1881. ]} Glacial Phenomena in the Yellowstoue Park. .205 
table-land to the south. This table-land I have named the Park 
plateau ; it is wholly volcanic, and is separated from the base of 
the granite highlands on the north, by the valley of the Yellow- 
stone proper, and by the East fork, its geologic as well as topo- 
graphic continuation. It extends, with but few interruptions, one 
hundred miles to the south. We are here led to inquire whether 
or not there are evidences of former glaciers on this plateau. 
Such evidences do exist, but they are certainly not such as we 
might expect. Instead of well-defined moraines, an area dotted 
by erratic boulders and broad expanses of polished surfaces as in 
the Wind River and Teton mountains, we find only a few rocks 
other than those that may have been derived from the plateau 
itself. It should be remarked, however, in this connection, that 
the soft rhyolites which form the greater part of the plateau, 
would not retain glacial ne for any considerable length of 
time. 
An occasional small block of granite indeed is found, and some- 
times at unexpected levels, as on the slopes of the Washburn 
mountains many hundreds of feet above the general level of the 
plateau. A very few have been observed beyond Mt. Washburn, 
on the south side. The most remarkable example of these is a 
boulder resting upon the brink of the grand cafion, about a mile 
and a half below the great falls and nearly eighteen miles from 
the northern border of the plateau. 
On a stormy day in December I undertook to meander the 
grand cafion from the falls to the base of Mt. Washburn, and 
during a storm of rain and sleet took shelter under the overhang- 
ing edge of a great rock in the dense timber. Considerably to 
my surprise I discovered it to be a very compact coarsely crystal- 
line feldspathic granite. In shape it is. somewhat rectangular, the 
edges for the most part sharp and unworn, the result of spawling 
by the heat of forest fires. In cubical dimensions it will probably 
exceed two thousand feet. It is within a stone’s throw of the 
brink of the cafion and rests upon a sheet or a series of sheets of 
rhyolite, not less than one thousand feet in thickness, as may 
easily be determined by an examination of the section exposed in 
the cafion walls below. 
In seeking the possible source of this rock we naturally turn to 
the south, towards the sources of the Yellowstone. The plateau 
along the river’s course and around the lake is totally volcanic. 
. 
