462 Notices of Memoirs — Dundee, 1912 — 



and limestones contain a typical development of the molluscan fauna 

 of the Beekmantown Limestone, belonging to the western life-province 

 of North America. As the Beekmantown Limestone is succeeded by- 

 shales, with Arenig Graptolites, it follows, in accordance with British 

 classification, that these groups must be of Upper Cambrian age. 



5. The highest subdivision of the Durness Limestone (Durine) has 

 not yielded fossils of zonal value, and the members of this group are 

 not overlain in normal sequence by graptolite-bearing shale or other 

 sediments. 



Cambrian TalcBogeogiaphy between North America and North-W est Ei<i ope. 



In attempting to restore in outline the distribution of land and sea 

 in Cambrian time between North America and North-West Europe, 

 reference must be made to various investigators whose researches in 

 palfEOgeography are more or less familiar to geologists. Among these 

 may be mentioned Suess, Dana, De Lapparent, Freeh, Walcott, Ulrich, 

 Schuchert, Bailey Willis, Grabau, Hull, and Jukes-Browne. The views 

 now presented seem to me to be reasonable inferences from the 

 pala^ontological evidence set forth in this address. 



In the North-West Highlands there is still a remnant of the old 

 land surface upon which the Torridonian sediments were laid down. 

 There is conclusive evidence that the pre-Torridonian land was one of 

 high relief. As the Torridonian sediments form part of a continental 

 deposit it may be inferred that the Archaean rocks had a great 

 extension in a north-westerly direction. The increasing: coarseness of 

 the deposits towards the north-west suggests that the land may have 

 become more elevated in that direction. At any rate, the pile of 

 Torridonian sediments poirits to a subsidence of the region towards the 

 south-east, and probably to a correlative movement of elevation towards 

 the north-west. 



The sparagmite of Scandinavia is an arkose resembling the dominant 

 type of the Torridon Sandstone ; is of the same general age, and has 

 evidently been derived from similar sources in the Scandinavian shield- 

 In eastern North America coarse sedimentary deposits form part of 

 the newer Algonkian rocks, which are still to be found rising from 

 underneath the Cambrian strata in the region of the great lakes- 

 These materials were obtained from the great Canadian shield, which 

 must have formed a large continental area during their deposition. 



It is reasonable to infer that these isolated relics of old land surfaces 

 were united in pre-Torridonian time, thus forming a continuous belt 

 from Scandinavia to North America. During the period which elapsed 

 between the deposition of the Torridon Sandstone and the basement 

 members of the Cambrian system a geosyncline was established which 

 gave rise to a submarine trough, trending in an east-north-east and 

 west-south-west direction, both in the British and North American 

 areas. In the latter region it extends from Newfoundland to Alabama, 

 its south-eastern limit being defined by the old land surface of 

 Appalachia. The extension of this Appalachian land area in a north- 

 east direction beyond the limits of Nova Scotia and Newfoundland 

 was postulated by Dana and other American writers. This geosyncline 

 remainetl a line of weakness throughout Palceozoic time, both in Britain 

 and North i^bnerica, which resulted in the Caledonian system of folding 

 in Britain, and in the TaconiCj Appalachian, and Pennsylvanian systems 

 in North America. Hence it is manifest that the original shore-linea 

 of this trough are now much nearer each other than they were in 

 Cambrian time. 



The Cambrian rocks of the North-West Highlands were laid down 

 along the north-west side of this trough during a period of subsidence, 

 for the great succession of Durness dolomite and limestone, with little 

 or no terrigenous material, is superimposed on the coarser sediments of 



