244 PROCEEDINGS OF THE AMERICAN ACADEMY. 



crystiils seem to have attained a fair degree of perfection in their 

 crystal form. Cut parallel to the macropinacoid the crystals are 

 often nearly square. There is usually an elongation parallel to the 

 edge 001-010. Again the elongation is parallel to both this direction 

 and the prismatic axis yielding tal^ular forms. The prism faces are 

 often strongly developed as is also the y, (201) face, the latter tend- 

 ing to give acutely terminated crystals. As noted in the megascopic 

 description, the feldspars occasionally reach a length of 8 mm. or 

 more but the largest dimension usually seen in section will not exceed 

 3.5 mm. and the average is not much over 1 mm. Twinning is 

 common after the Carlsbad law, less common after the Manebach 

 and still less so after the Baveno law. Leaving aside later enlarge- 

 ments, one may observe many straight edges, but the outlines are 

 very generally curved and the terminations are commonly rounded 

 or irregular. A great many of the phenocrysts have been broken 

 by movements in the crystallizing mass, giving rise to irregular frag- 

 ments of varying sizes and shapes, and rendering it difficult to tell 

 just how far gradation in size as a result of normal growth extended. 

 It appears, however, certain that there was a continuous gradation 

 in size down nearly to that of the groundmass individuals. Upon 

 the margins of the phenocrysts is a narrow, but sharply marked 

 zone of orientated feldspar substance of later growth (see Figures 

 II and III, Plate 1). Outwardly this zone fades into the ground- 

 mass and while often of uniform width entirely around the plienocrysts, 

 it more often varies considerably especially where the contours of the 

 older crystal are irregular, broken or indented. This zone contains 

 many minute, rounded quartz grains, tiny microliths of aegirite and 

 sometimes riebeckite, and is clearly of groundmass age. As it is often 

 seen developed on the broken surfaces of the phenocrysts, its deposi- 

 tion clearly took place subsequent to the fracturing of the latter. 

 This enlargement on many of the smaller, more irregular feldspars is 

 proportionally much broader than on the larger phenocrysts, its area 

 often exceeding that of the enclosed grain. These smaller feldspars 

 grade downward in size practically to groundmass dimensions and 

 form in effect a part of the latter. The original feldspar material 

 includes a few early pyroxene crystals (or such replaced and man- 

 tled with aegirite), but is free from other inclusions. Where recry- 

 stallized or invaded by albite, the phenocrysts contain abundant 

 shreds and fibers of riebeckite and aegirite grains. 



With low powers, the phenocrysts appear fresh and homogeneous, 

 with higher powers considerable portions of the crystals and, rarely. 



