( 309 ) 



for their bafe. This Hone almoft always 

 fufficiently preferves in them its primitive 

 charaders ; but it is accompanied with a 

 principle, if I may ufs the expreffion, of 

 fibrofity and expanfion in its parts, which 

 exhibits no obfcure fign of its having been 

 ad;ed on by the fire. The lava with a 

 petrofiliceous bafe, of the crater of Vul- 

 cano *, is the only exception. On the con^ 

 trary, feveral of the Euganean petrofilices, 

 with refpedt to their texture, grain, com- 

 pa6tnefs, and fra£lure, fo exactly refemble 

 the moft perfedly characterized natural 

 petrofilices, that they might be miftaken 

 for them, if we did not find that they have 

 pafled into the ftate of lava. This peculi- 

 arity, however, is not confined to the Eu- 

 ganean volcanos ; it likewife appertains 

 to thofe of the ifies of Ponza, as has been 

 ihown by M. Dolomieu in great numbers 

 of lavas with a petrofiliceous bafe. I'hefe 

 lavas cannot but greatly excite our admi- 

 ration, as we find it impoffible to conceive, 



* See Chap. XIII. 



X 3 from 



