388 



REPORT — 1856. 



In the remarkable case sketched, the ridges and hollows assume a regu- 

 larity of wavy interruptions which appear the effect of concretionary forces 

 whose axes cross the bed, the concretions being subsequently pressed by 

 cleavage, so that the rock can sometimes be practically divided by art, and 

 in other cases is found actually divided by nature into irregular oblong 

 solids whose axis is parallel to the line of dip of the cleavage. Phaenomena 

 of this order are observable among the slaty rocks of Westmoreland (Win- 

 dermere Head, Bowness), and in some tracts of South Wales (Llandowror), 

 but they do not yield good slate. 



In some cases the irregular surface of the beds is apparently due to ori- 

 ginal ripple structure, which by the general movement of the mass of the 

 rock across the cleavage planes, have acquired superposed wrinkles parallel 

 to the cleavage edges. Thus in several cases may the planes of stratification 

 be clearly distinguished from joints. 



The steps thus placed for a mechanical theory of the series of changes by 

 which the structural characters and accidents of position in slate rocks might 

 be determined, were relaid with care, and strengthened by new observations, 

 by Mr. D. Sharpe*. 



In the quarries of South Petherwin, where argillaceous, ochraceous, and 

 calcareous beds occur, the former are wholly cleft, the latter partially so, or 

 rather cracked, the soft ochreous beds not marked by cleavage. In tlie argil- 

 laceous slate the thinner and more tender fossils are much changed in figure, 

 the encrinite columns not so. The distortion is greatest where the angle 

 between the planes of cleavage and stratification is least. The contraction of 

 dimensions in the plane of the strata on the line perpendicular to the strike 

 of cleavage, is estimated at one-fourth, and there is an expansion in the 

 plane of cleavage on the line of the dip. Mr. Sharpe's general result is 

 expressed in these distinct terms : — " From these and similar cases, we learn 

 that the shells have been compressed by a force acting in a direction perpen- 

 dicular to the planes of cleavage, and that the compression of the mass 

 between the cleavage planes has been counterbalanced by its expansion in 

 a direction corresponding to the dip of the cleavage." And again, "As 

 the expansion of the rock in one direction may have been caused by its com- 

 pression in the contrary direction, it follows that all the eff'ects yet described 

 may have originated in the compression of the mass of the rock in a direction 

 perpendicular to the cleavage planes." The oblique pressures which appear 

 to have affected many shells in the planes of stratification and produced such 

 extraordinary distortions as that of Spirifera disjuncta (Diagram 34 a, com- 

 pared with 34 b), " may always be resolved into the same two direct forces ; 

 one forwards along the plane of cleavage towards the intersection of the 

 cleavage and the bedding, the other downwards in a direction perpendicular 

 to the cleavage. When the bedding and cleavage exactly coincide at Tin- 

 tagel, the shells are flattened and drawn out considerably, even 50 per cent, 

 in one direction," — the direction 



being, doubtless, that of the line 

 of dip of the cleavage planes. 



Mr. Sharpe thus concludes this 

 part of his investigation : — 



" It may be asserted as probable, 

 that all rocks affected by that pe- 

 culiar fissile character which we 

 call slaty cleavage have under- 

 gone, — 



Fig. 34 a. 



Fig. 34 b. 



* See Geol. Proc. 1846 and 1848. 



