On ascending this canyon I easily found the parent rock of these pebbles and bowlders.
the It is a powerful outcropping ledge of beautifully striped siliceous slate, full of fissures and joints, and easily broken into blocks of all sizes, crossing the canyon about a half mile above the lake.

This rock is so peculiar and so easily identified that its fragments become an admirable index of the extent of the glacial transportation.

I have, myself, traced these pebbles only a little way along the western shores of the great Lake, as my observations were principally confined to this part; but I learn from my brother, Professor John LeConte, and from Mr.John Muir, both of whom have examined the pebbles I have brought home, that precisely similar fragments are found in great abundance all along the western shore from Sugar Pine Point northward, and especially on the extreme northwestern shore nearly thirty miles from their source.

I have visited the eastern shore of the Lake somewhat more extensively than the western, and nowhere did I see similar pebbles.

Mr.Muir, who has walked around the Lake, tells me that they do not occur on the eastern shore.