PALEOZOIC TIME — UPPER SILURIAN. 541 



In Illinois, the limestone underlies the city of Chicago and constitutes 

 the gray " Athens Marble " and the gray and buff " Joliet " building-stone. 

 In the Mississippi valley it often contains flint or chert in nodules and is 

 dolomyte. In Wisconsin, it has distinctly the features, in some places, of an 

 old coral-reef. Porty species of Corals have been described from it. Some 

 large coral masses " stand erect in the rock, precisely as they grew," making 

 up, along with fragments and sand derived from broken corals, shells, and 

 Crinoids, the coral reef-like limestone bed. Between and about what look 

 like true barrier reefs, there are accumulations of coral fragments, becoming 

 finer and finer on receding from the reef, and thus the rock graduates into 

 ordinary limestone. (Chamberlin.) 



In New England, the St. Lawrence Bay of the Niagara period extended 

 far south along the Connecticut valley ; for Niagara beds, with their fossils, 

 occur at Littleton, N.H., resting unconformably on older beds. They occur 

 also in northern Maine and New Brunswick ; and on the coast of Maine in 

 Penobscot Bay, near Machiasport, in Cobscook Bay ; along the Acadian 

 trough; they exist also in Nova Scotia. In Anticosti, a thick limestone 

 ranges continuously from the Hudson to the Clinton groups. 



The Rocky Mountain region has few outcrops of Upper Silurian rocks. 

 The Niagara beds have been observed in the Black Hills of South Dakota, 

 near Deadwood, but not in the Wasatch Mountains ; and they have not been 

 identified in Arizona at the Colorado Canon, nor over the Great Basin. They 

 are doubtfully identified in the Eureka district, Nevada, in the upper part of 

 a limestone stratum which is Trenton at base — a Halysites occurring in the 

 beds. The formation has a wide range in Arctic seas, and occurs on some 

 islands in Hudson Bay. 



Mineral oil exists in large quantities in the Niagara limestone at Chicago, 

 though not capable of being collected to advantage. Worthen says that 

 a portion of the limestone is " completely saturated with oil." 



The distribution of the rocks of the Niagara period sustains the conclu- 

 sions presented on a preceding page with regard to American geography at 

 the opening of the Upper Silurian. They show that the waters over the 

 state of New York shallowed toward the Hudson River, and thickened west- 

 ward, thus according with other evidence as to the emergence of the Green 

 Mountain region in connection with the making of the Taconic Mountains. 

 The shallowing was toward the emerged mountain belt. They prove also, 

 through the abundant arenaceous deposits, that while in the earlier part of 

 the period the region of the Eastern Interior Sea was shallow, at a later date 

 deeper and clearer seas prevailed, even from Hudson River to and beyond the 

 Mississippi, in which Corals and Crinoids were growing abundantly ; yet they 

 were not necessarily deep seas, since 150 feet of depth is enough for all the 

 work of the modern reef-making Polyps. 



1. Medina Group. — The Oneida conglomerate is a thick-bedded formation, and the 

 rock so hard as to stand out boldly in rocky ledges and ridges. The Shawangunk grit was 



