WARREN. — ALKALI-GRANITES AND PORPHYRIES. 249 



small adjacent grains and it may l)e that the clusters represent loosely 

 connected poikilitic growths, the indi^'idual members of which, have 

 been for the most part separated, but only slightly or not at all de- 

 orientated by slight movements in the groundmass. Upon alteration 

 they become opaque, apparently due to the formation of magnetite 

 or ilmenite, or else they alter to limonite (?) blurs. In some instances 

 they are seen accompanied by a development of astrophyllite fibers 

 as is also the case with the aenigmatite of the granite. 



The groundmass, aside from the later growths upon the feldspar 

 phenocrysts and the poikilitic portions of the hornblendes and the 

 aenigmatite, all of which are clearly of contemporaneous age with the 

 groundmass and therefore properly belong to it, consists of a micro- 

 crystalline, inequigranular mixture of quartz, microperthite and aegi- 

 rite with some accessory hornblende, mostly of secondary origin, 

 magnetite, hematite, dusty particles, and calcite. The average 

 grain of the c^uartz and feldspar will probably lie a little under 0.02 

 mm., the range from about 0.06 to 0.00.5 mm. The microperthite 

 grains though apt to be sub-rectangular, particularly when enclosed 

 in the hornblende, are usually quite irregular in shape, the quartz 

 often shows a tendency to round outlines while the aegirite is scattered 

 abundantly among the other minerals in the form of very irregular, 

 small grains whose longest dimension rarely reaches 0.02 mm. and is 

 usually measured in thousandths of a mm. 



Texture. — Following the descriptive terms as proposed by Cross, 

 Iddings, Pirsson and Washington, ^^ the rock is sempatic and skedo- 

 granophyric, and in addition the groundmass is in part poikilitic. 



Variations from the normal type on Rattlesnake Hill. — A portion 

 of the greenish-grey porphyry is characterized by the presence of a 

 reddish discoloration in the feldspar and a slightly darker color. 

 In such the microscope shows the presence of abundant deposits of 

 hematite along the cracks in the feldspar, of more abundant blue 

 hornblende shreds in the groundmass, while the groups of poikilitic 

 hornblende are bluer in color and often show secondary blue fibers 

 and shreds developed about and upon them. 



The strongly marked streaks of dark-grey porphyry are in the nature 

 of indefinite interlaminations in the grey variety and blend rapidly 

 into it. They differ from the lighter type in the presence of very 

 abundant, fine, black dust in the groundmass and in tiie feldspar 

 phenocrysts, particularly about the margins and along cracks; in the 



36 Texture of Igneous Rocks, Journ. Geol., 14, No. 8 (Nov.-Dec, 1906). 



