TRANSLATORY MOVEMENTS 175 



slip, the displacement of a bed in that direction would be the slip. Other 

 methods must be used with other data. 20 



THROW AND HEAVE 



The generalization of "throw" and "heave" presents peculiar difficul- 

 ties. They apparently first came into use in the coal fields of Great 

 Britain, where in general the strata are nearly horizontal or the faults 

 are strike faults. If a fault were encountered when a coal seam was 

 being worked, it would be important to know how far a drift should be 

 run horizontally and how far a shaft should be opened vertically to reach 

 the other part of the disrupted seam. The necessity of expressing these 

 distances succinctly seems to have given rise to the two terms; the 

 "heave" to represent the horizontal distance, the "throw" to represent 

 the vertical distance, between the two ends of the disrupted seam. The 

 choice of the words themselves evidently depended on the fact that the 

 distances to be expressed were exactly equal to the relative horizontal 

 and vertical displacements of the two sides of the fault, and the words 

 were used indifferently to express the distances mentioned and the dis- 

 placements. When, however, we have to do with oblique faults, a glance^ 

 at figures 13 and 14 will show that the vertical distance between the ends 

 of the disrupted bed will vary with the horizontal direction between the 

 points to be used for its determination, and that, in the case of oblique- 

 slip faults the stratum on the downthrow side of the fault may be above 

 that on the upthrow side, and vice versa. Therefore the two uses of the 

 word "throw" become completely incompatible, and. it becomes necessary 

 to limit the word to one of the displacements mentioned. It seems im- 

 possible to separate the word from its relation to the strata and apply it 

 to the relative displacement of the sides of the fault, for miners would 

 never accept the change; and, moreover, with the great majority of 

 faults the true relative displacement of the sides would never be known 

 and the word could not be used at all ; when this displacement is known 

 it can be properly designated by the words "slip" or "shift." Tt is 

 therefore advisable" to restrict the word "throw," and with it the word 

 "heave," to the displacements of the ends of a disrupted stratum; and m 

 order to extend the use of the words to oblique faults, we must define the 

 vertical plane in which the two points of the disrupted stratum lie which 

 are to be used to determine them. These considerations lead to the fol- 

 lowing definitions : 



20 General methods of determining the slip and shift are given in Geometry of Faults. 

 Bulletin of the Geological Society of America, vol. xx, pp. 171-100, and In Tolnmn's 

 Graphical Solution of Fault Problems, loc. clt. 



