STEATIGRAPHICAL GEOLOGY. 9 



that transpired during its deposition on the ocean-bed; and these, when placed 

 together, in their order of deposition, constitute the true geological column. 



"The Upper Silurian rocks are subdivided in ascending order, as follows: 



1. Oneida Conglomerate, which takes its name from Oneida county, New York. 



2. Medina sandstone, which takes its name from Medina, New York. 3. Clinton 

 Group, which takes its name from Clinton, New York. 4. Niagara Group, 

 which takes its name from its development at the Falls of Niagara. 5. Onon- 

 daga Salt Group, which takes its name from Onondaga, New York, where the 

 salt springs have been extensivelj' wrought; and 6. Lower Helderberg Group, 

 which takes its name from the Helderberg JNIountains of New York. 



The Oneida Conglomerate has been called the Shawangunk Grit, tlie Shawan- 

 gunk Conglomerate and the Millstone Grit. The word Shawangunk, signify- 

 ing, in the Indian language of the aborigines, White rock, has been regarded as 

 quite appropriate, because it is expressive of the character of the roclis. The 

 greatest thickness of this Group in New York and Pennsylvania is about 500 

 feet. The rocks are of such a character that they have not preserved the fossil 

 remains with the exception of imperfect fucoidal impressions. 



The Medina sandstone is usually of a red color, with the exception of a gray 

 band near the top. Between the mouth of the Niagara river and Lewiston, it is 

 350 feet thick, though at Barton, Canada, it is 618 feet thick. Its dimensions in 

 Pennsylvania are much increased. In the latter State it is subdivided into three 

 Groups on lithological grounds; the lower, a compact greenish gray sandstone 

 about 400 feet thick, the next a soft argillaceous red and brown sandstone and 

 shale 700 feet thick, and the higher a white or light gray sandstone and shales, 

 450 thick; making a total thickness of 1,1£0 feet. Like other sandstones, it 

 usually contains but few fossils, but in some localities it is highly fossiliferous, 

 especially in the upper part. 



The Clinton Group is only estimated, in Ohio and other Western localities, at 

 50 feet or less in thickness. In New York and Canada from 50 to 400 feet, and 

 on the Island of Anticosti at 610 feet; but in Pennsylvania it reaches the great 

 thickness of 1,620 feet. (Geo. of Penn., vol. i., p. 106.) 



Prof. Hall says of this group: " In the Western portion of the State (N. Y.) 

 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 sandstone 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 pre- 

 cisely like the Medina sandstone. The general character of the marine vegeta- 

 tion 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. When 

 we examine the Clinton Group in the central part of the State, its analogies are 

 chiefly with the Medina sandstone: and it is there a powerful and important 

 formation, presenting, however, great variation in its successive beds and charac- 

 ters, in every respect truly protean. In its Western extension, the Clinton Group 

 assimilates in character to the Niagara Group, and in the Western district has 

 nearly lost the character which it presents in Oneida county. At the same time 

 that the Group assumes a more calcareous character in its Western extension, it 



