PRINCIPLES CONTROLLING ORE DEPOSITS 733 



rocks are readjusted to deforming stresses. 1 If the movement 

 be slow and the temperature that of moderate depth the stress 

 does not need to accumulate so that it shall be greater than the 

 crushing strength of the rocks. Under such conditions, long 

 before the crushing strength is reached, the contained water 

 begins to act upon the material of the rocks and re-arranges it 

 by continuous solution and deposition; so that it behaves as a 

 plastic body. At all times the rock is a solid except for the 

 infinitesimal amount held in solution ; and yet it continually 

 adjusts itself to the deforming stresses. A great many rocks 

 which have been thus deformed under deep-seated conditions 

 have a laminar structure which is analogous, not exactly similar, 

 but analogous, to the leaves of a book. To make the analogy 

 exact it would be necessary to suppose that the leaves are 

 welded together, i. e., held firmly by the molecular attractions 

 between them. What has happened in the case of these laminar 

 rocks ? They have been transformed from a massive to a lam- 

 inated form by recrystallization, but in many cases combined 

 with mineral granulation and differential movements of the min- 

 eral particles. During the process of recrystallization, for each 

 mineral particle, material is continually taken into solution on 

 the sides where subjected to greatest stress and deposited on the 

 edges where the stress is less, until the laminar structure is pro- 

 duced. The process of adjustment largely and in many instances 

 mainly by continual solution and redeposition is rock flowage. 

 Now rocks in which this process has taken place are found at the 

 surface at many places. Moreover these rocks are frequently 

 those of great strength. In many places it is certain that the 

 amount of material which has been removed by erosion since 

 the rocks were recrystallized is not more than 2000 or at most 

 3000 meters. Since, therefore, the process of rock flowage often 

 takes place at much less depth than that at which rocks are 

 crushed, it follows that large openings are not likely to exist at 

 depths so great as above calculated for the closing of openings 



1 Metamorphism of Rocks and Rock Flowage, by C. R. Van Hise : Bull. Geol. 

 Soc. Am., Vol. IX, pp. 269-328, PI. XIX. 



