2G4 W. LINDGREN — TWO NEOCENE RIVERS OF CALIFORNIA^ 



tion A A. In this province the volcanic flows are not conspicuous. It 

 is probable that the higher parts of the foothill region remained above 



them, and erosion to a great extent removed them from the lower parts. 



Second, the middle slopes, consisting chiefly of more or less altered 

 sedimentary rocks, the auriferous slates. In this region the broad tables 

 of Neocene lavas have largely effaced the pre-volcanic topography. 

 Often, indeed, ridges of older rocks rise here also above the top of the 

 gently sloping volcanic table-land, but as a rule they arc not prominent. 



Third, the region of high bed-rock peaks adjoining the divide, in which 

 the character of a table-land, frequently noticeable even here. bec< >mes 

 modified by prominent points of ante-Tertiary igneous and sedimentary 

 rocks projecting conspicuously above the level of the Neocene flows. A 

 glance at the map, on which only peaks of the older rocks are marked 

 with their elevation in numbers, will make clear this distinction. At 

 the divide there are many volcanic peaks, culminating in the extinct 

 volcanoes of mount Lola. Castle peak and others which exceed 9,000 

 feel in height. The elevations of these volcanic peaks are not given on 

 the map. Were the later volcanic masses removed along the divide the 

 lowest passes would still be about 7,000 feet high. 



( 'ondition of the Sierra X< vada before and during the gravel Period. — From 

 the evidence accumulated it cannot be doubted that during the gravel 

 period or the later part of the Tertiary the Sierra Nevada in this region 

 formed a mountain range as distinct, if not as high, as at present. The 

 two Neocene rivers headed near where the corresponding modern rivers 

 begin now, in a region of lofty peaks and ridges. Their watersheds cer- 

 tainly did not extend further eastward than the first summit, and in fact 

 corresponded pretty closely with those of the modern rivers. On the 

 Truckee sheet, at least, the Neocene divide coincides very nearly with the 

 divide of to-day, and only unimportant changes can be noted. East of 

 the divide there was an escarpment of moderate slope, and which is now 

 exposed in many canyons to a height of 1,000 to 2,000 feet; it probably 

 was much higher than this, but below a level of from (},<>( )0 to 7. "(Hi feel 

 above the sea its slope is completely hidden under the immense accumu- 

 lations of lavas Lying between the two summits north of lake Tahoe. 



The total height of this escarpment as it was before the eruption of 

 the Neocene lavas should perhaps be measured from the bottom of hike 

 Tahoe to the summit of the western divide, approximately 4,000 or 4,500 

 feet. There is, on the Truckee sheet, no evidence of any important post- 

 volcanic fault along the western summit, nor is there any decided evi- 

 dence that the steep eastern slope just mentioned represents a fault 

 formed shortly beforcor during the volcanic period. 



This is illustrated by sections G G and L L (plate 7). .Many similar 

 ones could be selected from the Truckee sheet. In G G a contact be- 



