108 THE STRUCTURE OF THE [Ch. VI. 



Now on what principle can it be decided which of these 

 two directions, exhibited by the fissures or joints traversing 

 the schistose rocks, is the true dip of the strata. In the 

 secondary rocks, the laminae of the slate are frequently 

 found at considerable angles to the strata, as determined by 

 the alternation of the slate with other rocks ; and, therefore, 

 it may be given in favour of the greater angle inclining 

 towards the granite, as in the above diagram : but it may be 

 stated that, in adopting the other, we are not singular ; for 

 Professor Sedgwick, in his excellent communications, pub- 

 lished in the Transactions of the Cambridge Philosophical 

 Society, has assigned the same dip to the Cornish strata. 

 This subject will be reverted to again hereafter ; at present, 

 we must continue the description of the primary slates. 



Complicated as the structure of these rocks may have 

 appeared, in the foregoing details they have hitherto beeen 

 only considered under their most simple forms ; for, in 

 addition to what has been already said, it must be stated that 

 all the schistose rocks are more or less curved, not only in 

 the direction of their beds, but also at right angles thereto. 

 These curvatures are sometimes contorted, but never to that 

 degree which is so characteristic of gneiss and mica-slate. 



In looking down on the slate district from a granitic 

 eminence, the surface will be found to consist of two systems 

 of curves, which cross each other at right angles; and 

 wherever the subjacent rocks are laid bare, they exhibit a 

 corresponding curvature. These curves are not, indeed, 

 always parallel with the surface ; for the latter has been more 

 or less modified by the operation of atmospheric and other 

 agents : thus, the sides of some hills are serrated or irregular, 

 by the protrusion of some strata more indestructible than those 

 with which they alternate ; and some valleys are not concave, 

 but flat, from the accumulation of detrital deposits. But such 

 variations may be easily allowed for, and, indeed, are not 

 perceptible when viewed at a distance : the general external 

 appearance, though modified, accords well with the internal 



