TRANSLATORY MOVEMENTS 169 



vertical shift, and this expression is self-explanatory. When the fault is 

 a fairly clean-cut fracture and there is no bending of the strata, the slip 

 and shift are identical. 



Separation, which indicates the distance between the surfaces of the 

 two parts of a disrupted bed, vein, or of any recognizable surface, meas- 

 ured in any indicated direction. The separations will usually be meas- 

 ured, the slip and shift calculated. 



Throw and heave, which refer to the displacement of the edge of a 

 disrupted bed. 



By keeping in mind the general meaning of these words, all confusion 

 in the uses of the proposed nomenclature can be avoided. 



There is no generally accepted word in present use to denote the slip. 

 Willis and Tolman use "displacement;" Spurr uses "throw." We have 

 reserved displacement for general use, and the word "throw" is at present 

 used in quite a different sense. Becker used "slip measured on the dip of 

 the fissure." 10 The generalization of this word to comprehend any move- 

 ment in the fault surface is natural, and the word suggests its own mean- 

 ing. The word "shift" also suggests the meaning attached to it ; there is 

 no distinctive word now in use to describe the shift. 



In mines, where the fault surface itself is visible, the slip will generally 

 be determined; it is of paramount importance in mining. In field sur- 

 veys, where the fault is studied by the dislocation of the outcrop of strata, 

 or dikes, often at a considerable distance from the fault, the shift is de- 

 termined. The shift is of greater importance in the larger problems of 

 geology than the slip. The distinction between the slip and the shift is 

 important; it has not heretofore been recognized in the nomenclature of 

 faults. 



The separations are very useful terms. The perpendicular or strati- 

 graphic separation is of the greatest importance. It is frequently the 

 only important displacement determined, and in strike faults all the dis- 

 placements in a vertical plane at right angles to the fault strike — thai is. 

 all the displacements which have heretofore been given the most atten- 

 tion — can be expressed in terms of it. The offset is often the most im- 

 portant surface measurement made. 



FAULTS IN STRATIFIED ROCKS 



Among stratified rocks the character of the displacement of the strata 

 due to a fault is so much influenced bv the relation of the strike of the 



'•"Geology of the Comstock Lode, p. 17(5. Monographs Of Hie !'. S. Geological Survey 

 vol. ill. 



