10 GLACIAL AND SURFACE GEOLOGY OF THE PACIFIC COAST. 



It is a mistake, however, to sujipo.se that striated surfaces cannot be pro- 

 ducerl by dr>j ruhhiny, or friction of surfaces against each other without the 

 aid of ice or water. Every mining geologist knows from his daily experience 

 that beautifully striated and polished surfaces have been produced in abun- 

 dance by the sliding of portions of the vein material against its walls, and that 

 this has often taken place where the amount of motion must have been very 

 small, the immense pressure being sufficient to bring about the result, even 

 when the distance over which the surfaces have moved cannot have ex- 

 ceeded a few inches. Excellent imitations of glacial striation are produced 

 by the passage of heavy vehicles with locked wheels over rock surfaces. 

 Pebbles also may be smoothed and striated by dry motion, provided they 

 are fixed in their places while the motion takes place. Where heavy rock 

 slides or avalanches have occurred, the bottom may occasionally be observed 

 to be not only striated, but even deeply grooved. It is decidedly unjustifi- 

 able in the geologist to presume on the former existence of ice at any epoch 

 or in any region from the occurrence of isolated smoothed and striated 

 boulders or pebbles in the rock masses. Only by careful comparison of all 

 the phenomena taken in their ensemble, over a considerable area of surface, 

 can the former presence of ice be clearly established in a region where the 

 present conditions are not such as themselves to throw light on the matter. 

 For instance, the finding of an occasional striated boulder in a conglomerate 

 of Devonian, Carboniferous, or Permian age would certainly not justify the 

 geologist in inferring that glaciers must have been in existence at the time 

 such rock Avas deposited. 



As a consequence of long-continued crowding over the surface of the icy 

 mass, which carries more or less detrital material imbedded in it, the project- 

 ing edges of the rocky strata gradually lose their sharpness, and become 

 worn into those rounded forms to which the term roches moutowiees is com- 

 monly applied. This peculiar kind of surfiice is easily recognized by an 

 experienced observer. But there are certain rocks which assume forms very 

 closely imitating those produced by ice. Much granite, for instance, has a 

 concentric structure, causing it to break under the influence of ordinary 

 meteorological conditions into spherical forms, and to weather in masses with 

 rounded surfaces, very much resembling the roches moutonnecs. Want of 

 knowledge of those facts has led to many mistakes of this kind, especially in 

 the Sierra Nevada. 



The conclusion to wliich the writer has arrived, after much examination 



