PARRY — ALPINE FLORA. 537 



period of winter storms, extending from early autumn to late 

 in the succeeding spring. But when the summer sun lays 

 bare the exposed rocks and smooth alpine slopes, the contrast 

 becomes less marked, and the line itself can be less regularly 

 traced by the eye. Barometric measurements show that this 

 timber line, with slight local variations, marks a very uniform 

 elevation, gradually diminishing with a more northern lati- 

 tude. Thus, Pike's Peak on the south shows a timber line 

 having an elevation of 12,000 feet above the sea level, while 

 Long's Peak, nearly two degrees farther north, marks the tree 

 limit at an elevation of 10,800 feet. Intermediate points, as 

 Gray's Peak and Mount Flora, give a comparative mean of 

 11,700 for their respective timber lines. 



It would be particularly interesting to know whether this 

 timber line, offering so peculiar and well marked a feature of 

 alpine exposures in different portions of the globe, is connected 

 with a certain range of mean annual temperature, and, if so, 

 what is the actual mean thus indicated. But sufficient data 

 have not yet been furnished for the solution of this interesting 

 problem. 



A more simple explanation of the phenomena in question 

 has been suggested by my observations in the Rocky Moun- 

 tains, and I am inclined to refer the limitation of tree growth 

 to a certain range of minimum temperature. The facts es- 

 pecially bearing on this point are these : 



In the first place, there is a very marked distinction in the 

 timber line itself, everywhere easily recognized, and, as we 

 may say, marking at each locality two distinct timber lines, 

 the one of very uniform elevation, consisting of thrifty, often 

 large trees, of upright growth, and which terminates sharply, 

 without any manifest dwarfing or stunting of regular growth, 

 though mainly confined to species of pine that higher up be- 

 come dwarfed and deformed in their struggle with the ele- 

 ments; this first and lowest I would designate as the true 

 timber line. But again above this, and straggling irregularly 

 up sheltered ravines and rocky slopes, is a class of depressed 

 tree growth, singularly deformed, often spread out in dense 

 mats on the ground, forming almost inextricable thickets. At 

 other points, it presents itself to view with blighted tips and 

 twisted, gnarled lower branches, and prostrate trunks, creep- 

 ing snake-like through rocky fissures — everything betokening 

 a struggle for existence. The arborescent forms here repre- 

 sented belong almost exclusively to Pinus aristata and Abies 

 Engehnanui, both of which, as far as known, are peculiar to 

 the Kocky Mountains. The actual elevation of this line being 

 determined altogether by the local peculiarities of surface, as 

 affording necessary shelter, is of course quite irregular and 

 undulating, and may therefore be properly designated as the 

 false timber line. 



