Basalt in Paleozoic Rocks in Central Virginia. 309 



olivine; the former are of a yellowish gray color with darker, 

 brown borders, the latter in particularly brilliant colors and 

 frequently with sharp, crystal faces. The groundmass is very 

 fine and consists of plagioclase, magnetite, augite, and a little 

 olivine. In the case of the olivines, there seems to be a grada- 

 tion between the coarse phenocrysts and the grains in the 

 groundmass. Considerable numbers of magnetite crystals are 

 included in the augites near their borders, and also appear to 

 a limited extent in the olivine. Slight decomposition has pro- 

 duced stains of limonite and small growths of chlorite in the 

 cracks of the olivine. The large percentage of magnetite in 

 groundmass renders it comparatively opaque. This rock bears 

 a strong resemblance to the section (No. 1) described by 

 Diller. 



No. 11. — This specimen is somewhat weathered, being taken 

 from the surface, and contains many faces stained with iron. 

 It is of a dull gray color and very finely crystalline. No por- 

 phyritic minerals appear and the only minerals which can be 

 detected by the magnifying glass are here and there small 

 feldspar laths or an occasional feldspar cleavage face. 



The texture of this rock, as seen under the microscope, is 

 uncommonly fine for the basic ernptives of this region, and the 

 mineral composition is likewise more than usually simple. The 

 bulk of the rock consists of a fine mass of plagioclase crystals 

 with a marked parallel arrangement. Intermingled and in part 

 included within these feldspars are great quantities of minute, 

 black crystals, the majority of which are magnetite. The only 

 other constituents which occur in any quantity are the small 

 individuals of mica, apparently biotite, which are compressed 

 between the feldspar crystals and many minute individuals of 

 augite. Considerable secondary limonite is present, probably 

 derived from the decomposition of the iron oxides. Certain 

 very small grains have the appearance of olivine, but are diffi- 

 cult of determination. The chief characteristic of the rock is 

 its high feldspathic content and the parallel arrangements of 

 the feldspars, which indicate flow structure. No traces of glass 

 were observed. 



No. 15. — This rock is of fine grain and dark gray to black 

 color. Weathering extends to a depth of half an inch from the 

 surface and produces a light gray color with considerable stain- 

 ing of iron. At the same time the attitude and size of the 

 feldspar crystals in groundmass is brought outbykaolinization. 

 In the fresh portions crystal faces and cleavage planes of the 

 feldspar are very frequent. From this groundmass porphyritic 

 crystals of augite of considerable size stand out prominently. 

 Smaller porphyritic crystals of olivine are present in less num- 

 bers. Fine black specks of magnetite are dotted through the 



