332 ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 



gards the part played by water seems very reasonable. "We can 

 readily conceive that it would be difficult for water, even under 

 considerable pressure, to obtain access by means of fissures or 

 otherwise through the solid crust of the earth to the smelted mass 

 beneath. As previously remarked, it would be impossible for it to 

 penetrate the highly heated rocks constituting the inner part of 

 the earth's crust. But it would seem yery possible, especially in 

 those volcanoes situated on the coasts of continents, for water to 

 obtain access to great depths in the craters and subterranean 

 canals. As to the cause of the rise of lava in these, Naumann 

 propounds the following theory : 



"The solidified crust encloses the fluid interior of our planet? 

 and at their junction the same solidifying process, by which the 

 crust of the earth was formed, must still be going on. Because 

 however imperceptible the radiation of the internal heat may now 

 be, it still continually takes place although in a lesser degree; and 

 jt cannot be doubted that on the inner side of the earth's crust fluid 

 matter is continually assuming the solid form. It is indeed the case 

 that the greatest number of fluid bodies experience a diminution of 

 their volume, and only a few of them, such as water and bismuth? 

 expand, while solidifying, but we must reflect that the relations as 

 to density of the bodies existing in the great depths of the earth 

 where vulcanism has its seat, must be essentially different from 

 those, which they possess on the surface, where we can experiment 

 with them. The pressure of the superincumbent masses must com- 

 press the materials existing at these depths. But fluid bodies are 

 gifted with a much greater degree of compressibility than solid bo- 

 dies, and therefore it can easily happen, that the most and per- 

 haps all fused material which solidifies on the inside of the earth's 

 crust experiences in this solidification an increase in its volume. 

 The unavoidable consequence of this can be no other than that 

 during this slowly progressing solidification a diminution of the 

 capacity of the earth's crust takes place, that consequently the 

 the space enclosed by it and filled with fused material is contrac- 

 ted. The next consequence will be, that a part of the fluid ma- 

 terial will be pressed up sometimes through one and sometimes 

 through another volcanic canal, until the weight of the column of 

 lava equalizes the pressure in the interior. In this way the first 

 conditions are given by means of which volcanic eruptions become 

 possible."* The objections to this theory lie in the following 



* Lehrbuch I. 289. 



