24 UNITED STATES AND MEXICAN BOUNDARY. 
the most arid tracts, and serves to give a somewhat stimulating property to the atmosphere. 
This odor is due not only to the abundance of artemisias, but to a great variety of plants, 
especially those of the Labiate order, such as species of Salvia and Audibertia. Most of the 
Composite found here are more or less aromatic. In all cases the aroma seems to depend upon 
a resinous exudation, which probably performs the office of checking evaporation, by closing 
the pores of the leaves, and thus enables the vegetation of these arid tracts to survive the long 
continued dry season. i т 
The effect of the peculiarities of the climate upon the vegetation of this region may be noticed 
here. The singular alternations of wet and dryness, heat and cold, produce a confused blending 
of seasons. Upon the immediate coast most of the annuals and those perennial plants having a 
succulent nature make their growth during the winter months, that being the season of rain. 
In this latitude vegetable growth meets with but very little impediment from cold, as in these 
months the temperature rarely falls below the freezing point. When the Novem7er rains com- 
mence falling vegetation puts on its spring aspect, every barren hill is soon clothed in green, and 
by midwinter flowers are blooming in profusion everywhere, and many have even passed their 
brief season and have gone to seed. At this time the larger trees and deciduous shrubs drop 
their leaves, and only resume them at the close of the rainy season. This seeming exception to 
the general activity of vegetable life during the winter months is accounted for by the fact that 
the class of plants alluded to is almost exclusively confined to the margins of streams, hence 
their growth depends less upon moisture from the atmosphere. As dryness advances, during 
the months of May and June, all the evanescent forms are swept away, and the profusion of 
spring flowers gives place to the scanty products of the arid summer. Finally, these two yield 
to a still more sparse autumnal growth, which is mainly confined to the courses of streams. 
On the mountains, however, we find the alternation of seasons more like that of temperate 
climates. Here there is a winter sufficiently cold to prevent and a summer sufficiently moist to 
favor vegetable growth, which continues throughout the latter season. The scanty vegetation 
upon the eastern side of the mountains and along the Colorado is but partially watered by the 
uncertain showers of summer, and depends mainly upon the regular overflowing of the river. 
As the waters recede, the inundated places are speedily covered by a very rapid growth. 
An enumeration of the plants observed, with precise and detailed accounts of the most 
interesting among them, will be found in the accompanying memoir by Professor John Torrey. 
AGRICULTURAL CAPACITIES. 
We will conclude this general sketch by briefly considering the agricultural capacities of the 
country, as indicated by its general geological features, its climate, and the natural botanical 
productions. 
Commencing at the coast, we notice a prominent wall of high tertiary stuff abutting on the 
sea. This portion, which is thinly clad with verdure at any season, presents an uninviting 
aspect. Hence it happens that to the traveller who views it from the sea it is forbidding in 
the extreme. Аба variable distance inland, however, where we find the line of settlements, 
the rounded hills are covered with a deep rich loam, which in the spring produces a luxuriant 
crop of wild oats. The river margins of this section are also of the same fertile character, and 
