There are other ways by which we may form some estimate of the time which has elapsed since the melting away of the great glaciers, besides that given by Croll. From measurements taken on Table Rock, at Niagara Falls, which we know has receded in post-glacial times from Lewiston to the place which it occupies at present, we are certain that Lyell was not far wrong when he estimated this to have taken at least sixty thousand years. Shaler, on entirely different grounds,--mainly the redistribution of certain angiosperms--has arrived at figures in excess of these. Calculations made upon the canyons of the Columbia, San Joaquin, and Colorado Rivers, all show the estimations previously given to be conservative. Of course, the figures given will apply only to the time which has elapsed since the melting of the American ice-cap, as we have no means of knowing that the American and European glaciers acted at all in unison in their retreat to the northward. The manner in which we can get some idea of the length of time required to account for the enormous quantity of work done in the Champlain period, is by taking into account the deposits which lie in almost all of the great river valleys which were covered by the glaciers, or whose watersheds were made into lakes by the subsidence of the land to the north, and the rapid melting of that portion of the ice-cap which contained stones, dirt, and other material picked up in the travels of the glacier across the country. The Rhine, the Rhone, and the Danube in Europe, and the St. Lawrence, the Connecticut, and the Mississippi in America, all flow through valleys lined with cliffs of loess. These accumulations overlying the coarser sands and gravels, and conforming to the river valleys, have been measured in the case of the Rhine, and were found to be about eight hundred feet in depth. It is unreasonable to suppose that these deposits being, as they are, material thrown down out of the water after the rivers had lost their transporting power, could have accumulated at a greater rate than that now going on in the rivers, such as the Mississippi and the Nile, to-day, and if this was the case, these deposits must have taken no less than three hundred and twenty-five thousand years to form. Inasmuch as this work was all done during the Champlain period, this figure can be safely taken as the minimum for the measure of the duration of that time.