I am convinced that nearly all our ancient formations, which are throughout the greater part of their thickness rich in fossils , have thus been formed during subsidence. Since publishing my views on this subject in 1845 , I have watched the progress of geology, and have been surprised to note how author after author, in treating of this or that great formation, has come to the conclusion that it was accumulated during subsidence. I may add, that the only ancient tertiary formation on the west coast of South America, which has been bulky enough to resist such degradation as it has as yet suffered, but which will hardly last to a distant geological age, was deposited during a downward oscillation of level, and thus gained considerable thickness.