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 

 volcanic 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- 

 canic rocks took place. 



3. Other Tlieories 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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