WARREN. — ALKALI-GRANITES AND PORPHYRIES. 239 



Though this last thickness may be exceeded in the western part of 

 the Blue Hills, it is thought to represent very roughly the present 

 thickness of the porphyry. 



The least altered material obtainable from the old quarries on the 

 east end of Rattlesnake Hill shows the granite-porphjTy phase to" be a 

 holocrystalline rock having much the appearance of a rather fine 

 grained granite (for which it is usually mistaken on first inspection) 

 of a prevailing light, greenish-grey color. Closer inspection shows 

 clearly that it consists of abundant phenocrysts of alkaline feldspar 

 and quartz embedded in a finely granular groundmass in which appear 

 numerous irregular specks or grains of black mineral. The feldspars 

 are the more abundant phenocrysts. They are elongated parallel to 

 the edge 001-010, have a somewhat tabular habit on 010 and although 

 usually rounded or broken on the ends, show nevertheless, a tendency 

 to form rather acute terminations. They may measure as much as 

 8 mm. long, 5 to 6 mm. in breadth by 2 to 3 mm. in thickness, but the 

 average is smaller. A chatoyancy, characteristic of the cryptoperthitic 

 feldspars, may sometimes be seen (this is well shown on polished 

 surfaces). The quartz forms rather inconspicuous rounded grains 

 usually smaller than the larger feldspars. The fracture of the rock 

 is much like that of the fine-granite and the jointing is likewise finer 

 than in the coarse granite, with a strong tendency to the formation of 

 sharply prismatic blocks. 



While reddish stains may be seen in a few of the feldspars of almost 

 any specimen, there are streaks and irregular patches in which red 

 spots are very abundant and characteristic. The grey variety of 

 the granite-porphyry is interlaminated, as it were, with streaks of 

 considerable regularity and persistence of a darker phase. This is 

 dark grey to almost black, and recalls the association of the dark 

 and light grey coarse-granite with which it is doubtless strictly 

 analogous. 



Alteration of the porphyry produces a whitening, generally accom- 

 panied by a slight brownish or reddish discoloration of the feldspar 

 phenocrysts, while the groundmass becomes grey or grayish blue 

 owing to the breaking up and dissemination of the dark constituents. 

 More advanced alteration produces a general breaking up of the 

 groundmass. 



Over large areas in the hills, the granite-porphyry appears in a form 

 that may more properly be termed in the field a quartz-feldspar- 

 porphyry than a granite-porphyry, for although, as sho\\n by the 

 microscope, the grain of the groundmass is but little finer than that 



