WHAT IS TERKA FIRMA? WILLIS. 393 



than that borne by another adjacent to it, then the base of the 

 heavier column will tend to spread with greater horizontal force 

 than that exerted by the lighter column; but in order to cause move- 

 ment, the excess of thrust from the heavier must be greater than the 

 strength of the rocks under the lighter load. The last conclusion 

 follows because the material against which the excess of horizontal 

 pressure is directed is held to the condition of a rigid solid by the 

 very load that crushes it. 



It may seem as though the approximate balance of lateral pres- 

 sures in the foundations of mountains, continents, and ocean basins 

 were sufficient to explain the apparent stability of terra firma. But 

 it will not have escaped attentive thought that the pressure beneath 

 the mass of the Tibetan Plateau is sufficient to cause rocks at its 

 base to spread near sea level. Or that the continental plateaus stand 

 so high that their weight approaches the crushing strength of ordi- 

 nary rocks near the level of the oceanic plateaus beneath the waters. 

 Any lateral pressures, which may exist at these levels, are not op- 

 posed by lateral stress from an adjacent mass and stability depends 

 upon the firmness of the rocks. Since the Tibetan Plateau and 

 others stand, and since continents are stable, at least during very 

 long periods of time, it would seem that rocks under these great 

 loads must be stronger than the same rocks in the testing machine. 

 This is no doubt to a certain extent true, and there is some experi- 

 mental evidence to show that the rigidity of rocks increases greatly 

 under high pressures. 



The resistance which any solid offers to a permanent change of form 

 is known to physicists as the viscosity of the solid, and it may safely 

 be said that the viscosity of a solid increases under pressures a]3plied 

 from all directions in some ratio for each particular substance that 

 is as yet unknown, but which, no doubt, gives the rigidity of steel to 

 rocks a few miles below the surface of the earth. 



Here we must introduce the idea of time. There is evidence to 

 indicate that the huge masses of continents are not firm enough to 

 maintain their altitude permanently; that in the lapse of ages they 

 do spread laterally with a glacier like motion ; and that the spreading 

 lowers the surface. When this happens to a continent that has 

 already been reduced by erosion to a low plain, the conditions are 

 peculiarly favorable for submergence of the land beneath the sea, as 

 has repeatedly occurred in the history of continents. 



There is, furthermore, abundant evidence to show that at other 

 times the bases of continents have been compressed laterally, squeezed, 

 as it were. This effect has long been attributed to a contraction of 

 the earth in cooling, as was first suggested by Dana, but the advances 

 of geologic knowledge have greatly strengthened an old objection — 

 namely, that contraction by cooling is inadequate to account for the 



