5° SIDNEY PAIGE 
from the area between their present limit and the mountains; 
(2) There may have been faulting by which the lowland was 
relatively lowered; or (3) The mountains may have been worn 
back and the lowland developed by differential erosion since the 
deposition of the gravel. 
Mr. Rich adopts the latter explanation and presents evidence to 
show that No. 1 is impossible. I wish first to indicate wherein the 
statements used as evidence to disprove No. 1 are inaccurate, 
and second to show that a combination of No. 1 and a part of No. 2 
is the more plausible explanation of the observed facts. 
Mr. Rich says: 
Opposed to the first of these alternatives is the fact that the gravel plateau 
ends abruptly along a relatively straight line. There are no outliers of gravel 
between this general line and the mountains. It is highly improbable that 
streams flowing nearly parallel and not more than a mile apart should strip 
all signs of the gravels from the upper four miles of their course, while in their 
lower course, where they flow across the gravel plateau, they should be in 
relatively narrow valleys with almost no tributaries and should have done little 
more than to cut their way through the plateau without having oe able to 
widen their valleys to any great extent (Fig. 2). 
A second objection is the fact that the line of contact between the gravels 
and the underlying rock slopes upward toward the mountains at such an angle 
that it would intersect the projected line of the plateau surface at a point not 
far within the present limit of the gravels (see Fig. 5). In other words, the 
gravels thin toward the mountains at such a rate that they would wedge out 
within a short distance from their present limit, and the lowland is accordingly 
developed in the bed-rock. : 
On that portion of the geologic map of the Silver City quadrangle 
shown by Mr. Rich two important outliers have been omitted. 
Their position is shown in Fig. 1, a and 6. Still another outlier is 
present at c, which position was just off the western edge of the first 
map. Nor should it be said that the gravel plateau ends abruptly 
along a relatively straight line, for as shown on the map the edge of 
the gravel plain has many of the characteristics of an erosion 
border. Further, it is not “highly improbable” that streams 
flowing nearly parallel and not more than a mile apart should strip 
all signs of the gravels from the upper four miles of their course, 
for a point which has failed to be considered here, though it is 
mentioned in another place (p. 500, line 3), is that an increased 
