[Footnote BH: American Journal of Science, vol. cxxxii, 1886, p. 77.] When, now, we attentively consider the combination of causes necessary to produce the climatic conditions of the great Ice age of North America, we shall be prepared to find far more extensive variations in the progress of the continental glacier, both during its advance and during its retreat, than are to be observed in any existing local glaciers. With respect to the arguments adduced in favor of a succession of glacial epochs in America the following criticisms are pertinent: 1. So far as we can estimate, a temporary retreat of the front, lasting a few centuries, would be sufficient to account for the vegetable accumulations that are found buried beneath the glacial deposits in southern Ohio, Indiana, central Illinois, and Iowa, while a temporary readvance of the ice would be sufficient to bury the vegetable remains beneath a freshly accumulated mass of till. Thus, as Dr. Bell suggested, the interglacial vegetal deposits do not necessarily indicate anything more than a temporary oscillation of the ice-front, and do not carry with them the necessity of supposing a disappearance of the ice from the whole glaciated area. Thus the introduction of a whole Glacial period to account for such limited phenomena is a violation of the well-known law of parsimony, which requires us in our explanations of phenomena to be content with the least cause which is sufficient to produce them. In the present instance a series of comparatively slight oscillations of the ice-front during a single glacial period would seem to be sufficient to account for all the buried forests and masses of vegetal _débris_ that occur either in the United States or in the Dominion of Canada.