386 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



The average optimum exposure of these groups as indicated by the 

 optimum exposure can, then, be expressed somewhat as follows : — 



Anthophyllite 2 minutes. 



Gedrite 2 



Tremolite 3 " 



Actinolite 2 " 



Richterite 1 " 



Astochite 1 " 



Edenite 2\ " 



Pargasite 2| " 



Common and basaltic hornblende l^^ " 



Glaucophane If " 



Riebeckite f " 



Arfvedsonite \ " 



Barkevikite \ " 



Aenigmatite ^ " 



Attackabilitt of the Amphiboles. 



Referring as they do to the appearance of best figures on the prismatic 

 cleavage, without regard to their .size, these tables do not express the 

 attackability of the various species. Thus the average time for the 

 development of the sharpest figures on anthophyllite and actinolite is in 



