112 HENRY S. WASHINGTON 



Point, near the water's edge, which was called by him a quartz- 

 porphyry, and was briefly described by Tarr. It has a width of 

 ten feet, with a strike of N. 2° E. It is cut by a narrow dike 

 of dense black diabase. 



The rock is compact, with practically no change in texture 

 throughout its width. Phenocrysts of white or yellowish alkali- 

 feldspar, up to 1.5 cm. in diameter, and smaller smoky bipyra- 

 mids of quartz are thickly sprinkled through a very fine-grained, 

 rather dark blue-gray groundmass. By the action of the waves 

 and the sea water, with which most of the dike is covered at 

 high tide, the groundmass has been dissolved, and on these sur- 

 faces the beautifully sharp and automorphic phenocrysts of feld- 

 spar and quartz stand out prominently. The feldspars are stout, 

 prismatic, parallel to the axis a; are frequently twinned according 

 to the Carlsbad and also the Manebach laws, and show a number 

 of planes. They will be examined crystallographically later. 



Under the microscope the sections present a striking appear- 

 ance. The sharp quartz phenocrysts are clear, with occasional 

 streaks of minute gas or liquid inclusions, and not infrequently 

 carry rounded inclusions of granular feldspar, or feldspar and 

 glaucophane, like the groundmass ; while here and there isolated 

 crystals of glaucophane are also seen. The highly automorphic 

 feldspars are uniformly microperthite, or microcline-microper- 

 thite, no plagioclase being seen. They are dusty with minute, 

 often rod-shaped, microlites of a colorless transparent substance, 

 the nature of which is difficult to determine, and are stained, 

 especially at the edges, with limonite. They carry inclusions of 

 glaucophane crystals, or small groundmass patches, which occa- 

 sionally show a well-developed micrographic structure. There is 

 little evidence of magmatic corrosion, especially in the feldspars, 

 though the quartzes show a tendency to rounded angles and shal- 

 low embayments. 



The groundmass is very fine-grained, and is composed of 

 minute needles of dark greenish-blue hornblende, up to .01 mm. 

 in length, strewn pellmell in a granular mass of feldspar and 

 quartz. Neither magnetite nor apatite was seen, nor is any 



