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RICHTHOFEN NATURAL SYSTEM 



These appear to be the most essential geographical and geological relations 

 regarding the mode of distribution and occurrence of volcanic rocks. It is perfectly 

 evident that no one of them singly was the chief cause of their occurrence, nor were 

 all of them together ; but each of them had a marked influence, either upon the direc- 

 tion and the location of the orifices of the fractures, or upon the mode of ejection. 

 Keeping in view these different relations, I shall dwell, in the rest of this chapter, more 

 particularly on the connection between the occurrence of volcanic rocks and the con- 

 figuration of the surface. This subject may appear to be beyond the scope of this 

 paper. But the examination of every question which relates to the inner connection 

 between the phenomena attending the ejection of volcanic rocks will aid in disclosing 

 the true nature of these, and promote the knowledge of the principles of their natural 

 system. We will endeavor to answer the following questions : Was the particular 

 structure of certain portions of the crust of the globe, which is indicated by the situa- 

 tion of elevated regions on its surface, among the causes of the eruptions of volcanic 

 rocks ? or were the inequalities of level on the surface, in the volcanic regions, due to 

 the processes attending and the agencies causing the eruptions? 



The answer to both questions must be in the affirmative. The peculiar structure 

 of the earth's crust, at those places where mountain ranges and highlands rise on its 

 surface, appears to have influenced in a great measure the distribution of the volcanic 

 rocks, because those among them which are accompanied by the latter, had been 

 elevated before the time of the eruptions, and the adjoining lowlands were not the 

 theater of eruptive activity. But, on the other hand, it is evident that the ejection of 

 volcanic rocks, or rather those subterranean processes of which they were one of the 



acter in rocks and scenery to that which that range has in other parts. It appears that a gap of more than one hundred 

 miles in length lias been formed in the region of the'two volcanoes, by the displacement of a portion of the Sierra Nevada, 

 which was probably bounded by two lines of fracture transverse to the direction of the mountain range, and has subsided 

 thousands of feet, and that then an immense accnmulation of volcanic rocks filled up the gap, and closed in building up the 

 two giant volcanoes. Other lines of dislocation which have given vent to volcanic rocks, and which have more frequently 

 been noticed, are directed parallel to mountain ranges. Of such nature appears to be the abrupt descent of the Sierra 

 Nevada towards the Great Basin, which has been the theater of violent eruptive action ; and probably the relations on the 

 western slope of the Rocky Mountains are of a similar nature. The Vihorlat-Gutin Range, in Hungary, offers a striking 

 illustration of an extensive accumulation of volcanic rocks along the foot of a preexisting mountain range, though the dis- 

 location is not conspicuous in that country, on account of the deposition of recent sediments which filled up the Hungarian 

 Basin. There may be some affinity between these modes of occurrence of volcanic rocks and the manner in which they are 

 met with in certain areas of flat or hilly countries, surrounded by ranges composed of ancient rocks. The best illustration 

 is afforded by the Basin of Transylvania. The undulating country of the interior is encircled by high ranges consisting of 

 ancient formations, which are lined on their inner side with volcanic rocks. Hungary itself affords a similar instance, though 

 less regularity is perceptible ; and the same structure is somewhat approached in the geological relations of Bohemia. We 

 may also mention, as recalling that mode of occurrence, certain depressions between the two summit ranges of the Sierra 

 Nevada, such as the Basins of Sierra \ r alley and Lake Tahoe, which are encircled, first by a ring of volcanic rocks, and 

 then only by the metamorphic and granitic rocks which form the bulk of the Sierra Nevada. Or the " Parks " of the 

 Rocky Mountains. Their geology, it is true, is almost unknown. An interesting description of the San Luis Park, the 

 greatest among them, recently published (see American Journal of Science and Arts, November, 1867), shows that the 

 elevated rim of its basin, which is estimated at eighteen thousand square miles, is made up of ancient formations, and that 

 volcanic rocks encircle more immediately the extensive plain forming its bottom, which is itself composed of volcanic 

 sediments, thns completing a structure that reminds of that of Transylvania, in more than one respect. If we look for 

 instances on a grander scale, we may find some analogy with the mode of occurrence of volcanic rocks as just described, 

 in the volcanic ranges encircling the Basin of the Pacific Ocean. And it may not be out of place if we call attention to the 

 similarity with these circular basins which is presented by the configuration of the surface of the moon. 



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