Ch. 13] STRUCTURE 231 



MAJOR FACTORS 



The major factors of composition, structure, and state of stress de- 

 termine the equilibrium of an earth mass ; the first two factors operate 

 indirectly, and the last directly. These factors set the stage for slid- 

 ing, and any change in one may itself disturb equilibrium or produce 

 changes in other factors to lead to instability. For instance, a change 

 in composition, such as increased water content, may change both the 

 internal structure and the state of stress and lead to sliding. The 

 variables comprising or influencing the major factors are commonly 

 so interrelated that a discussion of their separate effects is impractical ; 

 therefore, they will be only briefly listed separately and their joint 

 effects considered in more detail, with a number of examples. 



Composition 



Any sedimentary deposit consists generally of three phases, solid, 

 liquid, and gas. Important factors in the physical composition of the 

 solid phase are the size, size distribution, shape, area, and nature of 

 surface of the individual rock or mineral particles; the amount and 

 kind of cement; and the mechanical strength of the particles and the 

 cement. Some of these characteristics may be permanent, some are 

 variable. To be considered in the chemical character of the solid 

 phase are mineralogy, chemical stability, and surface effects such as 

 hydration and base exchange. 



The relative abundance of the liquid phase is critical in determining 

 the mechanical stability of fine-grained sedimentary deposits. The 

 liquid, of course, is water. Many of the physical properties of sedi- 

 ments are determined by the peculiar properties of water and the 

 state in which it is held; that is, whether or not it has a free surface, 

 or is held in capillaries, or bound into a semi-solid state around the 

 mineral grains, or actually enters into the crystal lattice of certain 

 clay minerals. Air or other gases within an unconsolidated deposit 

 may affect its physical and chemical properties. 



Steucture 



For convenience, structure may be classed as either gross or fine. 

 Gross structure includes stratigraphic sequence, attitude, homogeneity, 

 and discontinuities such as bedding planes, joints, and faults. Fine 

 structure includes the arrangement of individual particles into loose 

 or compact structures, resulting from modes of disposition, or the 

 interaction of surface forces of the solid and liquid phases. 



