THE HISTORY OF THE NORTH ATLANTIC 7 1 



period was probably limited, for the most part, to high lati- 

 tudes, and its aspect, though more rugged and abrupt, and 

 of greater elevation, must have been of that character which 

 we still see in the Laurentian hills. The distribution of this 

 ancient land is indicated by the long lines oi old Laurentian 

 rock extending from the Labrador coast and the north shore 

 of the St. Lawrence, and along the eastern slopes of the 

 Appalachians in America, and the like rocks of the Hebrides, 

 the Western Highlands, and the Scandinavian mountains. A 

 small but interesting remnant is that in the Malvern Hills, so 

 well described by Holl. It will be well to note here, and to 

 fix on our minds, that these ancient ridges of Eastern America 

 and Western Europe have been greatly denuded and wasted 

 since Laurentian times, and that it is along their eastern sides 

 that the greatest sedimentary accumulations have been de- 

 posited. 



From this time dates the introduction of that dominance of 

 existing causes which forms the basis of uniformitarianism in 

 geology, and which had to go on with various and great modi- 

 fications of detail, through the successive stages of the geolo- 

 gical history, till the land and water of the northern hemisphere 

 attained to their present complex structure. 



So soon as we have a circumpolar belt or patches of Eozoic^ 

 land and ridges running southward from it, we enter on new 

 and more complicated methods of growth of the continents 

 and seas. Portions of the oldest crystalline rocks, raised out 

 of the protecting water, were now eroded by atmospheric 

 agents, and especially by the carbonic acid, then existing in the 

 atmosphere perhaps more abundantly than at present, under 

 whose influence the hardest of the gneissic rocks gradually 

 decay. The arctic lands were subjected, in addition, to the 

 powerful mechanical force of frost and thaw. Thus every 

 shower of rain and every swollen stream would carry into the 

 * Or Archaean, or pre-Cambrian, if these terms are preferred. 



