216 PROCEEDINGS OF THE AMERICAN ACADEMY. 



scopic study, it is difficult to escape the thought that they must 

 affect somewhat the density of the rock, and they must certainly 

 render it more susceptible to the action of chemical alteration. 



The quartz is throughout highly xenomorphic in its outlines. The 

 undulatory and broken extinction in most of its grains show that all 

 of it has been subjected to strain. Much of it, particularly in sections 

 which show granulation and secondary recrystallization of the micro- 

 perthite, is broken and in extreme cases (granite from the Gold-Leaf 

 quarry) it has been reduced to a mosaic of small grains. All of the 

 quartz contains abundant cavities, the majority of the large ones con- 

 taining liquid with a movable bubble. They are usually arranged in 

 gently curving lines across the quartz and in many cases obviously 

 mark the direction of an old resealed fracture. -^^ Minute particles 

 of iron oxides are closely associated with the cavities. The quartz 

 also includes rather rarely small riebeckites, but this is generally, if 

 not always, along fractured or broken zones. The quartz commonly 

 includes, and is intergrown with, aegirite; the same is also true of 

 zircon, although the latter is of course much less abundant than the 

 aegirite. 



The hornblende, always with more or less aegirite, forms irregular 

 patches of approximately the same area as the feldspar grains. Two 

 or more crystals are often grouped together. Its crystals are usually 

 broad with a tendency toward prismatic elongation. Toward the 

 cpartz it often develops its prism zone, l)ut these are marked by 

 many projecting points and irregularities. Although it often pene- 

 trates the borders of feldspar it is also found wrapped about the end 

 of the microperthite grains, and seems generally to have been con- 

 trolled as to its external form by the more abundant and dominant 

 mineral. Along its prism zone where seen in contact with feldspar 

 and quartz it shows a highly ragged contact surface. Even when low 

 magnifying powers show a fairly well marked line, higher powers 

 resolve the contact surface into a series of irregular projections and 

 indentations. The hornblende is often separated to a greater or less 

 extent from the other minerals by a growth of aegirite. Although 

 the aegirite seems to have attached itself occasionally to an original 

 crystal surface of the hornblende, it is usually found interpenetrated 

 with the latter, the vertical axes of the two being parallel. Outwardly 

 the aegirite develops to the exclusion of the hornblende, particularly 



13 Compare with description of the pegmatite quartz where more details 

 are given. Warre^i and Palache, These Proceedings, 47, No. 4, (July, 1911). 



