Disintegration of Rocks, 

 geographical distribution of rock disintegration. 



Observations over very wide areas have shown that while rock decay ; s 

 most pronounced in warm and moist regions, rock disintegration, accom- 

 panied by the formation of talus slopes and alluvial cones, is most energetic 

 in arid regions and in northern latitudes — that is, where great variations of 

 temperature occur. High mountain tops in all lands are especially exposed 

 to the influences which promote rock disintegration. 



The general absence of great accumulations of shattered rocks in warm, 

 humid regions is undoubtedly due to a great extent to the rapid decay of 

 rock surfaces, but still the generalization that rocks disintegrate most rapidly 

 in regions where great variation of temperature takes place is abundantly 

 sustained by observation. 



In an arid region there is generally a great change in temperature between 

 day and night and between winter and summer, and, besides, both rock decay 

 and stream erosion are retarded. In consequence, subaerial deposits occur 

 in such situations on a scale that is unparalleled in more humid lands. 



In high latitudes the great variation in temperature from season to season 

 promotes the disintegration of rock surfaces, while the low mean annual 

 temperature retards decay. The rank vegetation covering large portions of 

 northern countries and the prevalence of frozen soils and subsoils retard 

 erosion and favor the accumulation of debris. Hence the records of rock 

 disintegration on a vast scale are to be expected in all northern regions 

 where recent glaciation has not taken place. 



OBSERVATIONS IN ALASKA. 



Debris Streams. — Streams of loose, angular debris occur in very many of 

 the high-grade gorges on steep mountain slopes throughout Alaska. These 

 streams of loose stones are especially noticeable on the higher portions of 

 the steep mountain sides along the Yukon. They are lighter-colored than 

 the adjoining moss and lichen covered rocks, owing to the absence of all 

 vegetation upon them. Motion in these streams probably takes place prin- 

 cipally during the winter when they are covered with snow, or in the spring 

 when the snow is meltiug. Many of them are situated where snow accumu- 

 lates most abundantly, and occasionally originate snow-slides and avalancli 

 but the downward movement of the debris is probably due principally to the 

 slow settling or " creep" of deep snow on steep slopes. 



In the glaciated region of southern Alaska, especially on the steep mount- 

 ain sides about the head of Lynn canal, streams of stones of the same char- 

 acter as those noticed in the Yukon region are a conspicuous feature in th< 



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