110 SUCCESSION 



arrangement of primary successions. The following classifi- 

 cation is proposed, based upon the soil groups established 

 by Merrill (1897: 300). 



3. Succession in residuary soils. Residuary 

 soils are always sedentary, i. e., they are formed in situ. 

 They show certain differences dependent upon the rock from 

 which they originate, which may be mixed crystalline shale, 

 sandstone or limestone, but the thoroughness of decomposi- 

 tion causes these differences to be comparatively small. 

 Residuary soils are typically eugeogenous : their successions 

 in consequence usually begin with mesophytes, and consist 

 of a few stages. The soluble salt content is comparatively 

 low, since all soluble matters are readily leached out. Suc- 

 cessions in these soils are especially characteristic of shale, 

 sandstone and limestone ledges or banks. Cumulose depos- 

 its, like residuary ones, are sedentary in character, but, as 

 they are produced by the accumulation of organic matter, 

 they will be considered under reactions of vegetation upon 

 habitat. 



4. Succession in colluvial soils. Colluvial de- 

 posits owe their aggregation solely or chiefly to the action 

 of gravity. They are the immediate result of the disinte- 

 gration of cliffs, ledges and mountain sides, decomposition 

 appearing 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 sur- 

 face wash. Boulder fields are to be regarded as talus pro- 

 duced by weathering under the influence of joints, resulting 

 in huge boulders 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 



