68 RICHTHOFEN — NATURAL SYSTEM 
ing this intricate subject, but only to show that natural occurrences may, with the aid 
of the theory here advocated, be explained without having recourse to any forced as- 
sumptions. There is no one of the processes pointed out which is not within the limits 
of those we are accustomed to consider as highly probable in regard to that part of 
voleanie action which is removed only a little way beyond immediate observation, and 
therefore more accessible to well-founded speculation than is the remoter connection 
between volcanic action and the fissures through which the massive eruptions of vol- 
eanic rocks took place. 
3. Other Theories respecting the Origin of Eruptive Rocks. 
The various theories which have been proposed in regard to the origin, not 
only of the volcanic but of all those non-foliated crystalline rocks which are made up 
of silicates, diverge in different directions. Most of them, however, leave unnoticed 
the most essential features of those rocks, such as their nature in regard to the details 
of chemical composition, their similarity in character in distant countries and different 
ages, the laws of their mode of succession and distribution, and the fact of their peri- 
odical emission after long periods of repose ; and no one undertakes to account for 
all of them. There may be distinguished two classes of these theories : the first com- 
prehends those which assume the original seat of eruptive rocks to have been beneath 
the sedimentary rocks, while the second embraces those which would have it to be 
within the shell composed of the latter. It was the purpose of our foregoing theoret- 
ical considerations to point out, that it is exclusively in the direction followed by the 
theories of the first class that we may at all look for a satisfactory explanation of the: 
relations presented by the eruptive rocks. But, though the views here advocated 
belong altogether to this class, the leading theories embraced in it have a very different 
scope. That form of them which was held by Buch, Humboldt and others of the most 
prominent geologists, and is still quite largely adopted, starts from the assumption that all 
eruptive rocks were ejected in the same condition in which they are supposed to have 
been when at their original place in the earth’s interior, that is, molten by dry heat ; 
while the contraction of the globe by loss of heat is regarded as the sole cause of their 
ejection. Among the weighty objections which may be raised against these theories, 
may be mentioned: that the eruptive rocks on their arrival at the surface have evi- 
dently not had a temperature which would be sufficient for their dry melting ; that 
they contain a certain proportion of water enclosed, which was formerly not brought 
into account ; that the ejection from the depth to the surface of masses molten by dry 
heat is a process impossible of explanation, and that, if it was possible, the rocks 
should have a different texture from that exhibited by granite, diorite or propylite ; 
that, finally, contraction alone is as little capable of furnishing an agent for the rending 
of fissures opening towards the surface, which is the prime condition of eruptive 
activity, as is the cooling of the globe of giving the conditions requisite for the process 
of ejection itself. The theories mentioned have been longest maintained on the 
European continent, where they are even now advocated by many. Though approach- 
ing nearest of all to the most probable mode of origin of eruptive rocks, the reasons 
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