After this slow retreat had been in progress for a considerable time a submergence, known as the “_Champlain stage_” set in, reaching a depth of at least 600 feet and opening the St. Lawrence regions wide to the Atlantic, which penetrated into Lake Ontario. The ice now retreated rapidly under the influence of a maritime climate little colder than the present. In phase this period corresponds to the second _Yoldia Sea_ stage of Scandinavia; in point of time it was probably somewhat earlier. This was followed by elevation, the first result of which was to cut off the warm water and cause a sharp fall of temperature exactly analogous to that of the Ragunda moraines, but a few thousand years earlier and probably more marked. The continuance of elevation brought on a long continental period of extreme aridity, when trees grew on the peat-bogs of the eastern States, while the lakes of the Great Basin further west were almost or wholly dried up. At the maximum of the continental conditions the summers at least were warmer than at present, as indicated by the northward extension of various species of plants and fresh-water mollusca. The winters were probably more severe. Possibly the great aridity of this period was partly due to a sub-glacial continental anticyclone obstructing the path of depressions across America from west to east. The drainage area of the Great Basin received hardly any rainfall and was a hopeless desert, but the Atlantic States were able to grow trees on the old peat-bogs, probably with rainfall derived from the Atlantic. By reference to the cutting of Niagara gorge, we can infer that the warm dry period began about 6000 B.C., so that it corresponds exactly with the continental phase (_Ancylus_ stage) of Europe. This period of aridity was finally ended by a fresh submergence, the “_Micmac_,” which carried the land about twenty feet below its present level. In Yukon and Alaska, where the glaciation was not nearly so severe as further to the south-east, the depression of the land by the ice-load and consequently the subsequent rise on its removal were not great. There were no complicated geographical changes, and correspondingly there appear to have been no fluctuations of climate, but only a gradual passage to present conditions.