16 STEATIGKAPHICAL 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 tlie Waverly (Jroup, named from the 

 quarries at A\'averly, Ohio. In ilirliigan, Prol'. Wincliell subdivided the Group 

 into the Marshall C!roup, Napoleon Group, Michigan vSalt Group, and Carbon- 

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

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

 and consists mainl}- ol'red shale; it thins out rapidly towards the Northwest, but 

 maintains a great thickness Smitliwardly through ^'irginia 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 

 Paheozoic rocks. A single l(icalit3' at Burlington, Iowa, in the Burlington Group, 

 has furnished about 350 species. Another hjcalitj- at Crawfordsville, Indiana, 

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

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



The genus JSfuutilus among the Cephalopoda is clearl}^ recognized and is 

 quite abundant, while the genus Orthoceras, whose perfection rc])resented an or- 

 ganization akin to the embrj'onic form of the Xuutilus has become correspond- 

 ingly rare. 



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

 and entirely thins out before reaching the Jlississippi river. It is onlj- 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 depnsited Avith 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 lieen from Sontlieast to Northwest, and probably 

 indicates the close proximity of tlie source in a S(]utlieasterly direction. In 

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

 Illinois the formation thins out from the "West lo\\ ard tlu' East. The character of 

 this formation, its manner of (le|josition, the currents which must have existed 

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

 the Carboniferous couglomeratc." 



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

 it is hence called the Millstone grit. 



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

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

 feet in iMissoui-i, 2,000 feet in Kansas, and a gi'eater thickness in Nebraska. 



This (iroup is souietinies divided into I'p|)er and Lower Coal ^Measures, a 

 separation that seems to be Ibunded upon the fossil contents in man}' places. 



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

 PsilMiiliyfoii pri)icejis, liecanu' abundant in the (^'oal Measures. Thej' are distri- 

 buted through the rocks, the shales, and the coal. JMarine 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. 



