NIAGARA GROUP. 51 



breadth. In Wisconsin, Illinois, and Iowa it is principally a magnesian limestone, 

 sometimes too porous or friable for building purposes, but suitable for lime, as at 

 Chicago and Racine; at other places having a good reputation for buildings, as at 

 Joliet. It sometimes occurs more or less saturated with petroleum, as at Chicago, 

 where it indicates the presence of shales immediately below it, and in some locali- 

 ties near its base it contains beds of hematite in small lenticular concretions, as at 

 Iron Ridge, in Dodge County, Wisconsin. The maximum thickness in Illinois is 

 640 feet, in Wisconsin 800 feet, and in Illinois and Iowa 600 feet. It occurs in south- 

 eastern Canada, in New Brunswick, Newfoundland, and Anticosti, where its maxi- 

 mum thickness is 800 feet. It occurs in nearly all the States to which the Appa- 

 lachian System extends. In crossing Pennsylvania, where it consists mostly of 

 shales, it has a maximum thickness of 1,600 feet. It occupies extensive areas in 

 Tennessee and Alabama ; and in the latter State that part of it which was originally 

 a porous magnesian limestone, subsequently became infiltrated with iron in solu- 

 tion, and now constitutes the celebrated fossiliferous iron ore of Alabama. It 

 forms a sub-circular belt of exposures from 5 to 60 miles in width surrounding the 

 great Lower Silurian area in the middle part of Kentucky, South-western Ohio, and 

 South-eastern Indiana, where it consists of hard, blue and gray limestone, yellowish 

 and whitish-yellow magnesian limestone, and shales, variously alternating and combin- 

 ing, with a maximum thickness of about 600 feet. In some places near the base 

 there is iron-stained chert. At Cedarville, near the top, the porous magnesian lime- 

 stone is used for the manufacture of lime, and the harder limestone at Dayton, St. 

 Paul, and other places is used for building and other economic purposes. It sur- 

 rounds the Lower Silurian and Taconic uplift in the southern part of Missouri, 

 and frequently occurs in the Rocky Mountain ranges. It outcrops far to the north, 

 in the Arctic regions north of British America. Fossils have been described from 

 its exposures on Beechy, Cornwallis, Griffiths, Seal, Napoleon, and Offley Islands, 

 from Capes Hilgard, Hotham, Louis, and other points. It is substantially the 

 equivalent of the Wenlock in England, and has its representative in Scandinavia, 

 Russia, Germany, and other European countries. Several species of fossils occur- 

 ring in the upper part of the Group at Waldron, Indiana, are identical with those 

 occurring at the equally celebrated locality on the Island of Gottland, in the 

 Baltic Sea. It is so constantly present where the rocks from the Lower Silurian 

 to the Devonian are exposed, that it is regarded as a universal Group underlying 

 nearly all the more recent rocks on this continent. 



§ 104. It is a deep-sea deposit, as distinguished from all mechanical, littoral, 

 shore-line, and marsh deposits, and, like most other undisturbed marine sediments, 

 is generally limestone. The ocean must have swarmed with invertebrate life dur- 

 ing the entire age, as the rocks are almost wholly constituted of their harder parts. 

 It is so thoroughly characterized by its fossils that a palaeontologist has little diffi- 

 culty in recognizing it wherever it exists. It is in this Group the earliest land- 

 plants occur — Psilophyton and Glyptodendron. The latter was founded upon an im- 

 pression of uncertain value in a magnesian limestone. Psilophyton is supposed to 

 have been a marsh-plant that drifted in the ocean and became imbedded in the 

 mud, which preserved its characters. Psilophyton princeps is the oldest fossil land- 

 plant in America. Fucoids are scarce; in striking contrast with their abundance 

 in the Clinton. Sponges were more numerous than in any preceding age. Coral- 



