52 PEOCEEDINGS OF THE NATIONAL MUSEUM. vol. 66. 



on the ends of the diopside prisms in parallel position and mtiny of 

 the hornblende fibers are completely inclosed in the clear albite crys- 

 tals which line the cavity and are later than the muddy albite of 

 the rock on which they rest showing that the albite was contempora- 

 neous with or later than the hornblende. 



Rock such as this may have originated either by hydrothermal or 

 magmatic processes and there has probably been a complex overlap- 

 ping of such processes in the exposures of the Goose Creek quarry. 

 While the material here described has many points in common with 

 the albitic pegmatites regarded as magmatic products, the minerals 

 formed secondarily are the same as those which were described as 

 developing in the normal diabase along the cracks which intersect 

 this particular mass. 



HORNBLENDIZATION OF NORMAL DIABASE PEGMATITE ALONG CRACKS AND SEAMS. 



Although a section was not made from the specimen illustrated in 

 plate 3 where it is cut by the hornblendizing seam, sections were 

 obtained from another larger mass of pegmatite where it was cut by 

 a similar altered streak. These sections show some interesting fea- 

 tures which may be described as follows. 



The seam, which is traceable across the section, seems to contain 

 two varieties of hornblende, one a brownish-green amphibole having 

 a large extinction angle, up to 27°, which is pleochroic the color of 

 mineral being in all directions of about the same proportions of brown 

 and green but absorption is so complete in one direction that the 

 mineral is almost opaque. The other is greener amphibole occurring 

 in bundles of prismatic needles of positive elongation with the extinc- 

 tion Z/^ c=17°, which is pleochroic in X = pale greenish brown and 

 Z= deep emerald green. These hornblendes also replace the augite 

 of the rock. Usually the replacement in its first stages is by the 

 brownest type hornblende which often surrounds an unreplaced core 

 of augite. The completely replaced pyroxenes have a center of the 

 brown hornblende, which is optically a unit, surrounded by the green 

 one which has a confused uralitic structure. A little chlorite is 

 locally associated with the hornblendes. 



One peculiar feature of the rock near the seam is that some of the 

 feldspars have altered by the development of a fine kaolinitic altera- 

 tion product which has later been replaced by the green hornblende, a 

 replacement which may be observed in all its stages. The end product 

 of this process where the feldspar of micropegmatite has been replaced 

 is a micrographic intergrowth of quartz and deep green hornblende 

 (pi. 5, upper). Most of the plagioclases are fresh and clear except 

 where they contain irregular and sporadic aggregates of coarse seri- 

 cite flakes. Iron ore occurs as large skeletons of octahedral form 

 largely replaced by titanite. 



