ILLUSTRATIONS OF GEOLOGICAL STRUCTURE. 167 



forming the lake shores. The general structural features of a built terrace 

 are shown in the section inserted on page 151. 



When a shore current bearing debris enters deep water, as illustrated 

 by a simple instance on page 94, it commences the formation of an embank- 

 ment, which is increased by the addition of gravel, sand, etc., in inclined 

 strata at its end and along its sides. A cross-section of a regularly formed 

 embankment should show a series of more or less perfect arches of deposi- 

 tion. 



CO]SrGI.OMERATES AKD BRECCIAS. 



In many of the bars and terraces described in this report the gravel of 

 which they are composed is firmly cemented by calcium carbonate into a 

 compact conglomerate. A similar action has taken place in some of the 

 alluvial slopes oace submerged beneath the waters of the ancient lake, 

 which at times has resulted in the formation of typical breccias. On the 

 west shore of Pyramid Lake, near Mullen's Gap, the immediate lake margin 

 is formed of sand and pebbles that have been consolidated into a firm con- 

 glomerate by the deposition of calcium carbonate. Similar beds were 

 observed on Anaho Island and about the Needles at the northern end of 

 the lake. In all these instances the conglomerate slopes lakewards at a low 

 angle, sometimes amounting to ten degrees. This in all cases is e^adently 

 of a very recent date and in places is still being deposited. Although the 

 youngest of the rock series, yet it is sufficiently consolidated to acquire a 

 polish from the constant attrition of the sand that is washed over it and 

 compact enough to be used for the ruder kinds of masonry. Similar con- 

 glomerates which appear also to be still in process of formation were 

 observed on the shores of Walker, Winnemucca, and Mono lakes. 



Breccias cemented by calcium carbonate are formed in alluvial slopes 

 of the Great Basin above the limits of the Quaternary lakes. These 

 deposits are usually less firm than the lacustral conglomerates, and fre- 

 quently differ in the fact that the cementing material is accumulated most 

 abundantly on the lower surfaces of the stones forming the deposit. The 

 precipitation of calcic carbonate in the interstices of alluvial slopes appar- 

 ently results fi-om the evaporation of the percolating waters and the conse- 



