220 ROCHESTER ACADEMY OF SCIENCE. [Oct. 27,^ 



origin of the rock. The lens-like masses sometimes blend horizon- 

 tally and form an irregular thin stratum, making what Dr. James 

 Hall fifty years ago named the " Pearly Layer." (") 



Again the silt gave place to limestone, but the change was 

 gradual and not complete. Thin beds of limestone with layers of silt 

 intervening make the i8 feet of the Upper Limestone of the Clinton 

 group. With the lime accumulation there was considerable silica, 

 and masses of chert are found and siliceous replacement of the fossils. 



The Clinton epoch in time is thus represented here by alternating 

 strata, two strata of shale and two of limestone, all requiring 

 deep or comparatively quiet waters. The alternation of muddy 

 water, producmg silt deposit, and clear water, allowing limestone 

 accumulation, became in the Upper Limestone rapid and sudden 

 changes, api)arently not dependent upon changes of depth, but upon 

 varying direction of the sea currents. When the currents reaching 

 this locality came from near some land the detritus was swept in and 

 shale accumulated, but a change in the direction of the currents 

 bringing in the wash from some coral reef or limestone beach 

 permitted the calcareous accumulation. 



From the limestone and silt of the Upper Clinton, the change- 

 was to a coarser silt or arenaceous clay. Of this firmer shale 80 feet 

 were deposited here, called Niagara Shale. This was succeeded and 

 buried by the Niagara limestone, these two strata making the Niagara 

 group. Further westward, as at Lockport, this limestone formed 

 as a veritable coral reef, and even here large corals are found 

 imbedded in the finer mass. 



With this Niagara limestone ends the visible hard- rock record of 

 our history. But it is certain that some later records have been 

 destroyed — that is, some higher strata have been removed. The sea 

 bottom would hardly be elevated so quickly as not to permit some 

 detrital deposits upon the limestone, and probably heavy beds of 

 shale and even limestone once capped the Niagara in this locality, 

 which has itself been deeply eroded. Our strata dip or incline 

 slightly to the southward. As we pass south we find in successively 

 higher horizons the shales of the Salina group, and the limestones 

 of the Onondaga and the Corniferous, all of which may have 

 extended northward so as to bury this locality. So we can never 

 determine the geologic date when sedimentation ceased here. But 

 there was a day, sometime during either the Upper Silurian or the 



(6) Natural History of New York, Part IV. Survey of the Fourth Geological District, by 

 James Hall, Albany, 1843. 



