xxvi PREFACE. 



acquainted with all their varieties. In re- 

 fp'et to the whole furface of the earth, but 

 few volcanos have been examined ; and it is 

 obvious, that different pre-exifting rocks and 

 mixtures, under various circumftances and 

 degrees of fuiion, conflagration, and cooling, 

 muft have produced as many different mafles. 

 Common loam, mixed as ufual with a 

 little iron and fand, produces, by an intenfe 

 heat -of fife in every kiln, a fpecies of 

 {tone and vitrification, which bears a great 

 refemblanee to common lavas. Let us alter 

 the mixturcj and add fome more fand .or 

 falts, the produce -will undoubtedly prove 

 very different. Let us think of Meff. Wedg* 

 ivwd's 'and Reniley s, or other china and 

 glufs teahufa&ories ; of the metallic- furnaces, 

 and of that infinite- number of 'poiFible^com-r 

 Binat-ions-; -and we '^Vail- not winder if, in 

 futu-re~ times, the greal^chemical fire, of Na- 

 ture ihotrld be fouiicl'to hav^e produced many 

 ifpe&es'^&f rocks, which -hitherto -we have 

 little-thoiight to be of a volcanic kindito oi 

 This leads me, 3, S.Qjhej'ub-iuarJne-V.okartas 

 the ncw-raifed ijlands. The former, 



though 



