512 
from beneath to the surface by extravasation ; 
and geologists postulate a constant transfer- 
rence of material beneath by subterranean flow, 
thus completing the cycle of transferrences. 
But transferrence of material laterally and 
vertically does not serve completely to explain 
all the history of geologic movement. Another 
hypothesis is yet necessary ; and this exists in 
the postulate of ever-changing density, arising 
from the following sources: first, changes in 
density due to chemical action, especially as 
exhibited in hydration ; second, changes in den- 
sity due to solidification from the melted state, 
and to liquefaction from the solid state ; third, 
changes in density due to pressure and to re- 
lief from pressure. A consideration of many 
geologic facts has suggested to the writer that 
it may be possible, that, when the rigidity of the 
solid state is overcome by pressure, the rate of 
condensation due to added pressure is in- 
creased at that critical point; or, stated in 
another way, that the passage of rocks from 
the fluid state induced by pressure, to the solid 
state by relief from pressure, is marked by a 
sudden expansion. Should experiments here- 
after give warrant to this conjecture, the chain 
of conditions necessary to the explanation of 
dynamic geology would seem to be complete. 
Early in the history of geologic research, a 
contraction of the earth, due to the loss of 
heat, was postulated to explain the deforma- 
tions of the crust. ‘This loss of heat occurs in 
two ways, — by conduction, and by convection 
from the interior to the surface. The convec- 
tion is accomplished by the heating of subfer- 
ranean waters, and their escape as hot water 
and steam from the multitudinous hot-springs 
and geysers of the world, by the steam dis- 
charged in large quantities from volcanoes, 
and by the lavas which come to the surface to 
be cooled. By this method of convection, cool- 
ing progresses at a high rate; for the lavas 
even of quaternary times are of vast extent, 
and the lavas of all geologic history are corre- 
spondingly vast in amount. How cooling by 
convection is quantitatively related to cooling 
by conduction cannot be stated with our pres- 
ent knowledge ; but, when all of this cooling is 
considered, the rate of condensation is insuf- 
ficient to explain the known displacement — it 
is necessary to resort to other agencies. 
For the fundamental theory of geologic dy- 
namics we have as conditions, first, a fluid 
interior of great specific gravity, in part due to 
compression ; second, a solid crust of irregular 
thickness, not continuous by molecular cohe- 
sion, but composed of small fragments mechani- 
cally arranged, and permeated by water from 
SCIENCE. 
above; and, third, an aqueous fluid and an 
atmospheric gas in motion over the crust. 
The agencies of change may be considered 
as primary and secondary. ‘The primary agen- 
cies are, first, general secular cooling by con- 
duction and convection; second, the heat of 
the sun setting in motion the air and water 
at the surface ; third, the astronomic agencies 
that produce stresses. ‘The secondary agen- 
cies are, first, local heating and local cooling ; 
second, local loading and unloading, having 
an augmented effect at the critical point of 
solidity ; third, chemical reactions arising from 
changes of temperature, pressure, and hydra- 
tion ; fourth, the expansion of water into steam 
by internal and local heating. 
The changes wrought are, first, general 
secular contraction; second, transferrence of 
material horizontally at the surface by aqueous 
agencies, and in the interior of the earth by 
flow, and vertically by subsidence and up- 
heaval, and from within to the surface by ex- 
travasation ; third, change in the chemical and 
lithical constitution of rocks, as seen in various 
forms of metamorphism ; fourth, local lateral 
compression of formations, exhibited in plica- 
tion and implication, and local stretching, 
exhibited in certain parts of flexures. 
Conjointly and severally, the conditions, 
agencies, and changes thus enumerated seem 
to furnish a fundamental geologic theory, in 
harmony with and explanatory of the multi- 
farious facts discovered in geologic research. 
Geologists widely accept the several parts of 
the theory save one; namely, that which as- 
sumes that the solid state is a critical condi- 
tion of volume. The general theory enunciated 
is modified by a multiplicity of minor condi- 
tions, agencies, and changes, to expound which 
a voluminous treatise on geology would be 
necessary. 
The correlation and interdependence. dis- 
covered to exist between volcanism, seismism, 
displacement, surface degradation, sedimenta- 
tion, and metamorphism, furnish important evi- 
dence in favor of the general theory. So far 
as research has progressed, regions of great 
and frequent displacement are found to be 
regions of great degradation and sedimenta- 
tion, of great extravasation and seismism, and — 
of great metamorphism ; while regions of small 
displacement are regions of small degradation 
and sedimentation, of small extravasation and 
seismism, and, so far as known, of small meta- 
morphism. ‘The evidences of correlation are 
exhibited in many and diverse ways. 
The agencies of change enumerated in the 
above theory are interdependent, so that the 
xi 
[Vou. III, No. 64. _ 
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