182 H. S. WILLIAMS — SHIFTING OF FAUNAS 



cend the series. The Hamilton is richest, and occupies the thickest 

 series of strata, in the eastern sections; the Ithaca fauna occupies the 

 second stage of these sediments in the Chenango and Ithaca sec- 

 tions, and does not reach much westward of the latter section at that 

 stage. The Chemung stage does not appear in the two easternmost sec- 

 tions, except in a few rare places where it is interleaved with the domi- 

 nant Catskill sediments ; it is dominant in the central sections, and al- 

 most disappears in the extreme western section. As a type of sediment, 

 it is wanting there, but a few of its species do appear in the 'fine shales 

 called Erie or Ohio shales. The fourth stage of this class of sediments, 

 in the western Pennsylvania and Ohio sections, includes the Waverly 

 fauna, and the eastern limit of the fauna scarcely reaches the Clean 

 section, though a rare specimen representative of the fauna is occa- 

 sionally seen as far east as McKean and Potter counties, Pennsylvania. 



a TEE FINE MUD SHALES 



Type C, the third type of sediments, is a very fine mud shale, some- 

 times arenaceous, but dominantly argillaceous, in fine, thin layers, and 

 very evenly bedded ; with a sparse fauna of small, frail shells ; the 

 brachiopods, chiefly lingulas, discinas, and chonetes ; a few delicate 

 lamellibranchs, but goniatites and cephalopods dominant, and in some 

 places very large, peculiar types of fish occur. The sediments are either 

 light gray or black in color ; when black, there are evidences of marine 

 plants, as if a floating sargasso sea were present which dropped its car- 

 bonaceous matter on the smooth bottom. This class of sediments is 

 recognized in the Marcellus, Genesee, Huron, Erie, Girard, and Cleveland 

 shales, and, slightly modified, in the Portage shales. This t3^pe domi- 

 nates the western sections; reaches farthest east in the Marcellus stage; 

 only to the Chenango section in the Genesee and Portage stages ; ap- 

 pears only in the western New York sections in Chemung time, and is 

 restricted to western Pennsylvania and Ohio at the top. These sedi- 

 ments dominate the whole of the Ohio section, and to the southward, 

 in Kentucky, Tennessee, and in southwestern Virginia they attain a maxi- 

 mum thickness of several hundred feet, forming a continuous black 

 shale from the horizon of the Onondaga fauna up to the Knobstones — 

 the first formation of the Carboniferous of that region. 



The Shifting of Faunas 



In the region represented by the sections of the chart, these three 

 types of sediments and their respective faunas always bear the same 



