MODE IV. FELSITE. 



colours, white, grey, red, or brown. The glo- 

 bules, whose growth may be traced from the 

 instant when, having a diameter of half a line, 

 they begin to be apparent, till they have acquired 

 an inch in size, are generally black, with an 

 aspect of horn, sometimes brpwn or red, but 

 always of a different colour from the base : they 

 are striated from the centre to the circumference, 

 and have almost always a small crystal of fel- 

 spar, or a grain of quartz, for a central point. 

 Sometimes the gland is not completely formed ; 

 its place is only marked by the circular spot 

 round the central grain. The greater part of 

 these glands adhere strongly to their base, per- 

 fectly incorporating with it : some, but not those 

 which are striated, may be detached, and leafe 

 their impressions. It seems to be a mixture of 

 steatite which favours their separation. These 

 globules, which are not always harder than the 

 paste which contains them, are affected by fire 

 like petrosilex; and only seem to differ from 

 their base by the kind of texture that a peculiar 

 aggregation has caused them to assume." 



Dolomieu, following the observations of Saus- 

 stjre, 1079, has demonstrated*, that the petro- 

 silex of the Swedish and French authors is a 



* Journal de Physique, new series, vol. i. p, 244. 

 M 2 



