GREGORY.] RHYOLITES. 153 
ROCK OF THE MAIN MOUNTAIN MASS. 
Macroscopic description. — The rock forming the main mountain 
mass appears in the hand specimen as an exceedingly dense material 
of uniform texture, having a dark-gray color with a greenish tinge. 
It is just such a rock as has been called felsite in this region as a 
field term. Under the hammer it breaks in splinters and chips hav- 
ing sharp, jagged edges. It breaks much more easily in some direc- 
tions than in others and reveals the presence of numerous shearing 
planes along which there has been a slight movement in more than 
one direction. These small planes can usually be traced but a few 
inches and are located so irregularly that it is difficult to find a flat 
even surface of more than a square inch in area. This character is 
markedly different from the Mount Kineo quartz-porphyry, which 
macroscopically it otherwise closely resembles. The more weath- 
ered material assumes a darker hue and is specked with black dots, 
which on microscopic examination are found to be alteration sur- 
rounding minute quartz aggregates. Occasional larger cavities occur 
which contain quartz crystals whose pyramid and prism faces are visi- 
ble to the unaided eye. In weathering the rock forms a thin skin of 
rusty -brown material separated from the unaltered portion by a dis- 
tinct boundary. 
Microscopic description. — Under the microscope a thin section ap- 
pears at first sight as a porphyry of broken quartz phenocrysts in a 
groundmass of feldspar. Both the quartz grains and the groundmass, 
however, present peculiarities worthy of consideration. The quartzes 
are not true phenociysts, as the following facts show. Although 
occasionally there is a small individual quartz grain located b}^ itself, 
there are usually five to fifteen closely interlocking grains occupying 
an area in the groundmass. The larger quartz grains are nearer the 
center of the aggregate, while the smaller are found about the edge 
and adjust themselves to the shape of the cavity by filling in the recesses 
with smaller and smaller grains. Tongues and veinlets of quartz pro- 
ject into the groundmass, filling the little cracks which are sparingly 
distributed throughout the section, but no included fragments are 
found in the quartz, nor are reentrant bays and corroded borders seen. 
The liquid inclusions in the quartz are not evenly distributed, but 
form lines and cusps and polygons, leaving part of the grain clear. 
These geometric forms are complete within the individual grains and 
are not broken at the contact between two grains of the aggregate, as 
would be expected if one were dealing with inclusions of a pheno- 
cryst which had been shattered into fragments. Furthermore, the 
quartzes do not show the wavy extinction characteristic of rock sub- 
jected to great strain. These facts point to a secondary filling of 
amygdaloidal cavities, and the supposition is strengthened by the 
