330 PROCEEDINGS OF THE AMERICAN ACADEMY. 



parts of the original fekispar substance, and these have not yet suffered reery- 

 stallization, having been of more stable composition than the interior. 



Fig. v.— Altered granite-porphyry, between Hemingway and Hancock 

 Hills, Blue Hills. This shows the remnants of two feldspar phenocrysts. 

 All that remains of the original crystals is a part of the margin. The white 

 streaks of albite, which mark the position of cracks in the original crystal, 

 remain (see figs. Ill and IV). Between these is now a mass of albite and micro- 

 cline grains which are scarcely distinguishable from the surrounding ground- 

 mass. This illustrates an extreme stage in the destruction of the feldspar 

 phenocrysts. The groundmass still shows most of the original aegirite, but 

 the hornblende has suffered much modification. 



Microphotograph, crossed nicols. Magnification about 2.5 diams. 



PLATE 2. 



Fig. VI. — Granite-porphyry from about 20 ft. back from the contact, east 

 of summit Pine Hill, West Quincy. Shows a feldspar phenocryst crossed by 

 streaks of albite and recrystallized to a curiously mottled microperthite. 

 About the end of the crystal is seen a micrographic intergrowth of quartz and 

 feldspar. The groundmass is here much coarser in grain than in the porphyry 

 of the Rattlesnake Hill type and the grain is fast approaching that of the gran- 

 ite into which this rock passes within about twenty feet. 



JVIicrophotograph, crossed nicols. Magnification about 25 diameters. 



Fig. Vila. — Quartz-feldspar-porphyry, from near the top of Hemingway 

 Hill, Blue Hills. This shows the greater part of a feklspar phenocr3-st which 

 has undergone a partial and unusual alteration. Considerable portions of it 

 are still a perfectly homogeneous, but about its margins, also along the edges of 

 a break which crosses it, it has been replaced by a narrow band of normally 

 •orientated lath-like crystals of albite and microcline. A large part of the in- 

 terior has been changed to small areas of microperthite of curious pattern 

 (see Figure Vllb). The groundmass has been forced in along the break 

 referred to. 



Microphotograph, cro.ssed nicols. Magnification about 2.5 diams. 



■Fig. Vllb. — Same section as Figure Vila. Shows a feldspar phenocryst to 

 ■some extent broken and entirely changed to a curious aggregate of small 

 microperthite areas. The structure of these areas is irregular and slightly 

 divergent giving the effect of a curious and beautiful tracery. 



Microphotograph, crossed nicols. Magnification about 25 diams. 



Fig. ^TIIa. — Rhombenporphyry Xenolith, Pine Hill Area, West Quincy. 

 Shows a characteristic phenocryst of soda-orthoclase (anorthoclase?) The 

 shape of the crystal is characteristic of much of the feldspar in the rhomben- 

 porphyry as a whole, likewise the minute rod-like pyroxene microliths, located 

 just outside the sharply marked boundary but lying in a band of feldspar 



