in review, the process of land sculpture was active and all varieties of relief have been shown, even in the same area, in successive cycles of denudation and uplift. Thus New England is everywhere mountainous in structure, although much of its surface is now reduced to hills, and to valleys whose bottoms are not far above base level. The Connecticut Valley, as it now exists, is rather a product of downwear along its course than of uplift on its sides. The same is true of the Hudson, and, indeed, of all the valleys of the Appalachian region. Much of the Archæan core of this Eastern mountain system is well worn, like the coastal parts of New England. Such a region is the Piedmont belt of hills and low mountains lying between the Blue Ridge and the coastal plain in the South Atlantic States. So, too, the Appalachian Mountains of post-Paleozoic age were perhaps raised to Alpine heights, but have been greatly reduced and given their present relief of ridge and valley by processes of wasting and the survival of more resistant formations.
The strata of the Mississippi basin have for the most part never been raised to considerable heights, and the streams have not had sufficient vigor to be the instruments of largest denudation. But among the mountains and plateaus of the Cordilleran region the land forms owe their reliefs to long continued denudation, conditioned by the composition and structure of the rocks. For example, in the Grand Cañon region, or more broadly, everywhere among the Colorado plateaus, vast thicknesses of the upper strata have been bodily removed. Those that remain present south-facing escarpments of ragged outline, and their masses are in turn profoundly dissected by the swift river and its branches. The true measure of denudation is not the cañons, however, but the amount of stripping that has been accomplished over the whole region. Similarly, the lava sheets have been deeply channeled, as by the Snake River, or have often been nearly removed, and volcanic cones are in all stages of decay, from the little marred Mount Shasta to the ‘necks’ that mark the complete disappearance of overlying cones.
The principal reliefs were given to the country prior to the glacial invasion. But within the field of glacial movement important changes were effected, and in some cases the combined effect of the wearing of hill and mountain tops, and of the filling of valleys, was to diminish the total relief by several hundred feet. The territory affected includes all of New England, the Middle States into northern New Jersey and Pennsylvania, and the Central States to lines not far from the Ohio and Missouri rivers. Eastern Nebraska was included, with much of the Dakotas, Montana, and the more northern Cordilleras. Small remnant glaciers are still found in the high Sierras, on the volcanic cones of the Cascades, and in Montana and Colorado. The general effects of the ice sheet were the grinding and transport of rocky waste, coarse and fine, the commingling of this material with the preëxistent soils, the formation of moraines and other bodies of drift during retreat, the blockading of ancient valleys causing innumerable changes of drainage, and the formation of thousands of lakes.
Since the departure of the ice, or during post-glacial time, many of the smaller or more shallow lakes have been filled by sediment, or by deposits of vegetable origin, sometimes forming beds of peat. In the larger lakes deltas and marshy areas have been formed in the same manner. The interrupted streams have resumed their flow along the lowest line of levels that they could find, and in so doing have often cut through the veneer of drift and worn deeply into the underlying rock. This is the origin of most of the gorges of the Northern States. They are young, post-glacial valleys, and the old, buried channels are often to be found not far away. These conditions have not only produced striking scenic results, in the gorges and waterfalls of the Northern States, but have created nearly all the available water power of the same region. Before the ice invasion many streams flowed at lower levels, in the deep and mature valleys of the time. But the obstruction of these valleys has compelled the streams to flow at greater altitudes, and the rocky masses and spurs encountered in their downcutting have caused a concentration of descent in rapids or falls, and thus have made the streams a source of power, while at the same time obstructing the otherwise open ways of commerce.
The soils of the country may roughly be divided into two classes by virtue of their origin. Outside of the glacial region they are known as residual soils, being in the main the product of the local rock disintegrating in place. In this case the more soluble minerals, such as calcium carbonate, have largely been removed. The chief movements of such soils have taken place along the course of rivers and by means of them. The soils of the Lower Mississippi are an illustration of this latter phase, having been gathered from all parts of its basin. The remaining soils are glacial. The ice sheet invaded the residual soils of pre-glacial time, plowed them up, pushed them forward greater or less distances, and thoroughly mingled them with materials often coarse, and mechanically derived from the rocks living in the track of the glacier. These soils, therefore, partake of the variety of the numerous rock masses from which they have been derived.
Fisheries. The United States fishing industry originated in New England shortly after its settlement, and was gaining considerable prominence until England attempted to place restrictions upon it. It also was seriously damaged by the War of 1812, as well as by subsequent conflicts between American and British interests. The extermination of certain species of fish has at various times seemed probable, due to a lack of definite knowledge regarding the habits of the animals, and the fact that there were no limitations placed upon the number which could be captured. Measures have of late years been enacted which to a large extent have obviated these difficulties. (See Fishing Laws.) The United States Fish Commission, by its establishment of hatcheries and its investigations and experiments, also, has aided greatly in the development of the industry. (See Fish; Fisheries; and Fish Culture.) The chief centres of fisheries are naturally those States bordering upon the Atlantic. Pacific, and Gulf coasts and Great Lakes, the Mississippi Valley, and Alaska. The following table shows the status of the industry for the years 1880, 1890, and 1900, the figures
