650 Part IV. Chapter 5. 
from the lower to the higher slopes), Q. oblongifolia and Arbutus ee The upland pal- 
metto Brahea duleis inhabits the precipitous ledges of limestone. About the base of the 
mountains, especially on limestone ledges associated with rg Greggü, u parvifolia, 
Diospyros texana, occur Amyris madrensis and Sargentia Greggii. Common in rock valleys among 
the foothills are Acacia farnesiana, A. flexicaulis, Ehretia er Pithecolobium brevifolium 
and occasionally Sapindus acuminatus. 
the Guadalcazar Mountains the principal arborescent species are Quercus undulata, 
Q. polymorpha, Juglans mexicana and Sargentia Greggii. In meager soil, partially covering ex- 
tensive ledges of lime-rock, grow Neopringlea integrifolia, while rooting in the fissures is Dahlia 
dissecta and on the ledges, such ferns as Pellaea pulchella, Notholaena sinuata, N. Aschenborniana 
and Cheilanthes leucopoda!). 
5. United Cordilleran Region. 
This region includes the confused mass of mountain ranges formed by the 
union of the western and eastern Sierra Madre chains. The topography of 
this region is rendered more complicated by the fact that a system of 
volcanoes is disposed transversely to the trend of the Sierra Madre Occidental 
and Sierra Madre Oriental between the Gulf of Mexico and the Pacific Ocean. 
High above this central Cordilleran system tower the snow-capped peaks of 
several volcanoes (see ante p. 113), of which Orizaba (Citlaltepetl) stands isolated 
from the others and rises from the tierra caliente to an elevation of 18,250 
feet. Several large valleys, such as Mexico and Toluca, are situated at a 
general elevation of about 7,500 feet and are almost completely hemmed in 
by mountain masses, which form elevated rims to these valleys filled with lakes. 
The flora of the valleys and surrounding chains will be presented first, and 
then, as far as the data will permit, a sketch of the vegetation of the high 
volcinöis will be given. 
ı. Valley, Lakes, Dry Ridges. 
The soil of the valley of Mexico being a fine, light loam, it has been 
under a high state of cultivation for many centuries. Little of the original 
flora is found in the cultivated areas, but the botanist notices some interesting 
species near the town of Tlalpam?). The valley of Toluca is the most 
elevated valley of much extent in Mexico, being 8,500 feet above the level of 
the sea, one thousand feet higher than the valley of Mexico. In its upper, 
or southern portion lie Lake Lerma and its marshes fed by mountain brooks, 
which tumble down from the Sierra de las Cruces. During the summer, parts 
of this valley are solid with corn, while during the earlier part of the year, 
wheat occupies the same Krön 
ake Formation. According to my observations, the following aquatic plants are found 
in the lakes of the valley of Mexico. In Lake Xochimilcho, I collected Nymphaea mexicana, N. 
tussilagifolia, Eichhornia azurea, Marsilia quadrifolia, while on the wet te or floating ee 
1) PRINGLE, C. G.: Notes on Mexican Travel: see Bibliography p. 85 
2) HARSHBERGER, JOHN W.: Botanical Observations 1898: see Bibliogr. p. 83. 
