NIAGARA FALLS AND VICINITY 33 



The Utica shales and the arenaceous Lorraine shales which fol- 

 low on the Trenton limestones show a return of land-derived de- 

 posits due probably to a shoaling of the water. This may have been 

 caused by an upward movement of the sea bottom or by a partial 

 withdrawal of the water into deepening oceanic basins. Some 

 abrupt change is indicated by the sudden transition from limestone 

 to black shale. Another abrupt change occurred at the close of the 

 Ordovicic era, as indicated by the marked contrast between the 

 Lorraine shales and the Oneida and Medina beds which immediately 

 succeed them. 



The Siluric deposits of this region began as shallow water accumu- 

 lations, the lowest bed being the Oswego sandstone^ which farther 

 east, is replaced by the conglomerates of Oneida county and the 

 Shawangunk range. The marls and shales of the Medina series 

 succeed these sandstones with an aggregate thickness exceeding 

 iioo feet. A heavy stratum of gray quartzose sandstone, varying 

 in thickness up to 25 or 30 feet, separates, in the Niagara region, 

 the lower from the upper Medina shales and sandstones, which have 

 an approximate thickness of 100 feet. The Clinton shales and 

 heavy limestones follow on the Medina, with a thickness averaging 

 30 feet. The Rochester shales, with a thickness of 60 to 70 feet, 

 follow the Clinton limestones and are in turn succeeded by the Lock- 

 port limestone, whose average thickness, obtained from well records, 

 approximates 250 feet in this region. . The Salina shales succeeding 

 the Niagara beds (Rochester shales and Lockport limestone) have 

 an aggregate thickness of less than 400 feet, and are followed by the 

 Waterlime and the Manlius limestone, the former averaging 50 feet 

 in thickness, the latter from 7 to 8 feet. The lowest Devonic beds 

 are absent in this region, the Onondaga limestone resting directly 

 on the Manlius beds, there being, as before noted, an important 

 though not very pronounced unconformity between the two. A 

 glance at the geologic map of this region will reveal the fact that the 

 lower strata rise from under the covering newer beds on the north, 

 and occupy a belt of country of greater or less width according to 

 the thickness of the beds. Where they come to an end, the next lower 

 beds make their appearance. The discontinuation of the higher 



