It was not until the end of Mesozoic times, when the dawn of the recent ages had begun, that the Rockies were elevated. No volcanic forces took part in the work. All the rocks that compose them were laid down as sediment on a sea bottom, mud and sand and gravel and the lime of shells accumulating until the beds were five miles or more in thickness and were slowly transformed into slate and quartzite and conglomerate and limestone, the building materials of the mountains that were to be.
What gave the signal for the raising of the new range no one knows, but after the Cretaceous sediments forming the coal-bearing rocks of the prairie provinces had been deposited, the thrust from the Pacific became irresistible, the earth's crust yielded and step by step the thick layers of rock were pushed inwards, rising as folds or breaking off strip by strip, tilted up and riding upon one another like ice-cakes when a great floe is driven ashore. We must not think of this process as taking place suddenly in one mighty convulsion, but very deliberately, a small push with its earthquake shock, followed by a long quiescence before the next instalment of elevation; so that age by age the mountains grew, perhaps are growing even yet.
During all this time of slow growth in height the destructive forces were at work, frost and weather and running water and ice, tearing down the structures just as they do now, though the constructive forces were more than a match for them, at least in the earlier history of the mountains. The present forms of the Rockies are due then to a balance between the upbuilding and the down-tearing influences which have been at work during the past millions of years since the end of Cretaceous times.
Having discussed general causes shaping the mountains, let us turn now to some of the special features. The fundamental structure of the Rockies is simple when
