Page:Popular Science Monthly Volume 3.djvu/308

296 ice, the result is a layer of unassorted drift overlaid by a thickness of handsomely-laminated fine clay and sand. This combination of circumstances must have occurred south of Lakes Erie and Huron, producing the Black Swamp and the Cottonwood Swamp in Ohio and Michigan, about the south end of Lake Michigan, and over an extensive fiat south of the Winnipeg basin.

All the phenomena of the drift in the Northwest are, hence, attributable to the approach, long duration, and slow disappearance of the glacier-ice of Prof. L. Agassiz. It certainly seems unwarrantable to propose upward and downward movements of the crust, involving the submergence of the continent, when one simpler cause can be shown sufficient to produce the known effects. The submergence of the New-England coast to the depth of about seventy feet is all that Prof. J. D. Dana finds warranted by a vigorous inspection of the drift-deposits about New Haven, Connecticut. The four-hundred-foot "beach," near Montreal, may have the same origin as the so-called "beaches" that rise several hundred feet higher in the State of Ohio. The Champlain and Terrace Epochs find no application to the drift in the Northwest, as those terms are defined and used in the East. There is abundant evidence throughout the West of a former higher stage of the rivers. This higher stage may, however, be explained by referring it to the large increase of water incident to the melting of the glacier only after reaching the latitude of a warmer climate. The terraces have not, moreover, in the Northwest, generally that system or uniformity of height and arrangement necessary to warrant their reference to successive reductions in the volume of water, but are usually due to a variable resistance offered by the banks or rocks in which they occur, arising from their stratification.

No well-authenticated fossil remains from the hard-pan drift have yet been met with. Statements have been published of the finding of fossil remains in the unmodified drift in various parts of the Northwest, but they are generally based on the reports of non-scientific observers, and must be taken with great caution, unless verified by a geologist who has definite ideas of what "modified" or "unmodified" drift is. It would not be improbable that, near the southern margin of the icefield, the remains of vegetation, and even of animals, should be involved in the drift undergoing transportation, but their structures are too fragile to withstand the grinding incident to the general progress of the ice, and would not bear transportation from northern latitudes. Much uncertainty is also thrown on the true age of vegetable remains reported from the drift in the Northwest, by the wide-spread but hardly distinguishable clays of the Tertiary and Cretaceous formations, which contain modern species of wood and leaves, associated with marine fossils.

Believing, therefore, in the glacier origin, directly, of the Post-Tertiary deposits of the Northwest, it is impossible to concur in the