212 PALEOZOIC TIME. 



of the surface of the continent; for this is indicated in the varying lim- 

 its of the formations, as well as the alternations in the kinds of rocks. 

 One marked exception to the general quiet occurred during some 

 part of the Canadian period, in the region of Lake Superior, where 

 there wei'e extensive igneous ejections (p. 185), — events probably 

 connected with the deepening of the Lake Superior basin. Another 

 case of disturbance has been noted in Newfoundland (p. 181). It 

 occurred in the course of the Primordial, the lower beds of this 

 period having been upturned before those following were laid down ; 

 in other words, those of the two being unconformable. Indications 

 of probable disturbances in the Rocky Mountains are mentioned on 

 page 182; and the wide extermination of species that several times 

 took place show that there was change and catastrophe. But still it 

 remains a fact that the Lower Silurian was an era of comparative 

 quiet. This quiet, moreover, was a very long one, — probably two 

 thirds as long as all of the time that has since elapsed. 



5. DISTURBANCES AT THE CLOSE OF THE LOWER SILURIAN. 



The rocks of the Lower Silurian having been laid down over the 

 New England and other North American areas, the long quiet was 

 finally interrupted, in some parts of the continent, by subterranean 

 movements and metamorphism, — not by sudden catastrophe, but, 

 after the ordinary style in geological progress, by slow and gradual 

 change. The principal regions of this change, now known, are that 

 of the Green Mountains, the northern extremity of the Appalachian 

 region, and that of the " Cincinnati uplift," from Lake Erie, over the 

 Cincinnati region, into Tennessee. 



Previous to the epoch of revolution, the Green Mountain area had 

 been a region of accumulating limestones, through the Canadian and 

 Trenton periods, and of beds of quartzose sands and mud, and prob- 

 ably some limestone, through the Cincinnati era. But here the rock- 

 making over the region ended; next came the upturning, in which 

 the same rocks were lifted and folded and crystallized, and the Green 

 Mountain region made dry land. 



1. The fact of the Green Mountain revolution is manifested in, — 

 (a.) The present position of the rocks. — The strata were originally 

 horizontal. They are now upturned over the whole of the wide region 

 described, some portions standing vertical, the larger part inclined 

 30° to 60°, yet varying, occasionally at short intervals, from 10° to 

 90° ; the beds rising and descending in great folds. Moreover, the 

 whole series of beds, to the very bottom of the Silurian, if not to 

 lower depths, were involved together in the upturning. 



