Dynamical and theoretical geology, again, inquires into the supposed principles and causes in which all these arrangements have originated, and by which they have been modified. The agencies, processes, and changes which we now observe in the existing course of nature, furnish the grounds and analogies by which alone we can speculate respecting the past condition of things, subject always to the consideration that, in proportion to the difference of effects and changes in the two periods, so are the causes and agencies which produced them, as well in relation to time as to force. The enormous magnitude of the results witnessed during the more ancient geological epochs, demonstrates the intensity of the causes then in operation; and, admitting these causes to be the same in kind with such as prevail at present, we are yet warranted to infer their more violent activity, as likewise to assume a more rapid increment in their effects. Thus the geologist, from observation of the laws of crystallization now manifested in the aggregation of the elementary particles of bodies, reverts to the existence of similar forces, whatever they be, which produced the crystalline texture of the primary rocks, their fissile structure, and the separation of all those materials which exist among them in the form of gems, agates and metalliferous veins. Igneous causes he still finds in operation, as volcanoes, earthquakes, and chemical agencies closely connected with both; and the same forces he hesitates not to connect with the elevation of mountain-chains, the vast masses of plutonic matter everywhere diffused through the earth’s crust, and the disturbance, dislocation, and other changes produced upon all the stratified rocks through which this matter has penetrated. A mighty power in continual action, he further perceives in the waters of the ocean, of the atmosphere, of the rills and rivers that issue from the mountains’ side: and to such aqueous causes, operative in the past as in the present day, he refers the transport, lamination, and detrital structure of the materials of all the sedimentary deposits. Nor does he leave out of estimation the effect of assumed cosmical changes upon the temperature of the planet, and the laws that regulate the distribution of heat over the surface: causes such as these the geologist sees greatly to influence, at present, the conditions of organic life in every quarter of the globe; and hence, he justly calls for their assistance in explaining the history of those remarkable organic remains which characterize the several geological epochs.