378 The American Geologist. June. 1904. 
covered by the detrital material of the cHnoplain. In some 
cases, where erosion has cut through both, it appears that the 
Tertiary deposits (usually tilted and intersected by ancient 
basalt dikes and sheets) have been planed to a level covered by 
a definite limiting deposit, as by lake action, while in others, 
the deposit from the mountains are mingled with those of the 
degraded Tertiary strata. In still other cases, benches of 
Tertiar}^ with the upper surface leveled protrude above the clin- 
oplains . 
The effect of the fluctuations of river level are obvious in 
the bluff-like projections of the clinoplains, on the bottoms, 
for they doubtless at one time debouched directly upon the 
flood plain as do the modern deltas. 
Variation in the amount of lateral drainage is also repre- 
sented by two sets of benches along some of the main lateral 
arroyos or canons. In Nogales arroyo, for example, leading 
from the main canon north of Socorro mountain and convey- 
ing the drainage of a vast area extending to the Magdalena 
range, the stream must have been much larger formerly. A 
bench some thirty or forty feet above the present level and 
forty to sixty feet lower than the top of the clinoploin is very 
evident, the slope nf the bench being less than that of the cHno- 
plain. In this arroyo the Tertiary deposits are exposed b}' 
erosion so as easily to be seen near the mountain. Their ma- 
terials are sand and gravel and marly beds and they are dis- 
turbed by dikes and sheets of much altered vesicular basalt . 
Two types of clinoplain are to be distinguished . First, the 
talus plain proper. This is a true talus, standing at a rather 
high angle which varies with the nature of the rocks and the 
hight of the range. At the Limitars, for example, the entire 
eastern exposure of which is of granite and allied rocks, the 
talus plain consists largely of rounded granite boulders and 
coarse sand of disintegration . The slope is cut by rather deep, 
but not precipitous arroyos and the inters^als between these gul- 
lies is crowned or arched into long radiating ridges. The 
general surface section is, however, remarkably rectilinear. 
In some places the talus plain reaches the bottoms, but gen- 
erally it passes abruptly into the plain of the second type, or 
clinoplain proper! This is very slightly inclined with per- 
fectly rectilinear surface section, i.e., it is a perfect plane. 
