324 



WATSON AND CLINE IGNEOUS DIKES IN VIRGINIA 



in opposite directions from the inner zone toward the center and from 

 the outer zone toward the margin. Twinning is common parallel to 100. 

 The augite is rarely as fresh as the hornblende, and more or less altera- 

 tion is noted in all individuals to chlorite, calcite, and pyrite, which takes 

 place first around the margin and along cleavage lines and cracks. 



The absolute identification of the feldspars is attended with difficulty 

 because of their altered condition. In some parts of the thin-sections 



microcline appears to be the 

 chief feldspar and in others or- 

 thoclase or andesine-labradorite. 

 Microcline is developed in grains 

 and in irregular masses some- 

 times of large size and showing 

 the characteristic microcline 

 structure and Carlsbad twin- 

 ning. Orthoclase is likewise 

 fairly abundant in some parts 

 of the thin-sections. Its occur- 



Figure 4. — Sketches illustrating zonal Struc- 

 ture observed in Hornblende of the Camptonite. rence and distribution are simi- 



(a) Hornblende idiomorph showing zonal lar to microcline, but it is prob- 

 bands with crystal outlines. (6) Hornblende M not g0 aDimc [ a nt and On the 

 idiomorph showing ellipsoidal zonal structure. 



whole does not occur in as large 

 individuals. The plagioclase is developed chiefly in prismatic forms 

 showing characteristic multiple twinning and large extinction angles. 

 It is of the calcic variety and corresponds in composition to andesine- 

 labradorite. 



Analcite occurs in all thin-sections studied, but not evenly distributed. 

 Its prevailing mode of occurrence is in the form of phenocrysts, which 

 are abundant in the coarser-grained rock. It is also developed in small 

 grains occupying the interstices between the feldspars and mafic silicate 

 minerals of the ground-mass. These phenocrysts as seen under the micro- 

 scope are composed either of a single individual, with or without crystal 

 form, or of a group of idiomorphic individuals, some of which have been 

 replaced by calcite and fibrous zeolites through alteration. In some cases 

 where the area is occupied by a single individual it shows idiomorphic 

 outline and sharp contacts with the surrounding minerals. In other in- 

 stances no crystal boundaries occur, but the analcite, although pure in the 

 central zone, passes outward into the interstices between the minerals of 

 the ground -mass. Where a phenocryst is made up of a number of indi- 

 viduals, the contact with the ground-mass is usually quite sharp, with the 

 analcite crystals in the border zone occurring as partial idiomorphic 



