140 GEOLOGICAL HISTORY OF LAKE LAHONTAN. 



boldt and the Truckee canons, and therefore do not require farther description. 

 The medial gravels, in common with lacustrine shore deposits in general, 

 are heterogeneous accumulations of worn and rounded sand, gravel, and 

 boulders, with occasional inclusions of finer delris ; cross stratification pre- 

 vails, and many of the beds were deposited in an inclined position. 



The upper lacustral clays in the Walker River section are more varied 

 and indicate more complex conditions of deposition than the similar expos- 

 ures that have been described in the preceding pages. The upper and 

 lower portions of the upper clays have the normal features of the deposit, 

 but an intermediate portion, varying 20 to 30 feet in thickness, is of a more 

 diversified character, and includes strata of sand and gravel which are fre- 

 quently iron-stained and in many places forai contorted and folded layers. 



This portion of the upper clays obtained the name of the "bone-bed" 

 in our field notes, fi'om the numerous mammalian remains that it contains 

 (see Chapter VI). It is exceptional in the Lahontan series, and evidently 

 must have been formed under peculiar conditions. The only hypothesis 

 which seems to furnish assistance in interpreting the jihenomena observed 

 assumes that the embankment dividing Walker River and Walker Lake 

 valleys formed a dam in late Lahontan times that obstructed the free cir- 

 culation of the waters occupying the two basins and caused the region 

 above the obstructions to become a swamp or a shallow lake in which the 

 iron-stained deposits of varying character containing mammalian remains 

 were accumulated. Afterwards the lake rose sufficiently to flood the 

 vallev and allow homogeneous, fine-grained clays to accumulate. In this 

 portion of the deposit the shells of Margaritana margaritifera are abundant. 

 The surface of the upper clays over large areas both in Walker Lake and 

 Walker River valleys is coated with an abundance of dendritic tufa, which 

 occurs both in mushroom-shaped masses that have formed about small 

 nuclei nnd in irregular vertical sheets which penetrate the clays and in 

 some instances inclose considerable areas. These sheets of tufa seem to 

 have formed on the sides of fissures, or perhaps on eroded surfaces which 

 had been submerged, in such a manner as to take an accui'ate cast of the 

 beds against whicii they were deposited. The upper clays in the Walker 

 River section correspond not only in their composition and arrangement 



