CLIMATE BETWEEN FORT FILLMORE AND THE CORDILLERAS. 171 



The two first may he looked on as one valley. The two last have the lower mountain lime- 

 stone and the Devonian sandstone conglomerates resting on primary rock. In these valleys 

 the geological circumstances are eminently favorable for artesian boring, and there water might 

 be had at 300, 500, or 600 feet deep, according to the distance selected from the margin of the 

 basin, provided the climatal conditions were favorable ; east of these basins the country is too 

 much broken up by faults and dislocations to be available for ordinary artesian wells. 

 2. The meteorological conditions may be thus expressed : 



The region lying between 30° and 33° north latitude is one of summer rains ; within 100 

 miles of the Cordilleras it is a rainless district ; but along parallel 32°, when it does rain, it is 

 only in summer. This, of course, applies only to the country between the Sierra Nevada and 

 the Rio G-rande, for in west Texas, under this parallel, it is a district of autumn rains, and 

 between the Sierra Nevada and the coast it is a region of winter rains. 

 The elevation of the district is as follows : 



Fort Fillmore, on the Rio Grande, altitude 4,000 feet. 



Basaltic belt and porphyritic district, average for 105 miles.. 5,000 feet. 



Small basin district, 130 miles 4,000 to 3,000 feet. 



From the last level, 3,000 feet on the San Pedro, the land falls until the junction of the Gila 

 and Colorado is reached, when it is only 105 feet above sea level. 

 The elevation of the mountain tops are : 



Summits of the Organ mountains 7,000 feet. 



Chiricahui, or Mount Graham range 6,800 feet. 



Sierra Nevada, on parallel 32° 6,500 feet. 



In the two first instances the mountain tops do not exceed 2,000 feet above the plain, and, 

 therefore, constitute but small points of attraction to rain clouds. The Sierra Nevada, on the 

 other hand, is elevated nearly 6,000 feet over the district, immediately to its east, and hence 

 becomes an impassable barrier. 



The annual fall of rain over the district is thus distributed : 



Annual fall at Fort Yuma under 3.00 inches. 



Annual fall at Fort Fillmore, mean of three years 9.23 inches. 



Annual fall at Fort Bliss, mean of three years 11.21 inches. 



Forts Yuma and Fillmore are the extremes of the line. Fort Bliss is inserted to show the 

 difference which a position 40 miles further south produces. The rain is accompanied by 

 south winds, generally from the southeast or along the course of the Rio Bravo, but occasion- 

 ally from the southwest. From the Rio Bravo the rain fall diminishes toward the west, until 

 it is almost nothing upon the Colorado desert. 



The rains at San Diego have no relation with those at Fort Yuma, although both places be 

 in the same latitude, and only 220 miles apart, the mountain chain of the Cordilleras altering 

 the climate of the two stations. 



The cause of the diminution of the rain westerly may be due to two causes: 1st. That as the 

 rain clouds come from the east, in passing over the district, they gradually become drier ; and 

 2d. As the district west of the San Pedro gradually drops toward the Pacific, and thus becomes 

 more heated by the sun's rays, the rain clouds are not condensed, but are raised further up in 

 the atmosphere by the heat of the lower regions. In the Colorado basin clouds from the south 

 were frequently seen drifting north, and when nearly overhead, gradually breaking up, and 

 being dissipated under the immense heat of the plain. The difi'erence in amount of rain fall 



