Washington — Kaersutite from Linosa and Greenland. 189 



pits formed on lustrous basaltic hornblende and in other par- 

 ticulars the etch figures on barkevikitic hornblende. The etch 

 figures on the faces of the prism zone prove with certainty that 

 the mineral is monoclinic, and that it belongs in the general 

 group of the dark, highly ferruginous, aluminous amphiboles, 

 which are commonly referred to the hornblendes. 



Optically, this amphibole is remarkable in several of its 

 properties. The color is an intense jet black, with highly vitre- 

 ous, splendent luster. The streak is light brown. In common 

 with amphiboles of this group, it is brown to pale brown in 

 transmitted light, and is strongly pleochroic : c = dark brown, 



Figs. 2, 3. Kaersutite from Linosa. Etch pits on 110, HF. Fig. 2, 

 x 360; fig. 3, x 240. 



almost opaque, b=brown, a— -pale olive brown or olive green; 

 absorption, c > b > a. The natural color of the mineral is so 

 deep and the pleochroism so strong that the normal inter- 

 ference colors are greatly modified, and the observation of 

 the optic axes and similar optical phenomena is considerably 

 hindered. 



Owing to the extreme brittleness of the cleavage fragments, 

 it was found difficult to prepare sections parallel to the clino- 



