CHAPTER I. 



INTRODUCTORY. 



THE FIELD OF STUDY. 



The region treated of in the present vohime embraces about 90,000 

 square miles in northwestern Nevada, together with small portions of south- 

 em Oregon and eastern California. 



The object of the explorations herewith reported was the study of the 

 Quaternary geology of the country visited, and particularly the geological 

 history of Lake Lahontan-^ — a lake, now extinct, which occupied many of 

 the valleys of northwestern Nevada at a very recent geological date. The 

 basin of Lake Lahontan is one of the many independent drainage areas of 

 which the Great Basin is composed, and its geology is a page in the his- 

 tory of the vast region lying between the Rocky Mountains and the Sierra 

 Nevada. 



The Great Basin is to-day an arid region, but during the Quaternarj- 

 its climate was probably colder and more humid than at present. The 

 Sierra Nevada and Wasatch ranges, now for the most paii bare of snow 

 during the summer, were formerly crowned with vast nevds from beneath 

 which flowed many magnificent ice-rivers; the desert ranges of Utah and 

 Nevada were also snow-covered, and some of them gave birth to local gla- 

 ciers. The valleys which are now dry and treeless, and in many instances 

 absolute deserts, destitute of any kind of vegetation over hundreds of square 

 miles, were then occupied by lakes, the largest of whicli wei'e comparable 

 in extent and depth with those now drained by the Saint Lawrence Some 

 of these old lakes had outlets to the sea and were the sources of considera- 



