

168 STRUCTURAL AND FIELD GEOLOGY 
the converse structure appears (Fig. 47, 7) for which there is 
no name in English.* 
Faults having the same trend, but with their downthiane 
in different directions, often coalesce, and either suddenly 
terminate at the point of junction, or one may die out and — 
the other continue with usually a diminished throw. But 
when approximately parallel faults, having their downthrows 
in one and the same direction, come together, they almost 
invariably continue as a single fault, often with an increased 
amount of downthrow. 
The amount of throw of normal faults is, as we have seen, 
very variable—not exceeding a few feet in some cases, while 
in others it may reach thousands of yards, and thus bring 
into juxtaposition rocks of vastly different ages. The 
smaller faults usually extend for very short distances, while 
the greater ones may continue for hundreds of miles. The 
course of a large fault is usually approximately straight or 
gently sinuous, but not infrequently it is curved. Such 
faults may begin as a mere crack, or as a series of two or 
more converging fissures, with little or no accompanying 
rock-displacement. But as it continues, the throw gradually 
augments until a maximum is reached, after which it usually 
decreases until finally the fault dies out as it began—in a 
mere crack or series of cracks. In many cases, however, the 
throw varies very irregularly from point to point. 
The phenomena presented by the two conjugate systems 
of strike-faults and dip-faults which are so characteristic of 
many regions, lead to the belief that. these faults are of the 
same age—that they came into existence contemporaneously. 
This is suggested by several considerations to which refer- 
ence will presently be made. But it may be pointed out 
here that strike-faults of contemporaneous age rarely or 
never cross each other; and the same is the case with 
dip-faults belonging to one and the same period of disturb- 
ance. One fault may, and often does, run into another, but 
it coalesces with it, and does not diagonally intersect it. 
When strike-faults are more powerful than the dip-faults of 
* If one were required, perhaps ridge-faults might serve ; but there 
are already so many superfluous geological terms that one hesitates to 
add to the number. 
