GENERAL STATE OF STRAIN IN ROCKS. 597 



of material in the same rocks. If originally arranged under air, they have 

 a very much less compact arrangement than they or the ordinary sediments 

 do if arranged under water. Thus many tuffs are in a condition in which 

 mechanical adjustment of the particles by pressure may give a considerably 

 more compact arrangement. Also, since between the coarse particles there 

 are many fine particles, the number of points of contact between the 

 particles maj'- be sufficient to give considerable coherence by welding, and 

 the strength is further increased by the adherent power of the water films, 

 precisely as in the case of the muds and silts. Therefore it is probable that 

 to a considerable extent tuffs may be consolidated by pressure. 



In general in the zone of fracture the coherent rocks are sufficiently 

 strong to support themselves, and mechanical consolidation is relatively 

 unimportant, However, the openings produced by rather rapid movements 

 may be subsequently gradually closed by the steady pressure of gravity or 

 thrust. In the massive rocks containing no openings mechanical consolida- 

 tion has no appreciable effect. 



STRAIN WITHIN ELASTIC LIMIT. 



All substances subjected to stress are more or less strained, and the 

 amount of the strain is proportional, although not directly proportional to 

 the stress. The rocks throughout the belt of cementation are all subjected 

 to stress, and therefore to strain; for none are free from the Aveight of the 

 superincumbent material, and in general they are subjected to compressive 

 or tensile stresses parallel to the surface of the earth. It has been seen 

 that in the belt of cementation the direct stress of gravity, and its indirect 

 effect through orogenic movements, is sufficient to mechanically consolidate 

 the rocks and to fracture them in the most extensive manner. It is clear 

 that in a given case of rupture, before fracture took place the rocks must 

 have been strained to the elastic limit. Therefore the amount of the strain 

 within the belt of cementation varies from an insignificant quantity to 

 strain to the elastic limit of the rocks under the conditions in which they 

 exist. It would be generally agreed that the intensity of the strain is very 

 important at moderate depths. But it is commonly supposed that rocks 

 near the surface, and especially in little disturbed regions, are not under 

 considerable strain; but that this conclusion is erroneous is shown by 

 observations which have been made by a number of men. Niles has 



