TRENTON PERIOD. 205 



epochs are still united by having several species in common, while no Calcife- 

 rous species pass into the Chazy; (3.) The genus Conocepnalus, Primordial in 

 Europe, is well represented in the Calciferous,- (4.) Gasteropods and Cephalo- 

 poda, not Primordial in Europe, are not by these means removed from the 

 Potsdam. 



2. TRENTON PERIOD (3). 



Epochs. — 1. Chazy (3 a), or epoch of the Chazy limestone. 

 2. Trextox (3 b), or epoch of the Birclseye, Black River, and 

 Trenton limestones. 



I. Rocks : kinds and distribution. 

 1. American. 



The Trenton period was characterized by a profusion of Brachio- 

 pods, Trilobites, and Orthocerata, and by the making of limestone 

 strata, almost of continental extent, out of the shells and other 

 calcareous relics of the living species. The rocks extend over a 

 large part of the continent east of the Mississippi, and beyond to- 

 wards the Rocky Mountains. 



The Chazy limestone (3a), or that of the Chazy epoch, is so 

 named from the town of Chazy, in Clinton co., N.Y., on the west 

 side of Lake Champlain, where the formation occurs. The rock is 

 mostly a grayish limestone, and the fossils are remarkable for being, 

 with few exceptions, quite small. 



• The Trenton limestone (3 b) derives its name from the well-known 

 locality of the rock along the gorge at Trenton Falls in central New 

 York. The rock is gray to black in color, the dark colors predo- 

 minating in New York, and the grayish in the West. 



The thickness of the whole series in northern New York and 

 Canada, towards the Azoic, where probably lay the ocean's border, 

 is generally from 100 to 300 feet ; yet in the region of Ottawa — a 

 great St. Lawrence Bay in the earlier Silurian era (see map p. 170,) 

 — it is about 800 feet. West of the Appalachians the thickness 

 averages about 300 feet. Along the Appalachian region, in Penn- 

 sylvania, it is from 300 to 500 feet. 



1. Chazy epoch. — (a.) Interior Continental basin. — The Chazy limestone 

 outcrops at different places in northern New York, in the vicinity of the Azoic 

 (though not along its more southern border) ; also in Canada, around the Trenton 

 limestone of the Ottawa basin, and from the head of Lake Ontario westward 

 to Lake Huron. The thickness in some parts of New York is 100 to 150 feet. 

 Occasionally it graduates into the next rock below, the Calciferous sandrock, so 

 that the two are separated with difficulty. In the region of the Upper Missis- 

 sippi, in Wisconsin, Minnesota, and Iowa, there is a sandstone called by Owen 

 St. Peter's sandstone, from a locality at the mouth of St. Peter's River. It 



