24 On some Geological Agents. [Sess. 
powers are much greater than that of cold water. Hence we 
must look to them as principal agents in dissolving and re- 
depositing the metals and ores that fill the mineral veins in 
Cornwall, Devonshire, Wales, and many other localities. 
In concluding my observations on water, I will mention in 
further proof of its solvent powers that springs in many parts 
of the world discharge thousands of tons of carbonate and 
sulphate of lime, silica, sulphur, chloride of sodium, carbonate 
of soda, iron, manganese, cobalt, &c.—none of which are found 
in the atmosphere. 
I have before said that decaying or decayed vegetation 
supplied carbonic acid to running water, and so became an 
important agent in the destruction of various rocks. The thin 
crust of lichen that incrusts the surface of the rock and the 
tiny moss that grows in the crevice are agents of change by 
harbouring moisture, which, as I have pointed out before, 
dissolves the union of the rock-building substances that go to 
make what we call granite, basalt, &c. Trees, by the bursting 
power of their roots, especially on hillsides, are agents of 
change in a small way; and no doubt the overthrow of a 
forest by the storm has been the beginning of many of our 
peat-bogs. 
But if plants are agents of destruction and dissolution, they 
are also agents of protection and reconstruction. Plants afford 
great protection to the surface of the earth against the action 
of rain and running water. A good covering of grass, heath- 
wood, &c., binds and secures the surface-soil from being carried 
away, except where a stream has cut a channel or bed for 
itself, which is only natural drainage. Plants are builders too ; 
coralline and chara secrete lime, diatoms silica, and almost all 
plants secrete one or more mineral substances; while the 
sphagnum or bog-moss contributes largely to the formation of 
our peat-beds. So you see plants have a compensating action: 
in destroying the rock, they are only supplying us with fresh 
soil—less what may be carried off by water, and even this 
loss is compensated for by the protection they give against a 
greater loss. 
Water as a Solid, or Ice.—This is a giant that has done mighty 
work in our land in the past. He is still active and powerful, 
working and producing changes every day that the thermometer 
