38 BULLETIN OF THE BUREAU OF FISHERIES. 



THE GEOLOGY OF THE LAHONTAN SYSTEM AND ITS RELATION TO THE ICHTHYOLOGY. 



The recent geological history of the Lahontan system has been worked out in a mas- 

 terly way by Israel Cook Russell, ■* the results appearing in a large monograph pub- 

 lished by the United States Geological Survey. Russell's account begins with Quater- 

 nary times, when Lake Lahontan covered a large and very irregular area, now mostly 

 included mthin the State of Nevada. This great body of water, larger than Lake Erie, 

 attained a maximum depth of 880 feet and received the discharge of numerous rivers, 

 many of which flowed through narrow and deep mountain canyons. The history is con- 

 tinued down to the present time, when nothing is left of the ancient lake but the detached 

 basins with which we are now concerned, mere desiccated remains scattered over a rock- 

 bound waste of desert sands. Unmistakable traces of the old lake are to be seen on every 

 hand, and they are particularly evident in the valleys of Pyramid and Winnemucca 

 Lakes, where the eye of the observer can follow the ancient shore lines as they distinctly 

 appear one above another for long distances up the mountain sides. 



In the presence of this great body of water, to which the small river and lake basins 

 of northern Nevada were all tributary at a relatively recent time, we may recognize a 

 direct and simple explanation for the present distribution of the fishes now found there. 

 But a part of Russell's investigation which also concerns the ichthyologist has to do 

 with the question of an outlet of Lake Lahontan and, also, with the degree of desiccation 

 to which the lake was subjected during its history. Quoting directly from Russell, and 

 italicising statements of particular interest in this cormection, we have (p. 250, chap, vii) : 



The history of the fluctuations of the Quaternary lake of northwestern Nevada is recorded in various 

 ways, as has been described in the last three chapters, which treat it from the physical, chemical, and 

 biological standpoints. In the present chapter it is our purpose to present briefly the conclusions based 

 upon these various lines of evidence. The phenomena observed have great diversity of character, but 

 when interpreted in terms of geological history they support and supplement each other in such a way 

 that the conclusions drawn are believed to be well sustained. Moreover, the facts observed in the 

 Bonneville and in more than a score of desert valleys throughout the northern half of the Great Basin 

 which contained contemporary water bodies harmonize with the interpretation of the Lahontan record 

 here presented. 



The fact that all the minor basins in the arid regions of the Far West are filled to a depth of many 

 hundreds of feet with alluvium and lacustral sediments, together with the occurrence of the beach lines 

 of the Quaternary lakes on the surface of the vast alluvial cones, leads to the conclusion that all these 

 basins were barren deserts before the rise of the Quaternary lakes. The pre-Lahontan condition of north- 

 western Nevada must have closely resembled its present character, but at times it -was probably com- 

 pletely desiccated. 



The change of climate admitting of the existence and gradual expansion of lakes in the various 

 valleys throughout the Great Basin caused a number of those situated in northwestern Nevada to rise 

 sufficiently to unite and form a single irregular water body 8,922 square miles in area. This was the 

 first rise of Lake Lahontan. Like all inclosed lakes it must have fluctuated in depth and extent with 

 the alternation of arid and humid seasons and risen and subsided also in response to more general 

 climatic ocellations, which extended through years and perhaps embraced centuries. Finally the 

 climatic conditions which favored lake expansion ceased, and a time of aridity, like that which preceded 

 the first rise, was initiated . The lake slowly contracted until its basin reached a greater degree of desiccation 

 than that now prevailing. This was the inter-Lahontan period of desiccation. 



During the first rise lacustral marls and clays were deposited throughout the basin; the depth of 

 these is unknown, but they certainly exceed 150 feet in thickness. The waters were saturated with 



o Department Interior, Monograph Geological Survey, vol. xi, 1885. 



