DISLOCATIONS OF STRATA.* 



Ill 



B m— 



120. It is seen from the figure that a single inclined stratum consisting of the 

 layers 1, 2, 3, 4 would have a surface-width (width at the earth's surface or on 

 a horizontal plane) of a b. But by means of the fault another portion is brought 

 up to the surface, and a b is increased to a c. So other faults might go on 

 increasing the extent of the surface-exposure. This is further illustrated in 

 fig. 111. Let A be a stratum 10,000 feet thick (a to c) and 100,000 feet long 

 (a to b). Let it now be faulted as in fig. B, and the parts uplifted to a dip 

 of 15°, — taking a common angle for the parts, for the sake of simplicity of 

 illustration. The projecting portions being worn off by the ordinary processes 

 of denudation, it is reduced down to fig. C, m n being the surface exposed to the 

 observer. The first error that might be made from hasty observation would 

 be that there were four distinct out- 

 cropping coal-layers (calling the -^*&- m* 



black layer thus), instead of one J a a 



and the second error, the one above 

 explained with regard to calculating 

 the thickness of the whole stratum 

 from the entire length m n in con- 

 nection with the dip. If the stratum 

 were inclined at 15° without fault- 

 ing, it would stand as in fig. D ; 

 and if then worn off to a horizontal 

 surface, the widest extent possible 

 would be cr, — less than half what 

 it has with the three faults. The 

 length of c r may be determined 

 from the thickness a c and the 

 angle of dip, the angles and one 

 side of the triangle being given to 



find the hypothenuse. "With a dip of 15° it would be less than four-tenths of a b; 

 with a dip of 30°, one-fifth of a b ; with a dip of 45°, less than one-seventh. 



It is plain also, without further explanation, that when a layer is 

 folded many times upon itself, as explained on p. 104, a large extent 

 of horizontal surface of tilted beds may be produced even when the 

 stratum thus folded has of itself little thickness. 



121. Unconformable strata. — Another consequence of the tilting or 

 displacement of strata is this : that deposits are often laid down upon 

 the upturned edges of older rocks. Fig. 112 represents cases in which, 



Fig. 112, 



after the rocks below had been folded or upturned, other strata were 

 laid down at a b horizontally on the inclined beds, being thus uncon- 



