198 PALAEOZOIC TIME LOWER SILURIAN. 



large Trilobites indicate that the beds are rather the accumulations 

 of sheltered bays than of deeper off-shore waters. The fossils of 

 Swanton, Vermont, suggest the same conclusion. The interca- 

 lated sandstones and conglomerates, and even the limestones 

 of the Quebec group, Taconic rocks, and the Primordial series in 

 other parts of the Appalachians, require, for each, moderate depths 

 at intervals, in the progress of the great formation. 



But, taking the facts as allowing of even a thousand feet of depth 

 for the deposition of a large part of the shales (instead of the one 

 to four or five hundred feet which we deem more probable), there 

 are still several multiples of this thickness to account for along this 

 Appalachian region. The facts, therefore, show that this border 

 portion of the continent must have been subject to great oscilla- 

 tions, resulting in a subsidence nearly equal to the thickness of the 

 deposit. The oscillations were such as would produce the alterna- 

 tions of rock in the series, bringing the land near or to the surface 

 for the accumulations of sandstones and conglomerates, and de- 

 pressing it again somewhat for the shales. These were early move- 

 ments in that system of change which resulted in the evolution of 

 the Appalachian chain. 



The transition from the region of these oscillations to the interior 

 basin is singularly abrupt, as shown by the wonderful contrast in 

 the thickness of the strata. This contrast has been made still 

 stronger in later time by the foldings and disturbances along the 

 Appalachians. The boundary between the two is the course of a 

 series of great faults in the strata, — as has been pointed out by 

 different geologists, — which follows the general direction of the 

 Appalachian chain. Directly on its course are the waters of the 

 Hudson and Lake Champlain. From the north extremity of 

 Lake Champlain, as Logan has mentioned, the line stretches north 

 to Quebec, on the St. Lawrence, thence follows the river, and, 

 finally, bends around to Gaspe on the Gulf of St. Lawrence. In 

 consequence of the faulting, the rocks of the Potsdam period on 

 the -east side were raised several thousand feet above the level 

 of those on the west. The existence of this line of faults was 

 predetermined in the oscillations ; but when it was made is not 

 ascertained. It was natural that the oscillation of the unstable 

 border of the continent against the more stable interior should 

 sooner or later have ended in such a catastrophe. 



(5.) Change of condition between the Potsdam and Galciferous epochs. — 

 In passing from the Potsdam to the Galciferous epoch, there was 

 a change in the condition of the continental seas, so that the rocks 

 afterwards made were to a considerable extent limestone: it is 



