57° ZONES AND REGIONS [Pt. Ill, Sect. II 



Carriere, Pinus ponderosa, Dougl., Abies concolor, Lindl. et Gord., and 

 Abies bracteata, Nutt., Libocedrus decurrens, Torr., and, in the south, 

 Sequoia gigantea, Lindl. et Gord. (Figs. 309, 310), the mammoth tree, which 

 appears principally in isolated groups, and still further south forms a more 

 or less continuous strip for several miles. In contrast with the more 

 northern forest on the western slopes of the Cascade Mountains, the 

 high-forest of the Sierra is almost free of underwood. The cause of this 

 difference is not yet explained. In valleys the forest is thin and formed 

 of oaks. 



Representing an eastern branch of the Pacific forest, and separated from 

 the western branch by the almost treeless desert of the Great Basin, there 

 extends along the Rocky Mountains and its southern outliers the narrow 

 Pacific Interior forest (C on Map 4), with which Sargent also associates 

 the forest on the eastern slopes of the Sierra Nevada. Owing to scanty 

 rainfall, this forest, for the most part, is stunted, thinly stocked, and without 

 underwood. It occurs only on steep slopes and in canons (Figs. 311, 312), 

 whilst the valleys, except in the immediate vicinity of the water-courses, 

 are treeless or nearly so. In the north, this forest consists very largely 

 of the dwarf Pinus Murrayana, Balf. ; to the south of 52 N. the forest 

 increases somewhat in luxuriance and in number of species, and includes 

 Pseudotsuga Douglasii, Pinus ponderosa, and Larix occidentalis. In the 

 most southerly part of the Rocky Mountains, on the summits of the 

 Colorado Mountains, at altitudes of 2,400-3,000 meters, more abundant atmo- 

 spheric precipitations induce a luxuriant forest growth (Picea Engelmanni). 

 The lower ranges bear forests of Pinus ponderosa and Picea nigra, whilst in 

 the river-valleys poplars, alders, and maples or Abies concolor predominate. 

 The outlying hills, on the other hand, assume a more or less desert- like 

 character, with scattered and stunted juniper and a small oak. The most 

 southerly part of the Pacific Interior forest extends from the tops of the 

 mountains of New Mexico to Texas and to West and North-West Arizona, 

 and in places attains considerable luxuriance. 



The Atlantic forest (Pig. 313), which covers a far greater area than does 

 the Pacific forest, extends where it is broadest over twenty degrees of 

 longitude, and is chiefly coniferous in its northern part and along the coast, 

 but in the interior it is a summer-green broad-leaved forest. The area 

 covered by the Atlantic forest everywhere enjoys a rich rainfall, and the 

 vegetation is correspondingly luxuriant. 



Sargent distinguishes three great provinces in the Atlantic forest, a 

 northern province (2 on Map 4) of Pinus Strobus, Linn., a South Coast 

 province (3 on Map 4) of Pinus palustris, Mill. (P. australis, Michx.), and 

 a zues tern province (4 on Map 4) of broad-leaved forest. 



The West Indian and Mexican tropical forest (D on Map 4) send each 

 of them an offshoot of very slight extent towards Florida (5 on Map 4) 



