184 J. LeConte—Ancient Glaciers of the Sierra Nevada. 
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 well 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 
now have, I will not hazard an opinion. 
c. Passage of slate into granite—From the commencement of 
the rocky cafion at the head of Fallen Leaf Lake, and up for 
about two miles, the cafion walls and bed are composed of slate. 
The slate, however, becomes more and more metamorphic as we 
ere diorite into granite may, I think, be best explained 
y the increasing degree of metamorphism, and at the same 
moraine matter, the water very deep close to shore, and the 
bottom composed of precisely similar moraine matter. In 
rowing along the shore, I found that the exquisite ultramarine 
