394 



PALEOZOIC TIME. 



time, they continued to act in the same general direction throughout 

 the Paleozoic. The action of the two systems of forces together 

 evidently produced the great amount of subsidence adjoining the 

 Canada Archaean, where the thick deposits of the Huronian and Lower 

 Silurian periods were formed, and where, finally, the basins of the Great 

 Lakes were made. These and many other lakes of North America lie 

 near the limit between the oscillating part of the continent and the 

 stable Archaean area, and to this fact owe their formation. 



5. Cotemporaneous movements in the American and European con- 

 tinents. — The fact that the continent of Europe was above the ocean, 

 and in that condition which was characteristic of the Coal period, at 

 the same time with North America, shows a cotemporaneousness in 

 the oscillations of the crust on the opposite sides of the Atlantic 

 Ocean. This concordance will be better apprehended, when it is con- 

 sidered that the land must have been but little elevated, and quite 

 uniformly so, — enough to drain the great salt marshes of their salt, 

 and not so high as to turn them into dry fields. It was not sufficient 

 that there should be land and Carboniferous vegetation ; for, without 

 the wet, swampy lands, — wet with fresh waters, and very wide in ex- 

 tent, — the great accumulations of vegetation and immense coal fields 

 would not have been made. 



There is a similarity between the continents, also, in the character of 

 the oscillations which occurred in the course of the Carboniferous 

 period, which submerged the land after material for a coal bed had 

 accumulated, and buried it for long keeping beneath sands, muds, or 

 clays, and then brought it again to the surface for renewed verdure 

 and another coal bed ; and so on, in many successions. 



The Millstone grit, which preceded the Coal-measures in Europe as 

 well as America, is evidence of a degree of correspondence in that 

 upward movement of the continents through the waves which ushered 

 in the epoch of the Coal-measures ; and the prevalence and wide dis- 

 tribution of the limestone of the Subcarboniferous period, which next 

 preceded, mark another cotemporaneous movement, — a very general 

 submergence, preceding the emergence just alluded to. Moreover, in 

 both continents, some thin coal beds were formed in the Subcarbon- 

 iferous period. 



Contrast between America and Europe. — While the two continents 

 were at times concordant in their general movement, there was ap- 

 parently a contrast during the Coal period in the moisture of the two, 

 which may in part, at least, be attributed to climate. This is apparent 

 in the vastly larger coal fields of America. Guyot has called America 

 the forest-continent, a character it now bears because of its moist climate, 

 or more abundant rains ; and it is probable that it presented this 

 peculiarity with the first appearance of vegetation over its surface. 



