WARREN. — ALKALI-GRANITES AND PORPHYRIES. 279 



more aegirite. The phenocrysts of feldspar when present — which 

 is usually the case — consist of a fine microperthite, and show, as in 

 all of the xenoliths previously described, an inner core about which is a 

 later margin enclosing few to many grains of the dark minerals or 

 quartz. Here, however, the texture of the core is about the same as 

 that of the rim. The feldspar and quartz of the groundmass are 

 sometimes entirely xenomorphic, in other instances the feldspar is 

 quite rectangular in outline, in others the quartz is characterized by a 

 distinctly round habit and then is apt to be poikilitically enclosed in 

 the feldspar. The pyroxene forms irregular elongated prismoids 

 lying between the feldspar and quartz, also commonly enclosing the 

 feldspar. The hornblende is wrapped about the feldspars, often form- 

 ing poikilitic groups of some size. Slender shreds and fibers of blue, 

 secondary hornblende may be abundantly distributed through the 

 rock. Accessory minerals are as usual, although, as in the case of 

 type (2), fluorite, zircon and titanite may be locally abundant closely 

 associated with the quartz and often wrapped about the feldspar in 

 very irregular masses. This suggests a later introduction of these 

 constituents. In certain of these xenoliths the pyroxene shows a 

 peculiar alteration and replacement. The centers of the majority of 

 the pyroxene crystals are replaced, sometimes by a material which 

 appears to be siderite, sometimes by a fibrous or foliated material of 

 medium, mean refraction and high double-refraction, probably mus- 

 covite, but usually by both materials present in varying amount. 

 Minute grains of magnetite and sometimes fluorite are associated with 

 these. Decomposition of the siderite develops a yellow stain, and this 

 can be seen in the hand specimen and is believed to account largely 

 for the yellowish color of so many of the fine-grained xenoliths particu- 

 larly of type (3). The replacement of the center of the pyroxene 

 seems to have been accompanied by more or less recrystallization of 

 the marginal parts which as a rule are granular and very irregular. 

 Material from these has spread out into the surrounding microper- 

 thite, particularly along the lamellae of the potash member. The 

 remaining pyroxene is in part aegirine-augite, though outwardly it 

 appears to be aegirite. As augite is abundantly present in so many 

 of the xenoliths of the contact types it is not improbable that the cen- 

 tral parts of the pyroxene in these cases was augite, and that it has 

 been decomposed and replaced by processes perhaps connected with 

 pneumatolytic activities in the enclosing granite. 



^Microscopic study of the xenoliths of type (2) show that they are 

 uniformly more quartzose and richer in aegirite than the others. In 



