That this great change occurred, and was attended with an obliteration of the wonderful reptilian and avian fauna of the mesozoic age, is most true: that it occurred suddenly does not appear. On the contrary, there is evidence to show that the prodigious folding up of mountain ranges involved could not have proceeded with sufficient rapidity to turn the course of a stream of water. It happened that one of those folds--one which, had no denudation been going on meanwhile, would have lifted its crest higher than the highest peak of the Himalayas--lay directly across the course of the Colorado river. The river held its course uninterruptedly, sawing its way through the uplift until six vertical miles of rocky strata had risen past it. At no time, therefore, could the rapidity of motion in the bulging strata have exceeded the capacity of the river to wear away the obstruction, and the bulge was fifty miles across! We do not know how rapidly a river may sink its channel through such a rising barrier; but we do know that a process of that nature cannot legitimately be described as swift or sudden. And surely it requires not less intellectual far-sightedness and imaginative faculty to carry the mind across the enormous stretch of time involved in such a change slowly wrought--a period during which at least three vertical miles of the rising mountain fold was worn down by rain and atmospheric abrasion--as to mass the continental doming, the mountain folding, and the attendant life changes together as a convulsive "catastrophe."