596 A TREATISE ON METAMORPHISM. 



time the mineral particles are mechanically readjusted in reference to one 

 another. In consequence of the two the volume of the material may be 

 considerably decreased. The process of consolidation may continue until 

 finally the particles are brought so close together that they cohere. As 

 explained by Becker," when the films of water between the particles 

 become very thin, they may become an important factor in the coherence 

 of the rocks. The molecular attraction of the water films and the adjacent 

 particles, or their adhesion, and the cohesion of the molecules of the films 

 may be sufficient to give the rocks a certain amount of strength. As the 

 process of squeezing out the water goes on, many of the particles are 

 brought so close together as to be within the sphere of molecular attraction 

 independently of the separating films of water. So far as this is true the 

 particles are welded by the pi-essure. The welding and the adherent 

 influence of the water films may give the mucls and silts marked coherence. 



During the process of consolidation, as the water escapes the pressure 

 to which it is subject gradually becomes less. Therefore if saturated or 

 nearly so, during its escape it may deposit a portion of the material in 

 solution, and hence cementation dependent upon mechanical consolidation 

 may promote induration. 



In proportion as the particles of the mechanical sediments become 

 large and clean the process of mechanical consolidation becomes less 

 important, for the points of contact rapidly decrease in number as the 

 sediments become coarse, and where they are very coarse such contacts are 

 so few that the rocks gain little strength as a result of close contact and the 

 adherent effect of the films of water. Moreover, as shown by Professor 

 Slichter, 6 coarse mechanical sediments deposited by water are usually 

 arranged in a somewhat compact manner. This natural arrangement, 

 while not the most compact possible, is so compact that experiments 

 show that subsequent mechanical disturbances sufficient to displace the 

 grains in reference to one another ordinarily result in decreasing- the com- 

 pactness of the packing, and therefore in increasing the volume of the rocks. 



The tuffs are somewhat intermediate in character between the coarse 

 and the fine mechanical sediments. They ordinarily include both classes 



« Becker, Geo. F., The torsional theory of joints: Trans. Am. Inst. Min. Eng., vol. 24, 1895, p. 131. 

 ^Slichter, C. S., Theoretical investigation of the motion of ground waters: Nineteenth Ann. Kept. 

 U. S. Geol. Survey, pt. 2, 1899, p. 305. 



