68 GEOLOGY. 



water-courses, sometimes forming a continuously winding low bank, or terrace, on both sides, 

 and sometimes covering the slopes of the adjoining mountains, composed of metamorphic rock. 



On the hill-sides there are beds of this formation dipping towards the valley, and exhibiting, 

 by decurrent, undulating lines, a shaly, laminated texture. On the easternmost limits of 

 this valley the same formation is still more developed ; and it can be seen in all the valleys to 

 the south and southeast of the Sierra de Santa Barbara, which is a part of the cordillera 

 embracing the Sierra del Pajarito. The volcanic breccia, in many of the ravines, form walls 

 of from forty to fifty feet in height, varying from an angle of forty-five degrees to perpendicu- 

 larity. Pieces of this breccia, heated in a log fire, and then thrown into cold water, showed 

 much effervescence, without fracture. 



North and northwest of this mountain range, bearing east and west, the Sierra Janos rises 

 up in bold terraces of dark-brown amygdaloid trap and porphyry, the broader terraces being 

 nearer the base. These gigantic shelves are bordered with rocks projecting out in the most 

 fantastic shapes. They incline toward the main body of the sierra — deep and lateral valleys 

 intervening. A huge block, exhibiting on its south and west side gigantic walls, with distinct 

 stratification and cleavage intersecting at right angles, constitute at once the centre mass and 

 the peak. The rectilinear fissures are visible at a distance of ten or fifteen miles, and giving 

 the igneous walls more the appearance of mason-work than the result of volcanic action. 



This sierra's vernacular name, "Janos," bears no reference to its petrographic character. 

 The word signifies, in the language of the Papago Indians inhabiting this country, an arbores- 

 cent shrub of the bignoniaceous order, belonging to the genus "Chilopsis." Its frequent 

 occurrence here in the water-beds in this vicinity may have originated the name. 



On the northern slope of the Sierra Janos another group of mountains occur, known as the 

 Sierra Atascosa. Its bearing is the same as that of the Sierra Janos, and its longitudinal axis 

 is common to this sierra and that of Sierra del Pajarito. All these three links of the cordillera 

 have both dip and strike alike, the dip being to the east. Its petrographic character is similar 

 to that of the Sierra Janos, and, being closely connected orographically, may be considered its 

 twin. The cordillera formed by these three mountains terminates with the Sierra Atascosa, 

 which is separated by a narrow and rugged valley from the Sierra del Babuquibari, lying to 

 the northwest. This valley is of some importance, not only for its valuable fresh- water springs, 

 but also as affording the only means of communication between the settlements of the Santa 

 Cruz River valley and the coast regions along the Gulf of California. A rancho was once 

 established at these springs, bearing the Papago name of Aribaca, or, more properly, Aribac. 

 The settlement has, however, been abandoned long ago, in consequence of the repeated depre- 

 dations of the Apaches. The northwestern part of the sierra is composed of igneous rocks, 

 towering up into peaks of the most grotesque form, and bearing, not inappropriately, the name 

 of "Malpais." 



Atascosa means "miry," which probably has reference to a previous state ; it now presents 

 the appearance of being an upheaved, boiling, volcanic pool. This sierra and that of Janos 

 have about the same elevation above the Santa Cruz valley. 



Springs abound about the Sierra del Pajarito, but their drainage being, for the most part, 

 below the surface, it requires a well-practised eye to detect their presence, particularly during 

 the dry season, which occurs in April, May, June, and sometimes July. There is a considerable 



