44 PROCEEDINGS OF THE CALIFORNIA 
Now the great lake, also, has a similar structure. It also has a beautiful 
sand and gravel beach all along its upper shore, and a sand flat extending 
above it; while at its lower, or northern end, thickly strewed in the shallow 
water, and along the shore line, and some distance above the shore line, are 
found in great abundance boulders of enormous size. May we not conclude 
that similar effects have been produced by similar causes—that these huge 
boulders were dropped by the great glacier at its lower end ? Similar boulders 
are also found along the northern portion of the eastern shore, because the 
principal flow of the ice-current was from the southwest, and in the fullness 
of glacial times the principal exit was over the northeastern lip of the basin. 
b. Origin of Lake Tahoe.—That Lake Tahoe was once wholly occupied by 
ice, I think, is certain, but that it was scooped out by Lake Valley glacier is 
perhaps more doubtful. All other Sierra 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 afterwards modified by glacial agency, instead of a pure 
glacial-scooped rock-basin. In other words, Lake Valley, with its two summit 
ridges, may well be regarded as a phenomena 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. 
ec. Passage of slate into granite——From the commencement of the rocky 
canon at the head of Fallen Leaf Lake, and up for about two miles, the canon 
walls and Led are composed of slate. The slate, however, becomes more and 
more metamorphic as we go up, until it passes into what might be called 
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 Soda Springs. From this point the cafon 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 por- 
phyritic diorite into granite, may, I think, be best explained by increasing 
degree of metamorphism, and at the same time a change of the original sedi- 
ments 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. Itis 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 débris. Just at the 
same junetion, in Fallen Leaf Lake Canon, burst out the waters of Soda 
Springs, highly charged with bicarbonates of iron and soda. 
d. Glacial Deltas.—1I have stated that the moraines of Cascade Lake and 
Emerald Bay glaciers run down to the margin of Lake Tahoe. An examina- 
tion of this portion of the lake shore shows that they ran far into the lake— 
