ART. 2. PETROLOGY AT GOOSE CREEK SHANNON. 27 



However the albitic rocks have been formed, the most noteworthy 

 thmg about the association of the two kinds of pegmatite is the fact 

 that the feldspars vary abruptly from labradorite to practically pure 

 albite without any intermediate members of the plagioclase series or 

 any definite composition gradation. In the normal diabase pegma- 

 tite the micropegmatite is for the most part a quartz-labradorite 

 intergrowth and this rock does not seem to contain any albite. An 

 alkali feldspar occurs interstitially as micropegmatite but. judging 

 from the analytic results as to potash, this seems to be entirely the 

 potassium feldspar, orthoclase. 



APLITIC ALBITE ROCKS. 



White to pinkish sugary granular aplitic rocks occur occasionally 

 as narrow persistent nearly vertical dikes averaging only about 3 cm. 

 in width. These seem to follow the east-west fractures and intersect 

 both diabase of normal grain and diabase pegmatite. They are 

 usually though not invariably accompanied by more or less alteration 

 of the adjacent rock, although this is not believed to have been due 

 to the aplite as such, but rather to reopening of the fracture to permit 

 the action of later solutions. One such dike showed absolutely no 

 alteration of the adjacent diabase and, on the other hand, where the 

 walls are altered the aplitic rock itself is affected by hydrothermal 

 processes. 



The aplites are variable in their amount of quartz and micropeg- 

 matite. The narrowest dike examined in thin section was about 6 

 mm. in uniform width. This was bordered on either side by an 

 altered band of diabase 15 mm. wide which was probably altered after 

 the aplite solidified by solutions moving along a later crack which 

 intersects the zone, as the aplite is itself greatly altered. The aplite 

 dikelet consists of muddy altered interlocking albite grains with 

 abundant interstitial pale green spherulitic chlorite. Neither quartz 

 nor micropegmatite was observed. 



Another persistent dike was traced up the wall of the quarry for 

 about 1.5 feet with an average width of about 2 cm. and intersected 

 both diabase and diabase pegmatite. This also was found in thin 

 section to be a granulitic aggregate of interlocking grains of albite, 

 in this case with less quartz and with light colored transparent 

 diopside. This dike, like the other, was intruded along a fissure which 

 had later been reopened, and the aplite contained later seams filled 

 with diopside and a little axinite, etc. Another typical aplitic dike 

 is illustrated in contact with normal diabase in plate 2, upper. This 

 is cut by later cracks of two periods, those of the first period being 

 filled with diopside, while the latest crack, along which the specimen 

 is broken, is coated with laumontite. Like many of the dikes this is 

 porous from the presence of minute miarolitic cavities like those of 

 the coarse albitic pegmatite, though much smixller. 



