SURFACE FEATUEES. 91 



treatment, and permit four general divisions of the area: 1. The northeast- 

 ern, including the Mount Lincoln massive, which, as shown in Plate IX, 

 stands out quite by itself. 2. The middle- eastern region, or from Buckskin 

 to Horseshoe gulch, inclusive. 3. The southern, including both sides of 

 the range south of the line of Horseshoe and Empire gulches. 4. The 

 northwestern division, including the area on the west side of the range north 

 of the line of the Leadville map ; the middle area, which comes within the 

 limits of this map, being described in a separate chapter. Each of these 

 four divisions presents a general type of geological structure peculiar to 

 itself. 



The numbers after rock descriptions are the catalogue numbers of the 

 specimens in the Leadville collection of the United States Geological Sur- 

 vey. 



Surface features. The whole region treated of in this report may be divided 

 as regards its general supeificial characteristics into three belts or zones: 

 (1) The bare summits and high ridges above timber-line; (2) the belt of 

 forest growth covering the mountain slopes below timber-line ; (3) the open 

 grass-grown and treeless valleys. 



The elevation of timber-line can only be given in a most general way 

 as the average height at which tree-growth stops on the spurs where sur- 

 face conditions are favorable.' The bare glacial amphitheaters in the in- 

 terior of the range and the almost perpendicular walls of the canons present 

 conditions unfavorable to tree-growth even at points below the timber-line, 

 in spite of which the line is often well marked. Below an average elevation 

 of 11,700 feet the flanks of the mountains are. covered with coniferous trees 

 of the more hardy Alpine varieties, such as the Douglas fir and Engelman 

 spruce, which in favorable situations often form a dense forest by no means 

 easy to traverse, owing to the abundance of dead and fallen trunks, relics 

 of former forest fires. The lower limit of tree growth is even more sharply 

 defined; not, however, by its elevation above sea-level, but by the change 

 of surface slope to the low angle which characterizes the valleys. Whether 

 it be the bottom of a little mountain stream, a hundred feet wide, or the 

 broad expanse of the South Park, almost as many miles in extent, the down- 

 ward spread of forest growth is arrested with equal suddenness, provided 



