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THE MOUNTAINS ỌF COLORADO. 73 
ting steps for a long distance in an icy acclivity, and its scalp is 
always snow-clad. The pines and larches disappear at five thous- 
and, nine hundred feet, while the mosses and lichens continue up 
to the line of perpetual snow. The cerealia are not grown higher 
than three thousand, eight hundred, or four thousand feet, but in 
one sheltered place, Skala, barley ripens at five thousand, nine 
hundred and fifty feet above the sea. 
In order to produce glaciers there must be a marked relief and 
depression of the surface and a marked vicissitude between the 
summer and winter temperature. While the Andes in the tropics 
rise into the region of perpetual congelation, there is not that va- 
riation of temperature which is necessary to produce nevè, that 
aggregation of large crystalline facets, so different from river-ice, 
which make up glaciers. Many parts of Siberia and North Amer- 
ica are within the line of permanent ground frost, and yet no gla- 
ciers are formed. In the Alps, according to Forbes, the summer’s 
thaw percolates the snow to a great depth with water. The frost 
of the succeeding winter penetrates it far enough to freeze it to 
at least the thickness of one year’s fall; or by being repeated in 
two or more years, consolidates it more effectually. The glacier 
commences near the line of perpetual snow, and renewed by the 
accumulation of each winter descends to a lower level, its extrem- 
ity being constantly dissolved by the summer’s heat. 
In the Colorado region the conditions of relief and depression 
of surface are sufficient to maintain glaciers, but the temperature 
is not sufficiently low to maintain a line of perpetual congelation 
on which they depend for their existence. ; 
GuactaL Action.—Two enquiries naturally suggest themselves ; 
were these mountains formerly encased in ice? Were these plains 
subjected to that erosive action so conspicuously displayed in New 
England and the region of the Great Lakes? 
The western limit of the Erratic block group, as observed by 
me, is in the immediate valley of the Missouri, between Leaven- 
worth and Lawrence. The western limit of the striated rocks, 
as observed by Hayden, is at Plattsmouth, also in the immediate 
valley of the Missouri. 
In crossing the plains, which expand to more than six hundred 
miles m width, there is an absence of all drift phenomena, such 
as boulders, gravel knolls, and planed surfaces, until Denver 
