In the first place, when we see a deep river valley in which the present stream is doing an almost infinitesimal amount of deepening, we are not to infer that this represents all its work past and present. In times of unusual flood it may do in one week more than in many previous years. Farther, if there have been elevations or depressions of the land, when the land has been raised the cutting power has at once been enormously increased, and when depressed it has been diminished, or filling has taken the place of cutting. Again, if the climate in time past has been more extreme, or the amount of rainfall greater, the cutting action has then been proportionally rapid. Perhaps no influence is greater in this respect than that which is known to the colonists in Northeastern America as "ice-freshets," when in spring, before the ice has had time to disappear from the rivers, sudden thaws and rains produce great floods, which rushing down over the icy crust, or breaking and hurling its masses before them, work terrible havoc on the banks and alluvial flats, depositing great beds of gravel, and sweeping away immense masses that had lain undisturbed for centuries. Now we know that in Europe the human period was preceded by what has been termed the glacial age, and as it was passing away there must have been unexampled floods and ice-freshets, and a temporary "pluvial period," as it has been called, in which the volume of the rivers was immensely increased. Farther, it is an established fact that the period of the appearance of man was a time when the continents in the northern hemisphere were more elevated than at present, and when consequently the cutting action of rivers was at a maximum. This was again followed by a period of depression, accompanied probably by many local cataclysms, if not by a general deluge; and there are strong geological reasons to believe that this convulsion was connected with the disappearance from Europe of Palaeocosmic man, and many of the animals his contemporaries. This view I advocated some time ago in my "Story of the Earth;" and more recently Mr. Pattison, in an able paper read before the Victoria Institute, has developed it in greater detail, and supported it by a great mass of geological authority. If the Palaeocosmic period was one of continental elevation, when the greater seats of population were in the valleys of great rivers now covered by the German Ocean and the English Channel, and when the valleys of the Thames and the Somme were those of upland streams frequented by straggling parties and small tribes, and the seats of extensive flint factories for the supply of the plains below, and if this state of things was terminated by a diluvial debacle, we can account for all the phenomena of the drift implements without any extravagant estimate of time.