Basalt in Paleozoic Rocks in Central Virginia. 311 



first attacks the albite phenocrysts, which are kaolinized, as a 

 rule, even in the comparatively fresh specimens. Thus two 

 classes of phenocrysts are distinguishable among the feldspars 

 on the hand specimens, these being the weathered albites and 

 the clear, unaltered orthoclase. These general characteristics 

 are shared by all of these acid eruptives thus far discovered, 

 and the chief variations occur in the coarser or finer texture of 

 the phenocrysts. 



No. 18. — In this specimen is found the coarsest crystallization 

 of these acid eruptives. The groundmass is of the same dove 

 color as in the preceding rock and has an equally fine grain. 

 The proportion of phenocrysts of biotite and feldspar is 

 unusually great, and perhaps one-third of the rock is composed 

 of them. The biotites appear in flat, hexagonal crystals with sharp 

 outlines. The division of the feldspars into kaolinized and 

 fresh individuals is strongly accented. The rounded exteriors 

 of the fresh feldspar phenocrysts are so clear and transparent 

 as to strongly resemble quartz ; on their broken surfaces, how- 

 ever, the cleavage faces stand out prominently. A few pheno- 

 crysts of augite are also to be seen, but are very far inferior in 

 numbers to the other minerals. The phenocrysts of feldspar 

 and biotite are about equal in amount. 



In the corresponding thin section the minerals are in a much 

 fresher condition than in most of the sections and more satis- 

 factory determinations can be made. The general appearance 

 of the rock is the same as in the other sections, and its simple 

 composition is admirably shown. It is seen to consist of a 

 microcrystalline groundmass, with phenocrysts of orthoclase, 

 plagioclase, and biotite. In the arrangement of the crystals of 

 the groundmass no order or system is to be discovered. The 

 phenocrysts are perhaps slightly larger and more frequent than 

 common in these rocks, but they consist of the usual minerals, 

 feldspar and biotite. The outlines are sharp and clear and the 

 crystallographic planes are, in many cases, well defined. The 

 biotite crystals are of a greenish brown color and strongly 

 pleochroic, and occasionally they contain small portions of the 

 feldspathic groundmass. A slight tendency to parallelism also 

 appears in them. In one case the biotite and feldspar pheno- 

 crysts are intergrown. The feldspar in this case appears to be 

 orthoclase. The groundmass is a microcrystalline aggregate, 

 chiefly of feldspar, quartz, and many extremely small crystals 

 of magnetite or ilmenite. Such of the feldspars as can be 

 determined comprise both plagioclase and orthoclase. 'No traces 

 of glass are to be seen. There appear to have been practically 

 no disturbances of a dynamic nature in the body of the rock, 

 as the cracks in the larger minerals are very few, and the 

 extinctions are not wavy. One biotite crystal shows a slight 



