A Lthough we are probably well acquainted with all the chief agents of change, the long-continued action of which has shaped the surface of the earth into mountain and valley, and laid down and metamorphosed the deposits forming those portions of its surface which natural or artificial sections or excavations have enabled us to examine, there still remains considerable difference of opinion, not only as to the particular agent or agents which have formed many deposits, but also as to the rapidity with which changes have been brought about by them in the past.