DEPOSITION OF INCLINED BEDS. 127 



served is that the beds were deposited by currents in the position they still 

 retain during the time that the Humboldt Valley was occupied by the 

 waters of Lake Lahontan. lu other words, the inclined strata seen in the 

 canon walls are sections of current-formed embankments that have been 

 buried beneath the upper lacustral marls. They are arches of deposition, 

 and not plications due to orographic disturbance. 



The formation of gravel embankments across narrow straits in such a 

 manner as to completely close them has already been referred to in describ- 

 ing a structure of this nature at the lower end of Humboldt Lake {ante, 

 page 108). The study of current-formed gravel deposits in a large number 

 of the desiccated lake-basins of the Far West strongly favors the conclusion 

 that the inclination of tlie strata exposed in the walls of the Humboldt 

 Canon is due entirely to their mode of accumulation. Could the gravel 

 embankments familiar to us in many narrow valleys become buried beneath 

 lake sediments and then exposed in cross section by eros'on, they would 

 furnish examples of strata increasing in thickness at the same time that they 

 became inclined and arched, which would in many ways be the counterpart 

 of the phenomena illustrated by the section on Plate XXHI. 



The hypothesis that the dip in th , sections we are considering was due 

 to jjlication, in the manner common to many older rocks, suggested itself 

 early in the investigation, but did not find support in the facts observed. 



The contacts of the lower and upper clays with the medial or gravel 

 member of the series are nearly always unconformable. In many instan- 

 ces the surface of the lower marls was eroded into hollows and channels 

 previous to the deposition of the gravels, and these are now filled with 

 current-bedded debris in the manner illustrated by section K, Plate XXIH. 

 Similar unconformities by erosion are also to be observed at many points 

 where tlie contact of the medial gravels with the upper marls is exposed, 

 as shown at I, on the same plate. Other illustrations of unconformity in 

 the contact of the medial gravels with the evenly stratified beds above and 

 below them are shown in the remaining detailed sections of Plate XXIII. 



A comparison of the upper and lower clays indicates that they are 

 very similar in their nature and were probably accumulated under nearly 



