Great Coalfield of Shropshire. 195 



angle of about 6°. It is composed of the usual members, namely, 

 of quartzose sandstone, of indurated clay, of clay-porphyry, of slaty- 

 clay, and of coal, alternating with each other without much regu- 

 larity, except that each bed of coal is always immediately covered by 

 indurated or slaty-clay and not by sandstone. The series Is the 

 most complete in the deep of Madeley colliery, where a pit has been 

 sunk to the depth of seven hundred and twenty-nine feet through all 

 the beds, eighty-six in number, that compose this formation. 



The sandstones which make part of the first thirty strata, are fine- 

 grained, considerably micaceous, and often contain thin plates or 

 minute fragments of coal. The thirty-first and thirty- third strata 

 are coarse-grained sandstone entirely penetrated by petroleum ; they 

 are, both together, fifteen feet and a half thick, and have a bed of 

 sandy slate-clay about four feet thick interposed between them. These 

 strata are interesting, as furnishing the supply of petroleum that 

 issues from the tar-spr'mg at Coalport. By certain geologists this 

 reservoir of petroleum has been supposed to be sublimed from the 

 beds of coal that lie below ; an hypothesis not easily reconcilable to 

 present appearances, especially as it omits to explain how the pe- 

 troleum in the upper of these beds could have passed through the 

 interposed bed of clay so entirely as to leave no trace behind ; it is 

 also worthy of remark, that the nearest coal is only six inches thick, 

 and is separated from the above beds by a mass ninety-six feet in 

 thickness, consisting of sandstone and clay strata without any mix- 

 ture of petroleum. At the depth of four hundred and thirty feet 

 occurs the first bed of very coarse sandstone or grit ; it? thickness is 

 about fifteen feet. The next bed of sandstone deserving notice 

 occurs at the depth of five hundred and seventy-six feet, is about 

 eighteen feet thick, is fine-grained and very hard, and often mixed with 

 a little petroleum j the name given to it by the colliers is the bigfinU 



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