82 GEOLOGY. 



The streams of southern California are, in truth, the life-blood of its agriculture, and the 

 means to be adopted to extend this supply can only be efficiently based on a clear understanding 

 of all their separate relations, both as to atmospheric conditions and geological structure. In 

 many of the old mission establishments extensive lines of masonry were constructed, by means 

 of which the streams were tapjied a short distance above their place of sinking, and a vigorous 

 irrigating supply conveyed to the lower portions of the valley, thus rendering productive lands 

 otherwise useless for all the common purposes of cultivation. 



It is therefore in the true character of these streams — with reference to their sources, their 

 beds, the elevation and geological structure of their banks and bottoms — that we are to look 

 for the fairest general idea of the agricultural capacities of this region. 



But it must further be remarked that it is not to these mountain sources alone that we must 

 look for the needful supply of water ; occasional springs in the lowest portions of valleys fre- 

 quently furnish a constant flow sufficient to meet the demands of cultivation over a limited area. 



Thus, the extensive mission of San Luis Eey, proverbial for its vertility, depended almost 

 entirely upon such sources of supply. Similar examjiles in other parts, though rare, may 

 furnish usel'ul indication, in directing the location of artificial means of supply, by the con- 

 struction of Artesian wells. 



Referring to the character of the streams on the eastern mountain slope, we have before 

 noticed their abruptness, also the rapid diminution of volume which they undergo in their steep 

 descent. The excessive dryness of the atmosphere, and the more absorbent character of the 

 strata through which they pass, serves to exaggerate all tlieir peculiarities, as compared with 

 the opposite slope. The streams, equally as vigorous at their sources as those of the other slope, 

 are quickly absorbed in their course^ and none at any time acquire sufficient vulume to be 

 entitled to the name of river affluents. Thus, though the existence of wide and deeply cut 

 stream beds show the occasional agency of powerful streams, derived from the rapidly embodied 

 force of copious rains, yet their rare occurrence and short continuance only serve, in the main, 

 to give an exaggerated feature of bari'enness and desolation to a region where, during the 

 greater part of the year, scanty supplies of water are only attainable from stinted and unwhole- 

 some springs. 



The point at which water ceases to flow is extremely variable, and exhibits a singular inter- 

 mittent character : thus, in the morning you mayoress over quite a large brook, and at the 

 same place, by noon, find it entirely dried up, to show itself again when the diminished 

 evaporation, at night, allows the ground, instead of the atmosphere, to receive its aqueous 

 tribute. Often you meet with streams, near the lower mountain slope, present at one point of 

 their course and absent at another, thus constantly varying, according to the relative absorbent 

 or retentive character of their beds. 



On the desert plains, the stream courses are marked by wide beds, with more or less abrupt 

 banks, cutting through strata of sand, marl, or coarse gravel. Near the mountain base they 

 exhibit steeply inclined plains, strewn with a variety of rounded and angular pebbles. 



In the re-entering angles, formed by the irregular projection of mountain spurs, these plains 

 often attain an elevation of nearly one-half the mountain height, and are taken advantage of in 

 the selection of passes. 



But the point of all others which has attracted most attention, in reference to the distribution 



