Partlil] —-—dDIP, STRIKE, OUTCROP. 513. 
dip. A set of beds dipping westward for half a mile (a to 6 Fig. 231) 
have a north and south strike for the same distance. If the dip 
changes to 8.W., 8., S.E., and H., the strike will bend round in a 
curving line (asat 8). In the case of a gud-qud-versal dip the strike 
forms a complete circle (as at A). The dip being ascertained gives 
the strike, but the strike does not certainly indicate the direction of 
_ dip, which may be either to the one side or the other. Two groups of 
strata dipping the one east and the other west have both a north and 
south strike. Strike may be conceived as always a level line on the 
plane of the horizon, so that no matter how much the ground may 
undulate, or the outcrop may vary, or the dip may change, the strike 
4 S > 
SS 
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SS SS > 
= 
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= 
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Fie. 231.—GzoLocicaL Map, sHowInc STRATA CONTINUOUSLY EXPOSED ALONG A 
BEACH AND OCCASIONALLY IN THE INTERIOR, 
will remain horizontal. Hence in mining operations it is commonly 
spoken of as the level-course or level-bearing. A “level” or underground 
road-way, driven through a coal-seain at right angles to the dip, will 
undulate in its trend if the dip changes in direction, but it may be 
made perfectly level, and kept so throughout a whole coal-field so 
long as it is not interfered with by dislocations. 
In Fig. 231, the strike and outcrop are coincident on the flat beach, but 
cease to be so the moment the ground begins to slope up into the coast- 
cliff. This is seen in the eastern half of the map, where the lines of 
outcrop slant up into the cliff at an angle dependent mainly on the 
amount of. the dip. A section drawn in the line L L’ would show the 
geolgoical structure represented in Fig. 232. By noting the angles of 
21 
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