316 DYNAMICAL GEOLOGY. —- [Boox II. 
to which the inclination rises, and from which it “hades,” is the — ; 
upthrow side. The details of these features of geological structure 
are reserved for Book IY. Part VI. 
Part I].—EPIGENE oR SURFACE ACTION. 
On the surface of the globe and by the operation of agents work- 
ing there the chief amount of visible geological change is now 
effected. This branch of inquiry is not involved in the pre- 
liminary difficulty regarding the very nature of the agents which 
attends the investigation of plutonic action. On the contrary, the 
surface agents are carrying on their work under our very eyes. We 
can watch it in all its stages, measure its progress, and mark in many 
ways how well it represents similar changes which for long ages 
previously must have been effected by similar means. But in the 
systematic treatment of this subject a difficulty of another kind 
presents itself. While the operations to be discussed are numerous 
and often complex, they are so interwoven into one great network 
that any separation of them under different subdivisions is sure to 
be more or less artificial, and is apt to convey an erroneous im- 
pression. While, therefore, under the unavoidable necessity of 
making use of such a classification of subjects, we must. bear always 
in mind that it is employed merely for convenience, and that in 
nature, superficial geological action must be viewed as a whole, since 
the work of each agent has close relations with that of the others, 
and is not properly intelligible unless this connection be kept in 
view. 
The movements of the air; the evaporation from land and sea; 
the fall of rain, hail, and snow; the flow of rivers and glaciers; the 
tides, currents, and waves of the ocean; the growth and decay of 
organized existence, alike on land and in the depths of the sea ;—in 
short, the whole circle of movement, which is continually in progress 
upon the surface of our planet, are the subjects now to be examined. 
It would be desirable to adopt some general term to embrace the 
whole of this range of inquiry. For this end the worde pigene may 
be suggested as a convenient term, and antithetical to hypogene, 
or subterranean action. 
The simplest arrangement of this part of Geological Dynamics 
will be into three sections :— 
I. Air.—The influence of the atmosphere in destroying and 
forming rocks. 
If. Water.—The geological functions of the circulation of water 
through the air and between sea and land, and the action of the sea. 
Il. Life—The part taken by plants and animals in preserving, 
destroying, or originating geological formations. 
The words destructive, reproductive, and conservative, employed in 
describing the operations of the epigene agents, do not necessarily 

