1898-99. | On some Geological Agents. 21 
V.—ON SOME GEOLOGICAL AGENTS. 
With ILLUSTRATIONS OF THEIR Work. 
By Mr A. CAMPBELL. 
(Read Feb. 22, 1899.) 
I HAVE here a few specimens of rocks as illustrations of the 
work of some geological agents that are daily changing and 
modifying the surface of the earth. The specimens are by 
no means all that I could wish; but where the material is 
from 24 to 3 times heavier than water, transport and time 
become a matter of consideration, 
And the agents: as the most important and universal 
geological agent, I will take water first. This agent operates 
in various ways—as vapour; as running water, doing a large 
amount of mechanical work ; chemically, as a solvent of great 
power in combination with carbonic acid or the carbonates 
of sodium or potassium ; and, if I may use the term, as an 
explosive force in the form of ice. 
The mechanical action of running water can be easily 
studied, even on the public road, during a heavy shower of 
rain. The mud—it was dust an hour ago, road metal a week 
past, and hard basalt or granite in the quarry before that— 
is now being swept off the surface and carried away by the 
little rills that have collected into two small streams on the 
sides of the road. These in their downward progress will 
join or be joined by other streams, gradually swelling in 
volume and strength, and in the course of a few miles flow 
into the main stream of the valley, the river which will 
ultimately deposit the mud in the sea. But this loose material 
—mud, sand, gravel—has been doing geological work as it 
travelled along with the stream, abrading the rocks and stones 
over which it passed, making more loose matter to be carried 
seawards in suspension. But a river in flood carries forward 
an enormous amount of matter apart from the finely divided 
mud or clay that discolours its waters. Sand and gravel and 
stones are rolled along the bottom, grinding and reducing 
each other into materials similar to what is carried in sus- 
