Chap. 45.] 'Strata of the Earth. 91 



remarked that the largeft mountains are chiefly granite. 

 Wherever, therefore, the firft of thefe ftrata appears 

 on the furface, the fecond lies certainly under it, the 

 third under the fecond, &c. ftill excepting the toad- 

 ftone, which, being a volcanic production, may be 

 fuppofed to be in fome meafure cafually inter- 

 pofed. 



The toad-ftone interfeclrs all the mineral veins, and 

 cuts off all communication between the upper and 

 lower parts of the fifliires, being continued horizon- 

 tally in one uninterrupted mafs. Toad-ftone is of an 

 extremely hard and clofe texture, fo much fo as even 

 to prevent water from filtering through it, at leaft in 

 any quantity. It is perfectly fimilar to Iceland lava 

 in appearance, and in being unafiailable by acids. It 

 has no fifTures, and frequently fills up the fiffures of 

 the other ftrata ; in fine, it being not univerfal, but 

 only an occafional appearance, there is the utmoft pro- 

 bability that it is a fpecies of lava. It being inferted 

 between the other ftrata feems alfo to afford a proof, 

 that it originally flowed from a volcano, the funnel or 

 ihaft of which did not approach the open air, but dif- 

 charged its fiery contents between the ftrata in all 

 directions. When the toad-ftone is dug through, how- 

 ever, and the vein or fiflure purfued, the miner is 

 never difappointed in meeting it again, as fcon as he 

 arrives at the ftratum of lime-ftone *. 



The ftrata of coal, argillaceous ftones, clay, &c. are 

 always incumbent on the ftrata of grit, male, and 

 lime-ftone. The former are feldom in ftrata; of above 

 twenty feet thick, and generally not more than four 

 or five ; the latter are in ftrata of from fifty to one 

 hundred and fifty feet in thicknefs or depth f. 



* Whitehurft, Chap. XVI. t Ib 



U * All 



