WARREN. — ALKALI-GRANITES AND PORPHYRIES. 215 



The total amount of these particles varies widely in different specimens, 

 in different feldspars in the same rock and even in the same feldspar 

 crystal. It may be said, however, that they are most abundant in 

 the granite having a darker color and are undoubtedly largely re- 

 sponsible for that color. The hornblende and aegirite microliths 

 occur together in varying proportions, or one may occur almost to 

 the exclusion of the other. No regularity can be discovered in this 

 variation. The microliths of both show a tendency to an arrange- 

 ment parallel to the direction of perthite intergrowth, to the direction 

 of the albite twinning plane, and to the cleavage directions. They 

 also occur commonly quite at random. The hornblendes are chiefly 

 of a deep blue color — riebeckitic — although in sections, showing the 

 green, cataphoric variety of hornblende in larger crystals, the micro- 

 liths have also a corresponding deep-green color. Their habit is 

 either that of short prismoid grains or more commonly of much 

 elongated prisms or even hair-like growths. They sometimes form 

 radiating clusters. The great majority are very minute and the 

 largest rarely exceed 0.02 mm. in breadth or length except the more 

 hair-like forms which attain a greater length. The aegirite micro- 

 liths vary from the most minute particles up to ones 0.01 or 0.02 

 mm. in their greatest dimension, which is ordinarily the direction 

 of the vertical axis. They commonly show a curious tendency to 

 arrange themselves end to end, with minute swellings and irregularities 

 along their sides. In color they are pale yellowish green or yellow 

 to almost colorless, a is always nearly parallel to the vertical elonga- 

 tion as it should be in aegirite, a characteristic which obviously 

 excludes their being epidote, a mineral they resemble in general 

 appearance and which they have apparently been mistaken for by 

 Dale. The minute black specks and grains, presumably iron oxides, 

 the indeterminate particles, probably sericitic material or kaolin, and 

 the cavities, show a great preference for the microcline member of the 

 microperthite in which they are astonishingly abundant. It has been 

 frequently noted in feldspars in which the black particles, etc., are more 

 than usually plentiful, that they are concentrated a short distance on 

 either side of cleavage cracks now healed with fresh albite material, 

 also about the later albite crystallizations mentioned above, but 

 never in them. The cavities seldom exceed a few thousandths of a 

 millimeter in their greatest dimension. They are round, ellipsoidal 

 or irregular in form, and with the exception of a little brownish black 

 or reddish material appear to have no filling. x\lthough one doubtless 

 gets a somewhat exaggerated idea of their total bulk from micro- 



