MECHANICAL CONSOLIDATION. 595 



acting directly or indirectly through otogenic movements and intrusives. 

 The direct pressure due to gravity increases with depth. It is therefore 

 increasing at areas where loading is taking place by sedimentation or igneous 

 accumulations, and is decreasing in areas where denudation is going on. 

 Heat is also a force producing mechanical effects. As a result of denudation, 

 by which process the rocks approach nearer the surface; by sedimentation, 

 by which process rocks pass deeper below the surface; by mechanical and 

 chemical action, and by the intrusion of igneous rocks, the temperature is 

 changed. Increase of temperature expands the material affected, and 

 therefore induces compressive strains and tends to decrease the openings 

 within the rocks. Decrease of temperature produces the opposite effect. 



The mechanical work of deformation includes the processes of consoli- 

 dation, strain within the elastic limit, and strain beyond the elastic limit. 

 Each of these processes is considered in turn. 



CONSOLIDATION. 



Mechanical consolidation is a process by which the separated parts of 

 a rock are brought closer together through pressure. The pressure may 

 be due to the weight of the superincumbent rocks, to the lateral thrust of 

 orog'enic movements, to the hydrostatic pressure of igneous rocks, and 

 to mineral growth during cementation and metasomatism. The very 

 idea of the zone of fracture, to which the belt of cementation belongs, 

 implies that the rocks and mineral particles are sufficiently strong- to be 

 self-sustaining. 



In proportion as the rocks show openings, and especially in proportion 

 as they are incoherent, mechanical consolidation is likely to produce an 

 important effect. The incoherent rocks comprise the mechanical sediments 

 and tuffs in their original condition. The coherent rocks comprise both of 

 the above classes which have been more or less cemented and the massive 

 rocks which may have openings due to various causes. 



For the present purposes the mechanical sediments are of two general 

 classes, the silts and muds, and the coarse sands and grits. The first of 

 these classes is the one which is most affected by mechanical consolidation. 

 In many cases the mineral particles of the fine-grained silts and muds appear 

 to be separated by water. In such a rock pressure gradually squeezes 

 out this water, bringing the mineral particles into contact. At the same 



