16 GEOLOGY. 



The natural explanatory inference from these facts is, that the internal force, here represented 

 in the continued mountain range, was diffused over a large space, and not centralized on one 

 particular line. Hence arises no great prominence of one central chain, but a number of inde- 

 pendent ranges, serving to equalize the general elevation and give the character of an elevated 

 plateau to the surface of the country. 



Again, the same irregular action of the internal force, and especially the preponderance of 

 recent eruptive products, favors a varied direction of the mountain ranges, by means of which 

 areas are circumscribed and basins formed for the reception of aqueous depositions. Here, then, 

 we see the origin of those extensive plains and stretches of table-land to which our attention 

 has been so frequently directed in the preceding sketch. 



These same characters probably apply more or less closely to many other localities connected 

 with the general dividing range, whether north or south of the point we are examining. 



We are now prepared to descend the western slope of this dividing ridge, and note the pecu- 

 liarities of feature presented on our route westward. 



Decending, then, by an equally steep slope as the eastern ascent, and about the same height, 

 we come upon the alluvial plain below. The lowest depression of this plain is composed of a 

 light alluvial soil, and thence sloping gently upward to the west, exhibits a gravelly deposit, 

 till, at a distance of about eight miles from the base of the mountain just left, we come upon the 

 abrupt descent of the Guadalupe Pass. 



This noted pass, which has been so frequently traversed on the line of emigrant travel to 

 California, is now so well known as hardly to need a detailed description. 



This pass has been properly characterized as the first step of considerable descent from the 

 Mexican plateau to the heads of valleys leading to the Californian gulf It has now been clearly 

 established that at a point farther to the north, near the parallel of 32° latitude, the descent 

 westward may be accomplished by a more gradual slope, and without leaving the basin of drain- 

 age pertaining to the Gila river. 



The geological structure exposed in this mountain pass is similar to that before noted as 

 occurring in the upper slope of the Sierra Madre, including feldspathic lava^ granitic in texture, 

 associated with basalt, stratified porphyry, and closely cemented breccias. 



These several forms, variously associated, serve to give a remarkable diversity and broken 

 character to the rock exposure, presenting a confused outline of mingled crests, peaks, and ra- 

 vines. Through these the road has to work its way by sharp turns and very steep descents. 

 On attaining a lower level we pass down a ravine, gradually widening, which finally spreads 

 into a small valley, watered by a fine running stream, and beautifully shaded by large sycamore 

 and cotton-wood trees. This valley is closely hemmed in by steep rocky walls, marked by 

 intricate ravines, and rendered picturesque by a varied assemblage of live oak, cedar, and other 

 verdant shrubbery. In emerging from the higher points of the mountain range, the walls of 

 this caiion exhibit various forms of stratified porphyry running into a breccia. The character 

 of stratification has, at several points, a close resemblance to altered sedimentary deposits, 

 showing a reddish color and a very uniform character of dip. 



We finally leave this valley, mounting up a steep bank, composed of gravelly table-land, rising 

 200 feet above the bed of the stream, thence passing by a gradual and continuous slope toward 

 the main valley of the San Bernardino. The table-land here has all the usual characters of this 



