374 GEOLOGY. 



the Oneida, appear to have been accumulated in shallow water, or 

 possibly on land." This is shown by the cross-bedding, the ripple- 

 marks, etc., which affect the surfaces of its layers. In view of these 

 facts, and in view of the physical similarity of the two formations and 

 of the conformity of the younger on the older, it is inferred that the 

 deposits of the Medina stage followed those of the Oneida without 

 interruption where both formations are present. Like the eastern 

 Oneida, the eastern (Appalachian) Medina seems to have been deposited 

 in a basin which had no connection with the interior. Beds classed as 

 Medina are also found south of Lake Ontario, where they overlie the 

 Oneida of the same region. They appear at the surface as far west as 

 eastern Ohio and Ontario, where they thin out and disappear. West 

 of this longitude, the formation has not been identified. 



The Oneida and Medina formations south of Lake Ontario contain 

 marine fossils, showing that an interior sea existed in this region in 

 early Silurian time. The distribution of the two formations indicates 

 a more extended sea in the later than in the earlier epoch, and points 

 to a subsidence of the eastern interior during the early part of the 

 Silurian period. By the close of the Medina epoch, as the distribution 

 of the Medina formation shows, the area of deposition had been extended 

 at least as far west as Ohio (the Cincinnati arch). 



The Medina formation, which has but a narrow outcrop in northern 

 New York, is probably continuous beneath younger strata over most 

 of the state south of Lake Ontario and the Mohawk valley, and west of 

 the Appalachian basin, and farther south over considerable areas west 

 of the Appalachians. Nothing is known of the sediments which must 

 have been accumulating along the eastern border of Appalachia, nor 

 of those about the other land areas of the time. Though the Oneida 

 and Medina formations are unknown both in the interior west of Ohio 

 and an the western part of the continent, it is possible that strata 

 which are their time-equivalents exist in some parts of this vast ter- 

 ritory. If they anywhere appear at the surface, they have not been 

 identified. 



In Pennsylvania, the eastern (Appalachian) Medina formation has 

 a maximum thickness of something like 1800 feet. This, it is to be 

 remembered, was near a great land-area to the east whence the sedi- 

 ments were doubtless derived. West of the Appalachian belt the 

 formation is much thinner, having in Ontario a thickness of 300 to 400 



