76 GEOLOGY. 



trachyte, containing numerous large crystals of glassy feldspar. The singular aspect of this 

 mountain is produced by the protusion of crystalline rocks through a bed of black vesicular trap. 



The morphological features of these walls of rock bear a resemblance to the ice formations of 

 the Polar seas. Similar causes have effected similar results ; there, we have the consolidation 

 of aqueous masses ; here, the crystallization of pluto-volcanic rock. Similar in outline, there 

 are, on the one hand, ice-fields, hummocks, packs, and icebergs ; on the other, vast beds of 

 trachytic lava, contorted peaks of porphyritic or amygdaloid rocks, upheaved edges of immense 

 beds of metamorphic masses forced upon each other — broken, crushed, and shattered — and 

 formed over again. 



The whole of both the icy and rocky world, each one floating half submerged upon an ocean — 

 the one upon the salt waters, and the other upon the residue of a quaternary sea. The moving 

 medium is also somewhat analogous to the masses acted upon. There are the oscillatory move- 

 ments of the sea with one, and the folding of the earth's crust with the other. 



That metal is to be looked for in mountains like that of Sierra del Tule is doubtful. 

 A piece of copper ore, however, was picked up by one of the party off one of the highest — 

 almost inaccessible— peaks. Our duties were such as would not permit of an examination for 

 ores of any kind ; yet had their been any indications of their occurrence — such as oxides and 

 sulphates of copper or lazur and malachite — they would certainly have attracted our attention. 

 (For analysis of piece discovered, see page 25.) This, consisting like the last-mentioned 

 sierra of several ranges, would be more properly called cordillera. 



The petrographic features of these sierras are similar, and there is not much doubt but that 

 they originated from a common upheaving focus. This sierra is the last of the ranges traversing 

 the State of Sonora. Westward from its crest a few rocky peaks only are visible, rising out of 

 the diluvial main like out-posted reefs along a seacoast, and are in all probability the tops of 

 submerged sierras. The tinajas altas, or water-holes, in the volcanic crevices of this mountain 

 are famous ; they are the principal places in the surrounding country where the traveller 

 between the Colorado and the springs of Sonoyta may expect to find water. 



After leaving this sierra, the Colorado desert proper is entered upon, stretching in an unbroken 

 sheet of drifting sand to the foot of the California Cordilleras, a distance of about 130 miles. 

 The distance in a direct line from the Tinajas to the Colorado is about 45 miles ; about midway, 

 there is a slight swell of the sand traversing the desert, and which may be considered an under- 

 ground sierra. 



Keviewing the mountain ranges passed over, we find that they invariably dip to the east, 

 with their strike facing west ; each sierra and cordillera may, therefore, be considered as one 

 page in the great book of creation. Few of them have been fully opened so as to permit a 

 satisfactory reading of their pages ; whilst their greater number still remain closed, with just 

 one edge turned up. Our belief is, that when the time for further revelations come, the axis 

 of disturbance will develop itself in the eastern base of the California Cordilleras, and these 

 mysterious sheets will be turned from west to east. 



The sierras Santa Cruz, Pajarito, and Santa Barbara, have disclosed a part of their geological 

 history, while others, especially those on the confines of the desert, have hardly commenced to 

 do so. 



Earthquakes are not uncommon in the basin of the California Gulf. There are two solfataras 

 now known at the eastern foot of the south California Cordilleras, both still in activity; and 



