ART. 2. PETROLOGY AT GOOSE CREEK SHAXXON. 45 



of about 16° or 17°. The index varies slightly but the range of the 

 mean index is between 1.65 and 1.67 indicating a rather high iron 

 content. This hornblende is the same, essentially, as that occurring 

 in the zeolite-bearing veins which is described in somewhat greater 

 detail below. 



CHALCOPYEITE. 



Chalcopyrite is of rather frequent occurrence in the cavities as single 

 crystals which are too striated to yiekl good measurements. These 

 reach a maximum diameter of about 3 mm. and are later than the 

 Cjuartz and albite crystals of the lining of the cavities. In the altered 

 rock the chalcopyrite is often more or less decomposed on the outside 

 and along cracks to a dark brown to black material of brilhant pitchy 

 luster. This alteration product under the microscope is transparent, 

 golden-brown and completely isotropic, and has an index well above 

 1.82. It is doubtless limonite. 



Epidote occurs rather sparingly in the miarohtic cavities as minute 

 greenish yellow crystals often aggregated into branching groups or 

 strung, beadwise, on a thin fiber of hornblende. In some cavities 

 larger greenish black crystals elongated on the h axis and showing 

 terminal planes were found. These were made up of a large number 

 of smaller crystals in parallel growth and did not yield good signals 

 on the goniometer. 



CHLORITE. 



Chlorite was seen in some cavities associated with the larger epi- 

 dote cr^-stals as small deep emerald green spherules resting on albite 

 crystals and made up of groups of folia. Under the microscope the 

 mineral is found to be: Biaxial negative, vvdth 2V small, 0° to 10°. 

 The acute bisectrix is perpendicular to the perfect cleavage. Indices 

 of refraction; «= 1.630, /3 = 7= 1.637, 7-«= .007. Pleochroism X = 

 pale greenish-brown, Y = Z = deep blue green. Absorption X < Y = Z. 

 This is similar to the chlorite described below as a vein mineral of 

 which an analysis, made on a small sample, is given. It is fenta- 

 tively referred to aphrosiderite. 



HYDROTHERMAL ALTERATION. 



As has been stated in an earlier part of this paper, the bulk of the 

 diabase is entirely fresh and such alteration, aside from surface weath- 

 ering., as has been observed is confined to the immediate vicinity of 

 definite cracks, shear zones, and fractures in the normal rock or the 

 diabase pegmatite. These seams which are accompanied by altera- 

 tion are of several types and may be advantageosly described sepa- 

 rately although they doubtless intergrade. The alteration, in most 

 cases, is confined to a narrow and straight-walled zone parallelin;- 



