132 
GILA RIVER-PROSOPIS GUM. 
intersected by veins of jasper and layers (strata) of silicious jaspery rock. The base of these hills, 
and the plain close to the base, is covered with jaspery pebbles, but they disappear rapidly on 
leaving the hill.* 
At this locality, the first camp on the Gila, there were found growing some trees, which, at 
first, were taken for the ordinary mesquite tree. By accident, looking closely at one, the older 
branches and upper part of the stem was found incrusted over with tears of gum, of which 
there was such an abundance that in fifteen minutes one pound was collected by one hand ; the 
gum was not found adherent to the young shoots but below, where several branchlets had been 
given off and were cemented longitudinally in fissures, which the growth of the tree had formed 
in the bark ; some of the tears were moist and tenacious, but the majority was brittle and hard, 
and easy of removal; as the tree is beset with spines from half inch to one inch long growing 
on the old as well as the new branches, and, as the tree droops over like a willow, it was a diffi¬ 
cult matter to approach sufficiently near to pick the gum off; thus only a small portion of the 
whole was removed—there could not have been less than seven or eight pounds upon the whole 
tree, which was about fifteen feet high, dividing at five feet from.the ground into numerous 
drooping branches, altogether a very ungraceful “mesquite.” The gum thus collected had all 
the appearance and character of gum arabic, insipid, soluble in water and weak alcohol, but not 
soluble in strong alcohol, and yielding to chemical tests the same reaction as the gum of the 
acacia. Portions of the tears collected were brownish, like gum Senegal, but the larger por¬ 
tions were colorless, and by care in picking a very pure gum could be selected. Four of these 
gum trees were observed here, and from the purity and value of the gum, it is to be hoped that, 
at no distant period, these trees may be cultivated along the margins of the Gila bottom, and 
that gum gila may become one of the staple commodities of the district. 
Eighteen miles east of the first camp lie a series of isolated ridges, trending northwest and 
southeast, the most northerly approaching the river, and being about three miles long ; it is 
made up of quartzose conglomerate, dipping southwest 8°. Angular fragments and detritus 
of this rock were found scattered near the base and in the vicinity of the range, but none carried 
to any distance. The hill itself was covered with small rounded masses of jasper and trachyte, 
neither of which were observed in place here. 
Big Horn, or Goat mountains are a series of parallel ranges running nearly northwest and 
southeast, and overlapping each other ; the trail turns round the northern edge of the range, 
which presents there a crescentic outline, with high mesa land sloping down to the river ; over 
this mesa the trail leads and exposes a surface of granitic syenite, with here and there a felspar 
porphyritic rock, (leucite.) 
Upon this granitoid basis reposes thick beds of jasper and silicious rock, the latter a con¬ 
glomerate of white quartz pebble, which, further up, becomes distinctly stratified, and con¬ 
stitutes the mass of the mountain above 1,000 feet high. 
The dip being 60° or 80° southwest, and the thickness of the conglomerate beds two hundred 
feet, the line of junction of these conglomerates, with the underlying (metamorphic) silicious 
rock, is well marked on the eastern side of the mountain. 
Sixteen miles east of these hills the western limit of the basaltic region is met. This region 
extends north and south, in a well marked and almost unbroken line, with its escarpment 
westward. The basalt mesa is raised sixty to eighty feet above the river bottom. The upper- 
* A portion of the soil of the desert a mile away from the bottom margins, where no overflow reaches, was selected 
and preserved for analysis, the result of which is communicated in the chapter on Chemical Analysis. 
