THE UNSTABLE LAND. 107 



level cities? I venture to affirm, with Mallet, Oldham, 

 Schmidt, Hottinger, and Bocardo, that it is a sequence of accu- 

 mulated strains resulting from lateral pressure in the earth's 

 crust. There are two assignable causes of enormous lateral 

 pressure. First, as maintained by Constant Prevost, the solid 

 crust formed around a cooling molten globe, becoming too large 

 for the shrunken nucleus, strives to adapt itself to the dimin- 

 ished interior (molten or solid). It is, therefore, laterally 

 pressed. Relief is obtained, in part, by the development of 

 wrinkles, as in the skin of a shriveled apple, and in part, by 

 a process of crushing together. The strains are temporarily 

 resisted, but soon the crust must yield. As the crust is not 

 homogeneous, there must be stronger and weaker portions. 

 The motion which results, in the crisis of yielding, is accumu- 

 lated in isolated spots. If the pressure is a direct and simple 

 crushing pressure, then heat results from the crushing, lava is 

 formed and the pressure existing squeezes it, or the formation 

 of steam lifts it, to the surface. If the pressure has not a 

 simple, crushing tendency, there may arise a fracture. Then, 

 in. an instant, the strain is removed; the rocks recoil, and the 

 vibratory motion is generated. 



These lateral strains are augmented and localized by the 

 attractions of the sun and moon, which cause real tidal eleva- 

 tions and subsidences, and thus bring the crust to a snapping 

 tension, where the slow processes of terrestrial contraction had 

 not yet reached it. These tidal strains are greatest when the 

 moon and sun are nearest the earth, and also when they act 

 together, as at new and full moon. 



To add another word. While a tornado or cyclone is at 

 its acme of violence, the barometer is low; the pressure of 

 the atmosphere on the earth is diminished at the spot, and 

 elsewhere correspondingly increased ; the terrestrial crust must 

 therefore, tend to develop movements of the nature of tides; 

 and the predisposition to earthquake actions must be aug- 

 mented. Observation indicates the frequent actual coincidence 

 of earthquakes and cyclones. Similarly, a connection has 

 been observed between the pressure of the atmosphere and 



