Broadly considered, the continental sediments are all those which have been deposited elsewhere than in the sea. By this definition we must include among them not only the deposits made by rivers and other agencies working on the land, but also the sediments laid down in lakes and marshes. Between marine and continental types there is, of course, no sharp line of demarkation, but a transition zone occupied by the deposits made in estuaries, tidal lagoons, and sounds, on beaches, deltas, etcetera.
It would be gratifying if we could establish a few simple rules for distinguishing between marine and continental deposits; but with certain exceptions this is impracticable. There are but few satisfactory criteria of general application for separating continental from marine sediments.
In the past reliance has often been placed on the fossils; but in the continental sediments fossils are generally wanting, and not uncommonly they are rare even . . .