NOME I. COMPACT LAVA. 319 



kinds of grey compact lava, with siderite, augite, 

 felspar; and red compact lava with leucite and 

 felspar. But by his immediate transition to the 

 lapilli, the sand, and the powders, he would rather 

 seem by the term compact, to imply a vague dis- 

 tinction from the loose substances, than a strict 

 application of the word : and this, among a thou- 

 sand instances, may show the necessity of austere 

 language, and the most precise definitions in mi- 

 neralogy. 



Faujas used to indicate five differences between Opinion of 



Faujas. 



trap and compact lava. 1. Trap is soft, and may 

 be scratched by a knife, which on lava loses edge. 

 2. Trap attracts iron, but lava is a magnet. 3. 

 In electricity, lava acts like glass. 4. There is no 

 olivine in trap, but it is common in lava. 5. Trap 

 in a furnace becomes a transparent glass, but lava 

 remains opake. These distinctions will not, how- 

 ever, be admitted by the Neptunists. In Bron- 

 gniart's opinion, compact lava always presents a 

 grain somewhat crystallised, in which it differs 

 from trap*. If basaltic columns be found on 

 Etna, their origin may still remain dubious ; for, 

 according to Gioeni, the radical parts of that 

 mountain are basalt, which is only concealed by 



* i. 551. 



