1907] 



YOUNG— FOREST FORMATIONS OF COLORADO 



349 



formations 



I 



tremuloides 



been given in detail in the discussion of these formations. Pinus 

 aristata is a southern form not occurring in this region so far as I 

 know. 



+ 



Fig. II.— Timber-line on Mt. Arapahoe, south slope; approximate altitude 

 3450"!; to the right is a small clump of spruce and fir. 



Invasion and succession 



The soil of the mountain sides is the result of disintegration of 

 the talus slopes with which they arc covered. It is a loose gravel 



composed of particles of rotten gra 

 and forms an unstable foothold for 

 its way downward into the canons. 



It is continually working 



defined 



stages wathin any of the forest formations. The first invaders of a 

 new or denuded soil are determined primarily by proximity, second- 

 arily by mobility* On rough talus slopes, where there has been as 



i 

 * 



* 

 V 



