WARREN. — ALKALI-GRANITES AND PORPHYRIES. 269 



The groundinass consists essentially of microperthite (rarely a little 

 beparate albite) augite and ilmenite or magnitite. The feldspar is 

 in excess and now contains more or less abundant secondary horn- 

 blende, epidote, etc. The feldspar grains are xenomorphic and roughly 

 equidimensional and of somewhat variable size. The most commonly 

 observed dimensions are from 00.1 to 00.2 mm., the range from per- 

 haps 0.05 to 0.3 mm. The augite lies between the feldspar, indents 

 it, and is often included in the larger grains as well as in the ground- 

 mass additions to the feldspar phenocrysts. They have the habit 

 either of rounded grains, short irregular prisms, or of considerably 

 elongated (parallel to c') prismoid forms with irregularly developed 

 edges. A strong tendency is shown for the smaller prisms to grow 

 end to end forming a small train, and many of the prismoid crystals are 

 little more than loosely joined shorter crystals grown end to end. 

 This habit is doubtless due in part to the growth of the augite along 

 the direction of the perthite intergrowth in the feldspar, and it thus 

 also happens that the augite crystals have a parallel orientation over 

 small areas. Ilmenite or magnetite grains are commonly present in 

 or about the augite. The augite throughout shows a strong tendency 

 to alter into a green or bluish-green hornblende often accompanied by 

 a lighter green, micaceous mineral apparently a more or less altered 

 biotite. The alteration appears to be a complicated process which 

 involves not only the augite but the magnetite or ilmenite grains 

 usually found with it, and the adjoining feldspar. The resulting 

 products gradually replaced the augite and spread out into the feld- 

 spar particularly along cracks and crystal boundaries. The horn- 

 blende is mostly of the finely prismatic, aggregated type, though some 

 appears in the form of more massive crystals; the other principle 

 product consists of fibers or plates or is closely felted with a radiate 

 structure. It is strongly pleochroic in light yellow to pale green tones, 

 shows a parallel extinction and a strong double refraction. It appears 

 to be some form of biotite with a chloritic alteration. It often forms 

 patches occupying the position of original augite and ilmenite. In or 

 about these areas are more or less hornblende, epidote prisms and some- 

 times well formed crystals of titanite. Occasionally these patches may 

 be observed 3 or 4 mm. across, and these doubtless represent not only 

 the replacement of augite, etc., but point to an accumulation of the 

 secondary products about centers of alteration and replacement. In 

 the more syenitic types of this rock (Pine Tree Brook area) the horn- 

 blende has been recrystallized into good sized crystals and aggregates 

 of hornblende which replace the already small amount of groundmass 

 and enhance the highly granitoid appearance of this phase of the rock. 



