Gregory.] TBACHYTES. ]('»1 
aresplit and uneven and at times become merely a bundle of fibers, 
while little branch threads cling to the sides of the shreds. Some- 
times straight feldspars are seen, but more often they arc bent or 
curved so much as to form a half circle. Besides the expansion struc- 
ture so well exhibited about the vesicular cavities, parts of the slide 
show a well-developed flow structure. The rock is plainly a vesicular 
rhyolite lava with dacitic facies. 
TRACHYTES. 
Under this head will be described all the rocks of this region whoso 
principal component is alkali feldspar, whether they occur as extrusive 
volcanics or as intrusive masses or dikes. For the purposes of this dis- 
cussion they will be grouped as trachytes, doubtful trachytes, and 
quar t z - trachy tes . 
HEDGEHOG TRACHYTE. 
The specimens of this rock found in place are all from the cliffs and 
sharp comb at the very highest points on the mountain, but there is 
reason to believe that they fairly represent the whole mass. When 
seen in the ledge the rock appears as rough tabular blocks formed by 
intersecting cleavages. The color is very light gray on the more 
exposed surfaces and a rusty iron brown in the narrow cracks, but a 
blow of the hammer shows that this outer weathering is scarcely 
thicker than a coat of paint, and that the fresh rock is bluish gray in 
color, with a mottled appearance. The general texture is fine, uniform, 
not at all porphyritic, and, at one point on the mountain, slightly 
brecciated. The whole rock, however, contains abundant vesicular 
cavities, round or oval in shape, and varying in size from a pin head 
to a quarter of an inch in diameter. These cavities are usually filled 
with quartz, occasionally of the amethyst variety, and some contain a 
glistening white calcite. The rock is now thoroughly silicified, and 
the cavities and minute cleavages and fissures are filled with firm 
cement that in weathering melts down only as fast as the original 
material, or even more slowly, so that occasionally a quartz-filled vein- 
let projects slightly above tin' general level of the weathered surface. 
The rare empty cavities found in the altered portions were probably 
filled with calcite. 
The microscope reveals a rock made up of quartz-filled vesicles in a 
groundmass of feldspar laths. The quartz filling the cavities is com- 
posed of many grains, jagged and branched, with no optical orientation 
as a group, and only rarely containing inclusions. The minute fissures 
are filled in the same manner, and all the facts point to a secondary 
origin of the quartz. The sections of feldspar shown in the slide vary 
in form from long, rodlike shapes to squares, but Hie greater number 
Bull. 165 11 
