( 172 ) 



From this circumftance, and the nature of 

 their conftituent principles, their bafe ought 

 rather to be called a horn-ftone, of the na- 

 ture of which it at lead participates. This 

 analytical obfervation is likewife here very 

 opportune, as it is an additional proof that I 

 have properly called porphyritic the lavas 

 with a horn-ftone bafe, and containing felt- 

 fpars, which are fo numerous in thefe 

 iflands. 



The red Egyptian porphyries which I 

 analyfed are of two fpecies ; the firft has 

 been defcribed at page ii8 of Vol. JI. : 

 the fecond differs only in its colour being 

 of a lefs lively red, and in the abundance of 

 its feltfpars. It is evident, that to render 

 this experiment accurate, the fubftance of 

 thefe two ftones fhould be firft freed from 

 the Ihoerls and feltfpars which they con- 

 tain. 



Firjl Specks. 



Sllex, fomewhat reddifh, - 80 

 Alumine - - - 7 



Lime - - "3 



Magnefia 



