THE 
GEOLOGICAL MAGAZINE. 
NEW SERIES. DECADE III. VOL. VI. 
No. VII.—JULY, 1889. 
OR Gann Alii “Ack TiGiniaasS: 
I.—Suparriat Deposits or THz Artp Recron or Norte 
AMERICA. 
By Israzrn C. Russet, 
of the United States Geological Survey; Washington D.C., U.S.A. 
Part I. 
INTRODUCTION. 
HE subaérial deposits now accumulating in the arid portion of 
the United States may be divided into four classes: 1, Holian 
Sands; 2, Talus Slopes; 8, Alluvial Cones; and 4, Calcareous Clays to 
which no specific name has been applied, but which, for reasons 
stated below, will be called “adobe” in this paper. 
Kolian Sands.— Rounded hills of blown sands termed dunes give 
variety to the landscape in many portions of the arid region. These 
are local accumulations, but they sometimes have an extent of many 
square miles, and may attain a thickness of several hundred feet. 
They occur most frequently in sheltered valleys or in bay-like 
recesses Of large valleys, where topographic conditions favoured the 
accumulation of wind-borne particles. They are usually composed 
of quartz grains, which show by their rounded and worn appearance, 
when examined under the microscope, that they have travelled far 
during their uncertain wanderings. Dunes are constantly changing 
their forms, each wind that blows being busy in remodelling the 
contours of their rounded domes. Their internal structure is ex- 
tremely irregular, frequently showing cross-bedding and intricate 
contortions. Their surfaces are wind-rippled and many times 
covered with footprints. 
In some instances, especially on the Carson Desert, Nev., small- 
sized dunes may be seen which are formed of the cases of minute 
Crustaceans (Cypris) which have been gathered by the wind from 
the surface of the desert and accumulated in piles in the same 
manner as in the formation of ordinary sand dunes. 
Near Fillmore, Utah, another exceptional variety of dunes occurs, 
formed of small crystals of gypsum, which have been swept from a 
neighbouring desiccated lake-basin and accumulated in conspicuous 
white hills. 
The dunes of arid regions have the same characteristic forms as 
those occurring in more humid climates. They are steep on the 
leeward side and slope much more gently in the direction from 
which the prevailing winds blow. The sources from which they 
DECADE III.—vOL. VI.—NO. VII. 19 
