Prof. Sedgwick on the Structure of large Mineral Masses. 473 



cleavage of slate rocks is sometimes at right angles with the planes of stratifi- 

 cation : but I doubt the correctness of the statement, as 1 never observed such 

 a position of the cleavage ; and think that a mistake has probably been com- 

 mitted in confounding joints (which are often at right angles to the strata) 

 with true slaty cleavage*. 



In some of the largest quarries of Cumberland and Caernarvonshire the 

 cleavage planes dip towards the same point with the strata, but at a greater 

 angle. This rule has, however, too many exceptions to be of value ; for the 

 two planes not unusually dip to opposite points of the compass. There is, 

 however, one rule of position to which the exceptions are not, I believe, 

 numerous, viz., where the cleavage is well developed in a thick mass of slate 

 rock the strike of the cleavage is nearly coincident with the strike of the 

 beds. Where, however, the slate is coarse and thin-bedded, the direction of 

 the cleavage planes is often considerably inclined to the strike of the beds ; 

 and, ultimately, such planes will appear to pass into an obhque set of cross 

 joints. 



Besides the planes of cleavage, we may often find in large slate quarries 

 one or more sets of cross joints, which, combined with the cleavage, divide 

 the rock into rhombohedral solids. Should any one assert that this subdivision 

 of slate rocks into rhombohedral solids implies three planes of cleavage, we 

 might reply, that such solids are not capable of indefinite subdivision into 

 similar solids, except in one direction, viz., that of true cleavage ; and in this 

 way (even in hand-specimens) we may generally distinguish the true cleavage 

 planes from the joints. Respecting the angles of such solids, I have no rule 

 to offer that does not appear to be destroyed by the exceptions ; but as an 

 approximate rule, there is, I think, one set of joints often nearly transverse to 

 the strike of the beds. If there be any truth in this remark, the prevailing- 

 direction of such joints must be north-west, because the prevailing strike of 

 our slate rocks is north-east : but an assumption like this can only be re- 

 ceived under proper limitations. 



Of all places a slate quarry is often the very worst for determining the 



tification ; but through these beds there is an oblique "bate", or cleavage, parallel to which the 

 flags naturally break, so as to exhibit regular bevelled edges. 



Any one who is anxious to understand the structure of slate rocks would do well to commence 

 his study of them in Charnwood Forest. There is hardly a quarry on the east side of the Forest 

 which does not clearly show the difference between stratification and cleavage ; and in all the 

 larger quarries near Swithland, the peculiar stripes marking the stratification of this class of rocks 

 are very well exhibited. 



* Researches on Theoretical Geology, p. 108. 



