328 WATSON AND CLIXE — IGNEOUS DIKES IX VIRGINIA 



of the thin-sections studied olivine is abundant, but in a few it is devel- 

 oped only in small quantity. This is in accord with the microscopic ex- 

 amination of specimens of similar rocks from the Highland County area 

 by Diller and Keith. In addition to olivine, the other primary minerals 

 named in the order of abundance are plagioclase. augite. magnetite, a 

 little orthoclase. and apatite as inclusions in the feldspars. Usually 

 these minerals are quite fresh, but some alteration is noted, yielding the 

 secondary minerals serpentine, chlorite, calcite. kaolin, quartz, amphibole, 

 and magnetite. 



Plagioclase is developed in lath-shaped crystals up to 3 millimeters in 

 length and is of the calcic variety, corresponding to labradorite-bytownite 

 in composition. Orthoclase is usually present in minute quantity formed 

 in the interstices between the plagioclase and other minerals. Augite 

 occurs in irregular grains, nearly colorless to very pale brown, and as a 

 rule does not exhibit noticeable pleochroism. Like the feldspar, it often 

 shows wavy extinction due possibly to dynamic strain. Most of the au- 

 gite is quite fresh, but some alteration is observed, yielding chlorite, cal- 

 cite. sometimes a little hornblende in small prismatic needles, and magne- 

 tite. Olivine occurs chiefly in grains, whose outlines occasionally sul. 

 crystal form. Its period of crystallization was contemporaneous with but 

 near the close of that of plagioclase. while the period of formation of 

 augite was later. The fresh olivine is nearly colorless and the size of 

 grain ranges up to 3 millimeters. It usually shows some alteration, 

 which has progressed in the usual way along cleavage and fracture direc- 

 tions and about the borders. In a few cases the entire grain is altered, 

 but as a rule the alteration is only partial, the products being green fibrous 

 serpentine, calcite. and minute grains and crystals of magnetite. 



Magnetite as an original constituent occurs as crystals in all the thin- 

 sections, but in no case is it abundant. Apatite is developed only as 

 minute inclusions chiefly in the plagioclase feldspar. 



Chemical composition and classification in the quantitative si/stem. — 

 The chemical composition of the olivine diabase is well shown in the 

 analysis of the rock from a representative dike at Cross Keys. Augusta 

 County, given in column I of the subjoined table. The analysis of a 

 Piedmont olivine diabase from Davidson County. Xorth Carolina, is 

 added for comparison in column II. 



Analyses of Olivine Diabase 



I 



Si0 2 4S.10 



ALO a 15.25 



FeA 2.57 



la 



II 



.802 



47.66 



.150 



19.24 



.016 



1.83 



