174 



REID ET AL. REPORT ON NOMENCLATURE OF FAULTS 



might be called the gap and overlap of the bed, respectively; they are 

 measured parallel with the strike of the bed. 



The perpendicular separation is the distance between the planes of the 

 two parts of a dislocated bed or other surface measured at right angles 

 to these planes. 



The term stratigraphic separation may be used to denote the perpen- 

 dicular separation of the strata. The expression "stratigraphical throw" 

 was suggested by De Margerie and Heim 18 to represent what we have 

 called the stratigraphic separation. It was adopted by Willis 19 and. has 

 found its way into some text-books. It has no relation to the "throw," as 



Figure 5. — Plan of an oblique Fault. 



Figure 6. — Plan of an oblique Fault. 



defined below, and it seems unwise to use the same word to represent 

 displacements of quite different kinds. (For illustrations of the sepa- 

 rations see figures 15, 16, and 17.) 



It is extremely important clearly to distinguish between the slip and 

 shift and the separation. The first two refer to the actual relative dis- 

 placement of the two sides of the fault, the last to the relative displace- 

 ment of the surfaces of the two branches of a dislocated bed, etcetera. 



Movements of one side or of both sides of the fault parallel with the 

 plane of a bed would not alter the separation of the bed, but would ma- 

 terially alter the slip and shift. 



The measures which will be most commonly made are the offset at the 

 surface, and the vertical and horizontal separations in shafts and drifts, 

 respectively. In order to determine the slip or shift, it is necessary to 

 know the original and displaced positions of a point; this is not always 

 possible, and the slip and shift have rarely been determined with ac- 

 curacy. If striae on the fault surface should give the direction of the 



18 Loc. cit.. p. 18. 



19 Mechanics of Appalachian Structure. Thirteenth Ann. Rept. U. S. Geological Sur- 

 vey, 1891-1892, pt. ii, p. 224. He reduces the first word to "stratigraphic." 



