244 THE FORMATION 
295. Succession in colluvial soils. Colluvial deposits owe their aggrega- 
tign 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, 
