OF ARTS AND SCIENCES. 



289 



earth-like alteration product of the prehnite. Splinters of this variety 

 show under the loupe by transmitted light, nearly opaque, light-green 

 portions, separated by transparent white. The white fuses in tlie tlarae 

 of an alcohol lamp. 



The specific gravity of the brown part, taken where there were only 

 very small amygdules, was 2.80 ; and that of the green, 2.83. 



Paragenesis : — 



Fluid Magma. 



Lower Zone. 



Amygdaloid Zoke. 



(I.) 1. Plagioolase. 



(II.) 



2. Pyroxene and 

 Magnetite. 



1. Plagioolase. 



The characteristic 

 pseuilomorphs 

 after pyroxene. 



(III.) Chlorite in pseudomorphs 

 after plagioolase, and in 

 pse udo-amy gd ules. 



(IV.) 



Brown 

 matrix. ; 



Green 

 matrix. 



3. Besidpaky 



BASE. 



2. Gas 

 cavities. 



Chloritic substance 

 stained with 

 iron oxide. 



Pbehnite pseudomorphs 

 after plagioclase. 



Prehnite 

 amygdules. 



1. Grken EARTH-lilie sub- 

 stance (amorphous ) 



2. Quartz. 



HANCOCK MINE. 



A specimen of chocolate-brown amygdaloid, from the halvans of the 

 Hancock mine, contains beautiful amygdules of a dark green finely 

 scaly chlorite, each one surrounded by a narrow ring of quartz. The 

 matrix contains some grains that seem to be pyroxene, and some of the 

 feldspar crystals, though much altered, still show twin-striation. But 

 whole groups of the feldspar have been replaced by quartz in such a 

 manner, that the quartz polarizes the light as an integral individual 

 throughout the area of each group. The pyroxene grains and pseudo- 

 morphs within the areas of these groups have not been changed to 

 quartz. 



The chlorite of the amygdules is highly dichroitic, being green when 

 the longer axis of the laminae coincides with the shorter diagonal of the 

 nicol, and yellow when perpendicular to this. Portions revolve dark 

 between crossed uicols, as did also scales pressed in balsam between 

 glass. It is therefore uniaxial. The quartz which forms the outer 

 layer of the amygdules is connected with, and really forms part of, 



