JPirsson and Rice — Geology of Tripyramid Mountain. 271 



these gentlemen we desire to express our indebtedness for the 

 use of this material. The western two-thirds of the map has 

 been made chiefly from these sources, the eastern third is from 

 the older state map. We have added a few corrections of our 

 own. 



Nearly everywhere the mountain, and also for the most part 

 the surrounding area, are covered with a dense forest growth. 

 On the slopes of the mountain and on its top this is composed 

 of a thicket of small spruce trees which rise through a floor 

 mat composed of intermingled dead and fallen tree trunks, 

 more or less decayed, accumulations of spruce needles, shrubs 

 and moss, into which one often sinks to the waist, and through 

 which progress is extremely difficult. On the lower slopes the 

 kinds of vegetation are somewhat different but the character 

 of the thicket remains the same and the mantle is often 

 swampy in addition. Were it not for the slides and the chan- 

 nels of the streams running from them, the underlying rocks, 

 except on the summit, would be completely concealed by this 

 vegetable growth and deposit, which on a rainy day, to one 

 immersed in it, calls to mind Darwin's description of Terra 

 del Fuego. 



The Slides. — The most interesting features of the mountain 

 are what are locally known as the "slides." These are two tre- 

 mendous landslides, or avalanches, which have occurred, one 

 on its north, the other on its south slope. The North Slide has 

 left a bare face of underlying rock extending from the narrow 

 Ravine of Avalanches, which separates the mountain from the 

 next peak to the north, upward for half a mile along the slope 

 and with over a thousand feet of elevation, with an average 

 angle to the horizontal of 30°. Starting at a point not far 

 below the summit, it gradually widens until at its base nearly 

 the whole north face of the mountain, up into the head of the 

 Ravine of Avalanches, is exposed. The naked rock surface left 

 by this, which is about as steep and smooth as one can comfort- 

 ably climb upon, is interrupted here and there by piles and 

 trains of rock debris and lines of small trees and shrubs grow- 

 ing in crevices. The most conspicuous lanes of rock face 

 exposed are separated from several minor similar ones east of 

 them on the north slope and these from each other, by long 

 strips of soil and forest. The exposed rock of these smaller 

 eastern lanes appears quite weathered. Minor slides have also 

 occurred from the opposite slope of the neighboring elevation 

 into the Ravine of Avalanches, which appears to be well named; 

 see fig 1. A view of the chief double lane of sliding of 1885 

 is seen in fig. 3, taken from the opposite mountain side by 

 Prof. E. L. Rice. Small drainages pass down these lanes and 

 empty into Avalanche Brook, which heads below. 



