646 ZONES AND REGIONS [Pt. Ill, Sect. II 



Moreover, the Mexican deserts occur usually in high regions, where the 

 desiccating influence of the mountain climate makes itself felt 1 . 



Dr. G. Karsten, who with Professor Stahl visited the Mexican plateau 

 in the autumn of 1895, was kind enough, at my request, to communicate the 

 following description (see Figs. 387-391): — 



' The Cactus-Agave vegetation I studied chiefly near Tehuacan. The place has an 

 altitude above sea-level of about 1,700 meters. The summer has warm, sunny days, 

 little or no rain, and comparatively very cold nights. In winter there is even some 

 snow, but it does not remain lying. 



' The vegetation in question clothes the gently rounded ridges, which converge 

 from both sides on to the road to Esperanza ; the south and west sides are always 

 rich in vegetation, the north sides exhibit merely a few Mamillariae as the last 

 traces of vegetation. The soil is rocky, usually very rich in lime. 



' On such hills one can trace in the far distance the contour of the ground, for only 

 a quite low grey vegetation covers it. Isolated, or grouped together, the boss- like 

 individuals of Echinocactus ingens stand out, as do the large collections of Yucca 

 which occur more in the valley. 



' On closer observation this uniform grey vegetation is found to be surprisingly 

 rich : Agave, Dasylirion, and isolated terrestrial Bromeliaceae, species of Sedum and 

 Echeveria ; then numerous thorny shrubs, especially Mimoseae, a Cassia, many 

 white-haired leathery shrubs with rolled leaves, chiefly Compositae, which exhale 

 a strong odour frequently resembling that of insect-powder, and leafless bushes, 

 among them an Ephedra. The grass on the ground is thin and discontinuous. The 

 herbaceous vegetation is formed by green and white or whitish, often long-thorned 

 Mamillariae, and dense tufts of small species of Echinocactus, between which is 

 frequently Tradescantia navicularis. 



' It appears to me worthy of note that the shrubs, especially isolated large ones ot 

 Opuntia, were always overhung with little tillandsias with squarrose rosettes ; the 

 atmospheric humidity cannot therefore be so very low. The low fall of temperature 

 on clear nights— perhaps combined with the increased radiation owing to the form 

 of the tillandsias— may possibly cause a condensation of water to take place, even it 

 only to a small extent, every night. 



'On places richer in lime— almost pure white calcareous rock— above Carnero, 

 near Tehuacan, Echinocactus robustus occurs with Mamillaria mutabilis and M. mas- 

 chalacantha, also a leafless besom-shaped euphorbiaceous plant with small white 

 flowers, and Pedilanthus-scrub, which has likewise shed its leaves, isolated little trees 

 of Pereskia spathulata, Agave Corderoyi, and the white-margined Agave Gilbeyi ; 

 finally, numerous undetermined Bromeliaceae with sharp-thorned prickly leaves. 



' I saw a similar but poorer vegetation again on the plateau of Oaxaka from 

 St. Dionysio to Oaxaka itself (1,750 m.-i,6oo m.) : quite short silvery-white Mamil- 

 lariae in great abundance, springing from a sward of likewise short Bromeliaceae, 

 coloured grey to white. It was in November, and there was hoar-frost in the morning. 

 Maize and beans were frozen. During daytime a temperature of 15-20 C. prevailed.' 



1 See Section IV. The close connexion of the Mexican with the North American 

 desert formations induced me to treat them here and not with mountain formations. 



