COLORADO DESERT, NEW RIVER. 



127 



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The distance, by this pass, from Santa Isabel to San Felipe, is twelve miles, while the wagon 

 road between these two points is twenty-five miles, or more, in length. 



The geological formation exhibited along the eastern slope of the mountain range at this point 

 shows a very sensible change, and in place of the usual forms of feUl>itathic or quartz granite we 

 meet with a more prevalent character of micaceous granite, in which the scales of mica are fre- 

 quently of large size, and very confusedly intermixed. With this also occur mica and talcose 

 slates, traversed'by quartz veins. At this point, then, we have an approach to the gold forma- 

 tion, and in the section of country thus limited, exist the fairest prospects of mineral discoveries. 



The country thus characterized is, however, barren and desolate in the extreme ; water is 

 scarce, and pasturage of the poorest description. Thorny cacti and arid shrubbery usurp the 

 soil, not only of the mountain clefts, but also of the open valleys. At this point, indeed, we may 

 say, the desert proper commences'; for it is here we have the first appearance of the desert plants 

 Larrea Mexicana and Fouquiera Splendens. 



On leaving the last rocky exposures to enter on the open desert plain, we pass some distance 

 down the bed of Cariso creek ; along the course of which are exposed the high bluffs of sand, 

 marl, and clay, exhibiting a fine sectional view of the tertiary formation on which the desert 

 plateau is based. At the point where the road leaves the bed of the creek, to mount to the 

 desert table-land, some 150 feet above, fossil marine shells of Ostrca arc found, and gypsum 

 makes its appearance in extensive beds. The upper layer of the table-land shows a variable 

 thickness, composed of water-worn pebbles, derived from the adjoining mountains. Near the 

 mountain base, this plateau has a height of about 500 feet above the level of the Colorado river. 

 The surface extends in a gentle slope towards the Colorado, or eastward, about the distance of 

 twenty-five miles, where it reaches its lowest depression at the Lagoon or ''New river" basin, 

 which is in fact a part of the extended alluvial tracts belonging to the Colorado river. 



The proof of this latter fact is seen in the barometric observations, showing a depression at 

 this point, below the level of the Colorado river in high water, and also by tracing a direct 

 connexion between the overflow of this latter stream and the appearance of water at Now river. 

 The numerous depressions found along the course of this alluvial tract have, moreover, all the 

 character of the sedimentary soil of the Colorado bottoms, supporting, though more sparsely, 

 the same character of vegetation, and showing, frequently, fluviatic shells, identical with those 

 now found at lagoons and sloughs adjoining the river. 



This " New river" tract also receives the drainage of a large scope of desert country, which 

 is sometimes visited by heavy showers of rain. It retains this rain-water, and river overflows, 

 for several months ; when both these sources fail, it becomes a perfectly dry bed, or contracts 



into q^uaggy saline marslies. 



When we stopped here, in the latter part of September, copious local rains had filled these 

 lake reservoirs, which, with previous extensive river overflows, had enriched the soil and caused 

 a rank growth of annual grama grass. This afforded a fine grazing camp for our animals for 



two months. 



Directly sonth from our camp at this place, and about eight miles distant, hea a high mount- 

 ain range, having a direction nearly east and west. To the western and most prominent point 

 of this range the name of ■■ Signal Mountain" was given. This range is made up of a form of 

 sienitic rock, associated with recent lava. Its surface is bare, and presents a forbidding outline 

 of dark weathered rock, variously marked by furrows, and shows an irregular crest, gradually 

 sloping towards the east. 



