THE GASES IN ROCKS 563 
interior 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 
magmas 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 outbursts cannot be ascribed to the penetration of 
surface waters and gases, for the moon has neither appreciable atmos- 
phere 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 explo- 
sive lunar vulcanism, in the absence of any appreciable 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 percent. 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 crystallized, 
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 super- 
ficial 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 
