WARREN. — ALKALI-GRANITES AND PORPHYRIES. ZtO 



fine dark green to almost black groundmass, in which is enclosed 

 acutely terminated and often composite phenocrysts of feldspar. 

 In some, the phenocrysts are relatively few, in others they are very 

 numerous. In some of the xenoliths, quartz crystals are sparingly 

 developed, and these are always characterized by a halo of dark mineral 

 grains (usually hornblende) about them. Rarely small patches of 

 hornblende or pyroxene ma;)' be seen. In very fresh specimens the 

 feldspar phenocrysts are almost colorless and often show a fine cha- 

 toyancy. On weathering the feldspars whiten and the groundmass 

 becomes dull black and less clearly crystalline. 



Microscopic characters of the Xenoliths of the Contact Zones. — The 

 close resemblance to the rhombenporphyry is quite as obvious micro- 

 scopically as megascopically. The feldspar has precisely the same 

 characteristics, even to the sharpl^y marked rim of groundmass age, 

 including prismoids and grains of pyroxene arranged parallel to the 

 direction of perthitic intergrowth. The feldspar of the groundmass 

 is also the same. The same phenocrysts of pale brown augite occur 

 but these show a strong tendency to pass into a green variety, partic- 

 ularly about the margins. The pyroxene of the groimdmass has 

 about the same habit as that in the rhombenporphyry but is of a 

 light to rather strong green color often, with a weak pleochrism. Its 

 double-refraction is low, not exceeding that of ordinary augite, and its 

 optical properties otherwise seem to be those of augite, but it probably 

 contains some admixture of the aegirite molecule. Considerable 

 hornblende is also frecjuently present in the groundmass. It occurs in 

 part as a later growth on the augite, either in the form of minute prisms 

 or needles, or in a more massive form, and in part with a poikilitic 

 habit enclosing the groimdmass feldspar. Strongly pleochroic; a, pale 

 greenish-yellow or broAvn; /3, very dark green; 7, deep olive-green; 7 

 makes an angle of as high as 33 degrees on c', it appears to be an alkali 

 hornblende near catoforite. Biotite occurs as a finely foliated altera- 

 tion product of the augite and ihnenite or magnetite. The latter is 

 abundant in, and associated with, the augite and also scattered through 

 the rock. Magnetite forms grains and sharply bounded octahedra 

 and may be in part secondary, since it has been noticed that in the 

 rocks of this area the magnetite of secondary formation is apt to form 

 sharply bounded crystals. Apatite is present. Most of the xeno- 

 liths show considerable alteration resulting in the presence of calcite, 

 kaolin and ferruginous matter. 



The darker type of xenoliths pass on the one hand through all 

 gradations into a type lighter in color — greyish green — and with a 



