100 



LITHOLOGICAL GEOLOGY. 



hence the whole thickness is b d instead of c e. There may be many- 

 such faults, in the course of a few miles ; and each one would increase 

 the amount of error, if not guarded against. 



It is seen, from Fig. 110, 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 e. 



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 6). Let it now be 



B m — 



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 de- 

 nudation, it is reduced to Fig. C, m n be- 

 ing the surface exposed to the observer. 

 The first error that might be made from 

 hasty observation would be that there 

 were four distinct outcropping coal-layers 

 (calling the black layer thus), instead 

 of one ; and the second is the one above 

 explained with regard to calculating the 

 thickness of the whole stratum from the 

 entire length m n in connection with the 

 dip. If the stratum were inclined at 15° 

 without faulting, it would stand as in 

 Fig. D; and, if then worn off to a horizon- 

 tal surface, the widest extent possible would be c r, 

 the three faults. 



Fig. 111. 



D 



less than half what it has with 



Conformable and unconformable strata. — Strata are conformable, 

 when they conform to one another in superposition, that is, lie one 

 over the other with the same dip ; and they are unconformable when 

 one overlies the upturned edges of another stratum, with no con- 

 formity in dip or position. Fig. 112 represents cases in which, after 



Fig. 112. 



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

 down at a b and e f horizontally on the inclined beds ; these are 

 examples of unconformability. Below e f there are really two sets of 

 unconformable beds in a synclinal valley ; and, moreover, the lower 

 strata were much faulted and upturned, before the upper were laid 

 down upon them. The Connecticut River sandstone, like the latter, 



