HUDSON PERIOD. 217 



genera Tcxtularia, Rotalia, and Guttulina. Ehrenberg has also detected in this 

 rook great numbers of Pteropods (related to Theca), and made out ten new spe- 

 cies and four genera. *The rock derives its name from its most common fossil, 

 Obolus Apollinis (fig. 236), which is about as large as a small finger-nail. The 

 Siphonotreta unguiculata (fig. 235) is another of its fossils. The age of the beds 

 is either that of the Trenton or earlier. 



3. HUDSON PERIOD (4). 



Epochs. — 1. Utica, represented by the Utica shale (4 a) ; 2. Hud- 

 son River, or that of the Hudson River shale and some contempo- 

 raneous limestones (4 6). 



In contrast with the Trenton period, the Hudson was pre-emi- 

 nently a time of shale-making. Its surface-exposures in New York 

 State are shown on the map, the region being that marked 4. Its 

 life was abundant, and much resembled that of the Trenton period. 



I. Rocks : kinds and distribution. 



The Utica shale is the surface-rock along a narrow region in the 

 Mohawk valley, N.Y. (see 4 a on map, p. 170), following a course 

 nearly parallel with the outline of the Azoic farther north. The shale 

 is in some places three hundred feet, or more, thick. It extends 

 westward through Canada, and beyond, probably, into Wisconsin 

 and Iowa, though a very thin deposit at the West. Along the 

 Appalachians it occurs in Pennsylvania, and is from three hundred 

 to seven hundred feet thick. 



The rock is a crumbling shale, mostly of a dark blue-black or 

 brownish-black color, and frequently bituminous or carbonaceous, 

 — so much so as in certain places to serve as a black pigment. It 

 sometimes contains thin coaly seams; and much money has been 

 foolishly spent in searching for coal in this deposit. Thin layers 

 of limestone are occasionally interpolated, especially in the lower 

 part. 



The Hudson River shales are exposed to view through the centre of 

 New York, near the Mohawk, and westward from the northwestern 

 side of Lake Ontario across Canada (following the course in New 

 York) to Lake Huron (see map, fig. 205) ; also farther west, in Michi- 

 gan, Wisconsin, and Iowa. The rocks are slates or crumbling shales, 

 with some flagging-stone, in New York; but westward they become 

 more or less calcareous. 



The formation is represented in southern Ohio, about Cincinnati, 

 by a thick limestone, called "Blue Limestone, " but the beds are 

 much interlaminated with a soft shale or marl ; and this same rock 



