102 



an inch, in others assuming a dimension of many 

 yards. It is obvious that very thin strata cannot 

 be very persistent. Many different dimensions 

 are sometimes found associated in the same series 

 of successive strata. 



An apparently continuous general stratifica- 

 tion will sometimes extend for even many miles ; 

 but the surface of the earth is seldom in such a state 

 as to allow the same individual stratum to be 

 traced for a few hundred yards. 



A stratum is sometimes simply bent ; or un- 

 dulated, and in curves of greater or less magni- 

 tude. That undulation, or curvature, may be li- 

 mited to one direction, or another, or it may ex- 

 tend to two. In such cases it produces effects in 

 vary ing the apparent position of strata, which will 

 be considered in treating of their dip. 



The curvatures of a stratum are sometimes 

 such as to cause it to assume the form of a sphe- 

 roidal crust. They are also occasionally of such a 

 nature as to amount to contortions. In other 

 cases, when considerable, they are sometimes at- 

 tended by fractures more or less numerous. 



Strata are also frequently fractured, without 



