136 Elements of Mineralogy, 



I. VARIETY. 



Cellular Lavas. 



Thefe underwent only the firft and loweft 

 degree of fufion, being juft mollified and 

 heated fufficiently to expell the fixed air con- 

 tained in the argillaceous particles ; hence 

 they abound in fmall cavities arifmg from the 

 expaniion of that air after it had recovered its 

 elailic ftate. Their fpecific gravity is vari- 

 ous : fome are fo light, by reafon of their in- 

 ternal cavities, as to float for fome time on 

 water, and hence, have been miftaken for 

 pumice ftones, but they differ from thefe in 

 this, that their texture is never filamentous. 



Their colour is black, grey, brown, or 

 reddim brown, and their cavities are often fil-r 

 led with cryftalizations. 



Of this fort is the black cellular mill-ftonQ 

 of the Rhine. 



Thefe ftones contain from 45 to 50 per 

 cent, of filex, and from 15 to 20 per cent, 

 of iron, 4 or 5 per cent of pure calcareous 

 Earth, a&d the remainder argill, 



II. VARIETY, 



