246 PROCEEDINGS OF THE AMERICAN ACADEMY. 



of CTiicking, and again the fine material of the groundmass as a 

 whole has frequently been forced in along cracks. Occurring some- 

 times irregularly in the body of the phenocryst or orientated parallel 

 to the trace of the albite twinning, are often small, irregular to tabular 

 grains of albite. In a great many other cases the replacement of the 

 phenocryst by albite takes the form of curiously irregular masses,^^ 

 which project into the original feldspar substance often replacing as 

 much as a quarter or even two-thirds of the crystal. This albite is 

 sometimes finely twinned, though usually the twinning is confined to 

 a few stripes and is often lacking altogether. 



The quartz phenocrysts are less abundant, forming about 12% of 

 the rock. The largest grain measured was 3.5 mm. in diameter but 

 the average is much smaller being in the neighborhood of 1 mm. 

 The size most commonly seen is, however, somewhat larger than this. 

 While occasionally showing a fairly well marked crystal outline, the 

 quartz is usually much rounded and is often embayed by the ground- 

 mass. Its grains are sometimes granulated or, like the feldspar, 

 broken apart and almost without exception show abundant evidence 

 of crushing movements in the broken and strongly undulatory ex- 

 tinctions. The quartz contains the same inclusions of bubbles and 

 minute black grains as the quartz of the granite, but is practically free 

 from other inclusions except where fractured and then blue horn- 

 blende needles or small aegirites occur in the quartz. A very narrow 

 rim of later growth may frequently be noted about the quartz, but it is 

 much narrower and less conspicuous than the corresponding rim about 

 the feldspar and seems often to be wanting. 



The hornblende is relatively abundant and occurs in elongate masses 

 of irregular or rudely elliptical outline which are in great part poikili- 

 tic intergrowths with the feldspar and quartz of the groundmass. In 

 size, most of these areas are comparable with that of the smaller 

 phenocrysts of quartz and feldspar and rarely exceed 2 mm. in their 

 longest dimension. In the interior of many of these groups is massive 

 or nearly massive hornblende, frequently enclosing pyroxene (see Fig- 

 ure I, Plate !)._, The massive material passes gradually into the 

 poikilitic intergrowth and, while about the edges there are slightly 

 connected or unconnected grains, more distant from these masses 

 there is little hornblende present other than a few shreds of secondary 

 origin. 



The poikilitic hornblendes show a strong tendency to form along 



35 Compare albitization of feldspar in coarse-granite noted earlier. 



