INTRODUCTORY. 159 



that the diminution of precipitation, and the other physical changes con- 

 nected with the decrease of the water-surfaces and general drying-up of 

 the earth, are matters of more wide-spread occurrence, and of vastly greater 

 importance, than is the diminution of the glaciers in certain limited regions. 

 These ftxcts have never, as the writer thinks, been presented in their con- 

 nection with each othei'. or examined from the right point of view; while 

 " glacial geology," on the other hand, may be said with truth to have formed 

 the staple of the geological journals and text-books during the past few years. 



The problem first to be attacked, then, in the prosecution of this investi- 

 gation is. What means the desiccation which has been shown, beyond doubt, 

 to have taken place during the later geological ages, and to be continuing 

 during the historical period ? Is it something which is due to orographic 

 causes ; or to climatic changes, independent of size, position, and elevation of 

 the land masses ? Or, on the other hand, has this diminution of moisture 

 been the result of both climatic and orographic causes acting in concert with 

 each other; and, if so, is it possible to say what part each of these sets of 

 causes has taken in bringing about the indicated desiccation ? Still farther, 

 is it possible to assign a probable cause for such change of climatic con- 

 ditions as it may have become necessary to assume as having occurred ? 

 These are the principal questions to which it is proposed that our attention 

 shall be called in the present chapter. 



In Avhat has just been said the writer has not intended to suggest that 

 the phenomena of desiccation in certain parts of the world have not attracted 

 much attention, and been the object of more or less discussion. The quo- 

 tations given in the preceding chapter afford evidence enough that such 

 remarkable events could not tail to arrest the attention of observers. 

 What is asserted here is this : that these phenomena have not been consid- 

 ered in their ensonhle, but rather as simply local manifestations, indepen- 

 dent of any common cosmic cause ; or as being of a transient nature ; or, 

 again, as being something entirely exceptional, and not to be brought into 

 harmony with the ordinary course of nature. 



By far the larger number of writers who have discussed the subject of 

 desiccation have looked upon this remarkable change as something quite by 

 itself, and as having no connection with precedent changes in the geological 

 history of the world ! A few, however, have to some extent treated in a 

 more general way the remarkable phenomena which have been indicated 

 in the previous chapter : to the views advocated by both these classes of 



