overwhelming floods, Playfair and other exponents of the Huttonian school taught that mountains were slowly upheaved and slowly worn down. The simplicity of Playfair's argument finds excellent illustration in the often quoted passage regarding the origin of valleys: "Every river appears to consist of a main trunk, fed from a variety of branches, each running in a valley proportioned to its size, and all of them together 'forming a system of vallies, communicating with one another, and having such a nice adjustment of their declivities that none of them join the principal valley either on too high or too low a level; a circumstance which would be infinitely improbable if each of these vallies were not the work of the stream that flows in it." Descriptions of valleys should always recognize the share that rivers have had in eroding them, or else the "nice adjustment of their declivities" may pass unnoticed.
It should be noted, however, that to this day explanation is not always allowed an undisputed place in the treatment of the lands, however fully it is accepted as appropriate to the presentation of other divisions of physical geography. But the manner in which explanation is extending over a larger and larger part of the subject gives assurance that the geographers of the coming century will insist upon a uniformly rational treatment of all divisions of their science. The active phenomena of the earth's surface first secured explanation; it has long been considered essential to explain as well as to describe such phenomena as the winds of the air and the currents of the ocean; indeed, this is now so habitual that many geographers who may object to the explanation of a peculiar kind of a valley as a trespass upon geology, will nevertheless demand an explanation of rainfall and tides, although these truly geographical subjects are manifestly shared with physics and astronomy. Land forms of very elementary character, like deltas, or of rapid production, like volcanoes, have had to give some account of themselves all through the century; but it was not for many years after the announcement of Playfair's law, that the erosion of valleys by the rivers that drain them came to be regarded as a subject appropriate to a geographical treatise. Only in the later years of the century has the fuller treatment of this beautiful subject been attempted; even now much of it remains to be developed in the century to come.
The treatment of physical geography will be much more even, to the great advantage of its students, when explanatory description is applied to all its parts. The alluvial fans at the base of arid mountains should be accounted for as well as the dunes of deserts. The fault cliffs of broken plateau blocks and the weathered cliffs of retreating escarpments deserve to be considered as carefully as the wave-cut cliffs of coasts; the essential differences of these forms are reached most' easily through their explanation. The varied sculpturing of a moun-