AND STRATIFICATION. 69 



inclination. But the student must not imagine that 

 the stratum has a greater extent on this line than on 

 the reverse, or is of a long and narrow form : that can 

 only be said of its visible portion ; and a horizontal 

 bed, having no dip, can have no direction. 



Strata are subject to the various accidents of frac- 

 ture, displacement, flexure, and contortion ; and in 

 some very rare cases, they even lose their regular forms 

 and become shapeless masses. 



The forms of strata are far more perfect in the more 

 recent than in the antient series; and, in the latter, 

 they are sometimes either so obscure or so difficult to 

 discover, that their existence has been altogether 

 denied. Hence, in some degree, has arisen a most 

 pernicious confusion in geological descriptions and 

 reasonings, which it has here been attempted to re- 

 medy by separating the unstratified rocks from them. 

 The cause of this imperfection, in the older rocks, 

 will be found to lie in the changes of their positions, and 

 in the disturbance of their regularity, from flexure or 

 fracture ; in the less definite and frequent alternations 

 of rocks of different characters which they present ; 

 and, among the argillaceous schists, from confounding 

 the schistose structure with the planes of stratification. 



Most commonly, a single bed consists of only one 

 substance ; but the materials occasionally differ in 

 size in different parts, and that change occurs, either 

 laterally, according to the plane of the stratum, or in 

 the opposite direction, according to its depth. Changes 

 of the absolute quality of the rock in a stratum, are 

 far more rare. Many rocks are subject to be divided, 

 by joints, at some angle to the plane of the stratifi- 

 cation ; and these are often remarkably regular, so as 

 to separate the bed into cuboidal or other forms, of 

 considerable accuracy. Thus, in the sandstones, there 



