(4) _Beds of Marine and Lacustrine Origin._ If between beds of glacial drift there be found beds of lacustrine or of marine origin, such beds would indicate a recession of the ice during their time of deposition. Their position would be a minimum measure of ice recession. If such lacustrine beds contain organic remains, they will bear testimony concerning the climatic conditions which existed where they occur, at the time of their deposition. If the fossils in such beds denote a temperate climate, or a climate as mild as that of the present day in the same region, the ice must have receded so far to the northward as, in our judgment, to constitute its re-advance a distinct ice epoch. This line of argument may be even stronger than that drawn from remains of terrestrial life, since the ice would probably affect the temperature of the sea to greater distances than that of the land, and affect it to a greater degree within a given distance. The argument becomes stronger the further north the inter-drift marine and lacustrine deposits occur, since the ice must always have receded to a position still further north. If marine or lacustrine beds lying far north of the later ice limit contain proof of temperate climate, the argument becomes conclusive. The absence of marine and lacustrine deposits between beds of drift, would be no proof that interglacial epochs did not occur. Lacustrine beds could be made only where there were lakes, and lakes would be the exception rather than the rule. Marine beds in similar positions would rarely be known, except where a definite succession of changes of level has taken place. Both classes of deposits, if once formed, would be subject to destruction by the over-riding ice of a later epoch, if such there were. Neither would be likely to be preserved at all points where formed, and both may exist at many points where their existence is not known. The absence of these beds is at best no more than negative evidence.