602 PETROLOGY AND MINERALOGY 



remelted, and thereupon showed, in accordance with the above, an 

 increase of volume. 



The question has again attracted great attention because it plays 

 such an important part in the theory of vulcanism newly proposed by 

 Stiibel. Stiibel denies that the pressure of the contracting earth-crust 

 upon the actually glowing interior produced the volcanic phenomena. 

 He holds that the slowly freezing so-called "crust" retains impris- 

 oned within it relatively small, nest-like reservoirs of glowing molten 

 magma. The molten magma in these reservoirs escapes to the surface 

 by eruptive vents (Ausbruchskanal) , being forced out by the increase 

 in volume which the magma undergoes in the course of its cooling. In 

 view of previous results, he admits that the final result must be a con- 

 traction, but believes he may assume it to be most probable that in 

 the course of the cooling the molten mass passes through a transitory 

 phase of expansion or increase in volume. We have, however, abso- 

 lutely no experimental knowledge of such a phase. 



A further principle of physical-chemistry, which explains petro- 

 graphic processes within the sedimentary rocks, is the tendency to 

 reduce to a minimum the existent exposed surface (Oberfldche) of a 

 number of contiguous identical individuals. It appears that equilib- 

 rium between a saturated solution and the bounding surface of a 

 crystal in that solution is not established until the bounding surface 

 is reduced to its minimum. If one moistens the powder of a soluble 

 salt and then allows it to stand for some time, it is found that the mass 

 assumes a distinctive crystalline structure, composed of large individ- 

 uals, showing that a certain portion of the grains of the powder have 

 increased their own dimensions at the expense of the remaining por- 

 tion which may be said to have been consumed. In a corresponding 

 way, and obedient to the same laws, it seems that recrystallization 

 has produced similar effects in the structural character of those coarse- 

 grained marbles which represent former very dense limestones. In 

 this case it would seem that the small limestone grains, in the pre- 

 sence of carbonated water, possess the tendency to develop into larger 

 individuals by mutual assimilation and a rearrangement of their 

 molecules into parallel orientation. Moreover, we may thus come to 

 understand why the older saline deposits are sometimes so coarse- 

 grained, while deposits from salt lakes in recent times come down 

 almost crypto-crystalline (dicht} in character; and in like manner 

 the growth of the crystals of glacier ice, from the firn to the end of 

 the glacier, may be explained. 



Since petrography forms a part of geology, there is, of course, an 

 intimate connection between the two sciences, they being mutually 

 complementary. There can be no science of geology without petro- 

 graphy, nor can the science of petrography disregard discoveries made 

 in other branches of geology, but it is not necessary to discuss in this 



