278 A. C. LANE — GEOLOGIC ACTIVITY OF THE EARTH's GASES. 



Volcanics, M> T = P. That is, P and M are greater, more rapid than 



7\ ordinar}^ effusives. 

 >P. Some marginal forms of plutonics. 



The above suggestions show how a classification of rocks on other than 

 a chemical basis might be carried out. It might be extended a great 

 deal farther. Thus the difference between obsidians and pitchstones is 

 that in one M>T, in the other M <T. Personally, however, I should 

 prefer a classification based neither on mode of occurrence nor origin, 

 but on the textures, those textures being chosen, however, which are 

 most significant. 



Other possible Activities of Absorbed Gases. 



As the previous points have mainly engaged my thought, I should 

 prefer to stop here, but it would be unfair not to notice the fact that 

 escaping gas might have activities other than those already noticed. 

 Oozing up by a slow exudation, they might be supposed to produce a 

 general contact-zone corresponding to limited contact-zones around local 

 abyssal masses. They might produce mutual interaction between alter- 

 nation beds of different character, but there are very limited traces of 

 this sort of action. So, too, the crystalline schists have often a striking 

 mineralogic parallelism with contact-zone rocks. They might help in 

 charging many mineral waters where we do not suspect them, as, for ex- 

 ample, the carbonated waters of the Eifel. If they oozed up as a cooling- 

 water they would be charged with and deposit silica, and the silification 

 of sandstones into quartzites without crushing, which Van Hise* men- 

 tions, may have their help. 



Wadsworth ascribes many minerals and textures usually considered 

 primary to the action of percolating waters after consolidation. If he is 

 right, then these exuding waters probably play a greater role than I 

 suppose. 



It is also to be noted that, as Le Conte has remarked,t there would 

 be a continuous abstracting of material from below and adding it to the 

 surface. In a region much cut by dikes I once. estimated the area cov- 

 ered by them at six per cent. 



Finally, we see that in all these ways escaping gases would accelerate 

 the flow of energy from the earth's interior. They might assist very 

 greatly in promoting unequal cooling, and hence unradial contraction ; 

 and by thus abstracting heat a few miles doAvn and adding it to the 

 upper layers, both directly and indirectly they might, by accelerating 

 crushing, alter the thermal gradient appreciabl}^ It is evident their 



*Bull. U. S. Geol. Survey, no. 8. 

 t Am. Jour. Geology, 1893, p. 564. 



