VULCANISM. 69 



magmas in the interior, whether actually combined as water, or as its 

 elements held in solution, or chemically united in other compounds. These 

 gaseous elements form an integral part in the magmas, having been vital 

 factors in their development from the primitive planetary matter. That 

 this process of reworking has gone on to considerable depths, if we are to 

 start with typical meteoritic material, is evidenced by the fact that the 

 deep-seated plutonic rocks are characterized by micas and other hydrous 

 minerals, while mineral species of the meteoritic type are absent. 1 



The more restrictive phase of the problem of water will be discussed 

 under the head of vulcanism. 



VULCANISM. 



In the actual dynamics of vulcanism, provided the gases are original 

 in the magmas, the state in which they occur is not of vital importance, 

 except in so far as it determines the conditions under which the gases be- 

 come free, from occluded or chemical bonds, to perform their part in the 

 mobility of lavas, in the explosions which sometimes accompany erup- 

 tions, and in the phenomena of fumaroles and volcanic vents. The dis- 

 tinction between cavity, occluded, and chemically united gas, which is 

 made in the case of solid igneous rocks, can not be extended to the liquid 

 lavas. In the liquid lava the gas may be supposed to be imprisoned 

 mechanically, or else to form a part of the magmatic solution. On the 

 solidification of the mass, the gas, formerly existing in the free state, may 

 enter chemical combinations at the lower temperature, may be occluded 

 by the solid rock, or may become entrapped within the minerals last to 

 crystallize. So, too, it is possible that some of the gas dissolved in the 

 magma may, because of cooling and crystallization of adjacent portions 

 of the solution, reach a supersaturated condition and appear in the solid 

 rock also as gas inclusions. Otherwise, it would pass into the solid rock 

 occluded or chemically combined. The condition of the gases examined 

 in the laboratory need not, necessarily, correspond to a particular state 

 of occurrence in the lava before crystallization. 



Gases mechanically distributed throughout the lava would always be 

 an operative factor in vulcanism, while such gases as were chemically 

 combined in the solution would, presumably, only become free, and hence 

 fully operative, upon the lowering of the temperature and the relief of 

 pressure, 2 and probably but partially then. Since vapors and gases in the 

 free state are the cause of volcanic explosions, they can be traced as far 

 down in the conduits as explosions occur. From the nature of these explo- 

 sions, which appear to be due to the accumulation of vapor gradually work- 

 ing upward until suddenly able to relieve itself, it is fair to suppose that 



1 This statement should perhaps be qualified. The basalt at Ovifak, Greenland, 

 contains iron strongly resembling the meteoric metal, and in which the minerals cohenite, 

 lawrencite, and doubtfully schreibersite have been recognized. The occurrence of this 

 terrestrial iron would indicate that material of this sort still occurs at points within the 

 outer part of the earth. 



2 A falling temperature favors the liberation of hydrogen from water by ferrous 

 compounds (see p. 67), while carbonates are most easily decomposed at low pressures 

 (see p. 49). 



