COAL MEASURES. 



91 



4. Sandstones of different qualities, often in very 

 thin laminae. 



5. Coal, forming numerous beds or layers in the 

 sandstone. 



6. Sandstone, called millstone grit, because used 

 for millstones, containing vegetable remains. 



7. Calcareous slate or shale, from 300 to 600 feet 

 thick. 



8. Mountain or primitive limestone, the lower 

 strata containing marine animal remains, the upper 

 fresh-water deposites, with terrestrial vegetables. 



4. Coalbrook Dale Coal-bed. 

 Dr. Mantell, in his " Wonders of Geology," gives 

 the following account of this coal-field : '* It is situ- 

 ated on the east side of the range of transition rocks 

 forming the Wrekin and Wenlock Edge, and the 

 coal strata are superimposed on millstone grit. Beds 

 of ironstone occur, abounding in nodules, with or- 

 ganic remains. This coal-field is remarkable for 

 the dislocated and shortened state of the strata, and 

 the intrusion of volcanic rocks, which do not ap- 

 pear as dikes, as 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. Beds con 

 taining marine shells alternate with others abound- 

 ing in fresh-water shells and land-plants. The se- 

 ries of strata forming this carboniferous accumula- 

 tion consists of sandstone, indurated clay, slate 

 clay, and coal. A pit sunk in Madely Colliery, in a 

 depth of 730 feet, passed through 86 beds of alterna- 

 ting quartzose sandstone, clay porphyry, coal, and 

 indurated clay, containing argillaceous ironstone in 

 nodules. The sandstones of Coalbrook Dale are 

 fine grained and micaceous, and some beds are pen- 

 etrated by petroleum, which at Coalport escapes 

 from the surface in a tar-spring ; bitumen also oc- 

 curs in some of the shales. Plants, shells, and 



