ART. 2. PETROLOGY AT GOOSE CREEK SHANNON. 51 



Under the microscope the mineral is found to have a scaly struc- 

 ture and moderate pleochroism which, owing to the tangled character 

 of the aggregates, is not conspicuous, X= brownish green, Y = Z = deep 

 blue-green. The color and index vary somewhat, probably depend- 

 ing on slight differences in oxidation of the iron. The mineral is 

 biaxial and negative ( — ) with 2V very small ( ± 5°) ; acute bisectrix 

 perpendicular to the basal cleavage, |S= 1.600, varying .005. 



The mineral is soluble in boiling 1:1 hydrochloric and nitric acids 

 with separation of flocculent silica and is more slowly soluble in boil- 

 ing dilute sulphuric acid. It is probably best referred to diabantite. 



There is very little alteration of the adjacent rock along the 

 diabantite-coated joints. Even the fragments of augite and feld- 

 spar incorporated in the chlorite are relatively fresh and free from 

 alteration. 



ALTKKATION OF DIABASE PEGMATITE WHERE INTERSECTED BY DIOPSIDIC SEAMS. 



While the processes which have developed the various coarse albite 

 rocks seem in the quarry are somewhat obscure, they seem in part 

 to be the result of both magmatic and hydrothermal processes. At 

 one place where a number of diopsizing seams like those last described 

 were traced from the normal diabase (where they produced the nar- 

 row altered zones described) , into a coarse mass of diabase pegma- 

 tite, the seams became less well defmed and alteration apparently 

 due to them spread over a considerable part of the pegmatitic rock. 

 This rock seemed to grade into normal diabase pegmatite away from 

 the seams although the exposures were poor. In the hand specimen 

 the material of this altered portion is light colored with the texture 

 of the normal pegmatite and shows clear light green glassy prisms of 

 diopside in a base of coarse granular snow-white to pinkish albite. 

 Numerous skeletons of titanite pseudomorphous after magnetite are 

 easily seen. The rock contains small cavities of the type here 

 termed miarolitic, which are lined with albite crystals and partly filled 

 with later tufts of snow white fibers of hornblende and occasional crys- 

 tals of chalcopyrite. Such rock is common in the quarry. It may be 

 a magmatic differentiate of the type called albitic pegmatite, and 

 the fact that it is intersected by the diopside seams may not be 

 significant. 



In thin section under the microscope as shown in the photomicro- 

 graph, plate, 7 lower, this rock shows greatly sericitized and altered 

 feldspar which is apparently all albite, and large grains and crystals of 

 clear glassy diopside. No micropegmatite was seen. The pseudo- 

 morphs of titanite after skeletons of magnetite are abundant and 

 well defined and the space between the plates of titanite is filled with 

 spherulitic green chlorite. Where the small miarolitic cavities are 

 sectioned the fine colorless hornblende needles are seen to be grown 



