(on - 
. Fes 
"Parr IL. Secr. iii. § 1] ACTION OF PLANTS. 453 

humus, or organic portion of vegetable soil, consists of the remains 
of plants and animals in all stages of decay, and contains a complex 
series of organic compounds still imperfectly understood. Among 
these are humic, crenic and apocrenic acids. The action of these 
organic acids is twofold. (1.) From their tendency to oxidation they 
exert a markedly reducing influence (ante p. 832). Thus they con- 
vert metallic sulphates into sulphides, as in the abundant pyritous 
incrustations of coal-seams, shell-bearing clays, and even some- 
times of mine timbers. Metallic salts are still further reduced 
to the state of native metals. Native silver occurs among silver 
ores in fossil wood among the Permian rocks of Hesse. Native 
copper has been frequently noticed in the timber props of mines; 
it was found hanging in stalactites from the timbers of the 
Ducktown copper mines, Tennessee, when the mines were re-opened 
aiter being shut up during the civil war. Fossil fishes from the 
Kupferschiefer have been encrusted with native copper, and fish teeth 
have been obtained from Liguria completely replaced by this metal. 
(2.) They exert a remarkable power of dissolving mineral sub- 
stances. This phase of their activity has probably been undervalued 
by geologists... Experiments have shown that many of the common 
minerals of rocks are attacked by organic acids. ‘There is reason to 
believe that in the decomposition effected by meteoric waters, and 
usually attributed mainly to the operation of carbonic acid, the initial 
stages of attack are due to the powerful solvent capacities of the 
humus acids. Owing, however, to the facility with which these acids 
pass into higher states of oxidation, it is chiefly as carbonates that 
the results of their action are carried down into deeper parts of the 
crust or brought up to the surface. Carbonic acid is no doubt the 
final condition into which these unstable organic compounds pass. 
During their existence, however, they attack not merely alkalies 
and alkaline earths, but even dissolve silica. The relative proportion 
of silica in river waters has been referred to the greater or less 
abundance of humus in their hydrographical basins,” the presence of 
a large percentage of silica being a concomitant of a large proportion 
of organic matter. Further evidence of the important influence of 
organic acids upon the solution of silica is supplied by many siliceous 
deposits (p. 463). 
Wherever a layer of humus has spread over the surface of the 
land, traces of its characteristic decompositions may be found in the 
soils, subsoils and underlying rocks. Next the surface the normal 
colour of the subsoils is usually changed by oxidation and hydration 
into tints of brown and yellow, the lower limit of the weathered 
_ zone being often sharply defined. It has recently been proposed to 
ascribe mainly to the operation of the humus acids the thick layer of 
_ decomposed rock above (p. 338) noticed as observable so frequently 
1 This has recently been strongly insisted upon by A. A. Julien in a memoir on 
the Geological Action of the Humus Acids. Amer. Assoc. 1879, p. 311. 
2 Sterry Hunt’s “ Chemical and Geological Essays,” pp. 126, 150. 
