Fig. 33b.
Fig. 33c.
Fig. 33d.
With the growth of the continent, briefly outlined above, came greater and greater diversity in its relief, due
principally to the upraising of various mountains in a somewhat orderly succession from east to west.
The oldest mountains on the continent are the Laurentian Highlands of eastern Canada. Although the region referred to—the one mentioned above as being composed of Archean crystalline rocks—is not now of sufficient elevation or ruggedness to be termed mountainous, it shows in the nature and structure of its rocks that deep erosion has taken place. The inference is that truly great mountains have been removed, but the evidence may also sustain the interpretation that slow upheaval has been accompanied by erosion, and that at no time was the land conspicuously elevated.
Next in age after the Laurentian Highlands come the mountains of New England and the maritime province of Canada, which were upraised at the close of the Silurian period. The next great step was the crumpling into folds and upheaval of the rocks in the Appalachian region at the close of the Paleozoic era. The Park and Stony Mountains were upraised at the close of the Mesozoic era, and later came the Sierra Nevada and Cascades, followed by the Coast Ranges. Youngest of all, and in part for that reason the boldest and most lofty, are the magnificent mountains of southern Alaska, with a host of sublime peaks, like Mounts Fairweather, Logan, St. Elias, and perhaps McKinley. The last-named and highest peak of all, however, may be of volcanic origin.
In the above list showing the progressive westward movement of the birth of mountain systems, account is taken only of the elevations produced by upheaval. The mountains due to volcanic eruptions, which are still conspicuous, are all young, in comparison with the mountains situated to the eastward of the Sierra Nevada. The majestic cones of the northwestern portion of the United States, of which Mounts Shasta, Hood, Adams, Rainier, Baker, etc., are the most glorious, are of Tertiary or later age. The same is true, so far as known, of the still more lofty volcanoes in Mexico. The "pine-tree" forms of steam rising from the volcanoes of the Caribbees, Central