244 



THE FORMATION 



295. Sucxession in colluvial soils. Colluvial deposits owe their aggrega- 

 tion solely or chiefly to the action of gravity. They are the immediate result 

 of the disintegration of cliffs, ledges, and mountain sides, decomposition ap- 

 pearing later as a secondary factor. The masses and particles arising from 

 disintegration are extremely variable in size, but they agree as a rule in their 

 angular shape. The typical example of the colluvial deposit is the talus, 

 which may originate from any kind of rock, and contains pieces of all sizes. 

 Gravel slides differ from ordinary talus in being composed of more uniform 

 particles, which are worn round by slipping down the slope in response to 

 gravity and surface wash. Boulder fields are to be regarded as talus pro- 



Fig. 61. Talus arising from the disintegration of a granitic cliff; the 

 rocks are covered with crustose lichens. 



duced by weathering under the influence of joints, resulting in huge boul- 

 ders which become more and more rounded under the action of water and 

 gravity. This statement applies to those fields which are in connection with 

 some cliff that is weathering in this fashion ; otherwise, boulder fields are of 

 aqueous or glacial origin. The character of the successions in talus will 

 depend upon the kind of rock in the latter. If the rock is igneous or meta- 

 morphic, decomposition will be slow, and the soil will be dysgeogenous. 

 Successions on such talus consist of many stages, and the formations are for 

 a long time open and xerophytic. In talus formed -from sedimentary rocks, 



