
156 STRUCTURAL AND FIELD GEOLOGY 
occur. The amount of vertical displacement is known as the 
throw * of a fault, and is measured by protracting a line ina 
horizontal direction (as in Fig. 33), across the fault from the 
truncated end of some particular bed (a) until a perpendicular 
(«+—a*) dropped from the protracted line can reach the other 
end of the selected stratum on the opposite side of the fault. 
Miners seldom use the term /awlt, but speak of downthrows 
or downcasts, and upthrows or upcasts, according to the 
F F/ F? 

FIG. 33.—NORMAL FAULTS IN HORIZONTAL STRATA. 
direction in which they are working. ‘Thus the faults (F4, F?) 
shown in Fig. 33 would be described as downcasts or down- 
throws if they were encountered by a miner working in the 
direction from A to a, or from @ to A, but he would speak of 
them as upcasts or upthrows, if he approached them from the 
direction of © tog, or from, to a 
It is obvious. that in Fig. 33, representing tamees 
horizontal strata, the amount of throw is equal to the 
thickness of the beds lying between, 2—a? and +2!—a?; 
but this is not so’ aig 
case of inclined strata, aim 
Fig. 34, for example; yime 
amount of vertical displace- 
ment (@a—-+z) is in excess of 
the thickness of the strata 
measured as in the ie. 
ceding illustration (Fig. 33) at right angles to the planes of 
bedding (a1—a’), 
Strata cut across by an inclined fault are not only 
dropped to a lower level on the downthrow side, but the fault 

FIG. ' 34. 
* The amount of displacement varies indefinitely. Some faults are 
mere slips of a few feet or inches ; others are downthrows of several 
thousand yards. Between these extremes all gradations are met with. 
