666 THE WONDERS OF GEOLOGY. Lect. VII. 



situated on the east side of the range of rocks forming 

 the Wrekin and Wenlock Edge, the coal strata being 

 superposed on mountain limestone ; it contains beds and 

 nodules of iron-stone, enclosing organic remains. This 

 coal-field is remarkable for the dislocated and shattered 

 condition of the strata, and the intrusion of volcanic rocks ; 

 the latter do not appear as dikes or veins, in the fissures of 

 the beds, but rise up in mounds or protuberances. The 

 walls of the fissures are in some instances several yards 

 apart, the intervals being filled with- debris. Strata con- 

 taining marine shells, alternate with others abounding in 

 fresh-water shells and land plants, as in Derbyshire. 

 These alternations prove that these coal-measures were 

 deposited in an estuary, subject to occasional freshes 

 from a considerable river ; the frequent alternations of 

 coarse sandstones and conglomerates, with beds of clay 

 and shale containing the remains of the plants brought 

 down by the river, support this opinion.* The strata 

 forming this carboniferous series, consist of quartzose 

 sandstone, indurated clay, slate -clay, and coal. A pit 

 sunk in Madely colliery, to a depth of 730 feet, passed 

 through eighty-six beds of alternating quartzose sandstone, 

 clay-porphyry, coal, and indurated clay containing nodules 

 of argillaceous ironstone. The sandstones of Coalbrook 

 Dale are fine-grained and micaceous, and some beds are 

 penetrated by petroleum^ which at Coalport escapes from 

 the surface in a tar-spring ; bitumen also occurs in some of 

 the shales. Plants, shells, and crustaceans, are abundant 

 in the shale and iron-stone nodules; and the remains of 

 insects are sometimes met with.f 



6. Nature of coal deposits. — This brief notice of 

 two of the British coal-fields will serve to convey a 



* See a highly interesting memoir on this coal-field, by Mr. Prest- 

 M-ich; and Sir K. I. Murchison's " Silurian System," chap. vii. 

 \ Medals of Creation, vol. ii. p. 575. 



