Siliceous Genus. 135 



SPECIES XX. 



S/liceous Earth more or lefs perfectly united to 



from ~ to -j?- of its weight of Argill^ 0,3 



or 0,4 of Its iveight of Iron, and a fmall 



proportion of pure Calcareous Earth 9 but 



this isfometimes 'wanting. 



Lava. 



By Lava I underftand the immediate pro- 

 duct of liquefaction or vitrification by vol- 

 canic fire, which mould carefully be diftin- 

 guifhed from the fubfequent productions af- 

 fected by the water either in a liquid or fluid 

 ftate, which generally is ejected at the fame 

 time. 



All lavas are more or lefs magnetic, give 

 fire with fteel, are of a granular texture, and 

 fufible per Je. Moft are decompofable by 

 long expofure to the air, fooner or later, ac- 

 cording to the proportion of iron and calca- 

 reous Earth, and according as they were more 

 or lefs perfectly melted or vitrified. 



Mr. Bergman, in his elaborate treatife on 

 volcanic products, diftinguifhes three varieties 

 of lava ; the cellular or frothy, the compact, 

 and the vitreous. 



K 4 I. VARIETY, 



