DYNAMICAL GEOLOGY discusses the processes now in action upon the earth, whereby changes are made in the structure and composition of the crust; in the relations between the interior and the surface, as shown by volcanoes, earthquakes, and other terrestrial distii•btaces; in the distribution of oceans and continents; in the outlines of the land. and the form and depth of the sea-bottom; in climate; and in the races of plants and animals by which the earth is tenanted. It brings before us all the activities which it is the province of geology to study. The range of operations included within the space of inquiry in this lininch of the science may be regarded as a vast cycle of change, into which we may break at any point and around which we may travel, only to find our selves brought back to the starting point. Before any of the periods of which a record remains in the visible rocks, the chief source of geological action probably lay within the earth itself. The planet still retained much of its initial heat, and was doubtless the theater of great chemical changes, giving rise, perhaps, to manifestations of volcanic energy like those which have so marvelously roughened the surface of the moon. As the outer layers of the globe cooled, and the disturbances due to internal heat and chemical action became less marked, the influence of the sun, which must have always operated, would be relatively more efficient, causing a wide circle of superficial changes wherein variations of temperature and the circulation of air and water over the earth come into play.
While inquiring into the history and the present condition of the earth the geologist must keep his mind open to the reception of evidence for kinds ankdegrees of action which he has not imagined. Human experience has been too short to low the assump tion that the causes and modes of geological changes have been definitely ascertained.
Future discovery may produce evidence of former operations by heat, magnetism. chemical change, or otherwise, which may explain many of the phenomena with which geology has to deal. Of the influences, so many and so profound: which the sun exerts upon our planet, we can as yet perceive but little; nor can we tell what other cosmical influences may have given their aid in the evolution of geological changes. In the present state of our knowledge all the geological energy upon and within the earth must be traced back to the parent sun. There is, however, propriety and convenience in dis tinguishing that part of it which is due to the survival of some of the original energy of the planet, and that part which rises from the present supply of energy received day by day from the sun. In the former case we have to deal with the interior of the earth and its reaction upon the surface; in the latter we deal with the surface of the earth, and to some extent with its reaction on the interior. This distinction affords an oppor tunity to treat the subject under two divisions: I. Hypo,;ene, or Platonic the -changes within the earth caused by original internal heat and by--ch4rnical action.
II. Epigene, or 51.1?;faee 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.
In considering hypogene action we must call to mind a globe still intensely hot in its interior, radiating heat into space, and contracting in bulk. Molten rocks from the interior are from time to time poured out upon the surface; wide areas are raised up or sunk doWn; and in these movements remarkable changes are produced upon the rocks of the crust; they are broken, rendered crystalline, and sometimes fused. (See