THE WEATHERING OF ROCKS 317 
Action of roots. It frequently happens that roots 
find their way into crevices where the expanding force due 
to growth is sufficient to widen and deepen the fissure or 
even to wedge pieces off the rock. ‘The organic acids which 
exude from the roots or rootlets, moreover, have the power 
of dissolving or decomposing many rock ingredients. The 
chemical action of water is increased by the roots holding 
the moisture, and causing it to remain for a longer time on 
the spot. 
Under certain conditions, when for instance the under- 
lying rock is sufficiently close to the surface to be reached 
by the roots, the effects of root-action are very marked. 
The process by which the solution of mineral constituents 
is brought about by the agency of roots, can be observed 
on a small scale by placing on a slab of marble a few seeds 
of, say, cress, and covering these with a thin layer of moist 
earth. As the seeds germinate and the roots spread, the 
acids exuded by the root-hairs have the effect of dissolving 
those portions of the marble with which they come into 
contact. If the earth and seeds be now removed, the 
markings caused by the action of the roots can be seen. 
Norr.—The general process of weathering can be observed in 
most exposures of rock where the surface in colour, hardness, and 
_other properties will be found to differ from the clean fresh 
appearance presented by the same rock when broken into. 
The process should be observed in the field ; no amount of talk- 
ing will convey the same clear and lasting impression as a visit to 
some crumbling rock-mass, where perhaps the entire transition 
from rock to soil can be observed. 
The chemical changes should be illustrated by experiment when 
possible—thus oxidation can be shown by exposing the bright 
surface of a piece of iron (a) to the dry air of a room for a few 
days, and (6) to the moist out-door atmosphere for a couple of 
nights. From this will be learnt not only that oxygen unites 
with iron to form an oxide, but also that oxidation is much more 
rapid in a moist atmosphere—/.e., that water is the vehicle which 
carries oxyyel. 
