22 PROCEEDINGS OF THE NATIONAL MUSEUM, vol. 6C, 



This marked the initiation of the processes which, at a more advanced 

 stage, gave acid end-differentiates as further discussed following the 

 description of the albite-rich phases which follow. 



ALBITIC PEGMATITES. 



In addition to the normal diabase pegmatite last described, there 

 occur in the Goose Creek quarry, rocks very rich in albite which are 

 similar in structure and occurrence to the coarse plagioclase rock. 

 There are three principal types of these which intergrade, namely: 

 (1) Albite pegmatite having a structure identical with that described 

 as diabase pegmatite but in which the large crystals of feldspar are 

 albite as is all of the feldspar of the abundant micropegmatite which 

 they contain. In these rocks the pyroxene occurs in coarse-bladed 

 crystals with the curved branching habit, parting and lamination of 

 the augite of the diabase pegmatite, but the original purple augite has 

 been more or less completely replaced by pale green diopside so that 

 the present pyroxene is a pseudomorph of diopside after diallagic 

 augite. The skeleton magnetites have been largely replaced by 

 pseudomorphs of titanite. By gradual decrease in the proportion of 

 diopside pseudomorphous after augite this rock grades into: (2) A 

 relatively coarse albite rock containing abundant quartz-albite micro- 

 pegmatite. Diopside is present in greater or less amount but is in 

 glassy imperfect prisms which are original crystals and not altera- 

 tion pseudomorphs after augite. These rocks contain frequent 

 small miarolitic cavities, giving them a porous character, which are 

 lined with quartz and albite crystals. These types are not sharply 

 differentiated from: (3) A rock consisting of interlocking areas of 

 quartz-albite micropegmatite surrounding nuclear crystals of albite, in 

 which diopside occurs in branching fern-like graphic intergrowths 

 with the feldspar. 



The attitudes of many of the masses of albitic pegmatite are the 

 same as those of the normal pegmatite. Typical examples of the rocks 

 are shown in plates 6 and 8. The hand specimen shown in plate 8 

 composed essentially of albite and albite micropegmatite containing 

 long blades of augite narrowly bordered by secondary diopside is in 

 its greatest part typical of the first type, although the bottom of the 

 specimen grades toward normal diabase pegmatite. Plate 6 shows an 

 irregular mass of the micropegmatite-rich type containing dendritic 

 diopsides. 



A large part of the albitic rock is not of definitely demonstrable 

 origin but a small part of the material seems clearly to be a product 

 of post-crystallization hydrothermal alteration, while a similar small 

 part of the occurrences are seemingly incapable of explanation as other 

 than a product of magmatic consolidation. The greater part of the 



