114 G. H. Kinahan — On Denudation, 



cavernous parts of the Carboniferous or other limestones ; and 

 on the former flints will collect, while on lands over limestone, 

 pieces of chert or other hard substances. The calcareous class of 

 rocks, as they are in a great measure chemically formed, seems to be 

 more susceptible than other rocks, and succumbs easily to the chemico- 

 fluvial agencies, the carbonate of lime being carried off in solu- 

 tion, while the insoluble substances, such as flint, chert and clayey 

 matter remain. Besides, during part of nearly every summer, such 

 lands will be scorched and burned by the sun forming part of the 

 decomposed rock (the surface soil) into dust, and leaving it ready 

 to be carried away by the first wind or rain ; but grass land over 

 clays, shales, slates, igneous rocks, and most sandstones, will not 

 " grow stones." 



Engineers are well aware of the protecting power of grass, and 

 they grow it on the slopes of their railway cuttings and embank- 

 ments, by which means all denudation is stopped in two or three 

 years; after which the surface is gradually added to. The latter fact 

 is patent to any one who will "look and learn ;" for the platelayers 

 while repairing the permanent way always throw some of the old 

 bolts, chairs, or rails on the grass slopes, and if a bolt is left there for a 

 year it will be half covered, while after a few more years it will be 

 completely grown over ; for the chairs to disappear it will take five 

 or more years, and the rails proportionally more.^ It may be stated 

 that the covering up of these foreign substances is due to particles 

 of soil carried down the slopes. This, in a few exceptional places 

 may be the case, but in most instances it is due to the hond fide 

 growth of the grass, and it will occur not only on the slopes, but 

 also on flat level waste pieces of ground alongside railways, where 

 such substances are also thrown. It should be mentioned that the 

 growth of grass on railway slopes is unnatural, as previous to sowing 

 grass seed it is necessary to " soil the slopes," i.e. put a coat of 

 vegetable soil on them. Naturally new exposures of drift, or even 

 alluvium, take years before plants vegetate properly on them ; as 

 will be seen in railways where the slopes are not " soiled;" on slob 

 lands reclaimed from the sea, or a drained lake bottom, etc. In all 

 these places, if uncultivated, it will take years before a natural 

 protecting coat of grass will grow. This, apparently, is due to the 

 new soil being incapable of supporting vegetation until the surface 

 is partly decomposed by the atmospheric influences, and this action 

 usually goes on, extremely slowly at first, until plants begin to grow ; 

 afterwards it seems to be accelerated by the vegetation, and gradu- 

 ally to increase year after year ; and eventually all denudation must 

 cease when the vegetable clothing is matured. According to the 

 nature of the soil, the atmospheric influences will have acted to a 

 greater or less depth ; for if it be a stiff clay, they will not make more 

 than three or four inches of vegetable soil before the denudation 

 ceases, while in friable alluvium they will have penetrated to twice 

 or three times that depth. 



1 The disappearance of stones, etc., needs no growth of soil. It has been explained 

 by Darwin " On formation of mould," Trans. Geol. Soc. — [Ed. Geol. Mag.] 



