302 G. D. LOUDERBACK STRUCTURE OF THE HUMBOLDT REGION 



preserve the shoreline traces beautifully, although the incisions are very 

 shallow. 



It is probable that both valleys are underlain throughout by the 

 Lahontan sediments, but they have been covered dver and hidden by 

 the finer alluvial accumulations of Recent times. No stream exists in 

 the valley to the east of the mountains which might lay them bare at 

 some point, but in the Humboldt valley the river, opposite the northern 

 end of the range near Oreana, has cut through 50 to 200 feet of the strata 

 and given some fine sections. They are mainly unconsolidated and 

 horizontally stratified sands. They evidently unconformably overlie 

 the Truckee beds. 



Alluvial and recent deposits. — The most important of these deposits 

 are the great alluvial cones. The one encountered on the west side of 

 the mountains in the detailed section is about 8,500 feet long, and rises 

 300 feet, with a slope of 1 in 28, or 2 degrees 4 minutes. On the east 

 side of the range the section crosses one 7,000* feet long, which rises 350 

 feet, an average slope of 1 in 20, or 2 degrees 52 minutes. On approach- 

 ing Table mountain, which rises 3,400 feet above the valley (the Hum- 

 boldt mountains where crossed are only 2,100 feet), a cone was mounted 

 which is 14,500 feet long and which rises 700 feet above the valley — that 

 is, 1 foot in 20.5, or 2 degrees 46 minutes. The apex of the cone is taken 

 in each case outside the general range slope and not within the canyon 

 in which it rises. Some very fine material spreads itself out beyond the 

 cones for quite a distance, but produces no appreciable slope, the valley 

 being practically horizontal. As stated before, these cones are unmarked 

 by sharp shorelines or other shore features, showing that at least all of 

 the outer layers in all azimuths are post-Lahontan in origin. Distinct 

 shore marks on the cones in other parts of the Lahontan basin, however, 

 show that the bulk of each cone was formed in pre- Lahontan time. 



In the lower part of the Humboldt valley the river.- has deposited 30 

 feet or more of alluvium, and the deposits in Humboldt lake, which now 

 no longer exists, must also be considered Recent. No corresponding 

 deposits occur in the streamless valley of the Carson sink, where Lahon- 

 tan shore marks and tufa deposits can be seen still in the central parts 

 of the valley floor. 



The volcanic rocks — General character and relations. — The sedimentary 

 rocks just described are essentially valley deposits, and, as far as known, 

 nowhere occur on the mountains in question. The volcanics, on the 

 other hand, occur chiefly on the mountains, where they cover large areas. 



Volcanic rocks that are the successive products from several vents are 

 likely to be very irregular in their areal distribution and order of super- 



* The section crosses this some distance below its apex ; it is probably 8,000 feet long altogether. 





