96 GLACIAL AND SUEFACE GEOLOGY OF THE PACIFIC COAST. 



original foriuatioii. If tliey wei'e empty, their existence would hardly be 

 observed ; they might even escape notice altogether. The principal reason 

 why the water stands in these shallow basins is, that the rock, which is ex- 

 clusively granite, is impermeable : it holds up the water almost as well as 

 metal would do it. Again, the snpplj' of water to keep them filled is readily 

 forthcoming from the melting of the snow, which usually lies almost or quite 

 through the year at the base of the clift's by which these little pools of water 

 are surrounded. That there should have been inequalities existing in the 

 surface of the granite at the time the Glacial ejioch commenced, can certainly 

 not be a matter of surprise ; that the glacier, with the aid of the water pro- 

 duced by its melting, kept these depressions free from debris, or even, in 

 places, deepened them somewhat, is also a condition of things easily under- 

 stood. Tiie more difficult point seems to be, to find an answer to the ques- 

 tion why these hollows have not become filled up with the debris from the 

 adjacent slopes since tlie disappearance of the glaciers. This leads us to the 

 discussion of another interesting point which presses itself upon the attention 

 of the observer of the phenomena of past glaciation in the Sierra Nevada. 



The jioint is this : the more one studies the localities formerly covered 

 with ice in the I'egion in question, the more one is impressed by the fact 

 that but a short period lias elapsed since this ice disappeared. The Glacial 

 epoch — for California, at least — is only just over. This is shown in many 

 ways, by none, however, more satisfactorily than by the astonishing perfec- 

 tion with which the glacial polish has been preserved over large areas for- 

 merly covered by ice. That the surface should retain the striations and polish 

 for a long- time, and in great perfection, where the rock was, immediately 

 after their production, covered deeply with debris and soil, is not so remark- 

 able. There are man}' localities in New England and in the vicinity of the 

 Great Lakes where the striaj are almost as fresh looking as if just made ; but 

 in all such cases the rock surface has only been recently stripped of its 

 detrital covering by the hand of man or otherwise. All through the Sierra, 

 on the other hand, where the granite is entirely bare of overlying debris, 

 and appears to have remained so ever since the ice left it, the polish is appar- 

 ently as perfect as it was when first made. The only theory by which these 

 facts can be explained is this, that but a short period of time has elapsed since 

 the work was done. It might perhaps be argued that the surface had 

 remained covered with debris until a recent period, and that this has only 

 lately been removed. This, however, is not a tenable supposition in view of 



