SECONDARY ROCKS. 



85 



CHAPTER VIII. 



SECONDARY ROCKS. 



Carboniferous Group, 



Division of Secondary Hocks. — Coal Measures. — Vegetation ot 

 Carboniferous Formation. — Lower Secondary Rocks, how 

 Divided.— Millstone Grit — Its Mineral Contents.— Carbonifer- 

 ous Limestone — Its Kxtent in this Country.— Section of Coal 

 Measures in England.— South Gloncestershire Coal-basm. 

 — Coal-fields of l)erbyshire. — Coalbrook Dale. — Coal Meas 

 ures of North America. 



The secondary rocks have been divided into the 

 upper secondary and the lower secondary. The 

 lower secondary are equivalent to the coal formation^ 

 or, as it is sometimes called, coal measures. These 

 are composed of various beds of sandstone, shale 

 or slate, and coal, irregularly interstratified, and in 

 some places intermixed with conglomerates or 

 clay ; the whole showing a mechanical origin. Coal 

 measures abound in vegetable remains, and the coal 

 itself is almost universally referred to a vegetable 

 origin, being considered the accumulation of an im- 

 mense mass of plants. These have been distribu- 

 ted either by the agency of fresh or salt water 

 floods, over areas of greater or less extent, upon a 

 previously deposited surface of sand or argillaceous 

 mud, which afterward has been converted into 

 slate. After the distribution of these vegetables, 

 other sands or mud were accumulated upon them ; 

 and this operation, in some places, has been appa- 

 rently repeated several times. 



The vegetables which enter into the composition 

 of coal appear to belong to the lowest order chiefly, 



