268 \V. LINDGREN — TWO NEOCENE RIVERS OF CALIFORNIA. 



ridges of older rocks still rising above the Hows, and to them is to be 

 attributed the fact that the Neocene river system roughly corresponds to 

 the present one; but over the large stretches of volcanic tables the rivers 

 marked their new courses entirely independent of the older streams, now 

 following them, now crossing them in a most irregular manner.* 



The Neocene Yuba River. 

 the main river. 



Fromtln Sacramento Valley to French Coiral. At Smartsville, near Sacra- 

 mento valley, a stretch of channel three miles Long has been preserved. 

 Its character and grade are described in detail by Mr Pettee in the 

 "Auriferous gravels," pp. 379-383. At Sicards Mat, ahout two miles lower 

 down, on the northern side of the river, a fragment of the old channel 

 remains, and three miles further down, on the southern side of the Yuba, 

 the last trace of it is found, but it is not certain whether the elevation 

 given at the last place represents the lowest channel. From here on it 

 is buried under the more recent deposits of the great valley. It should 

 be noted, in this place, that the areas of auriferous gravels indicated 

 by Mr Pettee on the low rolling foot-hills west of Smartsville as Neo- 

 cene and below the volcanic flows are in reality Pleistocene and rest 

 on the andesitic breccia, and that they consequently have no signifi- 

 cance in tracing the Tertiary channel. 



The gravels at Smartsville and Sicards Mat do not belong to the oldest 

 Neocene deposits, for they contain a considerable amount of andesitic 

 pebbles. They are, however, certainly Neocene, for at Smartsville they 

 are covered by that sheet of andesitic tuffaceous breccia which, in the 

 region under consideration, marks the close of the Neocene period. They 

 must represent the Neocene river in its lower course, for both north 

 and south of Smartsville rise bed-rock ridges, and this place affords 

 really the only outlet possible for the channels of the upper course of 

 the Yuba; the Neocene river broke through the harried of the great 

 diabase area of Yuba county in a valley or canyon of somewhat gentler 

 profile, hut almost as accentuated as that of the recent river. The ridges 

 on each side rise to a height of one thousand feet or more above the old 

 channel (see section A A). It might he objected that the channel at 

 Smartsville was deepened during the intervolcanic period of erosion, 

 and consequently no longer represents the bed of the prevolcanic river. 



* In the region described postvolcanic faults are ran' : those found have seldom more than 1 • 



15 feet throw. The Tertiary deposits would greatly facilitate the recognition of any such faults of 

 considerable throw, and I think the probability very slight that the slopes shown in the sections 

 are to any noticeable degree influenced by such postvolcanic disturbances. 



