Page 151 - 1915, Springs of CA.

HOT SPRINGS. 13.9

house, shows that it is decidedly salty. The high content of silica
shown by the analysis is also worthy of note. A few other hot springs,
notably Arrowhead Hot Springs (p. 32), contain approximately as
much silica, but the siliceous deposit at the Morgan springs is
thought to be the largest spring deposit of this material in the State.
In fact the only other hot spring locality where notable amounts of
silica have been deposited is believed to be at Casa Diablo Hot
Pool (p. 147). LeConte and Rising mention the deposition of silica
at Sulphur Bank 1 (Lake 38) (p. 98), but it there takes place under-
ground and is not noticeable to the casual observer.
It is possible that the water is magmatic and juvenile that is, that
it was originally contained in the underlying rock masses and that
it reaches the surface of the earth for the first time in the springs.
The sodium, chlorine, and also the silica may therefore be derived
from the deep-seated rock magma or semifluid mass. Diller2 has
shown, however, that a strait or arm of the ocean once occupied this
area, which has since been uplifted and on which the mountain mass
of Lassen Peak has been built up, and that the Chico formation,
consisting of marine sediments, which outcrops 20 miles west and
southwest of Morgan Hot Springs,3 probably underlies the lava of
Lassen Peak. The suggestion, based only on the chemical character
of the water and the geologic structure that has been worked out by
Diller, is therefore offered that the Chico beds furnish the saline .con-
stituents of the water at Morgan Hot Springs. The solvent action
of this hot, saline water as it rises through the overlying lavas may
account for the unusually high content of silica in the water, although
literature concerning the solvent power of hot saline solutions on
silicates has not been found.
The slopes on each side of the meadow at Morgan Hot Springs are
covered with pyroxene andesite of Miocene or Pliocene age,4 but a
cemented conglomerate is exposed along the creek in the meadow
where the springs rise. The cementing material is siliceous and
probably has been deposited by the hot water.
Three-fourths of a mile northeastward, on a branch of Mill Creek,
is another hot spring over which a bathhouse containing several
compartments has been built. This spring also rises in a hard con-
glomerate similar to that at the springs in the meadow.
1 LeConte, Joseph, and Rising, W. B., The phenomena of metalliferous vein formation now in progress
at Sulphur Bank, Cal.: Am. Jour. Sci., 3d ser., vol. 24, p. 33,1882.
2 Diller, J. S., Geology of the Lassen Peak district: U. S. Geol. Survey Eighth Ann. Rept., pt. 1, p. 413,
1889; Tertiary revolution in the topography of the Pacific Coast: U. S. Geol. Survey Fourteenth Ann.
Rept., pt. 2, p. 423,1894.
3 Diller, J. S., U. S. Geol. Survey Geol. Atlas, Lassen Peak folio (No. 15), p. 1,1895.
4 Diller, J. S., idem, areal geology sheet.