DISTRIBUTION OF THE MOEE PEOMINENT 35TATITEAL OEDEES. 263 



often forming only dense thickets interwoven with numerous Convolvulacese and 

 climbing grasses. The hornbeam *, lime, willow, and cornel are met with in these 

 forests associated with a host of noble trees of the Laurinese. Above 7000 feet oaks 

 gradually decrease, being replaced by pines ; and at elevations of 8000 to 10,000 feet 

 they are only found scattered among the pines. Among the species here are Quercus 

 spicata, Q. reticulata, Q. chrysophylla, and Q. pulchella. Most of the epiphytes and 

 parasites of the lower regions have disappeared, though some species of Viscum still 

 abound. Mosses and lichens become plentiful, and Usnea barbata hangs in festoons 

 from the branches, replacing Tillandsia usneoides of the warmer zones. But oak- 

 vegetation does not entirely cease on the peak of Orizaba below an altitude of 

 12,000 feet. 



During the last 350 years there has been an enormous destruction of the oak-forests, 

 especially in the States of San Luis Potosi, Guanajuata, and Oaxaca, where the wood 

 was largely used for smelting silver. 



The oak-vegetation of the Northern States of the interior of Mexico is rich in 

 species, but almost all the trees are low and stunted — often only shrubby, and not 

 forming forests, but occurring in scattered groups on the steep mountain-flanks. 

 They are found chiefly at elevations of 6000 to 8000 feet, often completely covered 

 with Tillandsia usneoides. 



As an example of the large number of species found in a small area on the arid 

 mountains of the interior, Liebmann enumerates twenty-two observed by him in the 

 silver-mine district of Real del Monte ; and he adds that the same species inhabit all 

 the mountains of the interior from Zacatecas to Oaxaca. In the eastern valleys of 

 Oaxaca the oak descends to an elevation of a few thousand feet only. Among the 

 species found are — Q, salicifolia, Q. tomentosa, Q. petiolaris, and Q. skinneri ; the last 

 also inhabits the western side of Guatemala. On the higher peaks of the Cordilleras 

 of Oaxaca, as Sempoaltepec, Pelado, and Cumbre de Ocote, some species ascend to 

 elevations of 10,000 to 11,000 feet, where they form stunted shrubs two or three 

 feet high. 



The western cordilleras of Mexico, like the eastern, have their oak-forests, but they 

 are far from being so fine, in consequence of the dryness of the climate ; and they have 

 not been so fully explored. 



In North-western Mexico Seemann collected thirteen species of Quercus ; and he 

 states f that the first evergreen oaks were met with at an elevation of 1500 feet above 

 the sea-level (ascending from Mazatlan), but that the increase of the number of the 

 species as well as that of individuals was rapid, and at about 3000 feet they were 

 associated with some Coniferse and a rich mountain flora 



* Perhaps the hop hornbeam (Ostrya) is intended here, for we have seen no specimens of Carpinus from 

 Mexico, though the common North-American species has been recently discovered in Guatemala, 

 f Botany of the Voyage of the ' Herald,' p. 263. 



