GEOLOGY OF NORTH ATLANTIC GILLIGAN 213 



the underlying floor of Archean or Lewisian gneiss. This ancient 

 land surface can be seen rising into hills 2,000 feet high, with the 

 corresponding valleys, and the Torridonian infills the hollows and 

 mantles round the hills, from which it is again being rapidly 

 removed. 



The description of the constituents of the coarser beds given in the 

 Geological Survey Memoir on the Northwest Highlands points un- 

 doubtedly to the greater part of this vast accumulation of sediments 

 having been derived from some area of land differing in rock types 

 present from those of the older rocks of Scotland, upon which it 

 now rests. Amongst the pebbles are found many which have been 

 derived from sedimentary formations of an earlier age than the 

 Torridonian sandstone, as well as pebbles of volcanic rocks such 

 as spherulitic felsites and feldspar porphyries, some of which show 

 striking resemblances to the Uriconian volcanic rocks of Shropshire 

 and must have been derived from areas of volcanic rocks of which no 

 other trace is found in the northwest of Scotland. The principal 

 feldspars are microcline, microcline-microperthite, orthoclase, and 

 oligoclase, of which microcline is by far the most abundant and the 

 least altered. The heavy minerals in their order of abundance are : 

 garnet, zircon, magnetite, ilmenite, spliene and rutile. Monazite 

 has also been found by Doctor Mackie. 



Taken altogether the assemblage is quite unlike that which would 

 have been yielded by a land surface of Lewisian gneiss similar to 

 that found in Scotland. Where the Torridonian is best developed, as 

 in Southwest Ross-shire and in Skye, it has a total thickness of nearly 

 3 miles, and extends for 100 miles from Cape Wrath in a south- 

 westerly direction to the Isle of Skye, with outlying masses on the 

 shores of Broad Bay, Isle of Lewis. Doctor Peach was of the opin- 

 ion that the source of origin of the material lay to the northwest of 

 the present Scotland. 



In Scandanavia the metamorphosed rocks of pre-Cambrian age com- 

 prise a much more numerous suite of rocks than in Scotland, amongst 

 which are great thicknesses of altered sedimentary rocks, but here 

 again they are overlain by a group of red arkoses and sandstones 

 thousands of feet in thickness which cover a wide extent of country 

 in the heart of Norway, north of Oslo. This series of rocks is known 

 as Sparagmite and Dala sandstone and bears a striking resemblance 

 to the Torridonian of Scotland. The mineralogical composition of 

 these rocks proves to be quite unlike the older granites and gneisses 

 of the peninsula, and the assumption is that it, too, was derived from 

 some land surface, at present unknown, which lay to the northwest. 

 In the type region of North America there is still some doubt as to 



