134 
BASALT OVERFLOW—CHANNEL OF THE GILA. 
would not be a very expensive operation, but it is doubtful if there be sufficient wood at the 
locality for that purpose. A copy of the analyses of the ore is given in the chemical chapter. 
Twenty-one miles east of Gloat mountains the Gila emerges from a narrow valley bottom cut 
deeply through the elevated mesa land, which rises abruptly TO to 80 feet above the level of the 
bottom land. This is the western limit of the immense overflow of basaltic lava alluded to, 
which spreads over the whole country at this point from north to south. The trail which travels 
on the river bottom for a few miles ascends the mesa by a new route, the former having been 
washed away by the encroachments of the river altering its bed. The western edge of this lava 
overflow is abrupt, difficult to climb, and well defined, the large fragments not being trans¬ 
ported to any distance ; it is not regular, but deeply cut in, forming semi-lunar margins with 
cuts entering some hundred yards. In these gaps the trail ascends, it being the only ready 
mode of reaching the summit. 
This lava mesa rises by a very gentle slope to the east and north, and appears to preserve a 
horizontal line from north to south. Through a canon or break the Gila flows westerly, and 
has in many cases worn a recent way for itself through this basaltic region ; but as the valley 
of the river is very wide in some places it is difficult to imagine that the river could have 
formed this course for itself by wearing its passage silently for ages through this overflow ; it 
would rather appear as if there was a valley or gap in the flow, produced by some other cause, 
which the river now occupies. 
The hills north of camp (June 22) have flat basalt-capped summits, and appear to have a more 
easterly trend than the ranges further west. The outlines of the jasper and sedimentary quartz 
rocks are discernible, dipping S.W. The upper surface of the mesa is smooth, without any 
drift or travelled materials, and covered over by a light dust of its degradation. The river 
along this canon continually changes its bed, and terraces exist on each side of the stream, 
showing at least two different points of elevation. The overflow of the river in time of freshets, 
as shown by drift, is 14 feet, and lagoons are produced by the retiring waters. 
The basalt is a dark blue fine-grained augitic rock, which does not readily decay ; hence 
there is so little detritus found travelling over its surface. Yet, though not disintegrating, 
large masses are broken off and rolled down, accumulating at the bass, although there are not 
any drift or travelled fragments met with at a distance from the overflow. The fall of these 
large masses appears to be due to the action of the river, which, changing its bed, carries its 
waters close to the base, and undermines it by removing the soft yellow sand rock which under¬ 
lies the basalt. Perhaps it is by this action alone that the Gila has found its way through the 
overflow, and formed a channel for itself in some places over two miles wide ; for there can be 
little doubt that the lava north of the river, and that south, were originally connected, and, from 
the horizontally of the surface, it is evident it has undergone no action of local upheaval since 
its outpouring. Mechanical action has, therefore, been the means by which so deep and wide a 
chasm has been made in it, and there is no source so obvious as the action of moving water, 
although the Gila does not appear a stream sufficiently powerful to produce such effects. Look¬ 
ing south and east over the basalt mesa two conical hills, about 15 miles distant, have the 
appearance of being extinct volcanoes, the summits being crater-shaped. From these it is possible 
this lava may have flowed when the whole was as yet below the surface of the waters.* 
The underlying sandstone did not yield any fossils on examining it, and was in places 
rendered quartzose and metamorphic ; it is probably the extension of the conglomerates found 
a This basalt also contains small plates of salvadorite scattered throughout, and has small vesicular cavities filled with 
drnsic chalcedony. 
