94 RICHTHOFEN NATURAL SYSTEM OF VOLCANIC ROCKS. 



find only an explanation when such changes of the direction of the structural planes 

 are assumed to take place in depth. 



The tendency to simplify beyond their present conditions the general outlines 

 of the configuration of the surface of the globe, continues probably without intermis- 

 sion, though an era of comparative repose has followed the violent exertions of vulcan- 

 ism of the volcanic era. The great backbone of the Eastern continent, comprising the 

 Alps and the Himalaya, and the second great belt which comprises the Andes, and, 

 more properly, encircles the whole Pacific Ocean, are at the present epoch the main 

 features in the orography of the globe. Both have derived their prominent position 

 from the events of the volcanic era. The conclusions at which we have arrived in 

 these pages have been drawn chiefly from observations made in regions of prominent 

 interest in either belt, and it is for this reason especially that we believe that at least 

 some of them will be found to be susceptible of general application. 



I am fully aware how imperfect is this attempt to develop, in the correlations of 

 the volcanic rocks, the principles of their natural system. This system should be, as 

 we remarked before, not a classification of objects by certain conspicuous properties, 

 but a classification of objects by their definite mutual relations. We have not to make 

 the divisions, but to find them. "We shall be able to do this with greater or less per- 

 fection, in the same measure as those relations are studied and firmly established. 

 Much is needed to arrive at this end. It can only be achieved after evidence has been 

 accumulated by the combined and harmonious labors, in different countries, of geolo- 

 gists in the field as well as in the laboratory. The range over which both modes of 

 observation extend from year to year, and the field on which their results meet, is in- 

 creasing in a surprising degree. Yet eruptive rocks form ordinarily an object of less 

 exact observation than those of sedimentary origin ; and it cannot be denied that har- 

 monious observation, by which alone can be established the principles of comparative 

 petrology, is rendered nearly impossible, as long as no uniform system of nomenclature 

 is applied. These pages are the result of an attempt to add one more to the many 

 contributions towards establishing the foundation of both, and to show the value which 

 the study of eruptive rocks and of those mutual relations which comprise their natural 

 system promises to have for approaching the solution of some of the highest problems 

 of geological science. 



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