SIERRA MADRE OR NATER SHED. 15 



Westward we look down on another alluvial plain, less distinctly bounded by mountain 

 ranges, and extending to a great distance from north to south. On its western limits, at a 

 distance of about ten miles, this plain abruptly terminates by a slightly elevated terrace, the 

 descent from which to the lower level of the San Bernardino valley forms the well known Pass 

 of Guadaloupe. 



Here, then, we have the means of estimating the true character of this great water-shed, in 

 its connexion with the present line of boundary both to the north and south. 



Considerable confusion has arisen from the vague terms and expressions employed by writers 

 to describe the peculiarities of this part of the central axis of the North American continent. 

 There has been wanting in their popular descriptions the elements of a general principle, appli- 

 cable alike to all great dividing ridges. Geological science alone furnishes this element, giving, 

 in the general result of its observations, the best means of elucidating all the points involved, 

 and clearly explaining the several local jieculiarities exhibited. 



In most of the descriptions hitherto given of this portion of the dividing ridge, we hear in 

 frequent use the stereotype expressions that at or near the point under examination the range 

 of the Eocky mountains becomes " suddenly depressed," or " flattened out," to form the great 

 Mexican jilateau. Again, that at some imaginary point south of this great change of topo- 

 graphical features rises another distinct range, called the Sierra Madre, continuing thence to 

 form the line of Cordilleras extending to the extreme of the continent. 



Now, such descriptions as these embody no clearly defined principle of geological science, and 

 contain, moreover, errors of fact. 



The Spanish name of Sierra Madre (literally mother mountains) is the general term in use to 

 describe what is called a dividing ridge with us, and its special application to the range under 

 consideration is due to the important character of this divide as the mother range of the continent. 

 Now, it is well known that all extended continental ranges are due to a line of internal dis- 

 turbance, of varying intensity at different points, but in all alike characterized by the protrusion 

 of various igneous products, together with the iiplifting of adjacent stratified deposites, either 

 altered in texture by the action of internal heat giving rise to the various metamorphic products, 

 or showing the action of an uplifting force only in changes of inclinatian or dip of the strata. 



Most naturally, then, in view of the numerous and varied agencies at work, should we expect 

 changes of character at diflerent points of the same range, corresponding to points of greater or 

 less intensity of the internal disturbance, or the different products erupted or exposed to altera- 

 tion. Hence occur elevations and depressions, and variety of formation in the course of the same 

 continued range. 



With this principle in view, we have a ready explanation of all the peculiarities exhibited in 

 the portion of the range under examination. 



Thus the igneous products are mostly of modern origin, exhibiting various volcanic products 

 in the form of granitic lavas, porphyritic basalts, and amygdaloid traps. These products show 

 a very variable character of exposure, forming ranges irregular in their direction, and differing 

 in composition. 



These several mountain ranges cover more or less the entire face of the country, including the 

 dividing ridge only as one member of the general series. 



