WARREN. — ALKALI-GRANITES AND PORPHYRIES. 329 



PLATE 1. 



Fig. I. — Granite-Porphyry, Rattlesnake Hill, Blue Hills. At the bottom on 

 the right, is a pyroxene crystal surrounded by poikolitic hornblende enclosing 

 groundmass grains; above appears a cluster of aenigmatite grains; on the left, 

 below, is a cross-section of a pyroxene crystal with a small slightly darker 

 margin of aegirite; above is a massive hornblende becoming poikolitic out- 

 wardly and then showing enclosed groundmass grains. With the exception 

 of a small phenocrysts of feldspar, barely seen just above the center, the rest 

 of the section is all groundmass in which tiny microliths of aegirite can be 

 seen. 



Photomicrograph, plane light. Magnification about 15 diams. 



Fig. II. — Granite-Porphyry, Sassoman Pass, Blue Hills. On the right is 

 shown the edge of a large phenocryst of nearly homogeneous soda-potash 

 feldspar terminated bj' a sharply marked line outside of which is a narrow 

 margin of microperthite enclosing tiny groundmass crystals; near the bottom 

 is a small phenocryst of feldspar with a relatively broad, later rim; the rim 

 shows a delicate perthite structure which is also developed for a short distance 

 within the inner and earlier formed part. A similar phenocryst appears at the 

 top of the section. At the extreme lower side is seen a small phenocryst of 

 relatively late age enveloped by a relatively very wide rim of later age. In 

 this rim are minute enclosures of groundmass grains. The dark part of the 

 field is hornblende, poikolitically enclosing feldspar (microperthite) and quartz 

 crystals. 



Microphotograph, plane light. Magnification about 25 diams. 



Figure III. — Granite-Porphyry, Rattlesnake Hill, Blue Hills. This shows 

 a phenocryst of soda-potash feldspar surrounded by a narrow rim of later 

 microperthite enclosing groundmass grains. There is a faint perthite structure 

 developed in the inside portion of the phenocrysts next the rim but this is very 

 indistinctly showai with the magnification used. The phenocryst is crossed 

 by streaks representing cracks sealed with albite. A part of the body of the 

 crystal has suffered a recrystallization, shown as flower-like areas. 



Microphotograph, cros.sed nicols. Magnification about 25 diams. 



Fig. IVa. — Granite-Porphyry, Chicatawbut Hill, Blue Hills. This shows 

 one end of a largely recrystallized feldspar phenocryst. The boundary of 

 the phenocryst is marked by a sharp line outside of which is a rim of later 

 groundmass age. Old cracks, now sealed with albite, and having a deposit of 

 minute aegirite microliths on either side of them cross the phenocryst. 



Microphotograph, plane polarized light. Magnification about 25 diams. 



Fig. I\T). — Same as fig. IVa, but with crossed nicols. Nearly the whole 

 interior of the phenocryst is seen to be recrystallized to a fine aggregate of 

 feldspar-microcline and albite. A good part of the crystals stand normal to 

 the direction of the albite streaks which are seen to run into the marginal 



