Chap. 45.] Central Fire in the Earth. 29 f 



vttion allures us, that this heat increafes in proportion 

 as we penetrate below the furface of the earth ; on 

 the contrary, many experiments ferve to evince, that 

 it rather decreafes, (though never to lefs than thirty- 

 fix degrees) and that its variation at the fame diftance 

 below the furface conftantly bears a proportion with 

 the variation of the folar heat at the furface. 



The more general opinion is, that the ftrata origi- 

 nally lay horizontally, and were formed by a depofi- 

 tion from water. The arguments for this opinion 

 are forcible. The relics of a variety of fubftances, 

 which we now find only in the fea, are found in rocks 

 and mountains, at a very great diftance from it. In 

 ftrata of lime- ftone, every where diftant from the 

 fea, we find the remains of fhells, &c. the productions 

 of the ocean. Other circumftances prove, that the 

 fea has covered parts of the earth, which are now at 

 a great diftance from it, and that the various direc- 

 tions which the ftrata now have were not their direc- 

 tion at their firft formation. That the frame of this 

 earth has undergone fome violent concufllon is evi- 

 dent, as was already ftated, from the traces which ftill 

 remain. The ftrata we have feen are often broken 

 in different directions, in general perpendicularly ; fb 

 that he parts of the ftrata are feparated from each 

 other. The width of thefe rents is different, fome- 

 times a few inches, fometimes many yards. They 

 are very commonly filled up with fubftances different 

 from the compofition of the ftrata. In mountains 

 there is fometimes obferved the appearance of a white 

 ftone, which pafles through it like a vein. This has 

 been a rent filled up with a particular kind of ftone, 

 Thefe are very common in the ftrata of coal, They 

 are generally of considerable hardnefs, and in them 

 metallic fubftances are ufually fgund. When any of 

 U 4 thefe 



