a GEOLOGY. 
cribed, sometimes flows. This stream is formed by the surplus water of the Colorado, which 
occasionally, during the summer freshet, overflows the banks and runs northward towards the 
lowest portions of the desert, which are many feet below the surface level of the river. 
The soil of the area of which I have spoken is fine, rich, and deep, and wants as a supply 
of water to sustain a vigorous growth of vegetation. 
Along the banks of New river we saw the stems of ‘‘ Careless Weed,’’ wild sunflower, &c., 
which had grown up subsequent to the last overflow of the Colorado, a few years since, and 
which had attained a magnitude Ihave never seen elsewhere exhibited by any annual plants. 
At Alamo Mocho we reached the eastern border of the dry lake, and rose on to a terrace com- 
posed of fine stratified clays, covered with small rounded pebbles, which include representatives 
of many rocks subsequently found in place on the upper Colorado. Pebbles of this character 
cover the desert surface on both sides of the river, except where occupied by mountain chains 
throughout the lower five hundred miles of its course; and, as our subsequent observations 
— demonstrated, a large part of them have bane removed from the cafions through 
ich the main stream and its tributaries now flow, and have been transported by its waters 
rie many hundred miles from their point of origin. 
The route from Alamo Mocho to the Colorado exhibits few points of geological interest. The 
road runs either over the gravel-covered terrace just mentioned, or descending to a lower level 
traverses a surface of more recent material formed by the action of streams which have cut 
away the higher and older portions of the desert. The superficial phenomena of the Colo. 
rado desert, the large area which it includes, formerly covered by fresh water, but now dry, 
the character and origin of the transported materials, which strew the older desert surface, the 
physical conditions with which both were associated, are matters of peculiar interest and inti- 
mately related to the wonderful phenomena of the surface geology of the country bordering 
the upper Colorado, and in that connexion will be again referred to. 
