72 GEOLOGY. 



remarkably auriferous ; especially Soni, once a mining settlement of the Mexicans.. It was 

 abandoned at the time of the California gold fever. The latter subsiding, the old settlers, dis- 

 appointed on the Pacific coast, are now returning again. The Papagos claim this region, and, 

 from the time they first learned to appreciate the value of gold to the present day, have con- 

 tinued to prospect successfully. 



A desert of sixteen or eighteen miles in extent separates the Sierra de los Linderos from that 

 of la Nariz. Though these sierras are nearly related in petrographic character, there exists 

 one striking difference. The former is, at least where it was crossed, a true volcanic dyke, 

 bordered in some places by upheavals of crystalline strata ; its crest of eruptive rocks seems to 

 have been forced through a mass similar to itself. The latter, though consisting also of trachyte 

 and trapitic masses, appears only a simple upheaval; its crest, comparatively smooth, is the 

 upturned edge of a bed of igneous masses, dipping northeast ; its strike faces the west, at an 

 angle of 60 to 70 degrees. The surface (e) of the east slope is covered with a thick layer of loose 

 boulders, of a black or dark-brown vesicular trap. On the strike, stratification (a) is visible, even 

 at a considerable distance, the layers varying in thickness from five to twenty-five feet. 



VEKTICAL SECTION OF THE SIBiBA DE LA NARIZ. 



This sierra ranges in a slightly curved line from southeast to northwest, and joins about eight 

 miles north of the line the Sierra del Ajo, of which it is, in fact, only a spur. A little to the north 

 of where the line strikes, quite a depression occurs, the range here not being more than fifty 

 feet above the drift. 



A valley of about fifteen miles wide separates the Sierra de la Nariz from the Sierra de la 

 Laguna on the north. Its petrographic character seems to be similar to the de la Nariz, having 

 the same strike, dip, and stratification. Trap mounds accompany both sierras, cropping out 

 along their bases to a height of thirty or forty feet above the level of the valley. There are 

 no springs to be found about any of these mountains ; holes of water or ponds, (charcos and 

 lagiinas of the Mexicans,) formed in beds of clay, are the only dependence for water, and are 

 not to be relied on during the whole season. The surveying parties being unexpectedly fortunate 

 in finding an abundance of water here, the sierra was called La Laguna (de la Esperanza ;) 

 it seems but an eastern branch of the Sierra del Ajo. 



Northwest from the Sierra de la Nariz this latter sierra, a bold and high mountain range, 

 is visible. As we have been told, it takes its name, (del Ajo,) garlic, from its structure, 

 appearing as the consolidation of various branches forming a cordillera. Although composed 

 most probably of volcanic rocks, similar to that of ranges just referred to, it is quite different 

 in its other features. As seen from the southwest, a huge central block of metamorphic, 

 or, more probably, igneous rock constitutes the strike side. This block, exhibiting traces 

 of horizontal stratification, is divided into two almost equal parts by a protruding mass of rocks, 

 which, though lithologically almost the Same, show a vertical columnar structure instead of a 

 horizontal stratification. On the sides and about the base numerous isolated and volcanic peak 



