QU0GG1 JOE QUARTZ-TRACHYTE. 165 
slits are prominently brought oul by weathering, and occasionally tin \ 
are lined or even filled w ith quartz grains. 
Surf ace alteration takes the form of a thin brown coat, which forms 
a sudden and conspicuous change from the white rock beneath. In no 
case does this outer coating exceed one-eighth of an inch in thickness, 
and there is never a gradation from the weathered pari to the unaltered 
inner material, but immediately inside the brown coating the rock is 
as light colored and apparently as fresh as at the v< -r\ center of the 
specimen. The cross section of a white block which had been painted 
black would show no more marked and sudden contrast between the 
wood and the paint than is exhibited in the weathering of specimens 
from Quoggy Joe. r rhi> distinct method of weathering is e\ en w here 
an important characteristic of rocks of this type, and is ver\ useful in 
field determination. Occasionally this coating is stained with a shin- 
ing blue-black manganese-bearing material. 
Microscopic description. — The microscope reveals a rock composed 
of iron ore, albite, orthoclase, and quartz, with siderite, kaolin, and 
chlorite as decomposition products. The quartz is the only mineral 
which rises above the fine groundmass, and occurs as groups ^i grains 
usually surrounded by the iron carbonate. The individual grains are 
very irregular in their outline, and, as noted in the rhyolites, the 
groups have no common orientation. That some of the quartz is sec- 
ondary is indicated by the fact that it fills minute cleavage cracks 
which traverse the rock in all directions, and is mosl abundant in the 
little slitlike cavities connected by these cracks. The slits noticed in 
the hand specimen are found to be arranged along lines now filled 
with quartz grains, and are understood to be local enlargements of 
imperfect cleavage and to have been formed by a stretching due to 
unequal movement and pressure. 
The groundmass is composed of small, narrow feldspar microliter, 
arranged with typical trachytic structure, together with a great deal 
of quartz in finely divided particles. The feldspars are not easily 
determined, but they show many Carlsbad twins and albite twins, and 
these facts, taken in connection with their average extinction angle of 
17°, indicate orthoclase and albite or andesine, but the small amount of 
lime, as shown by the analysis, would prevent the formation of audi s- 
ine, and leaves albite as the probable plagioclase. The carbonate 
occurs in patches throughout the section, but is particularly abundant 
surrounding the partly quartz-filled cavities. Magnetite is found in 
specks and small dust aggregates, and the chlorite occurs in such forms 
and positions as to give no clue to the former mineral which it now 
represents. 
