THE PHYSICAL GEOGRAPHY OF THE LANDS. 159 



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 illustra- 

 tion 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 to- 

 gether '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 circum- 

 stance 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, how- 

 ever fully it is accepted as appropriate to the presentation of other 

 divisions of physical geography. But the manner in which explana- 

 tion 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 val- 

 leys 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 at- 

 tempted; 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 ap- 

 plied 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- 



