Page:Popular Science Monthly Volume 13.djvu/672

652 to Nantucket. These strata still probably underlie a large part of Long Island where they have been protected from erosion by the heavy beds of drift that cover them, while the shore-waves have eaten away all exposed portions. Evidence strongly confirmatory of the view that Cretaceous rocks have been scooped out of the basin of Long Island Sound is afforded by the fact that the drift of Long Island contains in immense numbers imperfectly-rounded blocks of a reddish-brown sandstone, filled with the impressions of dicotyledonous leaves—a rock nowhere yet found in place, but one which is probably the representative of the leaf-bearing Cretaceous sandstone of the Raritan River.

Whether the overlying Tertiary beds will be found on Long Island is perhaps doubtful, since they are not conterminous with the Cretaceous; but, from the fact that an outlier of this formation exists at Gay Head, Martha's Vineyard, it is highly probable that it was once continuous from Southern New Jersey.

On the preceding pages the history of the vicinity of New York has been traced backward for some millions of years. This history has been read from rock-graven records, which, although meagre and mutilated, give the generalities of the narrative with a truth and fidelity which shame all human history. It would be a pleasant duty to predict the future of this region, even in the same degree of fullness; but the future is as unknown to the geologist as to others. He learns, however, from his studies, that what we call terra firma is a type of instability, and that there is nothing stable but the law of change; and he can prophesy with confidence that in the distant future the history of the distant past will be, in part at least, repeated. Even now changes are in progress which, if they should continue a few thousand years, would very profoundly affect not only the aspects of this region, but its adaptability to human occupation. A number of facts indicate that the coast of New Jersey and Long Island is gradually sinking. From the marshes of New Jersey are taken the trunks of trees which could not have grown there except when it was drier ground, and on the shore stumps are seen, now under water, of trees which must have grown on land. So, too, the sea throws up in storms portions of turfy soil, once covered only by the air, and similar soil has been reached below the sea-level in pits dug through drifted sand along its margin. It is also said that the land boundaries have been changed and farms diminished even where the wash of the shore-waves produced no effect. The rate of this subsidence is very slow—only a few inches in a century—and it may at any time be arrested or reversed; but, should it continue, as it may, for some thousands of years, it would result in a submergence of land now valued at hundreds of millions of dollars, and a complete change of position in the seats of commerce and industry, which must always centre about this harbor. This possible catastrophe is, however, so uncertain and remote that it seems hardly sufficient to disturb the equanimity of at least this generation of inhabitants.