GRANITE PORPIIYKY. 343 



have a marginal zone of included quartz grains, the inner limit of the zone being 

 sharply defined. In places the inclosed grains are so numerous that the feldspar 

 crystals merge in the groundmass and tlieir outline is confused. The biotite, sur- 

 rounded l)y grains of iron oxide, is partly altered to green chlorite and epidote. The 

 hornblende is scarce and the crystallization of the groundmass granitoid. The next 

 three sections — 16, 19, 20 — should be considered together, since they are from the 

 same portion of the west side of the dike north of the last named road. No. 16 ia 

 fi'om a distance of 30 feet from the plane of contact with the limestone and is rich iu 

 sharply defined porphyritical crystals, the quartz being in perfect dihexahedrons. 

 There is an abundance of biotite and titanic iron with titanite, but no hornblende. 

 The groundmass has the coarse grained microj)egmatitic structure illustrated in Fig. 

 1, PI. VI. Xo. 19, from a distance of 10 feet, and Xo. 20, from the contact, ai-e still 

 more porphyry-like, having nuuh more groundmass with tiner grained micropeg- 

 matitic structure, of very homogeneous texture, the only pheuocrysts being feldspar 

 and quartz ; Diotite, hornblende, and the associated minerals are wanting. The feld- 

 spar is more altered than that of 16 and colorless potash-mica is more abundant. 

 Thin sections 21 and 22 are from the bottom of a gulch on the south side of the same 

 road. The first from a distance of 1 foot from the contact with limestone, shows a 

 porphyritic granite, rich in large, well defined crystals of feldspar and quartz, with 

 much biotite and hornblende. The groundmass forms but a small part of the whole 

 and is granitoid, the grains averaging 0-1 °"" in size. The second thin section is from 

 immediate contact and diflers from the first in having unichmore groundmass with the 

 same microgranular structure, the grains being only one- third as large as those at a 

 foot distance. The phenocrysts are smaller and more sharply outlined, the fresh 

 orthftclase having a Satin-like sheen in thin section. Hornblende is more abundant iu 

 minute crystals, and augite occurs sparingly. 



Thin section 24, from a local modification of the porphyry near its contact with 

 limestone on the north side of the road, requires special notice. The rather small 

 phenocrysts of quartz and feldspar have the same characteristics as those from other 

 portions of the same body, but the groundmass is very different. It is bluish gray iu 

 thin section, spotted with minute black specks; under the microscope it is seen to be 

 composed of irregular grains of quartz and feldspar having a granitic structuie. The 

 black specks are found to be angular microcrystalline patches, crowded with bla<'k 

 particles, and bearing slireds of colorless mica, and appear to be remnants of a base less 

 highly crystallized than the groundmass. Thin section 25 is from a breccia of por- 

 phyry and dark colored quartzite, the phenocrysts are angular fragments, the quartz 

 is rich in Huid inclusions, and tlic opaque, black i)ortioiis produce a very prominent 

 flow-structure. Both sections are free from biotite or hornblende. Thin sections 26 and 

 27 are interesting because they conje from the middle and side of a branch dike 

 not 30 feet wide. The former has a dense groundmass, bearing large quartz dihexahe- 



