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



nated structure, depending on the gradual deposition of the materials (and, 

 therefore, parallel to the stratification), as one of the cases of a slaty struc- 

 ture. Now, I contend that these structures ought never to be confounded. 

 They have very little in common ; can, in ninety-nine instances out of a 

 hundred, be distinguished even in hand-specimens; and ought to be desig- 

 nated by separate names. 



If the planes of slaty cleavage and st»*atification be inclined at indefinite 

 angles, some one may suppose that, among other varieties of position, we 

 may find cases of coincidence. 1 can only state in reply, that I never saw 

 such a case. The planes are, however, sometimes inclined at a small angle, 

 and, from their appearance in a c]uarry, might easily be supposed parallel ; 

 but a careful examination will, 1 believe, always correct the error. I have 

 seen a slate quarry in Denbighshire where the true fissile laminae were not 

 inclined to the strata at an angle of more than five or six degrees ; and in one 

 of the large quarries near Festiniog these planes are inclined at an angle 

 under ten degrees. Taking, however, the average throughout the Welsh 

 chains, the inclination of these planes is much more considerable, perhaps as 

 much as 30° or 40°; and nearly the same rule may be applied to the slate 

 system of the Cumbrian mountains *. It has^ indeed, been said, that the 



* I do not know who first published the general fact that true cleavage planes are never parallel 

 to the beds of the finer argillaceous schists. Dr. MacCulloch, in his description of the Western 

 Islands of Scotland, gives a striking instance of a fissile structure oblique to the stratification of clay 

 slate (vol. iii. Plate XXII, fig. 6.). Some years before the publication of that work, Mr. Bake- 

 well stated, " that slaty structure was," he conceived, " the eflfect of crystallization," and that " in 

 the slate rock of Charnwood Forest the slaty lamina? make an angle of sixty degrees with the 

 principal seam by which the rock is divided," (Introduction to Geology, Second Edition, p. 103); 

 and in the new edition of his work (published in 1833), he generahzes more boldly, and maintains 

 " that slate, unless it be of a soft or coarse kind, invariably splits in a transverse direction to that 

 of the bed, making with that direction an angle of about sixty degrees, and that it has frequently 

 two distinct cleavages." I agree with the most important part of this generalization ; but the 

 author is mistaken in supposing that the planes of cleavage and stratification make a constant 

 angle. The angle varies greatly even in the same chain ; and in different chains the average angle 

 would, I believe, be very different. 



Neither do I know any slate rock with two true cleavages. An ambiguity may, however, arise, 

 if we confound cleavage planes with planes of bedding, and with joints. For example, I entirely 

 agree with Professor Piiillips in the account he gives of the flagstone of Horton Scar (Geological 

 Transactions, Second Series, vol. iii. p. IG), though I should endeavour to describe the phseno- 

 mena in different language. What he calls the lanmice of false cleavage, I should call j)lanes of 

 true cleavage; and what he calls lam'mce of cleavage, I should call lamlncE of stratif cation. Be it 

 observed, there is no difference between the last-named author and myself, except in language. 

 The rock in question is a fine greywacke flagstone, and rises in beds parallel to the original stra- 



