THE DATE OF THE GLACIAL PERIOD. 611 



curred certainly centuries, but probably not many thousands 

 of years ago." * This opinion is sustained by the fact that 

 the erosion of present streams in these old beds is slight, and 

 by the fact that in the canons of the high Sierra, which were 

 once occupied by glaciers, "the smooth surfaces are still 

 scored with fine, hair-like lines, and the eye fails to detect 

 more than a trace of disintegration that has taken place since 

 the surfaces received their polish and striation. ... It seems 

 reasonable to conclude that in a severe climate like that of 

 the high Sierra it [the polish] could not remain unimpaired 

 for more than a few centuries at the most." To the same 

 effect is the testimony of Mr. Gilbert as to the date of the 

 last great extension of Lake Bonneville, of which he says : 

 % * The Bonneville shores are almost unmodified. Intersect- 

 ing streams, it is true, have scored them and interrupted 

 their continuity for brief spaces ; but the beating of the rain 

 has hardly left a trace. The sea-cliffs still stand as they first 

 stood, except that frost has wrought -upon their faces so as to 

 crumble away a portion and make a low talus at the base. 

 The embankments and beaches and bars are almost as perfect 

 as though the lake had left them yesterday, and many of 

 them rival in the symmetry and perfection of their contours 

 the most elaborate work of the engineer. There are places 

 where bowlders of quartzite or other enduring rock still re- 

 tain the smooth, glistening surfaces which the waves scoured 

 upon them by dashing against them the sands of the beach. 



" When this preservation is compared with that of the low- 

 est tertiary rocks of the region — the Pliocene beds -to which 

 King has given the name Humboldt — the difference is most 

 impressive. The Pliocene shore-lines have disappeared. 



" The deposits are so indurated as to serve for building- 

 stone. They have been upturned in many places by the up- 

 lifting of mountains. Elsewhere they have been divided by 

 faults, and the fragments, dissevered from their continuation 

 in the valley, have been carried high up on the mountain- 



* " Monograph XI," p. 273. 



