10 RICHTHOFEN — THE NATURAL SYSTEM 
classification is of especial importance, as it furnishes the key for deciphering the 
natural system of the ancient eruptive rocks. Volcanic rocks issue from volcanic 
vents under our very eyes, and the record of the history of those which have origi- 
nated in past ages, as preserved in their geological relations, is far more distinct than 
it is in regard to their ancient predecessors. No doubt exists in the mind of any ob- 
server in respect to the eruptive origin of all basalt ; while in regard to granite, very 
different views are entertained by distinguished geologists, and supported by weighty 
arguments. In the former case, we have conclusive evidence, while in the second 
speculation has a wider scope. The knowledge of volcanic rocks will, for this reason, 
facilitate the correct interpretation of the nature of rocks generated in remote ages. 
Volcanic rocks are widely spread over the face of the globe. It would be an 
object of great interest to lay down their geographical distribution on maps, and to 
explore the laws by which this distribution has been governed. This has been tried 
in regard to active volcanoes, and the importance and interest attaching to the results 
obtained are such as to have given rise at once to speculations as to the connection 
between volcanoes and other phenomena. The value of those results would be in- 
creased, if to the active craters were added the vastly greater number of those extinct 
volcanoes, the mode of preservation of which still allows us to recognize their former 
nature. Even then, however, the maps would convey but a remote idea of the gen- 
eral distribution of voleanic rocks; the areas comprised by which should be marked 
out with proper distinction of their main subdivisions. Besides an immediate bearing 
on more special geological questions, the knowledge of these subjects promises to be 
of high value for that entire department of geological science, by which the latter is 
most closely connected in scope with the science of physical geography. Many weighty 
problems, such as the causes of the present direction and extent of mountain ranges, 
of the outlines of continents, of the position and shape of groups of islands, of the 
secular oscillations of the surface of the globe, and many other questions, appear to be 
intimately connected with the geographical distribution of voleanic rocks and their 
mutual geological relations. The deduction from the latter of definite laws appears 
to be the initiatory step towards understanding the laws of eruptive activity of 
remote times, and, thereby, towards establishing a chapter in the history of the globe, 
which is among the obscurest and least understood. 
The following classification, in which existing names are retained, as nearly as 
could be done with convenience, is chiefly founded on observations made in the Carpa- 
thians and in the States of California and Nevada. In respect to the variety and the 
distinctness of the mutual relations of the volcanic rocks, these two countries are hardly 
surpassed by any in which this subject has, up to this time, been scientifically investi- 
gated. Until recently, all voleanic rocks, at least those of more frequent occurrence, 
were comprehended in the terms: trachyte, phonolite, trachydolerite, dolerite, basalt ; 
while, besides, separate names were used to distinguish modifications of texture, such 
as pumice-stone, obsidian, pearlite ; or varieties somewhat more distinct in point of min- 
eral composition, such as leucitophyre. The classification as given by Al. von Humboldt, 
in the fourth volume of the ‘‘ Cosmos,” may be considered as having represented the 
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