Evidence of the nature just considered is lacking, or, more correctly, surveys and soundings which would perhaps reveal the presence of submerged river channels have not been made about the shores of the more northerly portion of the continent, but instructive results in this connection are to be expected when that region is thoroughly studied. On the Pacific coast several transverse channels in the continental shelf, similar to the submerged valleys of the Hudson, have been discovered by soundings, some of which are thought to be true stream-cut valleys; others, however, start from the coast where there are no rivers entering, and may be due to other and as yet unknown causes. An exceptional feature in the geography of the Pacific coast to the south of the United States-Canadian boundary is furnished by the islands off the southern portion of California. These islands, of which Santa Rosa, Santa Cruz, Santa Catalina, and San Clemente are the most important, rise from water that is 300 or more fathoms deep to a height of from a few hundred to about 2,000 feet above the present sea-level. These islands are the summit portions of mountains similar to those which give a characteristic relief to southern California. One feature concerning the islands referred to which is of interest in connection with the study of the continental shelf is the presence on them of numerous terraces. These occur on a series of level, step-like areas, which sweep about the slopes of the islands, in a general way parallel to the present shore-line, and are records of an upward movement of the land. The highest well-defined terrace on San Clemente occurs at an elevation of 1,320 feet, but there are less distinct beach-lines up to 1,500 feet above the present sea-level. The evidence of movements of the land along the Pacific border of the continent might be multiplied, but enough has been stated to show that the western border of North America, like its eastern portion, is subject to fluctuation in reference to the level of the sea. The line which marks the passage of the solid earth beneath the waters of the sea is ever changing, owing to movements in the earth's crust. It is ever changing, also, owing to the action of waves and currents on the ocean's shores and other causes. _Life on the Continental Shelf._--No attempt need be made here to give an accurate description of the plants and animals which find a congenial home on the continental shelf. One needs, however, to have in mind a general idea of the wonderful abundance and variety of organic forms in the shallow waters adjacent to the continent to fully appreciate the changes in the relief of the ocean-floor, in which they play a leading part, and also the vast economic importance of these marine harvest-fields and pastures, as they may be termed.