AND STRATIFICATION. 71 



average, and not by measurements taken in a single 

 place, which are a frequent source of error. 



The disappearance of strata which is not the result 

 of extenuation, may arise from their dipping out of 

 sight in a gradual manner, or from those fractures or 

 subsidences by which they are entirely broken off. 

 This fracture is, of course, found at the elevated edge; 

 but the strata are sometimes renewed in a new position 

 near it, in consequence of the subsidence. 



In a series of beds, the same substances are some- 

 times repeated in succession ; at others, beds of dif- 

 ferent natures succeed in a certain order, or in repeated 

 alternations, more or less numerous or regular. Where 

 the beds are similar, they are sometimes separated by 

 mere planes, resembling fissures ; but it is more common 

 to find some slight change of texture, or some inter- 

 vening material, however small in quantity, indicating 

 the place of separation. When the beds are of dif- 

 ferent kinds, the mode of separation is generally very 

 distinct ; and, in some cases, in the secondary, it is 

 effected by the accumulation of animal or vegetable 

 remains in a thin lamina. The laminar disposition of 

 the beds themselves must be considered as, in itself a 

 result of stratification ; and I have elsewhere remarked 

 that it must not be confounded with the concretionary 

 structure. (Chap, xi.) 



Of the Positions and Relations of Strata. 



Consecutive and parallel order is a natural conse- 

 quence of the process of stratification. But this is 

 confined within certain limits, as noticed in the next 

 Chapter on the dispositions of strata. Near the unstra- 

 tified rocks also, that order ceases ; or any number 

 of the strata in one series, may be in contact with 



