MODE III. COAL. 565 



which descend two thousand four hundred feet, 

 or about half a mile. The semidiameter of the 

 globe is about 3500 miles ; so that our know- 

 ledge, comparatively, would only extend to the 

 outward texture of the paper, of a common 

 globe three feet in diameter. 



Mr. Kirwan has, with his usual accumulated Soils. 

 reading, discussed the various soils in which coal 

 appears; but an enumeration of the different 

 beds of clay and stone, would little interest the 

 general reader, while the scientific may consult 

 his work*. "The beds which immediately co- 

 ver coal, and are thence called its roof, are shale, 

 (a kind of clay-slate,) and argillaceous sand- 

 stone. Both contain impressions of vegetables, 

 generally such gigantic ferns and reeds as at pre- 

 sent astonish the traveller in the tropical regions. 

 The strata on which coal reposes, which are 

 thence called floor, sole, or pavement, are some- 

 times shale, or indurated clay; but more fre- 

 quently sandstone; and often the red ferrugi- 

 nous kind, which is esteemed most ancient. The 

 shells are chiefly those of rivers, and seldom those 

 of the sea. It is now well known, from the ex- 

 periments of Mr. Hatchet, that this substance is 

 of vegetable origin ; and it is a singularity, but 

 upon which no general theory can be con- 



* Geol. Ess. p. 290. 



