lakes which I have seen certainly
owe their origin to glacial agency. Neither do I think we
should be staggered by the size or enormous depth of this
Lake. Yet, from its position, it may be a plication-hollow,
or a trough produced by the formation of two parallel mountain
ridges, and afterward modified by glacial agency, instead of a
pure glacial-scooped rock-basin. In other words, Lake Valley,
with its two summit ridges, _may be regarded as a phenomenon
belonging to the order of mountain-formation and not to the
order of mountain sculpture_. I believe an examination of
the rocks of the two summit ridges would probably settle
this. In the absence of more light than I now have, I will not
hazard an opinion.[3]

[Footnote 3: This question practically has been settled by Mr.
Lindgren, and his conclusions are given in an earlier chapter.]

_c. Passage of slate into granite_. From the commencement
of the rocky canyon at the head of Fallen Leaf Lake, and up
for about two miles, the canyon walls and bed are composed
of _slate_. The slate, however, becomes more and more
metamorphic as we go up, until it passes into what
much resembles _trap_. In some places it looks like
_diorite_ and in others like _porphyry_. I saw no
evidence, however, of any outburst. This latter rock passes
somewhat more rapidly into _granite_ at Glen Alpine
Springs. From this point the canyon bed and lower walls are
granite, but the highest peaks are still a dark, splintery,
metamorphic slate. The glacial erosion has here cut through
the slate and bitten deep into the underlying granite. The
passage from slate through porphyritic diorite into granite
may, I think, be best explained by the increasing degree of
metamorphism, and at the same time a change of the original
sediments at this point; granite being the last term of
metamorphism of pure clays, or clayey sandstones, while bedded
diorites are similarly formed from ferruginous and calcareous
slates. Just at the junction of the harder and tougher granite
with the softer and more jointed slates, occur, as might
be expected, cascades in the river. It is probable that the
cascades at the head of Cascade Lake and Emerald Bay mark,
also, the junction of the granite with the slate--only
the junction here is covered with debris. Just at the same
junctio