6 PKOCEEDINGS OF THE NATIONAL. MUSEUM vol. 66 



now dull olive green. The glassy basalt at the contact with the 

 diopside rock is changed to this green color for a distance of about 

 5 millimeters. Under the microscope this green glassy basalt has 

 precisely the same appearance as the normal purplish glass and the 

 line of contact between the two can not be distinguished in the thin 

 section, although the glass is banded in more and less transparent 

 bands parallel to the contact. It may be recalled from the descrip- 

 tion of hydrothermal alteration of the diabase of Goose Creek that 

 the principal effect of the solutions was removal of some of the iron 

 of the original augite and the changing of this high iron pyroxene 

 to pale green or colorless diopside. The effect here on the basaltic 

 glass has probably been a similar substitution of bases although it 

 is not susceptible of proof by microscopic examination. 



Many specimens of the crystalline basalt of the dikes show nar- 

 row seams and veinlets cutting the normal rock. These have a 

 central white seam averaging i^ mm. in width bordered on either 

 side by a dense olive green layer about 1 mm. wide beyond which 

 is a bleached gi-eenish band from 1 to 2 mm. wide which shades 

 into the normal rock. Under the microscope these bands are not so 

 conspicuous. The central filling is composed of granular datolite. 

 The dense greenish band is largely pyroxene, apparently an enrich- 

 ment by enlarging the original grains of the rock. The outer 

 bleached streak presents no conspicuous difference from the adja- 

 cent normal basalt under the microscope except that the pyroxene 

 looks clearer and less colored while by comparison that of the 

 adjacent unaltered basalt appears brownish. It seems most prob- 

 able that this 'alteration is, like that observed in the Goose Creek 

 diabase, diopsidization of the augite. The outer band contains 

 scattered grains of pyrite. 



HYDROTHERMAL MINERALS REPLACING THE LIMESTONE 



As has been previously pointed out, large amounts of diopside and 

 diopside-garnet rock are developed adjacent to the basalt dikes or, 

 probably better, in and adjacent to the shear zone which is associated 

 with the dikes where they are exposed on the east side of the quarry. 

 These lime silicate rocks are fine grained and lusterless, with dull 

 green to brownish green and brownish gray colors. Their relations 

 are not clear, and perhaps the best method of describing them is to 

 describe the several specimens collected as typical of the several 

 variations. 



The writer's No. " Lb-3 " in the hand specimen is a sugary granu- 

 lar dull green rock showing no minerals clearly identifiable with the 

 unaided eye, except a little coarse calcite. It shows ghost outlines 



