154 • GEOLOGICAL HISTORY OF LAKE LAHONTAN, 



atmosphere at such times. Dust-storms are common on the deserts during- 

 tlie arid season, and impart to the atmosphere a pecuhar haziness that lasts 

 for days and perhaps weeks after the storms have subsided. Whirl-winds 

 supply a characteristic featui-e in the atmospheric phenomena of the Far 

 West especially during calm weather, as noted already, and frequently form 

 hollow dust-columns two or three thousand feet or even more in height, 

 which may many times be seen in considerable numbers moving here and 

 there over the valleys. The loose material thus swept about at the caprice 

 of the winds tends to accumulate on certain areas and forms dunes or drifts 

 that at times cover many square miles of surface. During its journey 

 across the country the material which finds a resting place in the dunes 

 becomes assorted with reference to size and weight, so that the resulting 

 sand-drifts are usually homogeneous in their composition, but are character- 

 ized by extreme irregularity of structure when seen in section. In the 

 Lahontan basin the subaerial dejjosits are usually composed of fine, sharp 

 quartz sand, but in some instances small drifts are principally formed of the 

 cases of ostracoid crustaceans. 



A large area buried beneath sand dunes of post-Lahontan date occurs 

 a few miles north of Winnemucca and extends westward from the lower 

 part of Little Humboldt Valley to the desert between Black Butte and the 

 Doiia Schee Hills. This belt of drifting sand is about forty miles long from 

 east to west b)* eight or ten miles in width. The drifts are fully seventy-five 

 feet thick and present their steeper slopes to the eastward, thus indicating the 

 direction in which the whole vast field of sand is slowly travelling. No 

 measurements of the rate at which these drifts advance has been made, but 

 their progress is evidently quite rapid, as it has necessitated a number of 

 changes in the roads in the southern part of Little Humboldt Valley during 

 the past few years. In some places in the same region the telegraph-poles 

 have been buried so deeply that they required to be spliced in order to 

 keep the wires above the crests of the dunes The sand is here of a light 

 creamy-yellow color, and forms beautifully curved ridges and waves that 

 are covered with fret-work of wind-ripples, and frequently marked in the 

 most curious manner by the foot-prints of animals, thus forming strange 

 hieroglyphics that are sometimes difficult to translate. 



