Chap. 41.] / Coal. 269 



a confiderable diftrict. This regularity, however, is 

 frequently broken by gaps filled by other matter, 

 which has evidently fallen in, in confequence of the 

 ftrata having been feparated from each other by fome 

 violent convulfion. The largeft gaps are called dykes, 

 and defccnd from the furface of the earth, fometimes 

 perpendicularly, fometimes obliquely, to the greateft 

 depths ever tried. On each fide of thefe gaps the 

 ftrata correfpond, but they are often funk feveral feet 

 or fathoms lower on one fide than the other, and this 

 is called a dip. 



With refpect to the origin.of pit coal, it is the opinion 

 of Dr. Black, Bifliop Watfon, and other philofophers 

 of high reputation, that the ftrata of coal were formerly 

 large collections of vegetable matter at the furface of 

 the earth. In diftant ages, Britain was probably al- 

 moft entirely covered with immenfe forefts and collec- 

 tions of peat mofs, which (according to the opinion of 

 thefe naturalifts) being covered with quantities of fand 

 or earth brought by floods, or by more gradual caufes, 

 as the falling of the fubftance of the neighbouring hills, 

 has been prefled and confolidated, in courfe of time, 

 into the fubftance called pit coal. The furface of the. 

 earth has alfo been probably rendered unequal in 

 various places by the action of earthquakes; this 

 would give rife to the formation of lakes in thofe 

 places which were deprefied. In this manner, a quan- 

 tity of vegetable matter would become covered by 

 depofition from water. Volcanic eruptions muft often 

 alfo have overwhelmed large collections of vegetable 

 matters. We even find vegetable matter in an inter- 

 mediate ftate between organized vegetable fubftances- 

 and coal ; for peat has ftill fome fmall remains of or- 

 ganic texture, but feems chiefly to confift of oily and 

 inflammable matter, which only requires time and the 

 preflure of fuperincumbent ftrata to convert it into the 



firm 



