Only a small portion of the surface of the earth has been geologically explored, and no part with sufficient care, as the important discoveries made every year in Europe prove.

No organism wholly soft can be preserved.

Shells and bones decay and disappear when left on the bottom of the sea, where sediment is not accumulating.

We probably take a quite erroneous view, when we assume that sediment is being deposited over nearly the whole bed of the sea, at a rate sufficiently quick to embed and preserve fossil remains.