102 THE DESICCATION OF LATER GEOLOGICAL TIMES. 



to them, and certainly without any very definite idea of their bearing as 

 forming a part of a connected series of events. The almost or quite universal 

 tendency has been, and still is, to consider climate as something on the whole 

 quite fixed, and in its very nature invariable, or else to invoke the agency 

 of causes of a periodical nature, involving cycles of events, l^ut not any 

 permanent change. That both these theoretical aspects of climatic persist- 

 ency are quite contrary to the facts developed by geological investigation 

 will be made apparent, as the writer believes, in the following pages. But 

 without further introductory remarks, the essentially important facts will 

 now be laid before the reader. 



Section II. — The Dn/ing-vp of the Lalccs of the Western Side of the North 



American Continent. 



It is not difficult to understand how it is that the decrease in dimensions 

 of a body of water like a lake is a phenomenon more striking in character 

 and more easily recognized than the gradual diminution of the volume of 

 water flowing down a river-channel. We are accustomed to see rivers vary- 

 ing their size in accordance with the progress of the seasons, and even shift- 

 ing their channels to a very considerable degree. The marks by which we 

 are able to recognize gradually occurring changes of volume are therefore 

 liable to be obliterated. And as long as a lake is only an expansion of a 

 river, while, in fact, it is in communication with the sea, its contents mov- 

 ing downwards with the river itself, only more slowly in consequence of its 

 greatly increased cross-section, so long any diminution of its volume which 

 may take place, unless it is very marked, will be likely to escape notice. But 

 let the lake be cut off" from its connection with the ocean, and become iso- 

 lated, by the increase of evaporation over precipitation, and the progressive 

 stages of a gradually lessening area will become very marked phenomena, 

 for the very causes which conspire to bring about this condition of things 

 are such as tend to preserve the records of their occurrence, which take the 

 form of ancient shore-lines, beaches, and terraces. A secular increase of 

 volume, in the case of both lakes and rivers, is natiirally more difficult to 

 make out than a diminution, because the rising waters conceal the work of 

 former ages. Forests, if submerged, gradually decay and leave few traces 

 behind. Tl)e works of man, on the other hand, are sometimes of a very 

 enduring character; but, of course, testimonials to change of water-level 



