W. J. UcGee—Dynamical Geology. 491 
and softened, and degradation is thereby accelerated ; by them the 
soft sediments are first lithified and then (sometimes) subsequently 
metamorphosed ; and by them the chemic complexity and structural 
heterogeneity of the terrestrial crust have been largely produced. 
The various processes are processes of alteration; and they comprise 
lithifaction and its converse (decomposition and disintegration com- 
bined) in their various phases, or rock-formation and rock-destruction. 
There is another subordinate category of processes which are 
intimately allied to those of the second great category, viz. glaciation. 
Only two clearly defined periods of extensive glaciation—both late 
Tertiary or Quaternary—are generally accepted, though others are 
recognized by different geologists; in general the tendency of 
glaciation is to obliterate surface-irregularity, both by grinding down 
elevations and filling up depressions, and thus to perfect the geoid ; 
but glaciation may also accentuate pre-existing irregularities of the 
surface, certainly by moraine-building and probably by basin-cutting, 
and must therefore be set apart as a unique agency in the modification 
of the external configuration of the globe. The general process 
comprises glacial construction and glacial destruction. 
The fourth subordinate category includes the effects of aerial 
circulation, directly upon land surfaces, and indirectly through wave 
and current-action under and about water surfaces. These pro- 
cesses have been in operation throughout geologic time, but so 
indolently that little trace of their products is found save among 
recent phenomena; in general the tendency is to reduce elevations 
and fill depressions, and thus to merge into gradation; but there is 
also a tendency to build dunes, beaches, and banks, and to excavate 
both subaerial and subaqueous basins, and thus to produce certain 
minor irregularities of the earth’s surface as well as to perpetuate 
others. The general process is wind action or eolation; and its 
subordinate processes are, like those of other categories, antagonistic. 
There is a final category which is in part allied to alteration, but 
which is in part unique, viz. the chemic, chemico-mechanical, and 
dynamic action of organic life. Hver since the terrestrial crust 
became so stable as to retain a definite record of the successive 
stages of world-growth, life has existed, and by its traces has 
furnished the accepted geologic chronology ; at first the organisms 
were simple and lowly and affected the rocks chemically through 
the processes of growth and decay, as do the lower plants and 
animals of the present; later, certain organisms came to contribute 
largely of their own bodily substance to the growing strata; and 
still later the highest organisms, with man at their head, have come 
to interfere with gradation, alteration, and eolation dynamically, 
and thus to directly or indirectly modify the various interrelated 
geologic processes—indeed it is probable that in populous plains at 
least the several natural processes combined are less potent factors 
in geologic development than human action alone. The vital forces 
are too varied in action to be conveniently grouped and compre- 
hensively named. 
This simple classification of elementary processes, which appears 
