I. C. Russell—Subaérial Deposits of North America. 291 
cones are more or less united and give to the contours of the moun- 
_tain base a scalloped outline.” 
Alluvial cones when best developed extend out from the mountains 
to a distance in many instances of several miles. Not infrequently 
the deposits on opposite sides of the valley unite along the centre 
of the depression, so that its profile is an unbroken curve; or, more 
accurately, is a combination of two curves, for each alluvial cone 
has a concave surface. 
The material forming alluvial cones varies in size from the finest 
powder to angular rock weighing many tons. It exhibits no regular 
bedding or stratification, but coarse and fine debris are mingled in 
endless variety. There is a well-marked gradation, however, to be 
seen as one travels from the apex of an alluvial cone towards its 
‘periphery. At the apex it is composed mostly of coarse, angular 
material, with fine silt-like clays filling the interspaces. Toward 
the periphery the fine material predominates, and in many instances 
forms a homogeneous unstratified deposit, without stones or pebbles. 
At other times coarse material occurs at certain horizons in the fine 
silt, thus recording periods of unusual precipitation, when the 
gravels from the mountains were swept out into the valleys to a 
greater distance than under ordinary conditions. At the lowest 
limits to which the debris is carried in large valleys it is always fine, 
and has a much smaller surface slope than the upper portions of the 
alluvial cones with which it is associated. 
The climatic conditions favouring the formation of alluvial cones 
might be discussed ; but as deposits of this nature are not the special 
subject of this paper, their further consideration will be postponed. 
Calcareous Clays (Adobe).—Widely distributed throughout the 
valleys of the more arid portion of the United States there occurs 
a peculiar calcareous clay which is used largely by the Indians,- 
Mexicans and others for the manufacture of sun-dried bricks, known 
by the Spanish name “adobe.” ‘The earth from which these bricks 
are made is also designated by the same name. We have therefore 
adopted it, with some extension of its popular meaning, as a con- 
venient term by which to designate the fine subaérial accumulations 
in general, exclusive of eolian sands, which occur in the region just 
referred to. 
J.—ApDoBE. 
Distribution.—The area over which adobe forms a large part of — 
the surface has not been accurately mapped, but enough is known to 
indicate that it is essentially co-extensive with the more arid portions 
of this country. In a very general way it may be considered as 
being limited to the region in which the mean annual rainfall is 
less than twenty inches. It forms the surface over large portions of 
Colorado, New Mexico, Western Texas, Arizona, Southern California, 
Nevada, Utah, Southern Oregon, Southern Idaho, and Wyoming. In 
this paper the great area here designated is termed the Arid Region. 
Adobe occurs also in Mexico and may there reach a greater develop- 
ment than in the United States, but observations concerning it 
south of the Rio Grande are wanting. 
