CHAPTER IV.. Of the Causes of Geological Phenomena.

OF THE CAUSES OF GEOLOGICAL PHENOMENA.

An exhibition of the composition and structure of the earth, together with an account, as far as there is reliable evidence, of the modifications which they have undergone, has been the object of the preceding chapters. They are mainly a collection and classification of observed facts. No reference has been made to causes or modes of operation, except in a few cases where it was necessary in order that a statement or description, might be intelligible.

If the facts have been given with sufficient clearness and detail to convey a correct general idea of the crust of the earth, we are prepared to inquire what are the agencies employed, and how they have operated in producing it. It is the province of the geologist to question every known power in nature, and to ascertain what geological effects each one is now producing; and, observing what effects are produced by given causes, he is to judge of the causes which have produced like effects in past geological periods.

Some of these causes are in their nature limited, and effects can be referred to them only within those limits. Thus, the congelation of water expands it by a certain proportion of its volume, and beyond that it can have no effect. But the expansive power of steam varies with the temperature; and hence the effects referred to it may be equally varied. Thus, we are not to expect exact uniformity of results in all past times, but the results will vary only as the circumstances vary upon which the operation of these causes depends.

Geological causes, in most instances, operate with extreme slowness; and therefore it will require a series of observations, continued for a long time, to ascertain what are the capabilities of these causes. But a single instance of their effects proves their capabilities thus far. Hence, one instance of the deposition of a stratum of salt in a salt lake; of the filling of a fracture with fluid lava; of a volcanic eruption, like that of Iceland in 1783; of the subsidence of a volcanic mountain, as that of Papandayang in Java; or of the rising of a large area of land, as in Sweden, as fully proves that natural causes exist capable of producing these effects, as if the effects were produced daily. As these effects increase in number, and careful observations are made and authentic accounts preserved, the means of correctly explaining geological phenomena will increase. The causes thus far known are Atmospheric Causes, Chemical Action, Organic Agency, and Aqueous, Aqueo-glacial and Igneous Action.