74 GEOLOGY. 



nated, in Spanish, as a cienaga. As the sierra heretofore referred to strikes the eye with a dark- 

 brown or even hlack aspect, on the other hand the sierras southwest, -composed of metamorphic 

 crystalline rock, (feldspar prevailing,) meet the view in a robe of glaring white, dazzling under 

 the rays of a Sonorian sun. It is common to the traveller to distinguish these two classes of 

 mountains as the hlack and ivhite, so opposite is their general appearance. At the western foot 

 of the Sierra Juchibabi, which bounds the valley on the south and near the old Mission of 

 Sonoyta Chloritic slates and greenstone appear — these are, however, but local. A branch 

 ranging north shows throughout the same light-colored feldspathic crystalline rock. The ridge 

 east of the cienaga is but a slight swell of the diluvial plains, and affords an open and travelled 

 pass to Presidio de Altar. 



Besides numerous deep charcos and even small lagoons in its lower part, this cienaga is 

 blessed with a small stream fed in its outset by a number of small springs. These springs 

 afford a constant flow of water, which proves their deeply-seated source. The water is clear, of 

 a bluish hue, but warm and slightly brackish. Notwithstanding this permanent suj^ply, the 

 little river of Sonoyta continues but about a mile as a running stream. There was a mission 

 founded here, but it has long since shared the fate of all similar establishments of this desolate 

 and remote region. The inhabitants now consist mainly of Papagos, who have a few miserable 

 huts, and irrigate a small patch of ground. 



The Sierra del Ajo, rising up at the northeastern corner of Cienaga de Sonoyta, is remarkable 

 for establishing, by a natural monument, a true boundary between the coast and the interior. 

 In its northwestern continuation auriferous and argentiferous copper ores abound. The gold 

 and silver product in this location is said to be sufficient to defray all expenses of mining and 

 assaying, leaving the copper a net gain. These mines have been long known to the Papagos 

 and Mexicans, but were not worked for want of capital and security against the Apaches. Some 

 Californians, under the name of "Arizona Company," have now "prospected" the country 

 between Sonoyta and the Gila. They commenced to build roads, make water-tanks, introduce 

 labor, and, notwithstanding these initiatory expenditures, anticipate a prosperous business. 



Following the bed of the Sonoyta river, a narrow but smooth pass leads to another cienaga, 

 which, having but little water, assumes the general aspect of the desert. The course of the 

 Sonoyta river is traceable through it, but the water, except in two or three places, does not 

 come to the surface, and it is necessary to dig for it everywhere during the dry season. Felds- 

 pathic crystalline mountains border also the cienaga except on the west, where the river finds 

 an outlet for the flood of water that may sometimes rush down its usually dry course. 



The sierra partakes very decidedly, in its physiographical character, the features of the Great 

 Colorado desert. It is a Papago name, and signifies little mountain gap or pass. Upon some 

 rising ground in the west end of the last-mentioned cienaga there is a settlement, or, more 

 properly, cattle rancho, the inhabitants of which are favored with spring water flowing out in 

 abundance from a dozen little springs. These springs come out in a line from a considerable 

 bank, which seems to have been formed by sediment, perhaps carbonate of lime, which they 

 themselves have precipitated. The water resembles, both in appearance and mode of issue, that 

 of Sonoyta, and there would be in all probability no error committed in assigning their thermal 

 and mineral properties to a common source. West of Quitobaquita the line passes over a broad 

 ridge dipping east and west before reaching the Cerros de la Salada. 



