740 DESCRIPTIVE GEOLOGY. 



trend, tlie liigliest points at the northern end attaining an elevation of over 

 3,000 feet above the valley. Although this group has been but little 

 studied, there appears to be a nucleus of syenite cropping out in one or two 

 places, and indicating a continuous body with that of Black Butte, which 

 it lithologically resembles, and in like manner does not reach the higher 

 summits. 



Observed strikes of the slates vary from north 35° to 45° east, dipping 

 steeply to the northwest. The low butte to the westward of the Eugene 

 Mountains is formed of the same slates, having a strike north 45° to 50° 

 east. 



Humboldt Valley. — That the Humboldt Valley, from the Havallah 

 Range southward nearly to Humboldt Lake, is underlaid by horizontal 

 Pliocene strata, seems evident from the great number of exposures in the 

 immediate region of the river-banks, and from the thinness, wherever ob- 

 served, of the overlying Quaternary material. Indeed, one noticeable fea- 

 ture of the valley is that there are no observations to warrant a belief in the 

 great thickness of the Quaternary deposits; for wherever they are penetrated 

 to any considerable depth, the upturned Miocene or horizontal Pliocene 

 beds are soon reached. That all the older formations are more or less con- 

 cealed by Quaternary is evident, but upon the mountain-slopes there are 

 no such talus accumulations as are to be found elsewhere ; the fine silts and 

 gravels of the plain serve as a mere covering to the Tertiary, while the allu- 

 vial bottoms are not only thin but limited in extent. The reason of this 

 thinness of Quaternary seems obvious : the whole region up to 500 feet 

 above the level of Carson Lake, or about 4,400 feet above sea-level, has been 

 so recently occupied by the waters of Lake La Hontan, which date back to the 

 Glacial period. The thin series of clays and sands, which form the mud lakes 

 and alkali bottoms, are of an altogether unknown thickness, and inasmuch 

 as they represent the bottom of these Post-Pliocene lakes, the Upper Qua- 

 ternary, which now covers the valleys, with the exception of these isolated 

 areas, is, therefore, only the accumulation since the desiccation of this great 

 sheet of fresh water. It is, therefore, evident that the very great accumula- 

 tions of Quaternary could not be expected in a region which, during the 

 longest portion of the Quaternary period, was submerged. 



