SIGNIFICANCE OF THE THREEFOLD STATE. 61 



bring up both helium and argon, proving the presence of considerable 

 quantities of these elements within the earth. This rather wide distri- 

 bution, when taken in connection with their supposed chemical inertness, 

 strengthens the presumption that occlusion, or some form of gas diffusion, 

 is prevalent in rocks. Though of much interest to chemists and physicists, 

 these gases, on account of their comparative scarcity, do not play a very 

 important role in general geological problems. Their presence in small 

 quantities within the rocks of the earth's crust being established, quan- 

 titative determinations become of less value. In the analyses made for 

 this paper, whose prime purpose was to determine the range and distribu- 

 tion of the common gases, the separation of helium and argon from nitrogen 

 was not usually attempted. These gases when present are included in 

 the figures given for nitrogen. In the case of pitchblende and carnotite, 

 however, helium was so important a constituent of the gas that its pro- 

 portions were determined. Carnotite produced 1.28 per cent of helium, 

 amounting to 0.04 volume, while pitchblende gave 38.48 per cent, or 0.37 

 volume. 1 In both of these cases nitrogen also was abnormally high. 



In general, therefore, helium and argon, together with at least as much 

 of the other gases as can be shown not to have been produced by chemical 

 reactions or the bursting of inclosing walls, are to be attributed to occlusion 

 or some form of diffusion not distinguishable from occlusion. In many, 

 and perhaps most, rocks this will not be the major part, for, of the three 

 gas-liberating processes, that by chemical interaction under the influence 

 of heat appears to be the dominating one. 



SIGNIFICANCE OF THE THREEFOLD STATE. 



GAS IN CAVITIES. 



While chemical reactions and the phenomena of occlusion imply that 

 gas exists in the interior of the earth, the presence of gas inclosed in cavi- 

 ties under great pressure adds the further implication that the gas often 

 exceeded the point of saturation of the magma, at least at the stage of 

 solidification. Cavity gases are most abundant in minerals of poorly 

 developed cleavage, pointing perhaps towards a strong tendency to escape 

 along cleavage planes during, or after, crystallization. The gas inclusions 

 in quartz may, however, owe their abundance not so much to the absence 

 of cleavage as to the fact that quartz is generally the last mineral to crystal- 

 lize out of a magma, and hence such absorbed gases as did not enter into 

 the other crystals would become concentrated in the siliceous residue and 

 might supersaturate it. 



It is possibly this freely-moving gas above the point of saturation which 

 contributes most to the mobility of lavas. Dissolved gases and vapors, 

 while favoring fluidity, would seem to be relatively less effective. But 

 the foregoing investigations imply that gases mechanically entrapped in 

 crystalline rocks are not very abundant, and suggest that perhaps the 

 theory of liquidity due to gas is overworked. On the other hand, it is 

 true that as the lava cooled down to the point where the last mineral crys- 



1 Analyses 93 and 94. 



