gaps, and breaches of continuity that could not be bridged over.  Everywhere they found themselves conducted abruptly from one system of deposits to others totally different in mineral character or in stratigraphical position.  Everywhere they discovered that well-marked and easily recognisable groups of animals and plants were succeeded, without the intermediation of any obvious lapse of time, by other assemblages of organic beings of a different character.  Everywhere they found evidence that the earth’s crust had undergone changes of such magnitude as to render it seemingly irrational to suppose that they could have been produced by any process now in existence.  If we add to the above the prevalent belief of the time as to the comparative brevity of the period which had elapsed since the birth of the globe, we can readily understand the general acceptance of some form of catastrophism amongst the earlier geologists.

As regards its general sense and substance, the doctrine of catastrophism held that the history of the earth, since first it emerged from the primitive chaos, had been one of periods of repose, alternating with catastrophes and cataclysms of a more or less violent character.  The periods of tranquillity were supposed to have been long and protracted; and during each of them it was thought that one of the great geological “formations” was deposited.  In each of these periods, therefore, the condition of the earth was supposed to be much the same as it is now—­sediment was quietly accumulated at the bottom of the sea, and animals and plants flourished uninterruptedly in successive generations.  Each period of tranquillity, however, was believed to have been, sooner or later, put an end to by a sudden and awful convulsion of nature, ushering in a brief and paroxysmal period, in which the great physical forces were unchained and permitted to spring into a portentous activity.  The forces of subterranean fire, with their concomitant phenomena of earthquake and volcano, were chiefly relied upon as the efficient causes of these periods of spasm and revolution.  Enormous elevations of portions of the earth’s crust were thus believed to be produced, accompanied by corresponding and equally gigantic depressions of other portions.  In this way new ranges of mountains were produced, and previously existing ranges levelled with the ground, seas were converted into dry land, and continents buried beneath the ocean—­catastrophe following catastrophe, till the earth was rendered uninhabitable, and its races of animals and plants were extinguished, never to reappear in the same form.  Finally, it was believed that this feverish activity ultimately died out, and that the ancient peace once more came to reign upon the earth.  As the abnormal throes and convulsions began to be relieved, the dry land and sea once more resumed their relations of stability, the conditions of life were once more established, and new races of animals and plants sprang into existence, to last until the supervention of another fever-fit.