GEOLOGICAL HISTORY OF NEW YORK. 643 



During the Palaeozoic ages, the New York ridge seems to have been 

 a land-surface ; for the Silurian, Devonian, and Carboniferous rocks were 

 deposited on both sides of it in New England, New York, and Penn- 

 sylvania, but no traces of them have been found upon it. In each of 

 these ages the sea flowed in over some portion of the continent, and 

 deposited on the inundated surfaces sediments containing more or less 

 complete representatives of the prevalent forms of life ; and these, now 

 fossilized, afford means for identifying and classifying the strata. 



In the Cambrian age the continent, composed of Lauren tian and 

 Huronian rocks, was broad and high, and the Cambrian strata (Acadian 

 group) were deposited only along its margin. 



At the beginning of the Silurian age the sea rose over its shores, 

 covering most of the land-surface, but leaving the Canadian highlands, 

 the Adirondacks, the Blue Ridge, with its New York spur, unsubmerged. 

 Then during all the thousands of years in which the Trenton limestone 

 group was accumulating by organic agencies, the slow growth and de- 

 position after death of the hard parts of animals, and the other thou- 

 sands of years in which the Hudson River and Utica shales were formed 

 in a shallowing sea, this old land was exposed to wear from rain and 

 wind, sun and frost. 



In like manner when the Upper Silurian and Devonian seas in turn 

 flooded more limited portions of the adjacent lands, covering them with 

 new layers of sediment, the old ridges and highlands which have been 

 enumerated, with large additions to their areas made in the Silurian 

 age, were suffering constant abrasion and reduction of altitude. 



In the Carboniferous age all the country for a great distance east, 

 north, and west of New York, was above the sea, but along the coast in 

 Rhode Island and Eastern Massachusetts were marshes where a luxuriant 

 vegetation was forming peat-beds that were destined, in after-times, 

 to become seams of coal; and in Pennsylvania, and thence westward in 

 Ohio and Illinois, were vast tracts of swamp half water, half land 

 which are now the most extensive coal-basins in the world. During all 

 these ages the belt of highlands which separates the valley of the Hud- 

 son from that of the Connecticut was probably much higher than now, 

 and stood as a witness of the varying phases of the unending war be- 

 tween land and sea, and saw the continent created and destroyed again 

 and again ; but in all these changes it took no part. 



In the latter part of the Carboniferous age the Alleghanies proper 

 were gradually elevated, the convex folds forming mountain-ridges, 

 the depressed or synclinal arches becoming the slowly-deepening coal- 

 basins. In the end all the country between the Atlantic and the Mis- 

 sissippi stood as a broad and elevated continental area. Subsequently 

 the sea rose and fell upon its margin, leaving there the record of its 

 oscillations in the deposits of the recent geological ages, but no con- 

 siderable portion of its surface has since been submerged. 



The Triassic age was a stormy one in the region about New York. 



