220 FIELD COLUMBIAN MuSEUM—GEOLOGY, VOL. II. 
spots. Although appearing of a fairly uniform: color and texture 
when seen in the rough, a polished surface makes evident that it 
is of fragmental origin. The fragments have angular or slightly 
rounded outlines, and average a few square millimeters in diameter. 
They vary in texture, some being fine-grained and compact, others 
scoriaceous. The border of each fragment is usually marked by 
a deeper red line. Some of the fragments show that the rock from 
which they were broken had a distinct flow structure. Irregular 
grains of transparent quartz are scattered through the whole mass 
of the rock, seeming, in some cases, to be a primary constituent of 
the larger fragments, and in others to be constituent grains. As they 
lack any distinct crystal outline, however, they are probably all frag- 
mental. A quantitative determination by Mr. H. W. Nichols of 
the amount of silica in this rock gave $i O, = 72.56 per cent. 
In the area covered by this breccia a rock, already mentioned 
as being of a chalk-like appearance, forms an extensive bed. The 
hand specimen of this (Mus. No. E 8186) is chalk-white in color 
and friable so as to soil the fingers when handled. It can be easily 
crushed in a mortar, and is used locally to make fire-brick. It is 
harsh to the feel, porous, and of lowspecific gravity. Before the 
blowpipe it is infusible. It is also anhydrous, giving off no water 
upon heating in a matrass. A chemical examination by Mr. H. W. 
Nichols showed the composition to be nearly pure silica, an approx- 
imate quantitative determination giving SiO, = 98 per cent. The 
rock is apparently a leached tuff, the leaching having removed 
nearly all other ingredients except the silica. Indications of a: former 
fragmental structure are afforded by small angular cavities in the 
rock partly filled by pumiceous remains. A peculiar feature noted 
during the chemical examination of the rock was that a heavy white 
fume was given off when fragments were heated in a closed tube. 
On breaking open the tube this. fume disappeared and a pungent 
odor, resembling that of SO,, was obtained. The fumes tend to 
form a ring within the tube, and when they are driven into the water 
the water becomes acid. 
The hand specimen (Mus. No. E 8185), taken east of this from 
about the middle of the northern side of the mountain, F. Plate LX VIL., 
is likewise a breccia, or coarse tuff, differing from that occurring 
west in being more plainly fragmental in character and in having 
a lavender color. The appearance of a polished surface is shown 
in Plate LXX., Fig. 1. In spite of its clastic origin, and although 
numerous irregular pores about the size of a pinhead are scattered 
through it, it 1s tough and fairly compact. The different fragments 
of which the rock is composed vary in size and color and shape. 
