52 SIDNEY PAIGE 
that the gravels did not extend farther northward, the following 
facts are submitted. The lava series (composed of igneous sheets 
and interbedded gravel deposits at each locality of essentially the 
same character) is found north, northwest, northeast, southeast, 
and southwest of this area. A deposit of gravel belonging to this 
series actually underlies the Pleistocene gravel at the eastern side 
of the map (see Fig. 1). Besides these facts there is clear structural 
evidence, which need not be considered here, which leads to the 
inference that great lava sheets once covered this entire area, and 
that the gravel which now occupies this space at the surface is the 
débris derived from the denudation of the great up-faulted portions 
of these sheets, of which such masses as the Little Burro Mountains, 
Lone Mountain, and the Silver City Range are the remnants. 
From this it is logical to conclude that the present frayed northern 
edge of the gravels occupies its position, not because it represents 
an old scarp line, but because it represents a position of stability 
established by a number of factors—gradient, rainfall, favorable 
location with respect to drainage, etc.—all acting on a gravel sheet 
which originally extended farther northward. After the edge 
was eroded back to this position of stability, it is conceivable— 
in fact, is probable—that differential erosion carved out the low- 
land lying north of it. 
To sum up: The gravels have been removed by erosion from a 
portion of the area between their present limit and the present 
position of the mountains, and the lowland has been developed 
in the old floor upon which these gravels once rested, by differential 
erosion. 
