SOUKCES OF ACCOMPANYING ILLUSTRATIONS. 2G3 



the genera] drainage system may be made out. In the case of an old 

 valley running across a recent creek or canyon, the angles of the contact 

 lines with the contour lines on the opposite sides of the present gorge will 

 easily and directly indicate the ancient trough. 



The accompanying map (plate 5) is reduced from atlas sheets of the 

 United States Geological Survey. The elevations are partly taken from 

 the very reliable observations of Messrs Pettee and Goodyear, partly 

 from the maps of the United States Geological Survey, supplemented for 

 short distances by my own aneroid determinations, and partly also from 

 the surveys of Mr Browne. The channels where obliterated by erosion 

 are marked by dotted lines; where remaining though generally hid- 

 den under volcanic masses, by heavy black lines. It must be under- 

 stood that in both cases the indicated position is only approximately 

 correct and showing the probable course of the deepest depression. 



The grades given in plate 6 are affected by errors in distance and eleva- 

 tion. The latter are believed not to be great, but the former are difficult 

 to ascertain. An attempt has been made in measuring the distance to 

 follow the probable curves of the rivers; nevertheless the grades are 

 probably all a little too steep on account of underestimating distances, 

 but thedill'crencesarenot, I think, large enough to be of much importance. 



The sections in plates 7 and 8 are taken from maps used in the field on 

 the scale of 1 : 62.~>i K), or nearly one mile to the inch. In most cases it 

 has been found necessary to enlarge the scale to 3,180 feet to the inch : 

 only section L L is drawn on the scale of 6,360 feet to the inch. In all 

 case3 the vertical and horizontal scales are equal. 



Outlines of Geologic History. 



Topography. — In this latitude the Sierra Nevada has two summits, 

 separated by the Truckee valley and the deep basin of lake Taboo. The 

 western summit, whose peaks rise from 8,000 to 10,000 feet above the sea. 

 is also the divide between the Pacific and the Great Basin, while the 

 Truckee river, draining lake Tahoe, has cut a deep canyon through the 

 second or easterly summit on its way to the depressions of the Nevada 

 deserts; on the eastern side of lake Tahoe the last-named summit at- 

 tains even higher elevations than the principal divide. With the easterly 

 summit and its escarpment this paper does not deal. 



On the Pacific slope, in the watersheds of the Yuba and American 

 rivers, one may roughly distinguish three provinces: 



First, the foot-hill region, most frequently consisting of prominent 

 ridges of diabase and amphibolite. Many of them, in Neocene times, 

 projected boldly above the river beds, as shown, for instance, in the sec- 



