gregoky.] RHYOLITE BRECCIA. L59 
Microscopic description. — The microscope brings out the porphyritic 
structure well and reveals the presence of phenocrysts of orthoclase 
and oligoclase. Quartz and an undetermined mineral arc present and 
rise in size above the general groundmass. Apatite occurs in long 
needles, which are broken and thrown out of alignment. Tin 4 ground- 
mass varies in character with the fragments of the breccia. The ortho- 
clase phenocrysts occur as plain laths of a somewhat square shape or 
as twins or basal sections nearly always with good crystal outlines, 
but they also occur with rounded edges or inclosed by a distinct zone 
of alteration. The oligoclase phenocrysts are fewer in number than 
the orthoclase, but exhibit the same general features. The feldspars 
measure 2 to 5 millimeters in length and are broken along lines where 
the pressure has been relieved. Alteration to kaolin and sericite has 
progressed so far that no clear pieces remain. Another mineral about 
one-fourth the size of the feldspar phenocrysts is abundant in the 
slide in limited areas. It is yellow-brown in color where freshest, but 
changes with different stages of alteration to a lavender-gray. Its 
shape and cleavage suggest a tetragonal mineral, and the high colors 
shown by double and single refraction suggest one of the rare acces- 
sory minerals — rutile, zircon, or cassiterite. The polarization is given 
lry an aggregate of grains, and the present altered state makes accurate 
determination impossible. The chunks of rhyolite pitchstone embedded 
in the mass yield a small amount of water in the closed tube and 
show a partially de vitrified structure under the microscope. Small 
aggregates of quartz grains, now occupy areas and cracks which appear 
to have been cavities that were filled by the quartz at a later date. 
Some fragments of the breccia have a groundmass composed of 
quartz and feldspar grains and laths arranged in well-developed flow 
structure. The streams separate to flow around the phenociysts, then 
reunite beyond. Other portions of the breccia show a considerable 
portion of partly devitrified glass in their groundmass, especially 
when the section is cut through the included glass fragments seen so 
prominently in the hand specimen. In one place a group of lathlike 
feldspar phenocrysts is found Avith the interstices occupied by altered 
glass, thus imitating the structure found in some diabases and termed 
mesastasis by Rosenbusch. Chlorite is now an abundant secondary 
product throughout the w 7 hole rock. 
VESICULAR LAVA. 
About three-fourths of a mile east of Haystack are a few scattered 
hummocks of coarsely vesicular lava. The surface exposure is of a 
rusty yellow rock, from which project angular knobs formed by 
weathering along the cleavage cracks. These cleavages are very 
numerous and intersect at all angles. 
