ART. 2. PETROLOGY AT GOOSE CEEEK — SHANNON. 23 



albitic rock showing the structure of the normal diabase pegmatite 

 is beheved to be a product of secondary alteration immediately fol- 

 lowing crystallization or, if such be conceivable, a product of essen- 

 tially hydrothermal action by a magma of extremely differentiated 

 composition rich in water. This is shown by the structure, which is 

 pseudomorphous after the pegmatite. The diopsidic pyroxene is here 

 pseudomorphous after the augite, the titanite clearly preserves the 

 form of the skeletal octahedrons of titaniferous magnetite and the 

 albite-quartz micropegmatite and the albite core crystals are of 

 precisely the form of the plagioclase-quartz micropegmatites with 

 plagioclase cores of the normal pegmatite. 



The albite with nonpseudomorphous diopsides, euhedral crystals 

 of titanite, and miarolitic cavities; and the micropegmatite rock 

 with plumose intergrown diopside, however, do not exhibit structure 

 clearly traceable to the normal pegmatite and may be assumed to 

 represent a true extreme alkalic magmatic differentiate, probably an 

 acid residuum from the crystallization of the larger masses of nor- 

 mal pegmatite. The specimen illustrated in plate 6 is of interest in 

 this connection and may be described in detail. The material of the 

 highly albitic mass seems to have been injected with its present com- 

 position into the cavity it now occupies since it is difficult to under- 

 stand how any extensive and thorough subsequent hydrothermal al- 

 teration which might have taken place could have confined itself to 

 the contents of the cavity and failed to produce any alteration of 

 the inclosing diabase which is perfectly fresh. It is equally difficult 

 to conceive an extremely sodic mass of this size having formed by 

 simple differentiation from the adjacent normal diabase. It seems 

 to have been injected from a short distance and probably these small 

 masses represent a little acid residuum squeezed from a considerable 

 mass of adjacent normal pegmatite. Where seen in place in the 

 quarry such small irregular white masses seemed always to be 

 connected by a stringer with considerable bodies of coarse normal 

 pegmatite. 



Near the borders of this mass the pyroxenes are bronzy augite, 

 like that of the normal pegmatite and the diabase, and these are 

 grown outward from the walls. At their tips the pja-oxenes are 

 changed to pale green diopside, clearly pseudomorphous after the 

 augite, wath abundant inclosed large grains of iron ore. Further 

 from the wall the bladelike diopsides inclose residual nuclei of 

 brown augite, immediately surrounding which the diopside is enor- 

 mously dusted with minute opaque grains. Around the exterior of 

 such diopside pseudomorphs there are grouped skeletons of iron ore 

 and crystals of titanite, the material of which was doubtless derived 



