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 vulean- 
ism of the voleanic 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. 
T 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. 
