PART I.
<<
Previous
|
Table of
Contents |
Next
>>
MOUNT MAZAMA.
LAVAS OF MOUNT MAZAMA.a
ANDESITES.
WATCHMAN ANDESITE AREA.
The area of which The Watchman
is part contains a number of separate flows, but upon the map they are not
distinguished. They are particularly well exposed by numerous cliffs, but in
much of the area they are covered by glacial material. Although the flows are
much alike when compared with one another, there is considerable variation
within each flow. The holocrystalline gray forms, like No. 60, pass into the
type like No. 6—dark, porous, soft, and crumbly, but rich in amorphous
matter—and No. 26, which is decidedly vitreous.
The composite character of the
flows may be best seen upon the inner slope, where their sections are exposed.
The andesites are generally gray, and may be reddish. They are seldom as
vitreous as No. 26. No. 60 has a decidedly platy structure, which is locally
well marked, although absent at other points. The plates are usually about
one-half inch to an inch in thickness, and strike N. 70° E. nearly parallel to
the spur, with a dip of 25° NW. The thickness of the plates at each outcrop is
rather uniform, but it varies between the outcrops.
At The Watchman and Glacier
Peak the layers of lava have a decided upward curve when viewed from the lake,
and suggest that the volcanic vent from which the lavas of that portion of the
rim issued was not central over the lake, but much closer to the western border.
This view is fully borne out by the character of the igneous material in Glacier
Peak. It is composed in small part of darker slaggy andesites and much red,
yellow, or whitish fragmental material which is highly colored, as if by the
escape of hot volcanic gases near the vent. From the lake these colored patches
are brilliant in the morning light. On the inner slope of Glacier Peak (Pl. VII,
A) are numerous columns, one of which is over 100 feet high. To call this
Glacier Peak is a misnomer, for the glaciation here is much less distinct than
on the hills farther south along the rim.
