THE BED-ROCK SURFACE AND THE CHANNELS. 



347 



Portions of the erupted material were almost entirely indestructible, and 

 remain to the present time very little affected by weathering, protecting the 

 various underlying detrital materials from being swept away, while they 

 themselves stand out in the well-known « table mountain " form, often rising 

 high above the surrounding country. The cause of such topographical 

 features is easily enough comprehended ; and were all the volcanic materials 

 as solid and indestructible as that of the Tuolumne Table Mountain, for 

 instance, there would be no difficulty in understanding how it is that these 

 lavas occupy so high a position. But, as Mr. Goodyear has shown in Ins 

 review of his notes, much of the erupted material appears to be of a kind 

 readily acted on by water, and more likely to be eroded away than the 

 average bed-rock itself. This is particularly the case in the region which 

 was the field of his special investigation ; but both north and south of his 

 district there is much solid basaltic lava, forming " table mountains," like 

 that of which the one in Tuolumne County offers a typical example* 



The great difficulty in regard to the formation of the present river canons 

 is to explain how they were started in the position which they now occupy. 

 Having once been begun, their present character can easily be shown to be 

 the result of the peculiar climatological and geological character of the region. 

 The gradually diminishing quantity of water which the streams carry pre- 

 vents their rising above the channel in which they are confined ; they must 

 therefore continue to excavate along one line. But as the volume of water 

 has decreased, so their erosive power has diminished; and the canons, 

 although gradually deepened, have become narrower, their whole form 

 being that which must result from the action of a force continually 

 diminishing in intensity. The sides of these canons have been kept free 

 from debris, as it appears, through the agency of the occasional sudden and 

 heavy " freshets," which still take place, as previously described, although 

 with, on the whole, gradually lessening intensity. 



At the present time the excavating power of the streams seems to have 

 diminished almost to nothing. Only the finer portions of the debris resulting 

 from the hydraulic washings is carried away by the current ; so that where 

 the bottoms of the canons have become filled to great depth with taxhngs, the 

 streams, under ordinary circumstances, hardly disturb them. Tt is only dur- 

 ing such winters as that of 1861-1862, when very extraordinary rises take 

 place, that portions of this detrital material are swept down into the Great 



* See Geology of California, Vol. 1. pp. 219 - 211, for section, and description* of some of these. 







