196 DYNAMICAL GEOLOGY. [Boox TIT. — 
ever open to the reception of evidence for kinds and especially for 
degrees of action which he had not before encountered. Human 
experience has been too short to allow him to assume that all the 
causes and modes of geological change have been definitively ascer- 
tained. Besides the fact that both terrestrial and solar energy were 
once probably more intense than now, there may remain for future 
discovery evidence of former operations by heat, magnetism, chemical 
change, or other agency, that may explain phenomena with which 
geology has to deal. Of the influences, so many and profound, 
which the sun exerts upon our planet, we can as yet only perceive 
a little. Nor can we tell what other cosmical influences may have 
lent their aid in the revolutions of geology. 
In the present state of knowledge, all the geological energy upon 
and within the earth must ultimately be traced back to the primeval 
energy of the parent nebula, or sun. There is, however, a certain 
propriety and convenience in distinguishing between that part of it 
which is due to the survival of some of the original energy of the 
planet, and that part which arises from the present supply of energy 
received day by day from the sun. In the former case the geologist. 
has to deal with the interior of the earth and its reaction upon the 
surface ; in the latter he is called upun to study the surface of the 
earth, and to some extent its reaction on the interior. This distinc- 
tion allows of a broad treatment of the subject under two divisions :— 
I. Hypogene or Plutonic Action—the changes within the 
earth caused by original internal heat and by chemical action. 
Il. Epigene or Surface Action—the changes produced on — 
the superficial parts of the earth, chiefly by the circulation of air and 
water set in motion by the sun’s heat. 
Part I. Hypocrene Action. 
An Inquiry into the Geological Changes in Progress beneath the 
Surface of the Earth. 
In the discussion of this branch of the subject it is useful to carry 
in the mind the conception of a globe still intensely hot within, 
radiating heat into space, and consequently contracting in bulk. 
Portions of molten rocks from inside are from time to time poured 
out at the surface. Sudden shocks are generated by which destruc- 
tive earthquakes are propagated to and along the surface. Wide 
geographical areas are upraised or depressed. In the midst of these 
movements the rocks of the crust are shattered, fractured, squeezed 
crumpled, rendered crystalline, and even fused. 
