26 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 6G. 



Another example of transition from normal diabase pegmatite to 

 the albitic rock if furnished by a narrow persistent dike exposed at 

 the north end of the quarry. This dike is from 10 to 15 cm. in width 

 and occupies a straight walled fracture in the normal diabase which 

 dips at a low angle, and is traceable for about 10 meters in the wall 

 of the quarry. At its upper end the rock of this dike is coarse diabase 

 pegmatite of dark color with large blades of diallagic augite, which 

 are conspicuously developed normal to the walls, with broad cleavages 

 of translucent greenish feldspar. Under the microscope this rock is 

 found to consist, as usual, of large blades of perfectly fresh pyroxene 

 of the ordinary purplish brown color, and crystals of clear plagioclase 

 showing fine albite and pericline twin lamellae, with interstitial 

 micropegmatite. The micropegmatite is sharp and its feldspar is all 

 transparent and limpid with an index above that of the balsam. Iron 

 ore occurs in the common skeletal form. All of the minerals are 

 fresh and unaltered. In the field this rock can be clearly seen to pass 

 by transition downward into a much lighter colored rock by decrease 

 in the size and number of the pyroxenes and by the increase in the 

 amount of a pinkish micropegmatite which is easily visible under a 

 lens. A specimen from this dike halfway down the face shows cen- 

 tral sharp crystals of feldspar which are largely sericitized but which, 

 where unaltered, show the albite and pericline twin lamellae, have an 

 index above that of the balsam and are apparently plagioclase like 

 that of the rock above. These are surrounded by broad areas of 

 micropegmatite, the feldspar of which is dusted with kaolin and has 

 indices below that of the balsam and is doubtless albite. The augites 

 of this specimen are partly changed to diopside, biotite, and horn- 

 blende, and the iron ore is to some extent replaced by titanite and 

 secondary biotite. A specimen from the lower end of this dike is like 

 the last in the lesser amount of dark minerals and in the predomi- 

 nance of the pinkish matrix. A section cut at the wall shows large 

 plagioclase and purplish augite crystals and interstitial micropegma- 

 tite, in contact with the normal diabase. This grades outward into 

 a rock made up of greatly kaolinized and sericitized feldspars and 

 augite largely replaced by diopside. A section from the center of 

 the dike here consists of albite crystals, muddy from the development 

 of sericite and kaolin, with largely diopsidized augites, in an abun- 

 dant matrix of quartz-albite micropegmatite. 



The albitic rocks can easily be distinguished in the hand specimen 

 after a little practice has been acquired, by the relatively lesser num- 

 ber and smaller size of the blades of pyroxene and by the white or 

 pinkish color and more opaque appearance of the feldspar. The micro- 

 pegmatite, when its amount becomes abundant, as in the albitic 

 rocks, can be readily seen under a lens. 



