(tkabau — Faunas of the Hamilton Gkoup. 



333 



Of those marked f,. Spirifer mucronatus is represented in the Lower 

 shales of Livonia by a small variety, which may have given rise to stouter and 

 larger individuals when the conditions became more favorable. The others 

 thus marked occur in the lower 147 feet of shale, associated with Marcellus 

 species, but as they do not occur in the intermediate eighty feet of shale, their 

 reappearance in the succeeding beds may be accounted for by immigration from 

 the west. 



The sequence of geological events in this region seems to have been as 

 follows : * 



After the limestone making epoch of the Corniferous period, a mud- 

 making epoch, with shallow water and a scarcity of life began, during which 

 the Marcellus shales were deposited. These conditions, as shown by H. S. 

 Williams, extended westward beyond the limits of the State of New York, the 

 black shales of Ohio and other states probably corresponding in part to the 

 Marcellus of New York. The shallow water and unfavorable conditions are 

 indicated by the scarcity and small size of the majority of the fossils, and by 

 the plant remains to which the shales owe much of their bituminous character. 

 In the Genesee Valley these conditions continued after the establishment of the 

 Hamilton fauna and thus Ave can account for the scarcity of the fossils in 

 the lower shales of that region, the preponderance of pelecypods, and the 

 similarity of the fauna as a whole, to the Marcellus. These conditions, how- 

 ever, soon gave way to more favorable ones in Western New York and Ontario, 

 allowing the development of a luxuriant fauna. To the south and southwest, 

 the conditions remained more nearly as they were, during the Marcellus epoch 

 of New York, and the deposition of the black shales of Ohio, Indiana, Ken- 

 tucky and other states continued. Toward the close of the lower Hamilton 

 in AVestern New York, a greater dispersion of the organic forms took place, 

 owing probably to an increased subsidence eastward, and the purification of 

 the waters by the diversion of the mud-bearing currents. At any rate a uni- 

 form condition of the sea existed at Livonia, Eighteen-mile creek, and perhaps 

 Widder, Ontario, with waters sufficiently pure to permit the growth of corals 

 and crinoids in profusion. That the waters were not very deep during the 

 formation of the Encrinal limestone, is shown by the comminuted character 

 of the organic remains especially the crinoids, indicating a certain amount of 

 wave action. The Upper Hamilton was inaugurated by the influx of muddy 

 waters, but the conditions now became reversed. Continued subsidence 



♦These generalizations are advanced tentatively, and may require modification after sections between those mentioned 

 here are studied. 



