280 Cincinnati Society of Natural History. 
of 130 feet. It consists of a dark, irregular, thick-bedded limestone, 
sometimes containing rough, flinty or cherty masses. It crosses over 
into Vermont, where it covers more surface than any other group of 
the Lower Silurian, and has a thickness of 300 feet. 
In 1863, the Canadian geologists, finding the limestone associated 
with shales and sandstones, called it the “ Chazy formation.” It covers 
a considerable area in Canada, reaching as far as the Mingan Islands, 
and sometimes exposing a thickness of 300 feet. It appears in the 
Lake Superior region where the bottom layers are arenaceous, and 
higher up they have an argillo-calcareous composition. It extends 
westwardly across the State of Wisconsin into Minnesota, where it is 
usually a quartzose, incoherent sandstone, and bears the name of the 
St. Peters sandstone, from its development on a river of that name. 
In this region it fills the deep hoitlows that had been worn out of the 
Calciferous group before its deposition. It sometimes varies from a 
foot or less to 200 feet or more in thickness, within a very short dis- 
tance—the variation of a hundred feet not unfrequently taking place 
within a quarter of a mile. 
In Tennessee, it consists of a blue, more or less, argillaceous lime- 
stone, having a thickness of 600 feet, succeeded by red and gray 
marble, valuable for building and ornamental purposes, 400 feet in 
thickness. It occurs in Missouri, Nevada, Utah and other places in 
the Rocky Mountain region. Though a group of no very great thick- 
ness, it has an extensive geographical range, and in some places is 
quite fossiliferous. In New York it so graduates into the Birdseye 
limestone that constitutes the base of the Black River Group, that 
some of the early geologists united them into one group. 
The genera, whose appearance is first known in this group, are dis- 
tributed as follows : 
Among the plants, Rusophycus occurs in the Black River, Trenton, 
Hudson River, and Clinton Groups. 
Among the Protista, Hospongia is peculiar to it. 
Among the Polypi, Streptelasma and Columnaria occur in the Black 
River, Trenton, Hudson River, Clinton and Niagara; and Bolboporites 
is peculiar to it. 
Among the Echinodermata, Rhodocrinus recurs in various groups as 
high as the Carboniferous; Hybocrinus, Palewocrinus, and Glyptocys- 
tites occur in the Trenton; and Blastoidocrinus, Malocystites and 
Pachycrinus are peculiar to it. 
Among the Bryozoa, Fenestella occurs in nearly every group as high 
as the Coal Measures; and Stictopora in every group to the Devonian. 
