|161«8,.] ^N THE FOR^fA-nON OF ROCKS, f^^ 



is a rouglmess and asperity in the structure^ owing to 

 their half vitrification, and numberless small pore^ 

 which they contain ; this asperity is softened down by 

 Age. Old lavas by infiltration^ and absorption of water^ 

 ivith the different substances that accompany it^ put oi^ 

 a milder and more unctuous structure, and approach 

 nearer the structure of NeptunisJi rocks. 



Currents of lava flowing from the crater as a centre^ 

 to^vards the circumference, are irregular and abrupt ijj. 

 their relativ*i position with suiTPuading fpi^n^tioijs 5 

 they have no marks of stratification; when divided U is 

 jalways by vertical fissures ; they 9-re found in detach- 

 <ed masses or long ridges, of a considerable thickness 

 in proportion to their widtli : occupying the iuequalites 

 of tlie surface of all formations whereon tliey lie, and 

 M ith which they seem to have nothing in common. 



Mineralogists liave divided the lavas into different 

 species, according to the different substances which 

 compose the mass of the rock ; others have classed them 

 according to the different crystals they contain ; but 

 these distinctions in no wise affect their origin, and are 

 foreign to the present subject. 



The total absence of metallic veins in lavas, forms a 

 mark of distinction between the two origins. Some 

 iron has been found disseminated in the cavities, in the 

 form of specular iron ore, which is evidently formed 

 by evaporation, but in small quantities scattered through 

 the porous lavas. 



Submarine eruptions are common, as is proved by 

 the number of islands thrown up under the evidence 

 of our senses ; and the still greater number of islands 

 that apparently, and by direct analogv, have been pro- 



Vol.L * Z ' 



