Parr VL] Oe 23 WAU LTS, - 533 
~ led to regard that fault as an important and dominant one, while it 
might be only a secondary dislocation in the near neighbourhood of 
a ereat fracture, for which the evidence would be elsewhere obtain- 
able, but which might never be seen itself. The actual position 
(within a few yards) of a large fault, its line across the country, its 
_effect on the surface, its influence on geological structure, its amount 
of vertical displacement at different parts of its course—all this 
information may be admirably worked out, and yet the actual 
fracture may never be seen in any one single section on the ground. 
A visible exposure of the fracture would be interesting ; it would 
give the exact position of the line at that particular place; but it 
would not be necessary to prove the existence of the fault, nor 
would it perhaps furnish any additional information of importance. 
The existence of an unseen fault may usually be determined by an 
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Fic. 265.—Map, ILLUSTRATING THE DETECTION OF AN UNSEEN FAvur. 
A, Field-map, showing the data actually obtained on the ground; B, completed Map, 
showing the geological structure of the district. 
examination of the geological structure of a distriet. An abruptly 
truncated outcrop is always suggestive of fracture, though sometimes 
it may be due to unconformable deposition against a steep declivity. 
Tf a series of strata (as in Fig. 265) be discovered dipping con- 
tinuously in one general direction at angles of 10° or more, and if at 
a short distance another different group be found inclined in another 
direction, the two series thus striking at each other, a fault will 
almost always be required to explain their relation. If all the 
evidence obtainable, from the sections in water-courses or other- 
wise, be put upon a map (asin A, Fig. 265) it will be seen that 
a, dislocation must run somewhere near the points marked / /, as 
there is no room for either series to turn round so as to dip 
below the other. They must be mutually truncated. The com: 
pleted map would represent them separated by a fault (FI, im 
