166 



time, insomuch that if they are lighted in the evening the fire will 

 keep alive during the whole night*." 



COAL is found at various depths in the earth, and interposed 

 between strata of different kinds. The veins of coal also vary con- 

 siderably in the thickness of the vein; sometimes being many yards 

 thick, and other times being merely a seam, little more than an 

 inch in thickness. 



The strata, which are generally interposed between the gravel, 

 clay, sand, and mould, at the surface, and the coal itself; or which 

 appear to be most generally connected with coal, have commonly 

 a considerable degree of hardness : but they differ from each other 

 very much in this respect, as well as in their thickness and their 

 other properties. Do not fear that, by endeavouring to-furnish you 

 with some information respecting these several strata, I shall engross 

 your time with the consideration of matters irrelevant to the grand 

 object of our inquiry ; on the contrary, be assured that it will de- 

 rive from this investigation a considerable degree of illustration. 



SAND-STONE is among the uppermost of the solid strata. It is 

 of a granulated substance, of various degrees of hardness and co- 

 herence ; and is formed by^ very small grains of various silicious 

 stones, and frequently of mica, imbedded in a calcareous argilla- 

 ceous, or even a silicious cement. In the latter instance they pos- 

 sess a great degree of hardness ; the hardness is also considerably 

 increased, in every species, by the admixture of iron which has 

 undergone but a slight degree of oxydation. 



RUBBLE-STONE is a species of sand-stone, containing also a bluish 

 slaty substance. 



LIME-STONE also frequently occurs among the superior solid 

 strata, and like the former is also found among the inferior strata. 

 It exists in very considerable varieties of hardness and coherence, 

 breaking with various kinds of fracture, according to the sub- 



* Marcus Paulus Venetus. Purchas's Pilgrimage, vol. iii. p. 88. 



