Ch. VI.] 



PRIMARY ROCKS. 



107 



extended surface ; but it may be in some measure deceptive, 

 arising from the formation of seams or fissures between the 

 laminae, as well as between the layers or strata. Adopting, 

 however, this method, the dip of these schists is commonly 

 found to be from the granite in the vicinity of this rock, and 

 at a distance therefrom, very frequently in an opposite direc- 

 tion : it is, however, very various ; and if the inclination does 

 follow any general rule, it is that of the great curvatures 

 of these rocks, with which it almost universally corresponds. 

 The angle of the dip scarcely, in any instance, exceeds 40, 

 generally fluctuating between 25 and 35 : and the amount 

 of this angle bears no relation to the vicinity of the granite ; 

 for next this rock it is sometimes great, at others, exceed- 

 ingly small. 



If the inclined surface of the layer, which dips in the 

 opposite direction, were adopted as the surface of the stratum, 

 then the slate of Cornwall would have an inclination varying 

 from 45 to 75, and even approaching still nearer to the 

 perpendicular. There is one consideration, that might induce 

 some to prefer this view of the case ; viz., that the layers of 

 massive crystalline and granitic rocks very commonly lie in 

 directions parallel thereto. Thus, the elvan courses, near the 

 granite, almost uniformly dip towards this rock, whilst the 

 laminae of the slate are disposed in an opposite direction, so 

 that they often appear to be intersected by the elvans. The 

 following diagram will, perhaps, render this arrangement more 

 intelligible : 



Fig. 5. 



a, porphyry in granite; b, the same rock traversing both slate and granite; 

 c, the same in slate ; d, d, d, compact rock in slate. 



