104 GEOLOGICAL HISTORY OF LAKE LAHONTAN. 



been brought within the reach of wave action many times. In this way 

 the records first made have been erased or obscured by subsequent additions. 



One of the best locahties in tlie basin for the observation of ancient 

 hxke-margins is at Terrace Point, near the northern end of Pyramid Lake.^^ 

 The water-hnes at this locaHty are drawn at nearly equal intervals, and are 

 approximately of the same strength. Even at a distance of several miles 

 they continue to form a conspicuous and striking feature in the scenery of the 

 region. These terraces are the result of both the destructive and constructive 

 action of waves and currents, and are largely composed of basaltic dehris 

 mingled with worn and rounded fragments of the different varieties of tufa 

 that sheath the interior of the basin. I'he j^resence of tufa in the terraces ren- 

 ders it evident that they were formed subsequent to the deposition of the 

 main tufa deposits, and, therefore, at least in part, belong to a very recent 

 chapter in the history of Lake Lahontan. These facts will receive further 

 consideration in the discussion of the chemistry of the tufas. 



The topography of terraced shores is well illustrated on Anaho Island, 

 Pyramid Lake, a map of which forms Plate X. The broad bench formed 

 by the lithoid terrace extends completely about the island and forms the base 

 for the tufa-coated crags that are apparently piled in huge pyramids upon 

 it. At an elevation of 320 feet above the lake surface the poorl}^ defined 

 dendritic terrace may be seen, and nearly at the top of the island is a faint 

 line marking the position of the Lahontan beach. At the time when the 

 ancient lake reached its greatest extension, Anaho Island stood but 15 or 

 20 feet above its surface, and during severe storms must have been com- 

 pletely buried by the dash of the waves. The modifications of topography 

 produced by terraces may also be seen on the Marble Buttes at the south- 

 ern end of Pyramid Lake, which at one time formed a group of small 

 islands in Lake Lahontan ; and, again, about the shores of Humboldt Lake, 

 a ]^ortion of which are shown in Plate XVIII. 



Although the terraces in the Lahontan basin are sufficiently distinct to 

 enable one to trace the outline of the ancient lake with accuracy, yet they 

 are by no means so well defined as the similar records made by the waters 

 of Lake Bonneville. In the former instance we have the result of the action 



■•■'Seuriai; IX. 



