392 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1910. 



earth's outer crust. The three- fourths which underlie the ocean 

 beds obviously are no exception to the conditions described. At 

 depths of 3 miles or more the rocks beneath the ocean basins must 

 also be loaded beyond the strength of rocks at the surface and must 

 approach a crushed condition. 



This crushed condition is not, however, that of rocks which fall 

 apart when crushed, for the foundations of continents and ocean 

 beds are part of the solid earth and are continuous all about the 

 sphere. There is, therefore, no space into which any crushed mass 

 may crumble. The strength of the rocks may be overcome, but they 

 can not fall apart. This condition has been reproduced experi- 

 mentally and it has been shown that marble and even the firmest 

 granite may be forced to change form, yet be held to a coherent solid. 

 The rock under these conditions may be compared to wax, if only 

 we bear in mind that it remains all the time a very strong solid. 



The zone of crushing without separating has been called the zone 

 of flow or flowage, because the movement of any rock mass under 

 such pressures is compared with that of a very stiff fluid. But the 

 word flow conveys an idea of mobility, and is thus misleading. It is 

 necessary constantly to insist that rocks in the zone of flowage are 

 rigid solids. 



Solution plays an important part in the flow of rocks. Not that 

 any large mass is dissolved at any particular time, but by the solu- 

 tion of a minute grain or molecule, which then flows from the point 

 at which it was dissolved to a point where it is redeposited. The 

 condition which causes solution is a slight excess of pressure or of 

 temperature or both ; and deposition from solution follows where 

 these slight excesses disappear. Rocks are composed of mineral 

 particles which differ widely in solubility and under adequate differ- 

 ences of pressure the less soluble may be granulated microscopically, 

 whereas those crystals which are soluble in any moisture or mineral 

 solution that may be present are dissolved and then recrystallized 

 on a point that is less hard pressed. The individual element of 

 motion is microscopic or even molecular, but the sum total of move- 

 ments may affect a mass of subcontinental dimensions during a geo- 

 logic epoch ; that is to say, during a million years or several million 

 years, more or less. 



Movements in the foundations of continents are exceedingly slow. 



In the zone of crushing, any rock mass of limited horizontal dimen- 

 sions may be regarded as the base of a column that reaches to the 

 surface of the earth. Being crushed by the weight of the superin- 

 cumbent mass it seeks to spread sidewise ; but it can not because each 

 adjacent mass, which is the base of an equally heavy column, also 

 seeks to spread in the same manner and to the same degree. If at 

 any depth in the zone of crushing one mass be under a heavier load 



