NIAGARA FALLS AND VICINITY 1 23 



indicate a high degree of saHnity of the waters of the early Medina 

 sea. If such was the case, it may have been accompanied by a more 

 or less arid climate, which favored the concentration of the sea 

 water. Thick beds of terrigenous material accumulated in the cen- 

 ter of the Medina basin reaching in the Niagara region a thickness 

 of over a thousand feet. These early deposits probably did not ex- 

 tend far west for, though in northern Ohio and Michigan, Medina 

 beds from 50 to 100 feet or more in thickness are known, these are 

 probably to be correlated with the upper Medina of the Niagara 

 region. 



Toward the close of the Medina epoch, the Siluric sea had en- 

 croached on the lands to such an extent as to effect a junction with 

 the Medina basin, whereupon normal marine conditions were again 

 established. This is indicated by the marine fauna and flora which 

 characterize the upper Medina beds. The first deposit in this re- 

 gion, on the reestablishment of normal marine conditions, was the 

 white quartzose sandstone which caps the red shale of the lower 

 series. Mud and sand now alternated, indicating an oscilla- 

 tion of conditions with numerous changes in the currents which 

 distributed the detrital material. Thin beds of limestones also 

 formed at rare intervals, chiefly from the growth of bryozoans in 

 favorable localities. In the Bay of New York the waters continued 

 moderately shallow, as shown by the well developed cross-bedding 

 structure in the sandstones. At intervals large tracts seem to have 

 been laid bare on the retreat of the tide, as indicated by the wave 

 marks and other shore features which give the surfaces of some 

 Medina sandstone slabs such a remarkable resemblance to a modern 

 sand beach exposed by the ebbing tide. In fact, we may not in- 

 aptly compare this stage of the Siluric bay of New York with the 

 upper end of the modern bay of Fundy, where the red sands and 

 muds are laid bare for miles on the retreat of the tide. 



After the last sandstone bed of the Medina stage had been de- 

 posited, the water probably became purer and deeper, and the 6 

 feet of Clinton shales were laid down in the Niagara region. In 

 the eastern part of the Bay of New York, sandstones were deposited 

 even during the CHnton epoch, while the conditions favoring the 

 deposition of Hmestone existed only during the short interval in 



