25S W. LINDGREN — TWO NEOCENE RIVERS OF CALIFORNIA. 



Page 



Bath to Ralstons 290 



The Damascus and Last ( lhance Tributaries 291 



< feneral character 291 



From the Ralston Mine to Summit Valley 291 



The lower Course 292 



The Duncan Peak Tributary 293 



The Canada Hill Tributary 293 



The upper Course 293 



Discussion of < rrades -"••I 



( inclusions -"- IS 



Introduction. 



Tlie investigations of the United States Geological Survey in the Gold 

 belt of the Sierra Nevada, carried out under the direction of Dr (i. V. 

 Becker, with whose consent this paper is published, have included the 

 geologic mapping of the country on topographic contour maps on the 

 scale of 1 : 125,000, or about two miles to the inch. During the course of 

 this mapping much information has been gained concerning the Neocene 

 river channels, now largely covered by deep volcanic flows or cut away by 

 subsequent erosion. The auriferous character of the accumulated gravels 

 gives, as is well known, great practical importance to these channels. 



A large number of the productive Neocene gravel deposits occur in the 

 watersheds of the present Yuba and American rivers, which were in- 

 cluded in the area assigned to the writer. It has been found that these 

 deposits are parts of two river systems which in a general way corre- 

 spond to the two modern rivers now draining the same territory. 



It is the purpose of the present paper to indicate buiefly the direction 

 of the principal forks of the Neocene Yuba and American rivers, to give 

 a more accurate idea of the Neocene topography within this district, 

 and to call further attention to certain channels which might prove re- 

 munerative if opened by mining operations. The continuity of sonic 

 of these can be asserted and their approximate position indicated. It is 

 not proposed in this place to enter upon any elaborate discussion of the 

 many and interesting questions connected with the accumulation of the 

 gravels, nor is it the intention to describe in detail the often complex 

 channel systems of any particular region* 



*The term "Neocene" has been used, < sistently with the nomenclature adopted by the Sur- 

 vey, in preference to " Pliocene." The N ;ene comprises tin- Miocene .-mil Pliocene periods of 



the Tertiary era, between which, in the Sierra Nevada, no definite line can be drawn. It is, in. I I, 



very probable thai the first period of erosion, the gravel period mm. I tin- volcanic period repre- 

 sent :i large pari of the time between the later Creti us and tin- later Tertian . bul there is no 



definite floral or faunal evidence t<> support this. 



