LESSONS IN GEOLOGY. 355 



difference is not very great. This age is divided into three parts, 

 the sub-carboniferous, carboniferous and permian. The sub-car- 

 boniferous rocks are mainly limestone, but in Ohio, Michigan and 

 Pennsylvania there are more sandstones and shales, all of marine 

 formation. The lowest strata of the Carboniferous is a coarse 

 sandstone called the Millstone grit. The other strata are thin 

 beds of coal, shale, limestone, sandstone, clay, iron, stone and 

 under clay. The permian rocks are an ill-defined group of sand- 

 stones, limestones, gypsum beds and shales, that mark a transi- 

 tion period from the palaeozoic to mesozoic eras. 



Coal occurs in separate areas or basins, as the Appalachian 

 coal field, the greatest in the world, extending from Pennsylvania 

 to middle Alabama, with an area of about 60,000 square miles 

 of work able coal; the central field, covering the greater part of 

 Illinois, part of Indiana and Kentucky, about 47,000 square miles. 

 The western field includes southern Iowa, and portions of Missouri, 

 Kansas, Arkansas and Texas, about 80,000 square miles. Then 

 about 6,000 square miles in Michigan, about 500 in Rhode 

 Island and about 18,000 in the region of the Bay of Fundy. 

 The carboniferous and sub-carboniferous rocks have a much 

 more extended area than the coal proper, as given above. 



As in the two previous ages, the invertebrates were well repre- 

 sented, and in addition to the marine forms there were many ter- 

 restrial and fresh- water mollusks, and some spiders and insects. 

 Fish were as large, abundant and varied as during the Devonian 

 age, and toward the close some amphibians appeared. But the 

 notable, the characteristic life of this age was its plants. Only 

 the cycads, a group of gymnosperms, were added to those known 

 in the Devonian age; but in general the plants of the carbonifer- 

 ous age were larger, more abundant and more varied in form; as 

 many as 500 species have been described from American rocks, 

 besides a large number of different ones from European rocks. 



It is generally believed that coal is of vegetable origin, and 

 that coal and its associated rocks were, in general, deposited 

 under the following circumstances : A large, nearly level, some- 

 what basin-shaped area of land has been covered with a bed of 



