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



istence of Neocene glaciers has thus far been met with in the highest 

 part of the range in this Latitude; nor is it likely, in view of the char- 

 acter of the Neocene flora high up on the flank of the range, that such 

 evidence will he found. 



Observations on Method ok Work. 



The help of a contour map is almost indispensable in order to ohtain 

 a correct idea of the Neocene drainage and topography.* 



Each point of the contact lines between the bed-rock ami the super- 

 jacent Neocene gravels or volcanic flows necessarily marks a point on 

 the old surface of the region such as it was before hidden under Tertiary 

 accumulations. A great number of these contact lines are usually ex- 

 posed b}^ the canyons and creeks eroded since the close of the Neocene 

 period, and each of them affords a section through a part of the Neocene 

 surface. It will easily be conceded that if the elevation of a sufficient 

 number of points on the contact lines were known, a contour map might 

 be constructed of the Neocene surface, showing the topography and 

 elevation above the sea, provided that no change in level or tilting had 

 taken place in the interval. Even in such a case the map would he 

 valuable as showing the relative topography, and if the existence and 

 amount of the disturbance of the old surface could he ascertained by 

 other means a correct map referred to the old sea level might he obtained 

 from it. The bed-rock points that have been above the surface of the 

 lava flows since the end of the Neocene — and there are many of them in 

 the Gold Belt region — have often suffered a degradation difficult to 

 measure, but probably in most cases not large. The flat tops of many 

 of them show them to have formed a part of the Neocene surface, and 

 the erosion, while scoring and furrowing their flanks, has not yet reached 

 their summits. 



Many of the topographic features of the Neocene region may be directly 

 read on the contour maps on which the geologic areas are outlined. 

 If in a certain vicinity all the contact lines between lava and bed-rock 

 run practically parallel with the contours and at the same elevation, the 

 conclusion is easy that the Neocene deposit rested on a horizontal surface. 

 provided no tilting has taken place since. If, again, the contact lines 

 cross the contour lines in an irregular way and at considerable angles, the 

 old surface was broken and irregular; with a sufficient number of contacts 



* Tin- part of the < told licit here under consideration Ins been mapped on tin- scale <>t l : 125,000, or 

 :ii ii mi two mill's to the inch, with a contour interval of one hundred feet. It comprises the Smarts- 

 vi lie, Colfax, Truckee, Sacramento, Placer ville, and Pyramid Peak sheets, and the topography has 



1 n executed by Messrs h. M. Wilson, A. F. Dunnington, B. 11. Mc Kee, and M . E. I louglas under 



tin' charge pf Professor A. 11. Thompson. The geologic maps of the larger pari of tic area are 

 finished and are now ready for publication. The Sacramento atlas short bas just been printed. 



