlie the quart- 

 zites and are separated from them by a strong unconformability 

 must be pre-Cambrian. It is thus established beyond any 

 reasonable doubt that two great pre-Cambrian systems of rock 

 exist in the north-west of Scotland. 



These two systems differ so entirely from each other that 

 their respective areas can be defined with minute accuracy. The 

 uppermost consists chiefly of dull reddish sandstones with con- 

 glomerates, and especially towards their base in Rosshire, some 

 bands of dark grey shale, the whole having a thickness of at 

 least 8,000 or 10,000 feet, though as both the base and the top 

 of the series are marked by strong unconformabilities, the whole 

 original thickness of deposits is nowhere seen. As these rocks 

 are well developed around Loch Torridon, they were named by 

 Nicol the Torridon Sandstone — a designation which has more 

 recently been shortened into "Torridonian." The lower system 

 is mainly composed of various foliated rocks which may be 

 embraced under the general term "gneiss." These masses pre- 

 sent the usual characters of the so-called "fundamental 

 complex" "Urgebirge," or "Archaean Series" of other countries. 

 The contrast between the thoroughly crystalline, gnarled, ancient- 

 looking gneisses below, and the overlying, nearly horizontal 

 Torridonian conglomerates, sandstones, and shales, which are 

 largely made out of their debris, is so striking that every ob- 

 server feels persuaded that in any logical S3''stem of classification 

 they can not be both placed in the same division of the geolog- 



