166 GEOLOGICAL HISTORY OF LAKE LAHONTAN. 



no instance is their presence indicated by a scarp crossing the surface of 

 the deserts. 



The existence of faults shearing unconsoHdated strata of sand and clay 

 can scarcely be accounted for by the hypothesis of tangential strain, as has 

 so often been done in the case of displacements in older and moi'e consoli- 

 dated rocks, as these beds are still horizontal and have not been subjected 

 to the pressure of superimposed accumulations. The strata on either 

 side of the planes of fracture are undisturbed. As the displacements are 

 local and unconnected with any general orographic movements and in 

 some instances die out as we trace them downwards, it seems safe to con- 

 clude that they have resulted from some change in the strata them- 

 selves, as is perhaps the case also with the joints occurring in the same 

 beds. The Lahontan sediments were water-laid and are now desiccated. 

 It may be that the contraction produced on drying will be found a sufficient 

 explanation of the faulting and jointing that has been observed. The dry- 

 ing of heterogeneous stratified beds must result in unequal contraction of 

 the various members of the series, at the same time that tlie unequal desic- 

 cation of various portions of the basin would complicate the resultant 

 stress. In the Lahontan basin changes of this nature have taken place and 

 have been accomplished by jointing and faulting. That these facts stand in 

 the relation of cause and eff'ect, however, is but a provisional hypothesis. 



STRUCTTJHB OF TERRACES AND ElVLBAlSrKMENTS. 



While describing the formation of terraces and sea-cliffs it was shown 

 that the loose material occurring on lake shores is sometimes swept along 

 by currents and deposited so as to form a horizontal shelf bounded by a 

 steep scarp on the lakeward slope. Owing to the mode of its formation, 

 the structure of such gravel-built terrace is necessarily irregular, but as a 

 whole is characterized by oblique stratification, especially on its lakeward 

 margin, and abounds in examples of current bedding. Its material varies 

 from accumulations of bowlders, sometimes two or three feet in diameter, 

 through all degrees of comminution to the finest of silt and marl, and is 

 usually of a heterogeneous character, dependent on the nature of the rock 



