igo8 ] THE PHYSICS OF THE EARTH. 225 



may have no hesitation in invoking the aid of many authors. If the 

 estabhshment of a great law of nature may be thus facihtated, surely 

 no one will doubt that the space utilized was devoted to a most useful 

 purpose. The extreme specialization characteristic of the science 

 of our day makes such summaries both useful and necessary for 

 the intelligent study of great problems ; and if more effort were made 

 in this direction it might contribute materially to the progress of 

 scientific research. 



(A) Accounts of Particular Mountain Systeais, and Their 

 Supposed !Mode of De\'elopment. 



§ 34. Tlic Andes. — We shall begin with the Andes of South 

 America, because this is one of the largest, simplest and most 

 typical of mountain systems ; and if a theory will not explain the 

 Cordilleras, we may despair of its explaining the more compli- 

 cated mountains of the globe. The reader should carefully bear 

 in mind not only what the author in question says from his own 

 point of view, but also how the facts he mentions accord with 

 the new theory developed in this paper. 



In the Encyclopedia Brifannica, ninth edition, under the article 

 " Andes," we find the following lucid exposition of Andean de- 

 velopment. It is not signed, but is supposed to have passed under 

 the review of Sir Archibald Geikie. 



" The formation of the Andes is due to several causes operating at dis- 

 tinct intervals of time. They consist mainly of stratified material which has 

 been more or less altered. This material was deposited at the bottom of a 

 sea, so that at some former time the highest portions were submerged, 

 probably in consequence to a certain extent, of subsidence of the sea bottom. 

 Since the latest deposits there has been upheaval .and denudation. The range, 

 then, has resulted from the accumulation of sediment on a subsiding area; 

 from the subsequent upheaval of such deposits, which have been increased 

 in height by the ejection of volcanic products; and from the operation of 

 denuding agents. 



" As far as our present knowledge goes, it appears to be probable that 

 the Andes mark an area on which sedimentary deposits have been accumulated 

 to a greater thickness than on any other portion of South America. It is 

 further demonstrable that these deposits belong to several geological periods, 

 the elevation having occurred at different periods, while their axes extend 

 in different directions. Hence it is a complex range of mountains formed 

 by the combination of several distinct systems of ridges. The width of the 



