Ifo.l.] GRAY AND HOOKER ON THE ROCKY MOUNTAIN FLORA. 3 



Our sketch must be, like our observations, a rapid and cursory recon- 

 naissince, noting some features wliicli arrested our attention, drawing 

 some comparisons, and suggesting inferences which seem to us probable. 



The phytogeography of the temperate portion of the North American 

 continent, in broad outlines is evidently this : An Atlantic forest region; 

 a Piciflc forest region ; and, between the two, the wide interior, mainly 

 non-forest, region — the special subject of our essay ; a region not easy to 

 name nor to describe succinctly, but of which the eastern half is a vast 

 woodless plain, gradually and evenly rising, so that its western margin 

 is about 5,000 feet above the sea-level ; then a mountain belt, the high- 

 est ridges and peaks of which rise from 11,000 to 14,400 feet ; then, shut 

 out from moisture by these mountains on the east and the Sierra on the 

 west, an arid interior district of plains, at an average of 5,000 feet 

 above the sea. This is mainly desert, and is traversed by many mount- 

 ain ranges, generally of north and south direction, and reaching an ele- 

 vation of 9,000 or 10,000 feet, or rarely higher. This whole interior, of 

 miles average breadth — like other great interiors not very exceptionally 

 favored — is marked by the scantiness or absence of arboreal vegetation 

 and of rainfall, the former being in great measure dependent on the lat- 

 ter. Its plains are treeless excei)t along water-courses ; the mountains 

 bear trees along sheltered ravines and on their higher slopes, upon 

 which there is considerable condensation of moisture ; but, whenever 

 the-y rise to a certain height (about 11,000 feet in latitude 37° to 41°), 

 they are woodless from cold and other hardship attending elevation, 

 although they enjoy an abundant condensation of moisture, mostly in 

 the form of snow. 



The Eocky Mountain region may be therefore divided vertically into 

 three botanical districts : 



1. An arid and woodless district, which occuj)ies far the greater part 

 of tlie area. 



2. A wooded district, in some places covering, in others locally adorn- 

 ing, the mountain slopes. 



3. An alpine uu wooded district t/oove the belt where trees exist. But 

 in some places, slopes woodless from dryness merge into tracts woodless 

 from cold, no proper forest belt intervening. 



These three botanit^al districts may be separately investigated. 

 The smallest in area — since it is restricted to mountain summits and 

 the least peculiar, is — 



I. — The Alpine Eegion. 



Botanically the alpine regions of the temperate zone in the northern 

 hemisphere are southward prolongations of arctic vegetation, almost 

 pure in the boreal parts, but more and more mixed with special types in 

 lower latitudes, these special types being a part of the flora which is 

 characteristic of each continent in those latitudes. 



