Calcareous Genus. 41 



fervefces with acids ; it is of different de- 

 grees of hardnefs, and readily burfts and 

 falls into powder in water, or by expofure to 

 the air. When the calcareous Earth is in 

 large proportion it burns to lime, and even 

 vitrifies in a ftronger heat. The proportion 

 of mild calcareous Earth is from 50 to 75 or 

 80 per cent, if greater it ihould rather be 

 clafled among chalks. Hence any Earth of 

 this fort, that lofes from 16 to 27 parts of 

 its weight per cent, by folution in acids, may 

 be deemed a calcareous marl, this being the 

 weight of the volatilized fixed air which 

 correfponds with the above proportion of cal- 

 careous Earth, Note alfo, that by clay I 

 mean common clay, that is, a mixture of 

 argill and filiceous Earth, in which this lat- 

 ter moilly predominates. See Dr. Withering 's 

 Diflertation. Phil. Tranf. 1773, p. z6i,and3. 

 p. 4. 



II. VARIETY. 



In a Stony Form. 



Pietra Forte, Putra Fongaia of 

 the Italians, Margodes. 



The two firft differ only in hardnefs ; they 

 are of a grey, blue, or yellow colour ; the 

 firft loofe and porous, the fecond harder ; the 

 third is a tufa, which, containing the feeds of 

 mufhrooms, produces them yearly, on being 



