82 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 
voleanic 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 has 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 accumulation 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 voleanic 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 preéxisting 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 Valley and Lake Tahoe, which are encircled, first by a ring of volcanic rocks, and 
then only by the metamorphie and granitie rocks which form the bulk of the Sierra Nevada. Orthe “ Parks” of the 
Rocky Mountains. heir 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 
voleanie rocks encircle more immediately the extensive plain forming its bottom, which is itself composed of volcanic 
sediments, thus 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 voleanic 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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