70 CHARACTERS OF STRATA 



are often produced subsidiary forms resembling those 

 of granite; and this, among other causes, has aided 

 in supporting the erroneous belief in the stratification 

 of that substance. In other cases, they confuse the 

 inexperienced observer, by misleading him respecting 

 the planes of stratification ; as happens particularly 

 in the argillaceous schists. In the Diallage rock of 

 Shetland, the fissures are so numerous and extensive 

 as entirely to suppress the appearances of the sepa- 

 rating planes of the strata; so that, in determining 

 the positions of these, recourse must be had to other 

 indications, derived principally from the more regular 

 adjoining rocks. 



When, by the repetition of beds, either of the same 

 or of different substances, there are formed successions 

 or series of strata, the knowledge which we obtain 

 of the various rocks, is derived chiefly from that ob- 

 liquity of position which is fully discussed in the next 

 chapter ; in consequence of which, united to its various 

 accidents, every member of a considerable series is some- 

 times brought into view, even within a narrow space. 

 By a careful comparison of positions, the same object 

 is effected, through many interruptions, and over dis- 

 tricts of great extent. 



It is sometimes observed that such a series preserves, 

 for a great space, a very accurate general parallelism ; 

 the different beds being parallel among themselves. 

 But, very frequently, the whole series is bounded, as 

 a single bed may be, by inclined planes; in conse- 

 quence of which it gradually vanishes by extenuation. 

 The other accidents of a single stratum are also true 

 of a whole series ; which may thus vary*, by undulation 

 or flexure, either in the line of its inclination, or in that 

 of its direction. Thus it becomes necessary that obser- 

 vations on these lines should be regulated by a general 



