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 volcanic 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 volcanic 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 volcanic 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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