455 CHAP. XXI. On the successive Forms of the Earth : Revolutions of the Globe. The subject of the present chapter is intimately con nected with a rational theory of the earth. It com prises the history of those revolutions of the surface, previously to its last, or present, condition, which have, at different times, involved the destruction of some, at least, of the organized beings by which it was inhabited. To these revolutions we are in debted for its most striking and important features; as to these it is owing, that the animals of former days are now known to us, and that their vegetables are treasured up for the supply of our wants. If the magnitude of the powers which these changes in volve, and the undefined ages which they demand, are alarming to those whose views have been con fined, by timidity or prejudice, to a narrow circle of obvious facts, let them recollect that nature every where displays the marks of enormous power more than once exerted; and that in the destruction which she every where exhibits, in the equally extensive and tedious successive loss and renewal of races of organized beings, and in many other geological phe nomena which I need not now enumerate, there is implied the necessity of a duration to which we dare not assign a boundary. In preceding chapters, the nature and the ap pearances of strata and of stratification, the changes of position which these have undergone, and the sources of the unstratified rocks, have been so far explained, as to enable the reader to follow without