Basalt in Paleozoic Rocks in Central Virginia. 313 



tic. The phenocrysts are of the same minerals, biotite, how- 

 ever, being comparatively scarce. The feldspar phenocrysts 

 also have a tendency to develop in rudely radiating bunches of 

 stubby crystals. These are, for the most part, plainly plagio- 

 clase, but there are many doubtful instances in which it is 

 probable that the mineral is orthoclase. Much the greater part 

 of the groundmass consists, as before, of minute plagioclase 

 crystals closely felted together and clamping small individuals 

 of biotite, quartz, and the iron oxides. In the latter considera- 

 ble decomposition has taken place, giving rise to limonite and 

 limonite stains, and outlining by these colors the network of 

 feldspar crystals. Occasionally inclusions of the groundmass 

 are seen in the feldspars, but they are more rare than in the 

 preceding sections. The feldspar phenocrysts are sharply out- 

 lined and the crystal faces seem to have suffered practically no 

 resorption. 



No. 21. — The material of this specimen is very badly decom- 

 posed, but owing to the striking texture of the rock, its nature 

 is seen as clearly as in the fresh specimens. The same grayish 

 groundmass appears, in which minute feldspar individuals can 

 be detected by their weathering. The usual phenocrysts of 

 feldspar and biotite are conspicuously developed and attain 

 sizes as great as in the preceding section. Only one or two 

 augite crystals, however, were detected. The decomposition of 

 the ferruginous minerals has stained the exterior of the rock, 

 and limonite has concentrated in those phenocrysts which were 

 most thoroughly decomposed, giving them a yellowish appear- 

 ance instead of the white color usual in the fresh rock. 



This section is of a strongly porphyritic aspect. The pheno- 

 crysts of biotite are large and well-shaped ; areas representing 

 probable feldspar were nearly worn away in the preparation of the 

 section on account of its weathered condition. The groundmass 

 consists of a finely felted mass of feldspar, chiefly plagioclase, 

 with many small crystals of biotite, ilmenite or magnetite, and 

 a little quartz. Owing to the weathered condition of the rock, 

 the ferruginous minerals have oxidized considerably, and stains 

 and bodies of limonite are frequent. Portions of the felds- 

 pathic groundmass are enclosed in the biotite crystals. These 

 latter are, for the most part, fresh and show slight decomposi- 

 tion to chlorite. The brown color and the pleochroism are both 

 strong. 



No. 22. — This section, like the preceding, is much weathered 

 and deeply stained with limonite. It has also a strongly por- 

 phvritic appearance. The phenocrysts consist of plagioclase, 

 doubtful orthoclase, biotite, and augite. The crystals are large 

 and the outlines are sharp and clear. Included in the augite 



