ry = - . r = - — 
340 ' DYNAMICAL GEOLOGY. — [Boox IIL — 
matter, but that wheat requires from 4 to 8 per cent.. To a 
geologist this organic matter has much interest, as the source of most 
of the carbonic acid by which so wide a series of changes is worked 
by subterranean water. The inorganic portion of soil, or still 
undissolved residue of the original surface rock, varies from a loose 
open substance with 90 per cent. or more of sand, to a stiff cold 
retentive material with more than 90 per cent. of clay. When this 
sand and clay are more equally mixed they form a “loam.” 
Reference has just been made to the thick accumulation of 
rock decomposed 7 sité observable in certain regions which, having 
been above the sea for a lengthened period, have been long exposed 
to the action of weathering. Where this action has been supple- 
mented by that of rain, widespread formations of loam and earth 
have been gathered together. These are well illustrated by the 
“ brick-earth,” “head,” and “rain-wash” of the south of England— 
earthy deposits, sometimes full of angular stones, derived from the 
subaerial waste of the rocks of the neighbourhood.? 
2. Mechanical Action.—Besides chemically corroding rocks 
and thereby loosening the cohesion of their particles, rain acts 
mechanically by washing off these particles, which are held in 
suspension in the little rain-runnels or are pushed by them along 
the surface. The amount and rapidity of this action do not depend 
merely on the annual quantity of rain. A comparatively large 
rainfall may be so equably distributed through a year or season 
as to produce less change than may be caused by a few heavy rain- 
storms which, though inferior in total amount of precipitated 
moisture, descend rapidly in great volume. Such copious rains, by 
deluging the surface of a country and rapidly flooding its water 
courses, may transport in a few hours an enormous amount of sand 
‘and mud to lower levels. Another feature to be kept in view is the 
angle of declivity: the same amount of rain will perform vastly 
more mechanical work if it ean swiftly descend a steep slope, than if 
it has to move tardily over a gentle one. 
Removal and Renewal of Soil.—Elie de Beaumont drew 
attention to what appeared to be proofs.of the permanence or lone 
duration of the layer of vegetable soil.* But the cases cited by him 
are not inconsistent with the doctrine that the persistence of the 
soil is true rather of the layer as a whole than of its individual 
particles.« Were there no provision for its renewal, soil would 
' Johnston’s Llements of Agricultural Chemistry, p. 80. 
? See Austen, Q. J. Geol. Soc. vi. p. 94, vil. p. 121; Foster and Topley, op. cit. xxi. 
p. 446. The vast extent of some superficial formations, like the “loess” above (p. 322) 
referred to, has often suggested submergence below the sea. But when, instead of marine 
organisms, only terrestrial, fluviatile, or lacustrine remains occur in them, as in the 
brick-clays and loess, the idea of marine submergence cannot be entertained. The 
remarkable “tundras” or steppes of Siberia, and the “black earth” of Russia, are 
examples of such extensive formations, which are certainly not of marine origin, but 
point to long-continued emergence above the sea. See Murchison, Keyserling, and De 
Verneuil’s “ Geology of Russia.” Belt. Q. J. Geol. Soc. xxx. p. 490; also posted, p.458. 
5 “Tecons de Geologie Pratique,” i. p. 140. 
* Geikie, Trans, Geol. Soc. Glasgow, iii. p. 170. 

