2.2, BOTANICAL FEATURES OF NORTH AMERICAN DESERTS. 
It was found convenient to make Tehuacan one of the bases of work. 
The name of this city is used to characterize an arid region consisting of a 
series of valleys and plains lying between abrupt limestone hills at an 
elevation of 4,000 to 5,400 feet, in latitudes 16° to 18°, on the eastern 
side of the continental divide, the drainage being through the Rio Salado, 
Rio Quitopec, and Rio Papaloapan into the Gulf of Mexico. 
The great valley in which Tehuacan is situated slopes to the eastern 
margin, where, lying against the base of the mountain, is the streamway 
of the Rio Salado, the waters of which are wholly used for irrigation. 
Throughout this valley springs occur on the upper terraces, which in 
many cases have sufficient volume to be of great service in irrigation. 
The water appears to be collected under or behind a tilted stratum of 
clay, and in many places where it does not break through, as in the 
springs, it is conducted to the surface by tunnels. So far as limited 
opportunities permitted examination, the construction of these tunnels 
was begun by digging a well or shaft at the lowest point on the slope or 
terrace below which water might be expected. If a supply was not 
obtained by the first, a second was sunk 100 to 200 feet up the slope, and 
this was continued until water in plenty was obtained. The wells were 
then joined by sections of tunnel at a level which would allow the water 
to flow toward the surface lower down, and when it emerged it was run 
in acequias of the ordinary type. 
Most of the shafts examined were from 20 to 4o feet in extreme 
depth. The excavations were all made by hand labor, the rock and dirt 
being brought to the surface by means of baskets and vessels, drawn up 
by a rope of ixtle, the only mechanical aid seen being a single forked 
branch planted in the ground at one side of the shaft or perhaps two on 
opposite sides with a cross-bar between. 
The accompanying analysis, furnished by Sefior Mont of the El Riego 
Hot Springs, shows that the waters are highly charged with sodium, cal- 
cium, and magnesium, besides small proportions of a number of other 
substances: 

LOUNGE) aaa ete chorale eee o. ooc0e4. | Carbonic acid. ©. .... «.. sae 0. 247872 
OCI, va as oles mate as ee 0. 204575 Sulphuric acid... .. » o-) seen 0. 059244 
Potassram tae did se ee 2 erat o.orgo38 4 INitridAcidn 3. 2 22. O. 009154 
LAGI RUST An ary ee a ree eas 0. 000902 Phosphoric acid......<. «0 cme 0, 00004! 
Ammoniim Oxide 3... 24.57 o 0. 000952 Arsenious acid. < 7.56 «se =e 0. OOOOSI 
CACM, 25. gio he eae aes SA 0. 179440 Chiorine, Js: <iwats sas oe 0. 131602 
SOTOMUINL oni seats ee 0. 000556 Bromine. siesees ee ee 0. 000187 
Baritinn py, wee ae te 0. Co0O81 Boracieacid: 745%. band es.2 ee 0. 001920 
Marnesitimius aac ccaael ieee 0. 064832 Silicisacid ser cunt 00. eee 0. 052925 
egwrey one} 4he (rege ef Oper ae 0. 000255 Residues. 5 occ5 sas ss lel eee Oo. 000009 
Manganese dioxide.......... 0, OOOOT4 —_—__—— 
Total, . dee +2 5..2e eee 0. 969654 
