74 THE GASES IN ROCKS. 



only come in contact with them, and only doubtfully then, in a very limited 

 portion of the throat of the volcano. 



These considerations seem to indicate that, for the most part, the volcanic 

 gases and vapors have not been supplied to the lavas by ground waters, 

 but are original constituents of the magmas. Doubtless at the beginning 

 of an eruption, following a period of quiescence, much of the steam merely 

 comes from such rain-water as may have accumulated in the crater and 

 upper part of the cone, but this does not account for the gaseous emanations 

 from the lava itself, nor from those volcanoes, such as Stromboli, and the 

 well-known Solfatara near Naples, which maintain a mild form of eruption 

 for long periods. Such meteoric water could contribute to the volcanic gases 

 little except some dissolved air, together with a trace of carbon dioxide, 

 and perhaps hydrogen from chemical action. Such soluble salts as this 

 water might dissolve from the crater walls were brought up from the in- 

 terior in the first place (making some allowance, however, for weathering), 

 and so have little bearing on the case. 



The hypothesis that the gases and vapors are originally from the mag- 

 mas, is greatly strengthened by the volcanic activity in the moon, if, as 

 is rather generally believed, the great pits on the surface of the moon are 

 craters produced by volcanic explosions; if not, of course the argument 

 does not hold. The gases and vapors which caused the tremendous out- 

 bursts can not be ascribed to the penetration of surface-waters and gases, 

 for the moon has neither appreciable atmosphere nor hydrosphere, and, 

 according to Stoney's doctrine, never could have held either, owing to its 

 feeble gravitative control. Such gases as are implied by these explosions 

 must be supposed to have arisen from within the interior of the moon. 

 The extent of this explosive lunar vulcanism, in the absence of any appre- 

 ciable atmosphere or hydrosphere, furnishes a strong argument against 

 the belief that surface-waters and atmospheric gases are essential factors 

 in terrestrial vulcanism. 



Thus far evidence of a negative nature has been brought forward to 

 show the difficulties in the way of thinking that surface-waters play a 

 prominent role in volcanic phenomena. But more positive evidence can 

 be presented to support the view that the hydrogen and water in the deep- 

 seated rocks are truly magmatic. Micas are prominent constituents of the 

 plutonic rocks. The immense granitic bathyliths, which were probably 

 formed beyond the reach of ground-waters, are characterized by this group 

 of minerals. In fact, micas are more abundant in the deep-seated rocks 

 than in the surface lavas of similar composition. Yet all micas contain 

 hydrogen (or hydroxyl) and yield water upon ignition. This varies with 

 the mineral species and locality, ranging up to 4 or 5 per cent. If these 

 micas in the massive intrusions are primary minerals, as they seem to be, 

 and were out of the reach of ground- waters until long after they were crys- 

 tallized, there appears no other alternative than to consider this hydrogen 

 as inherent in the magma itself. The general petrological principle that 

 plutonic rocks are micaceous and hornblendic, while their more superficial 

 equivalents are more frequently characterized by pyroxenes which are 

 less hydrous, may point toward the suggestion that the magmas originally 

 contain considerable water or the elements which can produce it, but as 



