34 T^s Interior of the Earth — Claypole. 
motion there, though very great, must be gentle rather than 
sudden, and must also be equally distributed. If moreover this 
layer be nearer the surface beneath the continents than beneath 
the oceans then a less thickness of the crust is subject to crush- 
ing than to "compressive extension"' in the former than in the 
latter case. This leaves a greater mass below the level of "no 
strain" vv^hich is suffering extension. The violent commotion 
that often disturbs the upper crust can not in the least affect the 
deeper masses. The only condition that seems competent to 
reduce the amount of extension beneath the land-masses v^ould 
be so great a depression of the level of "no strain" as to leave 
less material below it subject to extension, and this the nature of 
the case as already shown, does not admit. 
In reflecting on the subject we must bear in mind that the 
results above stated are only obtained by assuming as the tem- 
perature of original solidification of the crust a very high figure 
— in Mr. Fisher's investigation 7000 degrees F. This datum 
seems scarcely admissible when we recollect that material simi- 
lar to that which composed the primeval crust now solidifies at 
about 2000 degrees F. Even allowing for the effect of un- 
doubted greater pressure at that date and perhaps for some other 
conditions different from any now prevailing, it seems more in 
accord with physical laws that the original slaggy liquid cooled 
to a much lower degree before solidification took place. By 
changing this datum to z|ooo degrees F. Mr. Fisher obtains only 
0.7 of a mile as the present depth of the level of no strain, a re- 
sult yet more discordant with the views of physical geology 
than the former, and in the face of known facts scarcely tenable. 
Again, as said above, Mr. D. assumes an excessively high fig- 
ure for the duration of the cooling process — 175,000,000 years 
— a period which, though not impossible, is yet much beyond 
that which is usually believed to have elapsed since the epoch 
of consolidation. 
Considering all these points it may be well to hesitate before 
yielding full and implicit acceptance of these new results of the 
mathematician. The physical geologist is grateful for all con- 
clusions that his mathematical brethren can give him, but he 
1 Mr. J. M. Reade's term. 
