152 PKOCEEDINGS OF THE AMERICAN ACADEMY. 



parallel to the elongation and the extinction is very small. These 

 have apparently been called epidote by Dale ^ and do suggest that min- 

 eral in appearance as does much of the coarser aegirite of the pegma- 

 tites. Epidote is, so far as observed, altogether wanting, a fact quite 

 in keeping with the chemical character of the pegmatite. 



The minute black particles, like occasional larger black grains, are 

 probably mostly ilmenite or magnetite ; some are hematite. They fre- 

 quently give rise to slight reddish stains in their immediate surround- 

 ings. They are more common in the microcline than in the albite. 

 Minute cavities are also abundant in the feldspar. These may be 

 round, elliptical, almost rectangular, or amoeba-like in shape. Though 

 sometimes partially filled with black, reddish or yellowish stains, they 

 do not seem to have now any other filling. 



RiebecMte. — Macroscopically the riebeckite is characterized by a 

 black or bluish-black color, often rendered greenish by the associated 

 aegirite. The crystals of riebeckite are prismatic, showing only outlines 

 of the unit prism and occasionally a face of the clinopinacoid. Termi- 

 nal faces were not observed. Measurements of the prismatic cleavage 

 angle made on two fragments yielded identical values of 55° 5', an 

 angle considerably larger than that of common hornblende which is 55° 

 49'. A rather feeble tendency to part parallel to the base seems to be 

 present. With the exception of the inclusions in the feldspar and some 

 of the smaller crystals, the riebeckite is always intergrown with aegirite 

 to a greater or less extent in the manner previously described. Where 

 the riebeckite comes in contact with the feldspar and quartz, the two 

 are always to some extent intergrown and as already noted small shreds 

 or grains of riebeckite often lie near the contact sometimes with the 

 same orientation as the larger grain. Again shreds grow out from the 

 edge as if they were secondary growths. The riebeckite also usually 

 contains abundant black particles and larger black oxide grains, pre- 

 sumably ilmenite. Occasional grains of feldspar, fluorite, and zircon are 

 also included, and patches of carbonates, fluorite, ore grains, and zircon 

 are often seen. 



The deep color and strong absorption of the mineral makes the deter- 

 mination of its optical properties with any precision very difficult. 

 This difficulty is increased by the fact that it has been found impossi- 

 ble after many trials to obtain entirely satisfactory sections of the min- 

 eral across the cleavage owing to its extreme brittleness. By the study 

 of finely crushed material and thin sections the following characters 

 have been made out : 



' Loc. cit., p. 49. 



