NATURAL HISTORY OF CREATION. 65 



of its composition than mii^ht be expected. Some are 

 siliceous or arenaceous (sandstones), composed mainly of 

 fine grains from the quartz rocks — the most abundant 

 of the primary strata. Others are argillaceous — clays, 

 shales, Arc, chiefly deiived, probably, from the slate beds 

 of the primary series. Others are calcareous, derived 

 from the eai-ly limestone. As a general feature, they 

 are softer and less crystalline than the primary rocks, as 

 if they had endured less of both heat and pressure than 

 the senior formation. There are beds {coal) formed 

 solely of vegetable matter, and some others in which 

 the main ingredient is particles of iron (the iron Uack 

 hand). The secondary rocks are quite as communicative 

 with regard to their portion of the earth's history as 

 the primitive were. 



The first, or lowest, group of the secondary rocks is 

 called the Carhoniferous Formation, from the remarkable 

 feature of its numerous interspersed beds of coal. It 

 commences with the beds of the mountain limestone, 

 which, in some situations, as in Derbyshire 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 (anthra- 

 cite), the whole being covered in some places by the mill- 

 stone grit, a siliceous conglomerate composed of the 

 detritus of the primary rocks. 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 formerly suggested as having produced this sub- 

 stance. 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 conspicuous coal h^ds, alternating 



c 



