Art. IV.] Herrick, Geology of New Mexico. 79 
the margins, is reduced to five. The area over which this flow 
is undisturbed is only a fraction of the whole but in a large part 
of the area there has been extensive undermining of the soft 
material upon which the lava reposes. Thus the mile or more 
intervening between the lava sheet and Pallisade mountain has 
been transformed into a desperate kind of 7 nal pais in which the 
large fragments of lava are upturned in all directions, making it 
practically impassible. The source of the flow is apparently at 
a point at present separated from the main sheet, to the west 
of the “Cactus Plain” through which the rail road passes to 
Magdalena. The region of “ Indian Hill” is also a portion of 
this flow. In a deep canon in the last mentioned series of hills 
deeper parts of the flow, apparently in the channel of outlet, 
are exposed and here the nature of the rock is different from 
the superficial portions as elsewhere described. From this re- 
gion the lava seems to have spread to the north and east but it 
has been removed wholly in the valley of the stream that in wet 
times skirts the western border of the Socorro mountain region 
as well as over broad valley intersecting that range from the 
middle of its western margin and forming the southern bound- 
ary of the granite area. The history of the region from the 
end of the rhyolite period to the overflow of lava, so far as it is 
recorded, is preserved in a loose fragmental series the materials 
of which are apparently derived from the adjacent rocks. This 
is well exposed in the south-eastern part of the region where 
‘are a variety of lava-topped cliffs beneath which are sandstones 
and crags probably of quite recent origin. The flow extended 
for some distance north along the eastern face of the range and 
has left outliers in the form of lava-topped buttes of which the 
most conspicuous is near the springs which supply the city of 1 
Socorro with water. The underlying formation is here very 
largely derived from fragments evidently worn out of the earlier 
talus conglomerates. In the depressed area in the center of 
the region here under consideration, at the western foot of the 
Pallisade mountain, are remnants of the same horizontal strata 
Plate XI, Fig. 3, and their continuation below the large lava 
sheet to the west and south-west is easy to follow. These soft 
