PALEONTOLOGY OF NKW-IORK. 



MEDINA SANDSTONE, CLINTON AND NIAGARA GROUPS. 



These rocks have been described in the Geological Reports of the State as three distinct groups, 

 which distinction they are in truth entitled to throughout the State. When, however, we come 

 to investigate minutely the physical conditions under which they were formed, the line of 

 demarcation is not always so readily defined. The Medina sandstone, throughout the greater 

 part of its tliickness, is almost wholly devoid of fossils ; presenting the character of a nearly 

 homogeneous argillaceous sandstone, which, from its easy destructibility by atmospheric agen- 

 cies, usually presents a broad sloping plateau. In the banks of ravines, streams or rivers only, 

 the edges of the strata are seen, and here always in a state of decomposition. 



Towards the upper termination of the mass, its composition becon»es more purely arenaceous, 

 and it forms a distinct feature in the exposed edges of the harder strata. Here, also, in several 

 localities, it contains fossils in large numbers, though of few species. Between this point in the 

 Medina sandstone where marine shells appear, and the point we have left in the Hudson-river 

 group where similar fossils are found, there is a«tbickness, in some parts, of about one thousand 

 feet, in which no well defined forms are known to exist. Thus, the gray sandstone terminating 

 the Hudson-river group, the Oneida conglomerate and lower part of the Medina sandstone 

 separate the fossiliferous portions of the Hudson-river group from the point where we now 

 take up the subject, by the great amount of thickness named. It is true that in the eastern part 

 of the State these groups approach each other within one hundred feet, but the distinction 

 between them is much more strongly marked here than at the west. 



The upper fossiliferous portion of the Medina sandstone is generally of no great thickness, 

 though in the extreme western limit of the State it reaches a maximum of more than one hun- 

 dred feet. In the western portion of the State, the limit between the Medina sandstone and 

 Clinton group is well defined, and the materials very distinct ; but in the central part of the 

 State, we find the same conditions which operated during the deposition of the Medina sand- 

 stone to have been continued into the Clinton group. The latter commences by a shaly deposit, 

 which is soon succeeded by alternations of sandstone in character precisely like the Medina 

 sandstone. I have shown {Geological Report 4th District, p. 52) the existence of sea beaches, 

 with wave lines, drifted shells, etc. upon the surface of the sand. The same features are con- 

 tinued into the Clinton group ; and where the alternations of sand occur in the shaly deposits 

 of the latter, similar wave lines, drifted shells, and other evidences of beaches or shallow 

 water, such as the trails of shells or other animals, are found. 



The general character of the marine vegetation of the two periods is similar ; and a peculiar 

 type of plants commences its existence in the Medina sandstone, and terminates in the Clinton 

 group. These facts will be shown by the illustrations and descriptions in the successive plates. 

 Still it is convenient, and even necessary, to treat these two as separate and successive groups, 

 since in the main thev are each so well defined 



