TOPOGRAPHY OF THE PACIFIC COAST. 37 



This uniformity of gentle slope is enhanced in some cases, 

 especially in the region of the American and Yuba rivers, by the 

 broad, flat-topped lava flows which occupy the divides between 

 the canons. Sometimes it appears that the volcanics are thin, 

 while at other places, according to Whitney their thickness is 

 very large, quite often reaching 400 or 500 feet, and occa- 

 sionally much exceeding that amount. The plain, however, is 

 not limited to the areas occupied by volcanic rocks, but has a 

 wide distribution over areas of closely folded auriferous slates, 

 and cannot be attributed to the constructive effects of volcanic 

 eruptions. 



Mr. Gilbert was the first to call attention to the fact that this 

 uniform surface is due to erosion upon a system of plicated strata, 

 and "could only have been accomplished by streams flowing at 

 a low angle," ^ in other words, the plain must have originated 

 essentially as a baselevel of erosion. 



Judging from the topographic maps recently prepared for 

 the geological work in the gold belt, as well as from the obser- 

 vations of Whitney,^ Petty, ^ Goodyear,^ Lindgren,^ Turner, and 

 myself, it appears that the inclined plateau which now forms the 

 western slope of the Sierra Nevada was originally not worn 

 down to so complete a plain as that already described upon the 

 western side of the valley. 



Mr. Lindgren (1. c.) says, "that the Sierra Nevada, before 

 the accumulation of the gravels began, was a mountain range 

 greatly worn down by erosion, but not reduced to a baselevel of 

 erosion. It cannot even, on the whole, be regarded as a pene- 

 plain, above which isolated and more resistant hills projected. 

 The declivities and irregularities of the old surface are too con- 

 siderable for that, nor are the projecting hills invariably com- 

 posed of the hardest rock-masses." 



While some of the irregularities now recognized in the old 

 plain upon the western slope of the range are due, as urged by 



'Science, Vol. i, p. 195, March 23, 1883. 



^Auriferous Gravels of the Sierra Nevada of California. 



3 Two Neocene Rivers of California. Bull. Geol. Soc. of America, Vol. 4, p. 298. 



