132 GILA KIVER — PROSOPIS GUM. 



intersected by veins of jasj^er 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 arable, 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 obs^ved 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 jasj^er 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 tho soil of the di'seit a miie away from the hottoin margins, where no overflow reaches, was Belcc^ed 

 anil preserved for analysis, the result of whieh is eonmninieated in the chapter on Chemical Analysis. 



