490 W. J. McGee—Dynamical Geology. 
displacement of the solid or solidifying terrestrial crust in such 
manner as to produce irregularities in the surface of the globe. 
These are the movements involved in mountain-growth and in the 
elevation of continents; they have been in operation from the 
earliest eons recognized by the geologist to the present time; and 
their tendency is ever to deform the geoid and produce irregularity 
of the terrestrial surface. Such movements have been designated 
“displacement,” “diastrophism,” etc.; but for the present purpose 
they may be called simply deformation, and the quality of the move- 
ment may be characterized as diastatic. They are partly vertical 
(though there is perhaps always a horizontal element) and are most 
easily measured from a fixed datum-plane—such as sea-level—and 
are therefore commonly separated into elevation and depression. 
The second great category of movements comprises the various 
processes of aqueous erosion and deposition initiated by the primary 
deformation of the terrestrial surface. By these processes mountains 
and continents are degraded, and seas and lakes are filled with their 
debris ; they, too, have been in active operation from the dawn of 
geologic history to the present; and they ever tend to restore the 
geoid by obliterating the irregularities of the terrestrial surface 
produced by diastatic movement. Collectively the processes may be 
called gradation; and the antagonistic operations comprehended 
under the term are respectively degradation and deposition. 
A subordinate class of processes by which the rocks of the earth 
are formed or affected is the extravasation of lavas and other volcanic 
matter from beneath the surface, the outflow of subterranean waters 
containing solid matter in solution, and the escape of mineral- 
charged gases, together with the consequent collapse of cavities 
and other crust-movements. These processes have been in operation 
throughout geologic time, though perhaps with diminishing activity ; 
they have added materially to the superficial strata of the earth ; 
and they have modified the geoid not only by addition without, but 
by the commensurate loss within and consequent deformation and 
structural alteration. The various operations are commonly com- 
prehended under the term vulcanism ; and the two subordinate classes 
or processes of antagonistic tendency which it comprises are extra- 
vasation and its converse. The vibratory movements usually called 
seismic probably accompany both diastatic and volcanic movements 
under certain conditions. 
The second subordinate category of processes by which the rocks 
themselves and the operations of the second great category of 
geologic processes are modified, comprises the chemic and chemico- 
mechanical alterations in structure of the earth’s strata brought 
about by the action of percolating water, air, and other gases, the 
effects of frost and other variations in temperature of the exterior 
of the earth, the rise of the isogeotherms beneath the areas of 
degradation, the heat resulting from deformation, etc. These pro- 
cesses have affected the rocks ever since the solidification of the 
planet, but probably with progressively diminishing intensity ; by 
them the rocks exposed to degradation are disintegrated, decomposed, 
