108 Physlcdl Theoriefi of the Kartli — Eeade. 
for thinking so is that rocks from a much greater depth thau 
the calculations give for the position of the neutral shell have 
been forced up to the surface; the uudei'lying idea evidently 
being that only by the compression of the exterior shell of the 
globe through secular contraction can this have happened. Is 
this not begging tlie question in favor of what is called the 
"Contraction" theory of mountain formation and ignoring other 
possible explanations of the origin of the corrngations of the 
earth's surface? 
Professor Claypole seems to think that tlie strata immediately 
below the h?vel-of-no-strain must be in a state of "aqueo- 
igneous" or even "igneous plasticity.'' and therefore we are 
driven to place the foci of earthquakes and other disturbances 
in the strata above the level-of-no-strain, a limitation of depth 
for which we have no warrant in observation. I fail to see 
in what Avay the existence of the level-of-no-strain affects the 
question at all . It can no more do so than can the existence 
of a neutral axis in a bent beam affect the solidity of the wood 
of which it is composed either abov^e or below such a mathe- 
matical line. 
When I developed the idea of the existence of a leveWjf- 
no-strain, or neutral shell, it was partly with the object of 
showing that the current ideas of the effects of secular contrac- 
tion on the crust of our globe were undefined and fallacious, 
— "foggy" perhaps would be the correct word — and this though 
eminent mathematicians and physicists had been at work on the 
problem for some 50 years, curiously sliowing how a seemingly 
obvious result may escape minds that trust too much to mechan- 
ical modes of thinking. 
It indeed seems extraordinary that a mathematician and a 
practical engineer like the late Robert Mallet, who devoted 
years to the development of a theory of volcanic action depend- 
ent upon the heat evolved by the crushing of the crust of the 
globe, should have failed to discover in such a crust the exist- 
ence of a neutral zone. Had he done so, he would probably 
have abandoned his theory. Many pages of the transactions 
of the iloyal society and much good mathematics would have 
been saved; but no doubt the working out of such problems, 
even if founded on false data, is one of the necessary steps in 
the development of true ideas concerning the complicated 
operations of nature. 
