COMMENCEMENT OF THE DESERT. 71 



(rock or mountain.) Its great height, added to its spire-like top, causes it to act as a conductor 

 to the clouds, and thereby gather an unusual quantity of rain, which is retained for a longtime 

 in its numerous rugged and inaccessible recesses. (See outline sketch No. 39, azimuth line.) 

 Viewing the country westward from the Sierra Verde, a wide plain is visible, bounded at 

 a distance of fifteen miles by a mountain range traversing the country with the invariable 

 bearing southeast and northwest. 



The eastern half of this plain is favored with a more than usual cover of vegetable life — 

 abounding in grass, a dense growth of brushwood, and mesquite; the western part, a low flat, 

 was entirely destitute of vegetation, which seemed to have been destroyed by small trogloditic 

 quadrupeds of the order Bodentice. Although this plain had received copious showers of rain 

 a few days previous to our visit, singularly enough no life was given to the naked and barren 

 flat. A change, and not to its advantage, is here perceptible in the physiographical features of 

 the country, and becomes quite decided in the next mountain range, which is unlike all the 

 sierras eastward, and which presents an isolated group rising out of the diluvial main. 



Notwithstanding its lesser extension, the Sierra de la Union presents no peculiarity in its 

 petrographic character — being a compound of igneous and metamorphic rocks. The lat- 

 ter constitutes the greater portion. On the east slope feldspathic granite in a disintegrating 

 and somewhat metamorjihic state occurs ; on the west is a quaternary granite, similar to that 

 mentioned as occurring on the Sierrita del Granizo. The backbone or central mass is formed 

 of igneous amygdaloid and porpbyritic rocks, here and there overlaid and concealed by crystalline 

 strata. 



Thus far there has been but little room for the diluvial deposits, so broken up is this region 

 by the continuous succession of mountain ranges; but westward, great basins of quaternary and 

 alluvial deposits form the main in which the sterile mountains lie imbedded and completely 

 isolated by this vast sea of drift. Of the sierras ranging eastward, parts of them are so entirely 

 submerged as to appear detached and isolated mountains, their connexion being traceable only 

 by their general bearing. The country passed over may be viewed as a narrow strait, traversed 

 by long mountain reefs, and that, in part, as a c last of shoal water dotted with rocky islands. 

 The influence of the climate of the Gulf coast, as far as the Sierra de la Union, is quite apparent ; 

 on its west slope two leguminous trees, the Palo verde and Arbol de hierro of the Mexicans, 

 Cercidium floridanum and Olneya Tezota, three large Cerei, two gigantic Echinocacti, and other 

 desert forms, now appear in prevailing numbers. 



The line, after crossing a desert of about seventeen miles, strikes a comparatively low and 

 narrow sierra, composed chiefly of porphyry and amygdaloid rock. This sierra presents two 

 vertical peaks, rising up like a pair of horns, which constitute natural monuments for the line 

 as it falls between them. It is a northerly continuation of the Cordillera Cobota, so called by 

 the Papago Indians, who have several fixed settlements here. There is a cafion in this sierra, 

 near the line by which the west side is easily gained, and in which are seen masses of crystalline 

 rock; igneous strata, however, prevail. The name " Lindero " (boundary or landmark) was 

 given to this sierra, because of the line falling between the two conspicuous peaks before 

 mentioned. 



The sierras Arteza and Soiii lie, respectively, southeast and northwest ; the former in the 

 United States, the latter in Mexico, both well known and famous among the natives as being 



