Art. IX.] Herrick-Johnson, Geology of the Albuquerque Sheet. 229 
to be chiefly orthoclase though plagioclase twinning can be seen 
in many of them and it is probably unsafe to draw definite con- 
clusions from the absence of such twinning in small crystals. 
Next in importance is the bronze mica which occurs both in 
large crystals and smaller scales, but neither is abundant. In 
the smaller scales the mica is nearly completely altered. Iron 
lined cavities with ribs of the iron preserve the outlines of what 
may have been hornblende crystals but no recognizable speci- 
mens were seen. The granular magma is pale red or yellowish, 
evidently iron-stained. Scattered magnetite is present through- 
out the section. 
No. 574 from the central portion of the peak is a typical 
trachyte but evidently has suffered much alteration. Kaoliniz- 
ation of the feldspar is apparent to the eye while the mica 
stands out in evident scales. The section bears out the testi- 
mony of the hand sample. The mica is in delicate, often flex- 
uous plates and polarizes with almost unexampled brilliancy. 
The feldspars are mostly altered, often leaving cavities filled 
with kaolin, or if not so completely altered, the polarization is 
lowered so that the transparent contents react almost as if iso- 
tropous. Orthockse is the only identifiable', feldspar. 
Number 578 is a very porous light-colored rock lying near 
the contact with the penetrated sandstone. , The disintegration 
has been very complete so that the orthoclase has been nearly 
wholly removed leaving cavities that are usually unfilled. Under 
the microscope a few grains of the orthoclase remain visible 
and mica flakes are scattered through the relatively homoge- 
geneous magma. The dark ingredients are in minute dots in 
the magma and the magnitite is less abundant. The micas are 
paler and less pleochroic than in number 577. 
Number 576 is a very light brecciated phase near the 
sandstone and is like the above in a highly disintegrated state. 
The mica is nearly entirely altered to a black aggregate. 
Several other sections of various phases of these rocks show 
not only stages in the process of disintegration but show that 
proximity to the country rock was a very important element in 
that process. A section of the sandstone adjacent to the cone, 
