114 



STRUCTURAL GEOLOGY. 



128. 



129. 



B m- 



The figure (128) represents a fault at bg, so that the strata 1, 2, 3, 4 to the 

 left are repeated to the right ; and hence the whole thickness is hcl instead 



of ce. ah is the width at surface of 

 the strata 1, 2, 3, 4 ; but by the fault,. 

 ab is increased to ac. 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. 



So other faults might go on in- 

 creasing the extent of the surface 

 exposure. This is further illustrated in Fig. 129. Let A be a stratum 

 10,000 feet thick (a to c) and 100,000 feet long (a to h). 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 to Fig. C, nin being 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 (call- 

 ing 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 

 mn in connection with the dip. Very 

 often the beds have been shoved up over 

 one another in the making of a monocline 

 to such an extent that the faults are almost 

 or wholly obliterated. A calculation of the 

 thickness in such a case is impossible. 



If the stratum (Fig. 129 A) were in- 

 clined 15° without faulting, it would stand as in D ; and if then worn off to 

 a horizontal surface, the widest extent possible would be cr, which is less 

 than half what it has with the three faults. A block of the size mentioned 

 would require, in order to make it a monocline of 45°, that one end should 

 be dropped down 70,000 feet, or the other end raised as much, or that this 

 amount of change should be divided between the two ends ; and for a mono- 

 clinal block having a dip of 60°, the drop-down or upthrust would have to 

 be nearly 87,000 feet, or more than 16 miles. Calculating the thickness from 

 the dip in a region is liable, therefore, to enormous error. 



5. ConformdbilUy, Unconformahility. 

 Successive strata in a region may be conformable to one another or uncon- 

 formable. In the series of strata made over the earth's crust, the rocks 

 of successive periods and ages have, in large parts of the world, been made 



D 



