20 GEOLOGY. 



a journey of eighty miles, otherwise dreaded, was, by an agreeable disappointment, rendered 

 highly pleasant. Our journey was made in the first week in March ; doubtless another month 

 might have changed its features materially. 



Our course lies quite regularly to the north'west, a broken line of mountains lying on our 

 left, while to our right lies the extensive high mountain range northeast of Tucson. Directly 

 in our course is a singular pinnacled peak, being the "half-way point" between Tucson and 

 the Gila ; approaching this, we pass by a gradual ascent over a gentle ridge, forming a de- 

 pressed point in a continuous mountain range extending from the pinnacled peak, on our left, 

 northeast toward the Gila valley. Near the summit of this ridge we pass small alluvial tracts, 

 then occupied by a luxuriant growth of young grass, and cut up by deep gullies containing 

 abundant supplies of rain water. The rock exposure here has a more ancient appearance than 

 any before passed, indicating an approach to the granite ranges of the Californian Cordilleras. 



We descend the northern slope of this ridge, passing over extensive clay flats washed by 

 recent rains into frequent gullies, these finally centering in one form the irregular bed of a rain 

 stream leading direct to the Gila river. 



The portion of the Gila valley thus reached is where the river, emerging from the high 

 mountains occupying the mouth of the San Pedro, spreads out into the extensive alluvial bot- 

 toms, occupied in part by the settlements of the Pimo and Maricopa Indians. 



The gravelly table-land here forms a gentle slope, leading from the distant mountains, 

 and indenting the alluvial belt below. This latter consists of an upper level, supporting 

 a shrubby growth of mezquite, and a lower bottom subject to river overflows. On these upper 

 portions the Indians usually construct their dwellings, thus overlooking the lower cultivated 

 fields. The amount of land here capable of cultivation is quite extensive, forming a belt on. 

 each side of the river often several miles in width, and extending east and west for 20 miles or 

 more. 



The stream of water, then at its average height, (in early March,) measured about 40 yards 

 in width with an average depth of 2 feet, the volume, however, being considerably diminished 

 by the extensive irrigating ditches drawn from above. 



The line of the river bank is at this season set off" with lagoons and marshes, and everywhere 

 bordered with a dense willow growth, rendering it difficult of approach. 



The dams, which serve the purpose of drawing ofi'the irrigating water, are constructed of old 

 willow trunks and snags; these, in the course of time, entangling the loose soil and sediment 

 borne down by the river, furnish a bed for the willow growth, thus becoming more permanent 

 with age. 



From a rock knoll of true granite, abutting on the river on the American side, a fine view is 

 obtained of the general character and external features of this interesting locality. 



The character of the Gila valley, from this point down to its mouth, did not come under my 

 personal inspection. All accounts represent a great uniformity of general features already 

 sufiiciently detailed. 



Thus we have a succession of basins, limited by mountain barriers, through which the river 

 forces its way, forming caiions of greater or less extent. 



These basins are again occupied by more or less extensive stretches of gravelly table-land, 

 representing the desert features of this region ; through these are marked the alluvial tracts, 



