DESERT REGIONS OF NORTH AMERICA. 23 
When the waters of these springs are conducted through the irrigation 
channels the lime and magnesia are deposited in great quantity on the 
floor and walls of the ditches. These are constantly built up higher by 
the use of more soil, which in turn becomes highly charged with lime, and 
the ditches soon become raised much above the fields. In some cases 
they reach a level with the source of supply, and a new ditch is started 
nearby and parallel to the older one (plate 14). 
Even with care in the use of water, the soil soon becomes so heavily 
limed that its usefulness is soon destroyed, and extensive areas were seen 
upon which cultivation was no longer attempted. The information 
was also gained that, in some places in the neighborhood of the springs 
where small intensive gardens were kept, the necessary free and constant 
use of the water charged the soil so quickly that in about three years it 
was necessary to remove the surface layer and fill in with top-soil brought 
from the mountain valleys in which the only irrigation was by surface 
water of precipitation. Supplies of soil were being brought to the Haci- 
enda El Riego for this purpose during the visit to that place. 
So far as general information may be relied upon, the rainfall of this 
region comes in mid and late summer and can not exceed 15 inches, if 
the character of the vegetation may be taken as index, although the 
following record transmitted by the U. S. Weather Bureau shows that 
Puebla, 70 miles to the northwestward, at an elevation of 7,091 feet, 
1,683 feet above Tehuacan, has an annual average precipitation of 
36.34 inches. 





Average | Average | Average | Average | 
precipita- | tempera- | precipita- | tempera- 
Months. tion, 12 ture, | RBCS tion, 12 | ture; 
years. 8 years. | years. | 8 years. | 
eget y sc... ss. 0.20 i ae J Uc Sid Aki gehen et eee | 02.3 
February... .<@... oe 3% Bee SU oist fo. see 7erO e} e620 8 
00 rn 0.32 Sons. | September... 2.; Ovarrh Siders 
i r.20 65.0 WICtODEr 1... Beas 268) 160.8 
ee win as aaae 65.0 || November...... 1.06 | 57.6 
June 7.56 64.6 | December...... Oxos ee ea 3 
Annually: Ax, 36.34 60.4 








The difference in question might very well be due to the difference 
in altitude and to the topographical features involved. 
Among the agricultural products may be mentioned chilies, maize, 
the fruits of prickly-pears of various kinds, and sugar-cane. With the 
combination of desert conditions and the composition of the water, it 
might be expected that this region would offer some adaptations and fea- 
tures of distribution not encountered elsewhere. Cactacee and Liliaceze 
