32 ROCHESTER ACADEMY OF SCIENCE. [Oct. 8, 



region, the strata have not been conspicuously displaced. There is a 

 slight inclination or dip east of south, amounting at Rochester to less 

 than ioo feet per mile. This dip causes each rock formation to disap- 

 pear southward beneath the successively superior formations. The 

 strike of the strata is, therefore, nearly east and west, and the several 

 kinds of rock outcrop at the surface as bands stretching east and west 

 across the area. The width of these bands of outcropping rocks varies 

 from a fraction of a mile to several miles, according to their thickness. 

 Over most of the region, however, the rock is rarely seen, even in 

 stream channels, on account of the thick sheet of glacial and lake- 

 drift which overlies it. The finest exposure of strata, and one of the 

 most beautiful in this country, is seen in the lower ravine of the 

 Genesee river, where the river having lost its old preglacial valley 

 has cut for itself a new channel, eight miles long, through the sand- 

 stone, shales and limestones of the Niagara formation. The accom- 

 panying map and diagram will indicate the succession of the several 

 rocks which underlie the region. (See Plate 2.) 



The rocks of the whole of central and western New York are 

 unaltered sedimentaries, of marine origin, consisting of sandstones, 

 shales and limestones. They will be briefly referred to in the order 

 of superposition, beginning with the lowest. The lowest visible rock 

 is the Medina sandstone, which is at or near the surface in the north- 

 western part of the area of the map, and is extensively quarried. 

 This red Medina forms the rock bottom of the southern part, at least, 

 of lake Ontario and the rock bluffs at all points along the south 

 shore. Beneath Rochester the red Medina is over one thousand feet 

 thick, but here and throughout the region, except the north-western 

 portion of the lake border, it is buried under the shales and lime- 

 stones of the Clinton group. The entire section of the Clinton is 

 finely shown in the walls of the Genesee canyon at the lower falls in 

 Rochester. Here it rests on the gray top of the Medina, and in 

 ascending order consists of about 24 feet of the lower green shale; 

 14 feet of lower limestone, containing a bed of hematite iron-ore one 

 foot thick; 24 feet of upper green and purple shales; and 18 feet of 

 upper limestone. 



The Niagara group rests upon the Clinton and consists of 80 feet 

 of dark, gritty shales, exposed at the upper falls in Rochester, and 

 80 feet of limestone, upon which the city of Rochester is mainly built 



A very few miles south of Rochester the Niagara limestone 

 ceases to be the superficial rock, and well-borings through the glacial 



