254 PROCEEDINGS OF THE AMERICAN ACADEMY. 



now almost wholly recrystallized to an albitc-niicrocline microperthite 

 which occasionally takes on a very curious habit (see below), and al- 

 though there appears to have been some of the same albitization etc. 

 as has been described for the granite-porphyry, the changes in the 

 contact phases appear to have been more in the nature of a simple 

 recrystallization. The later rims of groundmass age are either lacking 

 or are developed to a slight extent. IVlany of the quartz crystals, 

 locally, show a well developed rim of later growth. Occasional 

 crystals or groups of hornblende seem to have been present as judged 

 by alteration products. There is often a strong clustering of the aegir- 

 ite microliths as if in an attempt to form a larger crystal, and there are 

 quite numerous small prismatic crystals of aegirite. Many aggre- 

 gates of aegirite crystals occur whose outlines and close packing 

 suggest that they were originally formed from a homogenous crystal 

 of pyroxene. 



The curious mode of alteration of the feldspar phenocrysts which 

 was referred to immediately above occurs rarely, and is not strongly 

 developed in the contact porphyry as a whole. The finest examples 

 were observed in slides from specimens taken a few feet east of the 

 aporhyolite contact of Hemingway Hill. It is so unusual, so far as 

 the writer's experience goes, that its peculiarities will be noted and 

 illustrated by microphotographs. For the most part, the phenocrysts 

 have been completely recrystallized into curious irregular areas of 

 slightly radiate intergrowths of albite and microcline, giA'ing the im- 

 pression of a delicate tracery (see Figure VHa and b, Plate 2). 

 Occasionally the Ijorders of the phenocrysts have been replaced, 

 wholly or in part, by a band of short albite laths alternating with 

 microcline (see Figure VHa). 



At the immediate contact and for a few inches away, the ground- 

 mass becomes extremely fine so that it appears almost isotropic with 

 low powers, and is only imperfectly resolved even with very high 

 magnifications. It consists of exceedingly minute prismoids of aegir- 

 ite mingled with a feebly polarizing aggregate of quartz and feldspar; 

 also with much fine dust, magnetite octrahedra and hematitic material. 

 These latter may be due to alteration which has affected to a greater 

 or less extent all of the specimens which it was possible to collect. 

 Replacements of what appear to have been small pyroxene pheno- 

 crysts may be occasionally seen. The feldspar and cpiartz pheno- 

 crysts are less numerous and smaller, and the proportion of quartz 

 relative to feldspar has increased (see Rosival measurements, table 

 p. 243, C'olumn III). The feldspar in all the slides examined is 



