274 Pirsson and Sice — Geology of Tripyramid Mountain. 



small amount of debris spread down the ravine below, com- 

 pared with the extent of surface denuded. 



The condition which has caused the slides appears to be the 

 steep and smooth rock surfaces on which the accumulating lay- 

 ers of largely organic deposits rested. When these became 

 heavy enough, at a time when they were saturated with water 

 after long and torrential rains, which also lubricated the under- 

 lying rock surface, they broke away and slid down. The steep- 

 ness and smoothness of the bed rock is occasioned by a certain 

 sheeting which it possesses and which is discussed later. Judg- 

 ing from the conditions, and from what has occurred, it seems 

 possible that other slides may occur in the future. The S$uth 

 Slide is in essential respects, as to size, height, etc., rpiite com- 

 parable to the North one, only in this case the thickness of 

 the debris of soil, rocks, etc., which moved, was apparently 

 greater. Thus the underlying rock is exposed only at the upper 

 part of the slide, the earth mantles the middle part and 

 increases in thickness as one descends, while the glen below in 

 which the Slide Brook heads is choked with the accumulated 

 material that moved down into it. This lower part has the 

 hummocky surface characteristic of landslides and is furrowed 

 by shallow ravines which the drainage has cut into it. Many 

 large blocks of rock, some of them 8-10 feet long, are exposed 

 in these ravines; most of them are of syenite from the mountain 

 above, but others are of black trap, porphyritic granite, dark 

 gabbro, etc., and are evidently transported glacial erratics. This 

 avalanche occurred on Oct. 4th, 1869, and a second one on 

 Aug. 13th, 1885, as a sequence to the terrible downpour which 

 also caused the largest North Slide. It can be just seen in 

 fig. 1 to the right as a white line showing through the dark 

 forest. An admirable account of these slides and the causes 

 which produced them has been given by Mr. A. A. Butler.* 



History. — The first mention of the geology of Tripyramid 

 Mountain that we have been able to find is in a description 

 of the South Slide by Prof. G. H. Perkinsf written shortly 

 after its occurrence. The mountain is called by him Passa- 

 conaway ; there appears at that time to have been some, con- 

 fusion in regard to the use of this name, and later it became 

 fixed to the mountain east of Tripyramid, which now bears it. 

 In his description of the slide he states that the upper part of 

 the mountain is composed of a gray syenite. As this term 

 was then used it bore reference to the fact that the rock 

 contained hornblende, it did not mean that it was free from 

 quartz, or nearly so. The name, however, proves correct as 



* Appalaclria, vol. iv, No. 3, p. 177, 1886. 

 f This Journal (2), vol. xlix, p. 158, 1870. 



