ACADEMY OF SCIENCES. 45 
that the lake has filled in two or three miles by glacial débris. On the east 
margin of Lake Tahoe, the water, close along the shore, is comparatively 
shallow, the shore rocky, and along the shore-line, above and below water, 
are scattered great boulders, probably dropped by the main glacier. But on 
the west margin of the lake the shore-line is composed wholly of moraine 
matter, the water very deep close to shore, and the bottom composed of pre- 
cisely similar moraine matter. In rowing along the shore, I found that the 
exquisite ultramarine blue of the deep water extends to within 100-150 feet 
of the shore-line. At this distance, the bottom could barely be seen. Judg- 
ing from the experiments of my brother, Professor John Le Conte, according 
to which a white object could be seen at a depth of 115 feet, I suppose the 
depth along the line of junction of the ultramarine blue and the emerald 
green water, is at least 100 feet. The slope of the bottom is, therefore, 
nearly, or quite, 45°. Itseems, in fact, a direct continuation beneath the 
water of the moraine slope. The materials, also, which may be examined 
with ease through the wonderfully transparent water, are exactly the same as 
that composing the moraine, viz: earth, pebbles, and boulders of all sizes, 
some of them of encrmous dimensions. It seems almost certain that the 
margin of the great Lake Valley glacier, and of the lake itself when this glacier 
had melted and the tributaries first began to run into the lake, was the series of 
rocky points at the head of the three little lakes, about three or four miles back 
from the present margin of the main lake; and that all lakeward from these 
points has been filled in and made land by the action of the three glaciers de- 
seribed. At that time Rubicon Point was a rocky promontory, projecting far 
into the lake, beyond which was another wide bay, which has been similarly 
filled in by débris brought down by glaciers north of this point. The long 
moraines of these glaciers are plainly visible from the lake surface; but I 
have not examined them. Thus, all the land, for three or four miles back 
from the lake-margin, both north and south of Rubicon Point, is composed 
of confluent glacial deltas, and on these deltas the moraine ridges are the natural 
levées of these ice-streams 
e. Parallel Moraines.—The moraines described above are peculiar and 
almost unique. Nowhere, except about Lake Tahoe and near Lake Mono, have 
I seen moraines in the form of pdrallel ridges, lying on a level plain and ter- 
minating abruptly without any signs of transverse connection (terminal moraine) 
at the lower end. Nor haveTI been able to find any description of similar 
moraines in other countries. They are not terminal moraines, for the glacial 
pathway is open below. They are not lateral moraines, for these are borne 
on the glacier itself, or else stranded on the steep cafon sides. Neither do I 
think moraines of this kind would be formed by a glacier emerging from a 
steep narrow canon and running out on a level plain; for in such cases, as 
soon as the confinement of the bounding walls is removed, the ice stream 
spreads out into an ice lake. It does so as naturally and necessarily as does 
water under similar circumstances. Thedeposit would be nearly transverse 
to the direction of motion, and, therefore, more or less crescentic. There 
must be something peculiar in the conditions under which these parallel 
ridges were formed. I believe the conditions were as described below. 
