AGATES, CARNELIANS, AND JASPERS. 155 
soluble in surface waters; so that when rain falls upon the 
ground, it usually takes up these acids in some variable 
but usually small proportion. Armed with these Humus 
Acids the surface waters gradually percolate downward into 
the rocks, slowly setting up important chemical changes as 
they go. In this, and indeed in many other natural pro- 
cesses with which the geologist has to deal, the essential 
feature appears to be that the work shall be carried on with 
the aid of very weak solutions, whose operation is prolonged 
through very lengthy periods of time. The weak solutions 
of Humus Acids in surface water slowly dissolve out certain 
of the constituents of the rock, and gradually transfer them, 
in very minute quantities at a time, to lower and lower levels 
within the solid rock. One of the earliest substances to be 
set free from the solid state is soda, which in an andesite 
may form as much as 7 per cent. of the whole rock. Now, 
even in domestic matters, it is well known that cut glass 
which has been kept in contact with water containing soda, 
for even a few days, becomes sensibly roughened. This is 
only another way of stating the fact that the surface of the 
glass has been slightly dissolved away by the soda solution. 
Chemists, dealing with the same subject in another way, are 
only too well aware of the fact of the solvent action of alka- 
line water upon glass; and they know that there will always 
be a certain quantity of silica in solution in alkaline water 
which has been left even a short time in a glass vessel. 
What happens to the artificial silicate, glass, when acted 
upon by water containing an alkali, happens also with the 
natural silicates, of which lavas mainly consist. It is true 
that in the course of a human life-time the quantity of rock 
dissolved by this means may be so small as to be almost 
imperceptible ; yet, given time enough, these alkaline solu- 
tions, originating through the action of bacteria, are competent 
to bring about the decomposition of whole mountain masses. 
As a matter of fact, they have been the chief agent con- 
cerned in accomplishing that end many times over in the 
world’s history. 
One of the difficulties that will occur to most people in 
thinking over this matter is how it comes about that water 
can percolate through the solid mass of a hard and close- 
