42 



CARBONIGENOUS ERA. 



COMMENCEMENT OF LAND PLANTS. 



THE next group of rocks is called the Carboniferous For- 

 mation, from the remarkable feature of its numerous inter- 

 spersed beds of coal. It commences with the beds of the 

 mountain limestone, which, in some situations, as in Derby- 

 shire and Ireland, are of great thickness, being alternated 

 with chert, (a siliceous sandstone,) sandstones, shales, and 

 beds of coal, generally of the harder and less bituminous kind, 

 (anthracite,} the whole being covered in some places by the 

 millstone grit, a siliceous conglomerate, composed of the de- 

 tritus of the earliest formation. The mountain limestone, 

 attaining in England to a depth of eight hundred yards, 

 greatly exceeds in volume any of the primary limestone beds, 

 and shows an enormous addition of power to the causes con- 

 nected with animal life, by which this substance is supposed 

 to have been produced. In fact, distinct remains of corals, 

 crinoidea, and shells, are so abundant in it, as to compose 

 three-fourths of the mass in some parts. 



Above the mountain limestone commence the more conspi- 

 cuous coal beds, alternating with sandstones, shales, beds of 

 limestone, and ironstone. Coal is altogether composed of the 

 matter of a terrestrial vegetation, transmuted by putrefaction 

 of a peculiar kind, beneath the surface of water and in the 

 absence of air. Some estuary shells have been found in it, 

 but few of pelagic origin, and no remains of those zoophytes 

 and crinoidea so abundant in the mountain limestone and 



