14 GEOLOGY. 



extending in its greatest length from north to south, and hounded on the west hy the clearly 

 defined range of the Sierra Madre. 



The course we travelled thence lies W.N.W., inclining towards this mountain range, and cross- 

 ing diagonally tlie wide basin plain intervening between this and the Janos range of mountains. 



About 10 miles from Janos we come upon a singular depressed valley, sunk some 50 feet 

 below the gravelly plain, having a lower alluvial belt about a quarter of a mile in width, which 

 is coursed by a limpid brook, and bordered by a scattering timber growth. 



This stream is said to have a lagoon source some three miles to the southwest ; thence flowing 

 northeast 10 or 1^ miles, it terminates in a marshy lake surrounded by mountains ; thus show- 

 ing a character similar to that before noticed in reference to Lake Guzman and Sta. Maria on 

 a smaller scale. 



From this point, following a continuous course W.N.W. (mag.,) the road passes over a gently 

 undulating swell, composed of gravelly table-land, thence crossing a wide, open, alluvial basin, 

 similar in character to those before described. 



We then approach the high mountain range of San Luis. . Our progress toward the moun- 

 tain base leads by a gradual ascent till a near approach brings to view deep gullied stream 

 beds, connected with the drainage of the mountain valleys, and terminating on the alluvial 

 I^lains below. Near their sources in the moimtains these ravines contain running water^ more 

 or less copious, according to the character of the season. 



On reaching the first rocky spurs from the main range, the country assumes a most pictu- 

 resque character. Clumps of live oak (Quercus Emoryi) edge the ravines, and are scattered along 

 the mountain slopes. Cedar of a shrubby growth is also frequent, and the usual mountain shrub- 

 bery serves to give a character of freshness and verdure to the scenery. 



Directly at the mountain base, and forming its projecting spurs, a reddish form of ijorphyritic 

 basalt makes its appearance, showing a precipitous columned face and tabled summit. 



In the recesses of the ravines, as exposed by the mountain torrents, a variable deposite of 

 igneous conglomerate is met with, flanking the central rocky mass. This central nucleus, as 

 exhibited along the sides and summit of the mountain range, is an igneous volcanic product of 

 quite recent origin, and characterized by Professor Hall as " feldspathic lava," exhibiting a 

 granitic appearance. (No. 86.) 



At the point where the old road crosses the ridge, called the " San Luis Pass," the ascent is 

 quite abrupt, rising from the plain below 800 to 1,000 feet. 



The summit crest commands a most extensive and grand view. Looking eastward, the eye 

 takes in at a glance the wide alluvial plain over which we have been passing, encircled by its 

 irregular mountain boundaries, showing plainly its basin character, and in which here and 

 there stand out isolated mountains, as islands in the broad expanse.* 



To the north and south is a continuation of the main ridge, more broken to the north, and 

 apparently forming slopes of easier ascent than the one passed over by us. Quite possibly at 

 several places there may be an easy transition from the jilains on one side to those of the oppo- 

 site slope. To the south the range is more continuous, of a rugged character, and increased 

 height. 



- This description was written before the line nndcr the treaty of 1853 was run. It will have been seen in the preceding 

 part of this work that good passes were found to the north and within the limits of the United States. W. H. E. 



