268 A. C. LANE — GEOLOGIC ACTIVITY OF THE EARTH S GASES. 



same time that elastic limit may be high. Now consider for a moment 

 what happens when a rock is crushed. Its elastic limit is passed and it 

 begins to flow as a plastic body. Then tensile strains are produced and 

 the stresses which these excite in consequence of its rigidity are soon 

 too much for its ultimate strength or cohesiveness and it flies in pieces. 

 If we return to Kick's experiments, we find that the brittle object exjDeri- 

 mented on is subjected under hydrostatic pressure to the stresses exerted 

 by the surrounding viscous medium. Now it has been proved* that 

 tensile shears, united with hydrostatic pressure greater than any one 

 of them, combine or reduce to mere pressure. The body is therefore 

 under pressure pure and simple, so that no cracks can be formed. It 

 would seem at first sight as if the substrata of the earth were in the same 

 condition, for certainly the pressure would increase within ten miles of 

 depth far bej^ond an3^thing that Kick used or that the rocks at the sur- 

 face stand. 



We must remember, how^ever, that we are dealing with hydrostatic 

 pressure, and not the vertical pressure of rocks, which would offer no 

 direct resistance to lateral tension ; therefore it is merely the w^eight of a 

 column of water that we have to consider. This, at a depth of 16,000 

 to 21,000 meters, would have a pressure equal to that required to crush 

 any rock. Above this point cracking is therefore theoretically possible, 

 and before we reach this depth we should have passed the critical point 

 of water and be dealing not with a liquid, but with a gas. The rocks 

 would, if we could see them, be already beginning to glow with a dull, 

 red heat; but as many lavas come to us much hotter than this we are 

 not satisfied with the possibility of cracks only thus far. The prob- 

 ability of deeper cracking depends on the greatness of lateral shears, and 

 whether intermolecular absorbed gaseous matter would convey the hy- 

 drostatic pressure. 



As to the last point, I think we can see a very material difference be- 

 tween the pressure of water from without, aiding cohesiveness, and the 

 pressure exerted by interpenetrating gas anxious to escape. Consider 

 two points on each side of a surface where rupture is nearly ready to 

 take place. It will take place when the cohesive force is not able to 

 resist those tending to separate them. Now, the effect of the absorbed 

 gas is to work against cohesion. It is precisely the opposite of hydro- 

 static pressure and may be treated as taking it up and neutralizing it. 

 The one tends to condense, the other to expand. 



Its Production by Stresses. — As to the tensile stresses produced we have, 

 first, those due to cooling and contraction. As these are generated slowly, 

 if the earth substance were a viscous fluid they would be yielded to by 



* Beoker: Bull. Geol. Soc. Am., vol. 4, p. 1.1 



