BIGSBY PALAEOZOIC ROCKS OF NEW YORK. 389 



"It is plain," says Hall (Rep. p. 254), "that the origin of the 

 materials of this deposition was to the east of Central New York, and 

 probably to the south-east, as is evidenced by the thinning of the 

 deposits, and the diminution of the sandy strata, at the west. The 

 increase of shaly strata in the same direction (which finally diminish 

 also) proves the same ; for this, being longer suspended, was trans- 

 ported beyond the sand. We have here a corroboration of the same 

 view as presented under the Hamilton Group, viz. a position in 

 Eastern New York near the margin of this ancient sea, while toward 

 the south-west we approach that part of more profound depth and 

 greater distance from shore. The evidence continues through the 

 Hamilton, Portage, and Chemung groups ; for in all these, and in 

 the intermediate beds of shale and limestone, we find a constant 

 diminution south-westerly. 



" Also in Eastern New York we find, both in the Hamilton and 

 Chemung, specimens of land-plants, or such at least as did not grow 

 beneath an ocean. These are rare in Central New York, one or 

 two fragments only having been found ; and at the south-west part 

 of the State and in Ohio I have seen nothing of similar character. 

 The inference naturally follows, that these were derived from land on 

 the eastern margin of this ocean ; and that some fragments floated 

 westward, and were deposited with the sand and mud. 



" Many of the thin sandy laminae throughout the district are often 

 almost completely covered with small fragments of carbonaceous 

 matter, apparently derived from terrene vegetation. These seem to 

 have been comminuted fragments of vegetables, brought down by 

 streams from the continent or islands on the east, and, being spread 

 evenly over the surface of the water, were distributed widely," as we 

 see now in lakes and bays. 



This indicates an approach to the great coal-measures. 



M. de Verneuil (loc. cit. p. 662) agrees with these statements. 

 He says that the predominance of sandstones and schists in the 

 eastern parts of New York shows that the continent existed on that 

 side, — from whence the rivers and coasts furnished their elements 

 to the sediments, just as the American geologist observed. 



The fucoids and ripple-marks, so abundant in every group from 

 the Potsdam to the Chemung, testify also to the proximity of bottoms 

 and shores ; so that, thick as it is, perhaps all the palaeozoic de- 

 posits were laid down in a shallow sea, whose bottom probably sank, 

 more or less continuously, to receive new sediments. 



The Chemung beds are sometimes treated separately, but without 

 good reason. One part is called the Ithaca group. It is a succession 

 of coarse hard shales and sandstones, of a dark colour, and without 

 any limestone. They are about 400 feet thick, and are best seen at 

 Ithaca in the county of Tompkins. 



Transition. — Except in a very few places, there is no strongly 

 marked dividing-line between the Portage, Chemung, and Ithaca 

 groups. The distinction is in the presence or absence of certain 

 fossils, and in the sandstone being coarser and more argillaceous 



