Oceanography has to do then, directly or indirectly, with a multitude of sciences and, more than any other, with geology. The present is at the same time the key of the past and of the future, especially in natural history. Man, in his investigations, works from the known to the unknown, from what he can see with his eyes, touch with his hands, measure with his instruments, to that of which he can perceive only the results; from phenomena present before him to those which were accomplished thousands of centuries ago. For a long time geology advanced in a rut out of which oceanography has forced it perhaps a little against its inclination. Old people and old sciences have their habits and a dislike to change, but old sciences, more fortunate than man, can grow young again.
Rocks are of igneous or metamorphic and of sedimentary origin. The former are the object of the researches of a special science, petrography, which studies their intimate nature and all the different branches of knowledge which relate to eruptive phenomena. Stratigraphy deals with rocks of aqueous formation, and, as the genesis of these is intimately connected with the order of their superposition, stratigraphists, in their investigations, consider together the constitution and the order of the sedimentary strata. Now, since these strata have been found beneath the water, nothing is more fitted to make their genesis clear than observation of the manner in which at the present time rocks are being formed on the bottom of our oceans. This task, to which it applies itself with ardor, is the duty of oceanography. When the particular character of the formations on the coasts or in the depth of the sea is known, when careful observation and exact measurement of actual phenomena shall have taught, for example, the necessary relation between the form and dimensions of a grain of sand and the exact velocity of the current which has transported it and affected its shape—angular when supported by force of the water, worn and rounded when simply rolled along the bottom among other grains; as soon as the presence, recognized quantitatively, of a fixed proportion of clay in the midst of a sandy deposit shall allow us, by means of physical and mechanical laws, to determine whether this deposit was formed in calm or agitated water; as soon as numerous measurements, repeated in different parts of the ocean, shall have established the generality of these relations—that is to say, made laws for them—we shall be ready to reconstruct the past. It will be sufficient to find the same characteristics in an old deposit to be able to call established relations to our aid. We may affirm that the point where the deposit was formed was at such and such a depth in the ocean, at such a distance from the shore. If, later, other sciences bring forward their cooperation and point out new relations, all the details will, one after another, appear. We shall then ascertain the size and form of the Silurian, the Carboniferous or the Cretaceous Sea, the force of its waves, its salinity, the temperature of its waters, the intensity and direction
