PALEOZOIC TIME — LOWER SILURIAN. 491 



are largely Lower Silurian and Cambrian. They are the Taconic series of 

 Emmons. The Eolian limestone of Vermont, and its continuation, the 

 Stockbridge, of Berkshire, Mass., with the intervening ridges of slates and 

 schists, are of this series, and also, the extension of the lines southward, 

 though interruptedly, into New Jersey and Pennsylvania; and it probably 

 comprises the interrupted series of limestone belts and the associated 

 schists which extend from Canaan, Conn., south through Litchfield County, 

 Conn., and Westchester County, IST.Y., to New York or Manhattan Island, 

 and part of this island, the rest being probably Archaean. 



1. Canadian Period. 



1. Calciferous Epoch. — The Calciferous formation, along the borders of 



the Archaean of northern New York and Canada, consists of a grayish lime- 

 stone which is often arenaceous and cherty, usually magnesian, and rarely 

 fossiliferous. It then extends southwestward through New Jersey and east- 

 em Pennsylvania. It includes in Missouri the first or uj)per of the four 

 Lower Magnesian limestones, with the underlying sandstone called the first, 

 or Saccliaroidal sandstone. Its equivalent is the *' Lower Magnesian" of 

 Iowa and Minnesota. 



2. Chazy Epoch. — The Chazy beds in New York consist mostly of lime- 

 stone. The formation was so named by E. Emmons, after the village Chazy, 

 in Clinton County, N. Y., where the formation has a thickness of 730 feet. The 

 limestone is gray to black in color, and is often recognizable, wlien in polished 

 slabs of black marble, by the presence of a large fossil shell three inches or 

 more across — the Maclurea magna (Fig. 634). The limestone is mostly 

 dolomytei. It occurs in Canada about the Ottawa basin. On the eastern 

 border of New York and the western of New England it makes part of the 

 Taconic series. The St. Peter's sandstone of the northern part of the 

 Mississippi valley has been referred to the Chazy epoch ; but it contains 

 few fossils of any kind, and none are characteristically Chazy. 



2. Trenton Period. 



The Trenton period is represented in New York, in its earlier part, by 

 limestones, and in its later part by shales ; and this division in the rocks is the 

 basis of a subdivision of the period into the Trenton and Utica and Hudson 

 epochs. This succession in the rocks implies that a time of clear open seas 

 first existed, in which Trilobites, Gastropods, and Bryozoans abounded, as 

 well as Brachiopods ; but that later, through some unexplained topographical 

 change, the waters lost much of their clearness, and bore along so much 

 sediment that mud deposits were made over the bottom, extinguishing life 

 that could not adapt itself to the new conditions, reducing Trilobites to a 

 few species, favoring the multiplication of Lamellibranchs and other Mol- 

 lusks, and causing many other changes both by migration and modification. 

 The change, moreover, was one of wide extent and influence. 



The name Trenton is derived from Trenton Palis, north of Utica, in 



