1848.] SHARPE ON SLATY CLEAVAGE, 111 



generic distinction. The specimens indeed reveal to us, in the 

 clearest manner, a feature in the physiology, — a mode of growth and 

 reproduction, which separates them mdely from all the modern ce- 

 phalopods ; or even from the ancient forms having the usual siphun- 

 cular and septate arrangement of recent species*. If the characters 

 there given can be reconciled with the theory of a large external soft 

 animal, enclosing the shell, the question may remain in its present 

 undecided state ; bearing on one side the array of facts, and on the 

 other the theoretical views of naturalists, deduced from those modern 

 cephalopods possessing few characters in common with these ancient 

 forms. 



2. On Slaty Cleavage (second communication). By Daniel Sharpe, 



Esq., F.G.S. 



In a paper read to the Geological Society on the 2nd of December 

 1846, on the cleavage of slate rocksf, I endeavoured to work out 

 certain general laws relative to the compression which such rocks had 

 undergone, to the position of the planes of cleavage, and to the con- 

 nection between their direction and the elevation of the beds. The 

 conclusions were founded upon observations chiefly made in North 

 Wales, Devonshire and Cornwall : I have since visited parts of West- 

 moreland and Cumberland, with the view of enlarging the field of 

 observation, and the result is contained in the following remarks, 

 which are thrown into the order adopted in the former paper. 



Compression of slate-rocks in a direction perpendicular to the 

 planes of cleavage. — In the former paper, p. 87, I stated that in all 

 the slaty fossiliferous rocks examined, the distortion of the fossils 

 proved that the mass of the rocks had undergone considerable com- 

 pression in a direction everywhere perpendicular to the planes of 

 cleavage, and some expansion in the direction of the dip of the 

 cleavage ; but that there was no reason to suppose that the rocks 

 had suffered any change of volume in the direction of the strike of 

 the cleavage. And it was inferred that these changes must be gene- 

 ral in all slaty rocks, although it might not be easy to find proofs 

 that they had occurred, where organic remains were absent. 



There is however more evidence of compression to be found among 

 the beds of unfossiliferous slate than might have been expected ; 

 and the examination of their mechanical structure affords quite as 

 strong proofs of pressure as those derived from the distortion of the 

 organic remains. 



In the neighbourhood of the roofing-slates there are frequently 

 found beds of a brecciated structure which cleave readily, but from 

 their irregular composition are liable to break. Such beds are 



* The modern Cephalopod, Nautilus Pompilius, the anatomy of which is so well 

 known from the labours of Prof. Owen, affords httle or no assistance in enabling 

 us to form conclusions as to the physiology and habits of animals like those in- 

 habiting the shells of Endoceras. 



f Journal of the Geological Society, vol. ii. p. 74. 



