DEEP-SEATED CRACKING. 267 



quakes lies in the sui)i)osabl\' plastic zone; often in fact just where we 

 might expect, below the level of no strain; that is, where lateral com- 

 presson ceases and radial contraction and lateral tension due to cooling 

 become dominant. Moreover, the common type of fault, the so-called 

 normal fault, cannot be explained by compression, but, as McGee^^ has 

 suggested and shown, may be due to difference of radial contraction, in 

 which case, of course, the crust cannot rest on a laj^er sulficiently viscous 

 or plastic to be unable to transmit and accumulate stresses so produced ; 

 and really when we look at the throw of the larger faults, like those of 

 the great bashi, it does seem natural that the crack should have extended 

 even more than five miles down.f 



So also, I i^resume, by similar methods to those applied to earthquakes, 

 the roots of volcanoes have been assigned to a depth not over thirty 

 miles ; J and, finallv, it may be remarked that whenever or wherever the 

 brecciated structure of obsidians and j^jorphyries was formed the rock 

 was then and there very capable of breaking. 



Us Possibility a priori. — Now, when we come to consider theoretically 

 the material properties of the earth as a whole, we find that in its astro- 

 nomic relations, that is, in its precession and mutation and in its resist- 

 ance to tidal deformation, it is rigid. Some geologists have been inclined 

 to reject the work of the mathematicians as involving too many hy- 

 potheses ; but it must be rememl^ered that a wide range to the assump- 

 tions has been given, and that mathematics never deals with real things 

 but yet has proved a great helj) in interpreting them. 



The rigidity spoken of refers to stresses which change in direction 

 every moment and accomplish a cycle everv twenty-four hours. They 

 therefore throw very little light on its plasticity and viscosity with refer- 

 ence to stresses of slower application ; that is, solidity or capacity for 

 accumulating strain. The upholding of the great continental irregulari- 

 ties points at least to a high degree of viscosity, even though that might, 

 if absolutely necessary, be otherwise explained through differences in 

 density, et cetera. 



Barns' experiments demonstrating that diabase solidifies under press- 

 ure § tend to show that it is solid; and all others which imply an ac- 

 cumulation of differences in radial contraction also stiggest a solid earth ; 

 yet it may be plastic, for matter may be said to be perfectly plastic if, 

 when flowage is started by exposure to a strain greater than it can stand, 

 it will continue to flow, if free to do so in the direction of strain, so long 

 as the strain is maintained ever so little above its elastic limit. At the 



* Am. Jour. Sci., vol. xxvi, p. 294. 

 tGeikie : Op. cit., 1893, p. 552. 

 X Geikie : Op. cit., 1893, p. 267. 

 § Am. Jour. Sci., vol. 46, 1893, p. 140, 



XXXVII— Bum. Geot, Soo. Air., Vol. 5, 1893. 



