Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/252

242 and violent changes, or the action of causes similar to what we perceive at the present day.

And if we reflect upon the manner in which the most delicate recent shells are buried when thrown upon the shore, in the sand and shingles of a beach, without being much mutilated, we shall not find it difficult to comprehend how very perfect specimens may sometimes be met with even in our gravel.

By attending to the revolutions that have happened to these upper strata, this subject will perhaps be less difficult to understand. Beds of gravel and sand containing fossil shells will be esteemed as the monuments of ancient changes though of different periods. But those shells grouped in families and contained in beds of foliated clay and marl, may be considered as still remaining in their original situations.

By this distinction we shall be able to separate real alluvial fossils, or such as have been washed out of regular strata from those that have never been disturbed; and the various states of violent convulsion, as well as of quiescence, which the ocean must have experienced during the several eras, the geological history of which we have been considering, are sufficient to account for these appearances.

In a work lately published by an eminent fossilist, I have met with an opinion, that all the spoils of terrestrial and submarine productions which we find buried in the strata in this country over the chalk, have been transported from distant climates, and have been deposited in a tumultuous manner by some great convulsion that blended them in one common grave. I should not have thought it necessary to allude to this idea, if I had not understood that it has been considered by some as demonstrating that all the strata over our chalk are alluvial.