( 27d ) 



fomewhat fcaly and conchoidal, of a grain 

 moderately tine, giving fcarcely any fparks 

 with fteel, and having, in the points, a very 

 flight degree of tranfparency. The feltfpars 

 and micas, however (of the former of which 

 the number is but fmall, but that of the lat- 

 ter very conliderable), do not form one ho- 

 mogeneous pafte with the bafe, but are both 

 in a flate of the greateft j^refervation. The 

 feltfpars exhibit a cryftallized form, but it is 

 impoffible to define it where the petrofilex 

 has not been altered. It is not the fame 

 with the fuperficial cruft, the bafe of which, 

 having been in part deflroyed, holds thefe 

 ftones but feebly ; fo that they may eafily 

 be detached entire with a knife ; when they 

 appear to be prifmatic cryftals, with tetra- 

 hedral rectangular facets. We cannot, there- 

 fore, in any manner, call this vein a glafs, 

 but only, at the utmoft, a vitreous lava. 



By the fide of this volcanic rock, there is 

 another, of a congenerous bafe, and con- 

 taining the fame micas and feltfpars ; but it 

 is of a pale green colour, and tolerably 

 I hard : 



