38 PALEOZOIC TIME. [CHAP. III. 



the opinion expressed by Sir R. Murchison, that a large 

 part of the Arctic region was land in Lower Silurian times, 

 because no rocks of that age had been discovered there. 

 Nor have any been found since this opinion was expressed, 

 though (Upper) Silurian rocks cover large areas in the 

 north-western part of the Arctic Archipelago. 



It seems probable, therefore, that the great northern 

 continent of early Cambrian times was partially broken up 

 by the submergence which ensued, and that an open sea 

 extending from North America to Scotland and Scandinavia- 

 was established for the first time at the beginning of the 

 Ordovician period; that this sea' divided the Cambrian 

 continent into two parts, the larger part lying over Green- 

 land and the northern regions, while a smaller mass lay to 

 the south, and stretched eastward through the north of 

 Ireland and into the south of Scotland, so as to separate 

 the northern sea from that which lay over the European 

 area. The thick conglomerates of Llandeilo and Bala age 

 which occur in the Girvan district may indicate a part of 

 its southern border, but of its northern shore we have 

 absolutely no evidence at present. Mr. J. E. Marr takes a, 

 somewhat similar view, 1 and has suggested the existence of 

 a ridge of pre-Cambrian land across the centre of Scotland 

 in Arenig times, separating the sea in which the Orthoceras; 

 and Durness limestones were formed from that of the 

 Girvan area. It is possible, therefore, that the land barrier 

 had a still further eastward extension, but until more is 

 known of the geology of the central Highlands this must 

 remain uncertain, and the form given to the land in the 

 accompanying map is of course merely a suggestion based 

 upon the preceding considerations. The map is in fact only 

 a pictorial representation of the theory suggested by the 

 facts which are known to us, and may require considerable 

 modification as our knowledge increases, and the same may 

 1 "Classif. of Camb. and Sil. Rocks," p. 68. 



