SUCCESSION 243 



293. Weathering. Practically all primary successions start on soils pro- 

 duced by weathering. This is also true of coral or volcanic islets and of lava 

 beds, for no terrestrial vegetation can secure a foothold upon them until the 

 surface of the rock has been to some extent decomposed or disintegrated. 

 Weathering, as is well known, consists of two processes, disintegration and 

 decomposition, which usually operate successively, though they are some- 

 times concomitant. Disintegration usually precedes, especially in rock 

 masses, and unless it is soon followed by decomposition, results in dysgeo- 

 genous soils. Decomposition often goes hand in hand with disintegration, 

 or it takes place so rapidly and perfectly that it alone seems to be present. 

 In either case, the resulting soil is eugeogenous. The relation of decompo- 

 sition to disintegration determines the size and compactness of the soil par- 

 ticles, and upon the latter depend the porosity, capillarity, and hygrosco-' 

 picity of the soil. These control iti large degree the character of the first 

 vegetation to appear on the soil. 



Another point of fundamental value in determining revegetation is the 

 disposition of the weathered rock. If it remains in situ, it will evidently 

 differ in respect to compactness, homogeneity, nutrition-content, water-con- 

 lent, disseminules, etc., from weathered material which has been transported. 

 An essential difference also arises from the fact that a rock may be weath- 

 ered a long distance from the place where the decomposed particles are finally 

 deposited, and in the midst of a vegetation very different from that found 

 in the region of deposit. The disposition of the weathered material affords 

 in consequence a satisfactory basis for the arrangement of primary succes- 

 sions. The following classification is proposed, based upon the soil groups 

 established by Merrill.^ 



294. Succession in residuary soils. Residuary soils are always sedentary, 

 i. e., they are formed m 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 decomposition causes these 

 differences to be comparatively small. Residuary soils are typically eugeo- 

 genous; 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. Successions in these soils are 

 especially characteristic of shale, sandstone, and limestone ledges or banks. 

 Cumulose deposits, like residuary ones, are sedentary in character, but as 

 they are produced by the accumulation of organic matter, they will be con- 

 sidered under reactions of vegetation upon habitat. 



* Rocks, Rock-weatherin&, and Soils, 300. 1897. 



