294 I. C. Russell—Subaérial Deposits of North America. 
an inch. In no instance could a definite lining to the tubes be 
distinguished. 
The adobe used for brick-making is usually light grey in colour, 
but this is not always the case. It is frequently light yellow, and 
has varying tints according to locality. Sometimes it has a reddish 
tint, caused by the prevailing colour of the surrounding rocks from 
which it was in large part derived. The grey colour of the adobe 
commonly seen in buildings is due in many, and probably in all 
cases, to an admixture of organic matter. Jts characteristic colour, 
when free from organic matter, is light yellow. In alluvial cones, 
and generally throughout the drainless valleys of the Arid Region, 
this is the prevailing tint. The organic matter which darkens the 
adobe in many instances may have been contained in the rocks 
from which it was derived ; this is certainly the case near Canon 
City, Col., where the adobe is formed principally from the wash of 
grey Jurassic shales. At other times the colour seems to have 
originated from the decay of vegetable matter buried in the deposit. 
_“ When examined under the microscope, the adobe is seen to be 
composed of irregular, unassorted flakes and grains, principally 
quartz, but fragments of other minerals are also present. An 
exhaustive microscopic study has not been made, but the samples 
examined from widely-separated localites were very similar. The 
principal characteristics observed were the extreme angularity of the 
particles composing the deposit and the undecomposed condition of 
the various minerals entering into its composition. It is to be 
inferred from this that the material was not exposed even toa very 
moderate degree of friction, and had not undergone subaérial decay 
before being deposited. Adobe collected at typical localities is so 
fine in texture that no grit can be felt when it is rubbed between 
the fingers ; in other instances it contains angular rock fragments of 
appreciable size/and, as we have stated in speaking of alluvial cones, 
may be intimately associated with rock-masses many tons in weight. 
When the rocks overlooking adobe-filled valleys are of easily eroded 
sandstone, the deposit in the valley below is sometimes so sandy 
that the distinctive features of adobe are lost. Other changes in 
colour and texture have been observed, depending on the character 
of the rocks from which the deposit was derived. 
Chemical Characters.—Analyses of several samples of adobe are 
given in the following table, which show that it not only has a 
varied composition, but differs in its chemical characteristics in 
different localities. 
The diversity of composition, as shown in the table, indicates 
that the deposits which we have classed under the same name 
on account of their physical and other characteristics vary widely 
in constituents. These analyses show that adobe is very distinct 
from residual clays, as is also proven by its appearance under the 
microscope. The table of analyses of residual clays from the 
Southern Appalachian region is here introduced for convenience of 
comparison. Residual clays are composed essentially of ferruginous 
silicate of alumina, and are remarkably free from substances which 
