Vol.55.] 



THE CRAVEN DISTRICT OF YORKSHIRE. 



341 



succession of the limestones appears regular, any individual band 

 is found upon inspection to be lenticular, and to die out in a few 

 yards, so quickly, indeed, that it is hardly possible to suppose that 

 one is examining lenticles produced by deposition.) 



I have various notes of folded structures in knolls formed of white 

 crystalline limestones in other areas, as for instance in the Ordovician 

 limestones of Keisley and Millom. 



Though knoll-structure is best developed in limestone, it, or an 

 analogous structure, is not absent from other hard rocks, such as 

 grits. Small examples of incipient knoll-structure in grits are 

 visible in the section by the canal-bank below Skipton Castle, though 

 the best examples of grit-knolls that I have seen are found in the 

 Middle Devonian rocks of Hagginton Beach, and the bathing-place at 

 Ilfracombe in North Devon. This structure is especially important, 

 as in this case there is no question of its production as the result of 

 accumulation of organic remains under special circumstances. 



Fij 



-Pseudostromatism (?) in limestone^ Otterhurn Valley. 



[TLe above is a diagrammatic view of a quarry-face, showing the apparently- 

 lenticular beds of limestone. This rapid alternation, however, seems to 

 be due to faulting along fissures which are but slightly inclined to the 

 bedding-planes, when they do not coincide with those planes.] 



Two types of rock are frequently associated with knolls of lime- 

 stone, and the association is probably of some significance, as it 

 indicates the occurrence of chemical changes which might readily 

 take place during the exertion of pressure : I refer to dolomites and 

 silicified limestones. Dolomitized limestone is found in association 

 with the other limestones of the Cracoe and Settle reefs, and fine 

 examples of silicified limestone are found in some reefs at the head 

 of the Stockdale Valley, east of Settle. The latter are specially 

 noteworthy, on account of the crystals of quartz which they contain 

 when the silicification is incipient. In other parts it is complete. 

 The silicification here seems identical with that recently described 

 to the Society by Mr. Arnold-Bemrose.^ 



1 Quart. Journ. Geol. Soc. vol. liv (1898) p. 169. 



