286 PROCEEDINGS OF THE AMERICAN ACADEMY 



fifth of the rock is occupied by amygdules averaging ^ inch diameter, 

 but sometimes much larger, and consisting chiefly of quartz covered 

 by, and more or less impregnated with, chlorite. In the thin sections, 

 the matrix consists chiefly of small plagioclase crystals, in which the 

 twinning is still more or less apparent in polarized light, but they 

 show marked alteration. The spaces between the plagioclase crystals 

 are filled with an impellucid mass of brown particles of iron oxide, and 

 confused, often radiating, long, slender, colorless, translucent crystals. 

 These crystals are feebly polaiizing, and at first sight appear like ap- 

 atite, but I observed no hexagonal sections. They often start out from 

 the end of a feldspar crystal and radiate from this, which, taken in 

 connection with their appearance, renders it quite likely that they are 

 feldspar microlites arrested during development into crystals. I noticed 

 no pyroxene, and only very isolated apparent pseudomorphs after it. 

 The impellucid substance between the feldspar microlites contains much 

 soft, green, chloritic substance. As pyroxene in these rocks shows 

 itself to have always crystallized iifter the feldspar, we should, perhaps, 

 not expect to meet with it where the rock solidified before the feldspar 

 microlites had united to form finished crystals. 



The amygdules have almost always sharply defined, smooth walls, 

 and are then bordered by a circumference of the unindividualized sub- 

 stance with its feldspar microlites, and these latter are then arranged in 

 a manner with reference to the amygdule that seems to clearly indi- 

 cate the exertion of a force by the cavity on a surrounding semifluid 

 medium. 



In places, the sharply defined outline and the unindividualized border 

 are missing, and the formation of a pseudo-amygdule has taken place, 

 often more or less enveloping the true amygdule. 



The amygdules consist chiefly of quartz in crystalline aggregates, 

 filling the interior, and surrounded by a mural lining of chlorite, con- 

 sisting of long, thin narrow phites, which are either orthorhombic or 

 imiaxial, and which bristle toward the interior. Long radiating tufts 

 of these plates penetrate far into the interior of tlie pellucid quartz 

 crystals, indicating that the chlorite lining is older than the quartz. 



Parasenesis :. — 



