620 GEOLOGY. 



in cases of free exposure at the surface; at any rate, such rocks con- 

 tain notable quantities of gases occluded in some way within them- 

 selves. Recent surface-lavas also contain gases of similar kinds, but 

 not in equal degree, so far as available analyses show. The gases are 

 in part held in numerous small cavities within the constituent minerals, 

 especially in the quartz. This is perhaps due to the fact that quartz 

 usually crystallizes late in the process of solidification, and its mother- 

 material becomes crowded with gases excluded by the previous crystal- 

 lization of other minerals. Analyses of twenty-five crystalline rocks of 

 various kinds from many typical localities by Tilden,^ gave an average 

 volume of gas, under ordinary atmospheric pressure, four and a half 

 times that of the containing rock. This shows the condensed condi- 

 tion in which the gases are held. Of these gases, the chief is hydrogen, 

 which much exceeds all the rest. Next in order of abundance is carbon 

 dioxide, followed by carbon monoxide, marsh gas (CH4), and nitrogen. 

 Water is frequently present and free oxygen almost universally 

 absent. The average ratio of hydrogen to carbon dioxide by volume 

 in these analyses is about 70 : 30. Five complete analyses gave the 

 following averages: H^, 52.134; CO^, 34.104; CO, 8.422; CH„ 3.224; 

 N2, 2.072. It will be seen that the gases contained in these rocks are 

 in proportions radically different from those of the atmosphere, and it 

 is doubtful whether they can be reasonably assigned to any other source 

 than the lavas from which the rocks were formed. It is to be noted, 

 however, that some sedimentary and meta-sedimentary rocks, such as 

 quartzite and quartz-schist, contain similar gases, but this may be 

 because the granules of the original rock retain them, notwithstanding 

 the secondary processes through which they have passed. Analyses of 

 meteorites show essentially the same gases in much the same proportions. 

 If evidence of this kind can be trusted, the standard original gases of 

 lavas are the elements or compounds of hydrogen, carbon, and nitrogen, 

 in the order named, while the chlorine and sulphur gases are to be re- 

 garded as accessory. Because of their intensely energetic and noxious 

 character, these latter gases make themselves disproportionately mani- 

 fest in the vicinity of active volcanoes. That they are really not pre- 

 ponderant seems to be implied by the fact that the volcanic rains, which 

 are extremely copious, are usually fresh, and only in rare cases is the 

 presence of the hydrochloric or sulphurous elements sufficient to pro- 

 * Chemical News, April 9. 1897. 



