16 STRATIGRAPHICAL GEOLOGY. 



The total thickness is about 1,200 feet. In Ohio the Group is subdivided into 

 the Cleveland Shale, Bedford Shale, Berea Grit and Cuyahoga Shale, these to- 

 gether constitute what is also known as the Waverly Group, named from the 

 quarries at Waverly, Ohio. In Michigan, Prol*. Winchell subdivided the Group 

 into the Marshall Group, Napoleon Group, Michigan Salt Group, and Carbon- 

 iferous limestone, the total thickness of which is 550 feet. In the Anthracite 

 Coal region of Pennsylvania this Group has a maximum thickness of 3,000 feet, 

 and consists mainly of red shale; it thins out rapidly towards the Northwest, but 

 maintains a great thickness Southwardly through Virginia and into Alabama, 

 gradually changing its character, however, to a calcareous limestone. 



The Lower Carboniferous rocks present us with a greater number of species 

 of Crinoids, and these in greater profusion than all the other subdivisions of the 

 Palaeozoic rocks. A single locality at Burlington, Iowa, in the Burlington Group, 

 has furnished about 350 species. Another locality at Crawfordsville, Indiana, 

 has almost a world-wide reputation for the great beauty, perfection and abun- 

 dancy of its crinoids. It is in the Keokuk Group. 



The genus Nautilus among the Cephalopoda is clearly recognized and is 

 quite abundant, while the genus Ortlioceras, whose perfection represented an or- 

 ganization akin to the embryonic form of the Nautilus Ims become correspond- 

 ingly rare. 



The Carboniferous conglomerate is 1,400 feet in thickness in Pennsylvania, 

 and entirely thins out before reaching the Mississippi river. It is only from 100 

 to 200 feet thick in Ohio. 



Prof. Hall says : " It was evidently formed from the fragments of older for- 

 mations, drifted, water-worn, rounded and deposited with the larger pieces at the 

 base, and the whole cemented together with smaller pebbles and sand. The 

 depth of the formation in Pennsylvania, and its thinning out to the North and 

 West, shows the current to have been from Southeast to Northwest, and probably 

 indicates the close proximity of the source in a Southeasterly direction. In 

 Michigan the thinning out is toward the South, or in a contrary direction. In 

 Illinois the formation thins out from the West toward the East. The character of 

 this formation, its manner of deposition, the currents which must have existed 

 to distribute it, all indicate that this continent was an archipelago at the era of 

 the Carboniferous conglomerate." 



In some places the conglomerate is a quartzose grit used for millstones, and 

 it is hence called the Millstone grit. 



The Coal Measures are 14,570 feet thick in Nova Scotia, 8,000 feet in Penn- 

 sylvania, 2,500 feet in Tennessee, 2,000 feet in Ohio, 1,200 feet in Illinois, 640 

 feet in Missouri, 2,000 feet in Kansas, and a greater thickness in Nebraska. 



This Group is sometimes divided into Upper and Lower Coal Measures, a 

 separation that seems to be founded upon the fossil contents in many places. 



Land Plants, which began their existence in the Devonian era, if we except 

 Psilophyton princeps, became al^undant in the Coal Measures. They are distri- 

 buted through the rocks, the shales, and the coal. Marine Vegetation, the growth 

 of the Marsh, and the Flora of dry land, existed in immense quantities, and was 

 widely distributed, but the higher orders of plants and forest trees were yet un- 

 known on the face of the earth. 



