114 GE; LOGICAL HISTORY OF LAKE LAHONTAN, 



one at A. It leaves the shore and returns to it, thus forming a loop inclos- 

 ing a cup-shaped depression 5 or 6 feet deep, now smoothly floored with 

 playa mud. The long, curved bar C has a smooth, evenly rounded top and 

 a nearly horizontal crest- line, which decreases slightly in height, however, 

 as we approach its southern end, where the curvature is more abrupt. Like 

 all the embankments in the series, this is composed largely of rhj^olite, with 

 some anamesite, all rounded and well worn-, but coarsest near its northern 

 end. At its southern end it becomes broader, as well as more sharply 

 curved, and shows three or four minor divisions, thus indicating that it is a 

 compound structure throughout. The area to the westward of this embank- 

 ment, once a lagoon, is now a playa, floored with smooth, horizontal, light- 

 colored mud, that is unclothed with vegetation of any kind. That the 

 embankment C is of a later date than the platform on which it rests is 

 shown by the same kind of evidence that pi'oves it to be of older date than 

 the bars superimposed upon it. This is an interesting conclusion, as the- 

 bar below C is incrusted and cemented with lithoid tufa, thus showing that 

 the higher bars were constructed after the formation of the calcareous 

 deposit. The bar C is a portion of the water-line designated as the lithoid ter- 

 race on page 101. The highest exposure of tufa is here about 25 feet below 

 the Lahontan beach. A corresponding relation of the lithoid terrace to the 

 highest beach line has been observed at a number of other localities in thfr 

 Lahontan basin, and will again claim attention when the oscillations of the 

 lake are considered. The outer margin of the platform on which the bar 

 C rests has been cut away by waves and currents so as to expose a steep 

 sea-cliff of cemented gravel, and furnishes the only example in the group 

 of a gravel structure that has suffered erosion by the wateis of the lake in 

 which it Avas formed. Below this horizon are other curved and sickle-shaped 

 embankments, the relative age of which is indicated by the manner in which 

 they overplace each other. The topography of these structures, and the 

 playas they contain, are faithfully repi'esented on the accompanying map. 

 The bars described above, with the exception mentioned, are unaffected 

 by erosion, and are as smooth and regular as if their elegantly curved 

 ridges were formed but yesterday. They afford beautiful examples of the- 

 symmetry of water-built structures. 



