262 J. LeConte-^ Origin of Normal Faults and 



this event seems, as shown by Russell to have been coincident 

 with the formation of the Basin ridges, and both of these with 

 the formation of the great fault-scarp on the western side of 

 the Wahsatch range. 



flow the Basin system was formed. — Now regarding the 

 Sierra and Wahsatch as belonging to the Basin system, we 

 may imagine how the whole system was formed. At the end 

 of the Tertiary the whole region from the Wahsatch to the 

 Sierra, inclusive, was lifted by intumescent lava into a great 

 arch, the abutments of which were the Sierra on the one side 

 and the Wahsatch on the other as shown in the dotted line 



Fig. 12. Ideal section, showing- mode of formation of Basin system. 



(fig. 12). The arch broke down and the broken parts readjusted 

 themselves by gravity into the ridges and valleys of the Basin 

 region, leaving the raw faces of the abutments overlooking the 

 Basin and toward one another. It must not be supposed, 

 however, that this took place at once, but gradually / the 

 lifting, the breaking down and the readjustment going on to- 

 gether pari passu / each readjustment probably giving rise to 

 an earthquake. 



Process still going on. — There are many evidences that the 

 process of adjustment of these crust-blocks is still going on. 

 In the Sierra we find evidence in the still deepening channels 

 of the rivers and especially in the occasional readjustment of 

 the walls of the eastern fault of the Sierra block. The Inyo 

 earthquake of 1872 was undoubtedly produced in this way. 

 Gilbert also finds evidences of recent movement of the 

 Wahsatch block in the faulted terraces of Lake Bonneville ; 

 and in this fact foresees the probability of destructive earth- 

 quakes here in the future. In the Basin region, both in Nevada 

 and in S. E. Oregon, Russell finds evidences of the same in 

 faulted lake terraces. Finally, during a camp of two or three 

 weeks in 1887 in Warner Mountains, where the structure 

 described by Russell is finely displayed, I found abundant 

 evidences of local subsidence still in progress. Many small 

 lakes in that region, probably of the type produced by block- 

 tilting, have apparently been formed during the present cen- 

 tury. In Blue Lake, for example, I found stumps of pines 

 standing in water fifty or more feet deep, rotted off level with 

 the surface, hut perfectly sound below. 



Two kinds of mountains. — I must not be understood as main- 

 taining that the Sierra, the Wahsatch and the Basin ranges 

 were entirely formed at the end of the Tertiary. Some of the 



