MATERIALS oi THE EARTH. 63 



inequalities of thickness in particular parts, or of 

 deficiencies of certain portions of some stratum. 



The case above stated, of a single uniform elevation, 

 conveys the simplest idea of a mountain ridge of stra- 

 tified rocks. But ridges are sometimes found, in which 

 the direction of the stratification is oblique, not 

 parallel. An example of this disposition occurs in 

 our own country, in Bute, and may be observed in many 

 other places. It is conspicuous, according to Hum- 

 boldt, in the great chain of Mexico. Even in these 

 cases however, where single ridges may not follow the 

 direction of the strata, that direction maintains its 

 general bearing ; and it is a remarkable fact, although 

 unwarrantably extended much further than it will bear, 

 that in many parts of the world, and over very large 

 spaces, the strata hold a parallel direction according 

 to their elevated edges ; and that if we take all that 

 have been examined, there are more tracts in which 

 that direction tends towards the North-east, than to 

 any other point of the compass. This is the case in 

 Britain ; and it is particularly sensible in Scotland, 

 where the heights of the elevations and the facility of 

 access, render this circumstance easily examined. 



But even where strata are thus prolonged on one 

 line of elevation, the quantity of that elevation varies 

 in different places ; and thus numerous other irregu- 

 larities are produced in the forms of mountain ridges. 

 As, lastly, the elevation of strata may increase till 

 they become vertical, other cases are produced, by 

 which the forms of mountains become still further 

 modified. 



It next happens, that instead of strata being simply 

 elevated in one continuous mass, they are raised by 

 successive intervals, forming stairs, by which the 



