106 THE STRUCTURE OF THE [Ch. VI. 



following statement: "In the course of the operations at- 

 tending the driving of levels and sinking of shafts in Crone- 

 bane and Tigrony, I have observed," says Mr. Weaver, " that 

 the slaty rocks (whose general range is nearly N. E. and S. W., 

 and dip S. E. at an angle of 65) are divided into great 

 beds, commonly about five fathoms thick, by parallel seams 

 or joints, which intersect the inclined plane of the clay-slate 

 at right angles, dipping 25 towards the N. W. These seams 

 are open fissures, which will sometimes admit one or two 

 fingers, and at other times scarcely the blade of a knife. In 

 their progress they pass uninterruptedly through all the beds 

 and contemporaneous veins included in the slaty rocks, 

 dividing them, and sometimes producing, as it were, a sensible 

 alteration in their disposition, though to no great extent. 

 This tendency towards a division into horizontal beds (in- 

 dependent of the slaty structure) may also be observed in 

 several parts of this district." * 



From the preceding observations on the arrangement of 

 the Cornish schistose rocks, it is evident that the dip of the 

 layers or strata may be referred to more than one direction : 

 which of these is to be regarded as the true one ? This is a 

 question not easily answered in a satisfactory manner ; and yet 

 we find that geologists have not often expressed any hesitation 

 in determining this point, though one of great importance, as 

 many of the arguments advanced in favour of the prevailing 

 theory depend on the direction and the inclination of the 

 primary strata. 



In Cornwall, the choice of the true dip lies between the 

 two largest faces or surfaces of the rhomboidal layers, which 

 run in the same direction, most commonly N. E. and S. W., 

 but dip at opposite angles. In the description of this county, 

 published in the Cornish Transactions, that surface of the 

 layer which is parallel with the laminae of the rock has always 

 been selected : this, indeed, generally appears to be the most 



* Geol. Trans., vol. v, p. 217. 



