268 PROCEEDINGS OF THE AMERICAN ACADEMY. 



tinctly micropertliitic. Albite or pericline twinning were not noticed. 

 Measurements made on cleavage fragments give extinctions nearly 

 or quite 0° on 001 sections and up to 13° or 14° on 010. The outer 

 part of the original phenocrysts is usually distinctly more perthitic 

 and giACs the impression of having a stronger double-refraction, the 

 effect doubtless of the distinctly crystallized albite. The close of 

 the porphyritic stage of growth is not so sharply marked as a rule as 

 in the granite-porphyry, but is nevertheless perfectly clear and ex- 

 tends around the individuals of a single group, showing clearly that 

 the clustering took place during the porphyritic stage of growth. 

 The later border zone of the phenocrysts is microperthite, similarly 

 orientated to the interior and including small augite grains of the 

 groundmass, often abundantly, and these are commonly orientated 

 parallel to the perthite structure. There is little augite that can be 

 truly said to be included in the central and earlier parts of the 

 phenocrysts. The feldspars have suft'ered more or less from alteration, 

 and besides sericite and calcite contain, often abundantly, minute 

 crystals of hornblende, epidote and magnetite. In the syenitic phase 

 of this rock exposed in the Pine Tree Brook area, the feldspar, while 

 retaining traces of the characteristics al)ove noted, is much more gran- 

 itic in habit and consists of a finely developed microperthite. 



The augite appears to have developed only to a small extent during 

 the phenocrystalline stage. A few crystals with, at most, only a feeble 

 attempt at definite crystalline form are found lying for the most part 

 entirely in the groundmass although some of them lie in the marginal 

 parts of the phenocrysts and penetrate perhaps for a short distance 

 into the older portions. These augites include, and are indented by, 

 the feldspar of the groundmass. They rarely measure 3 mm. in length 

 and are usually not over 1 mm. long by perhaps | or f as broad. Their 

 margins are irregular and have attached to them, particularly on the 

 ends, smaller grains like those of the groundmass generally. They 

 include apatite grains and magnetite or ilmenite. In common with 

 the augite of the groundmass they are of a pale purplish color and 

 appear to be ordinary augite although the chemical analysis of the 

 rock indicates ^hat they are rich in the "CaFe-" molecule. They 

 are occasionally polysynthetically twinned. Their alteration is exactly 

 the same as that of the rest of the augite. 



In the more highly porphyritic types it is only rarely that a pheno- 

 cryst of quartz can be found. When such occurs, the crystal is rounded 

 in outline and is always bordered by a strong development of augite 

 grains. 



