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454 DYNAMICAL GEOLOGY. [Boos III, — 
south of the limits of the ice of the glacial period, and the inference has 
_ been drawn that even where the surface is now comparatively barren 
the mere existence of this thick decomposed layer affords a pre- 
sumption that it once underlay an abundant vegetation, such as a 
heavy primeval forest-growth.1 Nor is the chemical action con- 
fined to the superficial layers. The organic acids are carried down 
beneath the surface, and initiate that series of alterations which 
carbonic acid and the alkaline carbonates effect among subterranean 
rock-masses (ante p. 348). 
3. Plants insert their roots or branches between the joints of rock 
or penetrate beneath the soil. Two marked effects are traceable to 
this action. In the first place large slices of rock may be wedged | 
off from the sides of wooded hills and cliffs. Liven among old ruins 
an occasional sapling ash or elm may be found to have cast its roots 
round a portion of the masonry and to be slowly detaching it from 
the rest of the wall. In the second place the soil and subsoil are 
opened up to the decomposing influences of the air and descending 
water. ‘The distances to which, under favourable circumstances, roots 
may penetrate downward are much greater than might be supposed. 
Thus in the loess of Nebraska the buffalo-berry (Shepherdia 
argophylla) has been observed to send a root 55 feet down from the 
surface, and in that of Iowa the roots of grasses penetrate from 
5 to 25 feet.? 
4. By attracting rain, as thick forests, woods and mosses, more 
particularly on elevated ground, are believed to do, plants accelerate 
the general scouring of a country by running water. The indiscri- 
minate destruction of the woods in the Levant has been assigned, 
with much plausibility, as the main cause of the present desiccation 
of that region.° 
5. Plants promote the decay of diseased and dead plants and 
animals, as when fungi overspread a damp rotting tree or the carcase 
of a dead animal. 
Animals.—The destructive influences of the animal kingdom 
likewise show themselves in several distinct ways. 
1, The surface soil is moved, and exposed thereby to attack by 
rain, wind, &c. As Darwin showed, the common earth-worm is con- 
tinually engaged in bringing up the fine particles of soil to the sur- 
face. He found that in fifteen years a layer of burnt marl had been 
buried under 8 inches of loam which he attributed to this operation.4 
Jt has been already pointed out that part of the growth of soil may 
be due to wind-action (ante p. 821). There can be no doubt, how- 
ever, that the materials of vegetable soil are largely commingled and 
fertilized by the earth-worm, and in particular that, by being brought 
* Julien, op. cit. p. 378. 
* Aughey’s “ Physical Geography and Geology of Nebraska,” 1880, p. 275. 
_* See on this disputed question the works cited by Rolleston, Journ. Roy. Geog. Soe. 
xlix. (1879). The destruction of forests is also allegcd to increase the number and 
scverity of hail-storms. 
Lrans. Geol. Soc. vy. p. 505. | 
Se 
