SECONDARY ROCKS. 



85 



CHAPTER VIII. 



SECONDARY ROCKS. 



Carboniferous Group. 



Division of Secondary Rocks. — Coal Measures. — Vegetation of 

 Carboniferous Formation. — Lower Secondary Rocks, how 

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

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

 Measures in England. — South Gloucestershire Coal-basin. 

 — Coal-fields of Derbyshire. — 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, 



