f 
120 FIELD CoLUMBIAN MuUSEUM—GEOLOGY, VOL. I. 
(about .o2 mm. in diam.) of a pyroxene. There is no well-marked 
flow structure such as is to be seen in the andesite of Popocatepetl, 
Of the phenocrysts the feldspars are the largest and the most 
numerous. They have usually well-defined crystal outlines. Many 
of them show mechanical deformation from pressure. This is usually 
only evinced under crossed nicols, but when so seen great cracks 
traverse the crystals and varied angles of extinction are exhibited. 
Some crystals likewise show distinct zonal structure. The crystals 
are as a rule very free from inclusions, but some contain scattered 
augite microlites and others fluid inclusions. 
Observations, according to Levy’s method, of the angle of 
extinction of the lamellae gave 30° as the greatest angle, thus 
determining the feldspar as labradorite. This determination was cor- 
roborated by the extinction of the Carlsbad twins which gave angles 
corresponding to labradorite of the composition Ab 3 An 4. The 
feldspar has been determined by Felix and Lenk* from its optical 
characters as bytownite, but by its specific gravity as labradorite. 
They account for this variance by the well-known fact that the 
peripheral portion of a feldspar showing zone structure is frequently 
richer in silica than the interior. 
The hornblende crystals are of considerable size, some of the 
sections across the prism measuring 2mm. Most of them are anhe- 
dral rather than idiomorphic and show much corrosion and resorp- 
tion. Frequently the alteration has been such that the entire crystal 
has been replaced by iron oxide and indeterminate grains and scales. 
These constituents usually, however, possess a zonal arrangement 
and conform to the bounds of the original hornblende crystal. 
The pleochroism, w greenish-yellow, g¢ dark-brown, is strongly 
marked, as well as the absorption g > WH >w. The extinction parallel 
to the prism is sometimes as high as 17°. 
Among other phenocrysts numerous hypersthene crystals are 
recognizable by their square or nearly circular outline when cut across 
the prism, as well as by their pleochroism, w, reddish-brown, g, 
olive green. They have an average diameter of about 0.7 mm. : 
Less frequent are crystals of augite, which are larger and distin- 
euishable by their color and prismatic cleavage. 
Among accessory minerals may be noted needles of apatite 1 mm. 
in length with basal cleavage and frayed outlines. They contain 
fluid and gas inclusions. Magnetite crystals can also be observed. 

*OZ. cit. p. 92. 
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