18 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.66. 



into spots of greater or less size which remained fluid after the main 

 mass of the diabase solidified and permitted the growth of large crys- 

 tals producing unusually coarse textures. In places the coarse rock 

 occurs in irregular and generally rounded to lenslike masses, some- 

 times of considerable size, isolated in fine homogeneous grained rock, 

 while elsewhere coarse grained fabric forms small spots so thickly 

 scattered as to make up approximately half of a hybrid mass of rock. 

 Most of the larger masses, however, are tabular bodies which occur 

 between parallel walls of the normal rock and appear to bear a truly 

 intrusive relation to it. These vary up to 50 cm. or possibly in 

 extreme cases to a meter in width. One such dike, which like most 

 of them, had a rather low dip, was some 10 to 15 cm. wide and was 

 traceable with little variation in width for 12 meters in the face of 

 the quarry. Both ends were concealed. The dikes follow fractures 

 of no great displacement and many of them terminate abruptly as 

 in the small but typical example illustrated in plate 1. Another of 

 the dikes is illustrated in plate 3. 



In considering the mechanics of this segregation of the residual 

 molten material which produced the pegmatites two hypotheses may 

 be considered: (1) The diabase solidified in areas, each given area 

 concentrically expelling its residual fluid molten fraction toward a 

 center which ultimately became a chamber of considerable size filled 

 with material which crystallized slowly, yielding rounded or irregularly 

 lenticular bodies of diabase pegmatite. Where a fracture developed 

 intersecting this mass previous to consolidation, the material was 

 forced along the crack and solidified as dikelike masses of the pegma- 

 titic rock; (2) the residual molten mineralizer-rich material was 

 distributed generally among the individual previously solidified crys- 

 tals of pyroxene and plagioclase of the diabase and, at the compression 

 of the mass and formation of fissures this residual liquor was pressed 

 into the fractures, there to solidify as the pegmatites. Definite proof 

 of either mode of formation can not be advanced and it is probable 

 that both were operative. That the material forming the pegmatites 

 must have been forced into its present position in the dikelike masses 

 in entirely molten form is shown definitely by the attitude of the 

 bladed pyroxenes. While it is clear that the order of crystaUization 

 was pyroxene-feldspar-micropegmatite, the long bladed augites have 

 in nearly all of the observed dikes grown in fingerlike arrangement 

 perpendicular to the walls of the dike and have oriented themselves 

 on grains of the constituent pyroxene of the wall rock. While bent 

 and split, they do not show any flowage arrangement. 



A typical sample of the diabase pegmatite was analyzed in the 

 museum laboratory yielding the results, ratios, and norm given below: 



