84 TRAXSACTIOyS OF THE A M ERIC AX IXSTITUTE. 



Mississippi, an entirely different set of fossils. JN evertlieless we can 

 trace these rocks throngh the entire length of tlie Apalachian chain, 

 succeeded by what we know as the Carboniferons formation the 

 characters of which have already been discussed before yon in the last 

 lecture. In the early part of the Devonian period — the Hamilton 

 group of New York — we begin to find land plants. Throughout the 

 series of rocks, many thousands of feet in thickness, from the beginning 

 of the Silurian period up to this point, we have seen no vestiges of 

 land plants,* The entire area had been covered by an ocean, and not 

 only this, but there had been no drifted materials indicating the exist- 

 ence of dry land in the region from which the sediments had been 

 derived. There have in earlier times been evidences of a shore line, 

 or of current lines in proximity to the shore, but no evidence of dry 

 land whatever. 



These dry land plants of the Devonian period, however, are mingled 

 with remains of shells and other organic bodies of marine origin, or 

 they occupy layers of the rock, above and below which the beds are 

 filled with marine organisms ; and from this we infer that these plants 

 have been drifted from their original position. 



The formations of the Devonian period are succeeded by what is 

 termed the Carboniferous period, in consequence of the strata then 

 deposited, being, in Europe and the eastern part of the American 

 continent, the great repository of coal. Yon have already been told 

 that coal is produced from land vegetation ; and when we see the 

 coal fields extending from Nova Scotia and New Brunswick over a 

 great part of the United States, you will be prepared to look for evi- 

 dences of dry land also. In the Hamilton and Chemung groups of 

 the Devonian system in New York, the land plants do not indicate 

 very close proximity of land, or at least till the later deposits of 

 that period ; while in the coal period we have evidence of its exten- 

 sion over large areas previously covered by the ocean. From the 

 earliest appearance of land plants, there seems to have been a con- 

 tinued accession of dry land until it had extended westward at least 

 as far as the longitude of tlie Mississippi Yalley. However, this 

 encroachment of dry land upon the area of the ancient ocean was by 

 no means constant ; for we find that both preceding and during the 

 period of the accumulation of the sands and clays which form the 

 Coal measure strata, there are beds of limestone of greater or less 



* From the earlier appearance of land plants in the north-east, we infer that dryland firet appeared 

 in that direction, and some of the fossil plants of that region may be of Silurian age. 



