400 



SLATY CLEAVAGE AND LAMIN/E OF BEDS COINCIDENT. 



whilst the finer planes between their courses of original deposit, split in lines precisely 

 and mathematically parallel, as if they had resulted from a crystalline action, subsequent 

 to the deposition of the sediment. 



In the above-mentioned quarries, where the lines of stratification are coincident with 

 those of slaty cleavage, the phenomenon is represented in the upper wood-cut, while 

 the discrepancy between cleavage and stratification, as is usually the case, appears in 

 the lower. 



80. 



Cleavage and beds coincident. 



81. 



Cleavage and beds divergent. 



It does not appear to me, that in the cases of coincidence, the term flagstone can be 

 applied to pure and good roofing slate, merely because the cleavage is not oblique to the 

 lines of bedding. 



It is indeed manifest, as first suggested by Mr. Lonsdale, (after the memoir 1 on this 

 subject had been read before the Geological Society) that there must be coincidences be- 

 tween the lines of cleavage and stratification, where the former maintain a fixed position 

 and direction, and the latter undulate in broad curves, as represented in this wood-cut. 



82. 



Diagram explaining how in undulatory strata, the cleavage and laminse of deposit must occasionally coincide. The darker 



portions of the mass are those where the coincidence occurs. 



Examples of both phenomena may be seen in the same range of slate quarries at 

 Panti-philip, a few miles south of Fishguard. 



» Proceedings of the Geological Society, vol. ii. p. 226 et seg. and Phil. Mag. vol. viii. p. 564. 



