ART. 2. PETROLOGY AT GOOSE CREEK SHANNON. 53 



HOENBLENDIZATION OF DIOPSIDE OF ALBITE-DIOPSIDE FEGMATITIC ROCKS. 



Many specimens of the coarse grained rocks consisting essentially 

 of albite and diopside have suffered a change of the pyroxene, the 

 glassy green diopside being replaced to various degrees by light fluffy 

 aggregates of white fibers of hornblende, which is entirely like that 

 described above as a mineral occurring in the miarolitic cavities. 

 These altered rocks also show thin asbestiform coatings on cracks 

 which have a pale bluish color, especially when wet. On dry speci- 

 mens the blue material varies abruptly to white. Under the micro- 

 scope this is seen to be composed of very finely matted fibers in which 

 the pleochroism, which is not pronounced, is from pale greenish brown 

 to pale brownish green. The extinction Zac is variable up to 20°. 

 The intermediate refractive index, /3, is about 1.670. A few plates 

 of "hour-glass" epidote are associated with this hornblende and a 

 few grains of an unidentified mineral with an index well above 1.67 

 and intense pleochroism in deep grass green and purplish brown. 

 These smears are entirely like the fibrous hornblende occurring in 

 veins and miarolitic cavities as described in other sections of this 

 paper. 



HYDROTHERMAL JOINT AND CAVITY FILLINGS. 



Wliat have been referred to throughout the paper as zeolite-bearing 

 veins are in reality small shear zones, usually only a few centimeters 

 wide, composed of a breccia of fragments of crushed diabase or of its 

 several differentiated phases, with the interspaces filled with second- 

 ary minerals deposited from solution. The earliest of these minerals 

 are essentially the same as those occurring in the miarolitic cavities 

 already described and are doubtless the product of deposition by the 

 same solutions. 



These shear zones with zeolitic minerals may follow either the 

 north-south joints or the east-west fractures and there is no essential 

 and constant difference between the minerals formed in shears of the 

 two directions. In general specimens from the two series of ruptures 

 can be distinguished by the darkening of the rock surfaces along the 

 north-south system by diabantite varnish while the rock cemented by 

 later minerals from the east-west fractures is lightened in color by a 

 slight alteration which has already been mentioned. 



Before discussing the origin and paragenesis of these veins the 

 several minerals will be described in detail. 



In this section are also described the minerals occuring as druses 

 coating the surfaces of basalt blocks along early-formed flat cracks 



In addition to the abundant albite which occurs as a magmatic 

 product in the later differentiates of the diabase and that which has 



