83 



Central New York has a thickness of nearly twenty feet, but 

 its average thickness is only about half that. Towards the 

 west it becomes thinner, until at Canandaigua Lake it is 

 represented by a calcareous band three or four inches thick.* 

 This limestone contains an association of fossils, which 

 cause it to be referred to the Genesee stage, of which it forms 

 the basal member. It is succeeded by the black Genesee 

 shales, containing a number of fossils which had been 

 characteristic of the black Marcellus shales. Twenty feet 

 of these shales occur in the Genesee Valley, while at Eighteen 

 Mile Creek there is scarcely a trace of them. At Section 1, 

 the thin bed of spore-bearing shale is the only representative 

 of this series, and even this does not occur everywhere. It is 

 very possible, however, that during the time that the TuUy 

 limestone and the black shale above it, were forming in 

 central New York, the transition beds of the upper Moscow 

 shale were deposited in the Eighteen Mile Creek region, and 

 that the Schizobolus fauna of these shales (if not the whole 

 Spirifer tullius fauna) was, in a limited sense, contempora- 

 neous with the Tully and lower Genesee faunae of Central 

 New York. 



After the close of the Moscow age, when the transition 

 beds had been deposited in this region, and the few inches of 

 spore-bearing shale were laid down upon them, a subsidence, 

 somewhat w^idely spread through Western New York, 

 occurred, which brought with it the purification of the 

 waters, which had hitherto been laden with fine carbona- 

 ceous silt. In the region about Eighteen Mile Creek, the 

 beginning of this purification of the water is marked by the 

 appearance, in a circumscribed favored spot, of a colony of 

 crinoids, which seem to have flourished there for a consider- 

 able period, so that their remains accumulated to a depth of 

 from two to three inches, as indicated by the thickness of the 

 " Conodont " limestone. That the water was not very deep 

 at this time, is shown by the highly comminuted condition 



»Hall, Rep't 4th Geol. Dist., p. 214. 



