162 GEOLOGICAL HISTORY OF LAKE LAHONTAN. 



exposed. That the arch represented on Plate XXV is the result of depo- 

 sition and not of mechanical disturbance is clearly shown by the horizontal 

 stratification of the material above and below it. 



UNCONFORMABIIilTY BY EROSION AND DEPOSITION. 



Wherever the junction of the medial gravels with the lower or upper 

 clays is exposed, one is nearly always sure to find evidence of unconforma- 

 bility, resulting usually from the erosion of the older strata. Examples of 

 this phenomenon are shown in nearly all the accompanying illustrations 

 which include the junctions in question. On Plate XXVII, the cross-strat- 

 ification of the gravels filling eroded hollows in the lower clays is admirably 

 shown by Figs. A, C, and D. On Plate XXIII, Fig. G illustrates the man- 

 ner in which the strata filling eroded hollows are sometimes thickened ; 

 while Fig. J shows a thinning of similar beds when deposited over a pro- 

 tuberance of the bottom on which they were laid down. Figure K, of the 

 same plate, furnishes an example of current-bedded gravels covering the 

 eroded surface of the lower clays, while a second line of unconformity, also 

 resulting from erosion, parts the gravels themselves. Sometimes the vari- 

 ations due to erosion and deposition are complicated by the eff"ects of sub- 

 sequent lateral movement, as appears to have taken place in D, Plate 

 XXIII. Unconformability by deposition alone, where erosion has but little 

 effect, is shown in Fig. B, Plate XXVIII, which illustrates the contact of 

 horizontal lacustral beds resting upon gravels that were deposited in in- 

 clined strata. Other examples of a similar character may be found in many 

 of the accompanying illustrations. 



JOINTING. 



The marly clays forming the upper and lower members of the Lahon- 

 tan series usually break into prismatic and cubical blocks on weatheiing ; 

 the vertical faces of the blocks are determined by joint planes, and the hor- 

 izontal by planes of laminatioJi. In many localities a more pronounced 

 jointing occui's, forming two approximately vertical sj'stems that are nearly 

 at right angles to each other. Judging from the number of instances 

 observed, at widely separated localities, the joints in question may be 



