» B * 



FIG. 89. — Diagram explanatory of dip measurement. 



FOLDS 233 



to EBF. The angle DAB is the one actually measured, but as 

 the two horizontal lines are parallel, that angle must be equal to 

 the angle ABE, which 



is the actual angle of C 4^ — — D 



dip. 



Strike. — The line of 

 intersection formed by 

 the dipping bed with 

 the plane of the hori- 

 zon is called the line 

 of strike and is necessarily at right angles to the line of dip. 

 (See Fig. 91.) If a piece of slate be held in an inclined position 

 and lowered into a vessel of water, the wet line will represent the 

 strike. As long as the direction of the dip remains constant, the 

 line of strike is straight, but as the direction of the dip changes, 

 the strike changes also, always keeping at right angles to the dip, 

 and in such cases as the Appalachian Mountains the lines of strike 

 are sweeping curves. 



Outcrop is the line along which a dipping bed cuts the surface of 

 the ground, and is, of course, due to erosion, which has truncated 

 the folds of strata. Except in the case of fractured beds, which 

 will be considered in the following section, if there were no erosion, 

 there could be no outcrop. When the surface of the ground is 

 level, outcrop and strike become coincident, because the surface 

 then is practically a horizontal plane. With the dip remaining con- 

 stant, the more rugged and broken the surface becomes, the more 

 widely do strike and outcrop diverge. For a given form of surface, 

 outcrop and strike differ more when the beds dip at a low angle 

 than when the dip is steep, for when the strata are vertical, outcrop 

 and strike again coincide, and the more nearly the strata approach 

 vertically, the more closely do the two lines come together. 



Having digressed to make these necessary definitions, we may 

 now return to the subject of folds. 



Folds present themselves to observation under many different 

 aspects, all of which may be regarded as modifications of two 

 principal types. 



