110 



LITHOLOGICAL GEOLOGY. 



under the pressure, are the causes which have produced these distortions. 

 Univalves and all fossils are liable to become similarly misshapen under the 

 same causes. 



119. Calculating the thickness of strata. — When strata are inclined, as 

 in fig. 110, the thickness is ascertained by measuring the extent 

 along the surface, and also the angle of dip, and then calculating the 



Fig. 110. 



thickness by trigonometry. The thickness of the strata from a to b 

 is b d, the line b d being drawn at right angles to the strata. Mea- 

 suring a b, and the dip, which is the angle bad, the angles and hypo- 

 thenuse of the triangle a b d are given to determine one side b d. Or 

 with the distance a e the side c e would be found. 



But it is important, for trustworthy results, that the absence of 

 faults be first ascertained. The figure (110) 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 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. 



