MODE III. BASALTIN. 49 



passage. In fact, like the courses of fluid mat- 

 ter, they have been subservient to all the laws of 

 hydrodynamics. The history of these currents 

 of lava is complete, and there is nothing left to 

 the imagination to supply. We behold the ori- 

 fice from whence they issued, the course they 

 pursued, the country they occupy, &c. 



" They flowed upon granite : their substance 

 then was either in or under that rock ; now these 

 lavas contain from 15 to 20 per cent, of iron; 

 the granite possesses scarcely any ; they do not 

 therefore consist of granite, fused and wrought 

 by volcanic agents: we must therefore, with 

 Dolomieu, seek under this rock, for the matter 

 which has yielded this substance ; but here we 

 can only form conjectures. The cause which 

 may have developed this subterranean fire, the 

 combustible matter which may have maintained 

 it, are entirely unknown to us. It is not coal, or 

 bituminous matter, for they are only found in 

 secondary regions, and never either in or under 

 granite: it does not consist in pyrites, because 

 pyrites, alone and enclosed in the bosom of the 

 earth, never decompose, and generate no heat. 

 As for the period when these lavas flowed, al- 

 though anterior to the history or tradition of 

 mankind, it is nevertheless very recent when 

 compared with those vast degradations which 



VOL, i. E 



