66 



GEOLOGY. 



Malpais — literallj, bad land; the "Mauvais terre" of the French. In Sonora it is exclu- 

 sively applied to mesas, lomas, or any more or less elevated plateau formed of igneous rock, 

 here mostly a compact or vesicular trap or basalt. 



Cienaga is a valley, or depression in a plain, where the water collects, and can only escape 

 by an obstructed outlet. Such a place is usually miry and boggy. 



Charco means a hole in clay, or stratum of rock, where water collects, and from which it 

 cannot run. 



Tinaja is a water-hole, found in the crevices of rocks and ravines, difficult of access. The 

 primary meaning of this word is an unglazed earthen jar, burned so as to allow exudation. 

 The water thus oozing through evaporates and keeps that remaining inside cool. 



Beginning at the intersection of meridian 111° and parallel 31° 20', we proceed to the con- 

 sideration of the various sierras crossed by the line. 



The Sierra del Pajarito (little bird) shows crystalline transition rocks, metamorphic and 

 unchanged ; also, trachytic strata, or metamorphic forms of granitic and syenitic rocks, (e.) Some 

 of the more elevated portions exhibit a rough cellular surface, whilst the lower are smooth and 

 more compact. The tint is light pink, or flesh color. This rock contains much glassy feldspar, 

 and, occasionally, particles of augite, indicating the frequent occurrence of a syenitic granite. 

 A fine-grained, white, metamorphic syenite, consisting of minute particles of hornblende and 

 white feldspar, occur on some of the cuchillas on the north side. On the lower parts of the 

 west slope talcose (argillaceous) and quartzose slates are met with, though trachyte dykes range 

 through in every direction ; in the bottom and slopes of the valleys the igneous rocks prevail. 

 In one place a solid mass of trachyte is cut through by an arroyo, forming a puerto, flanked on 

 both sides by vertical walls of eruptive (a and c) rocks fifty feet thick. The mountains on both 

 sides slope towards this gap at an angle of 35 to 40 degrees. Here, and other localities along the 

 foot of this sierra, pudding stone, volcanic breccia, feldspathic porphyry, and trapitic amygda- 

 loid rocks abound. Some of the water-beds are lined with a singular formation, (b,) and apparently 

 of a later age than those just mentioned. At first sight it may be considered a fresh-water 

 deposit, overlying or placed alternately with volcanic breccia. 



SECTIONAL VIEW OF THE VALLEY DE LOS JAN03. 



Occasionally traces of stratification, and even cleavage, are visible, especially in its upper 

 part ; its lower portion is cemented into a solid mass. Its color is a light brown, or dark ash- 

 gray. The outer crust looks as if it had been subjected to a process of calcination, for it readily 

 crumbles or exposes a marl or chalk-like substance that could be easily scratched out with the 

 finger. There were no means at hand to identify this as carbonate of lime ; yet we were 



