36 Mr. H. C. Sorby on Slat?/ Cleavage, 



crystals with portions of pei'oxidized brown-spar, and on chemical 

 analysis is found to be chiefly carbonate of lime with a little 

 magnesia and the oxides of iron and manganese. These crystals 

 have been developed in bands, apparently along such planes of 

 stratification as gave lines of facility for the change to occur ; 

 and hence I should call it an organic clay, foliated along the 

 planes of stratification with crystals of highly calcareous brown- 

 spar. I have not yet seen any limestone, in the district under 

 consideration, in which chemical changes have so occurred in 

 relation to the cleavage, that it may be said to be foliated with 

 dolomitic crystals in the true plane of cleavage, independent of 

 stratification, in a similar manner to what has taken ])lace with 

 respect to some other minerals in other localities ; but perhaps 

 such might be found by a more careful examination. 



These physical and chemical analyses will, I trust, suffice to 

 show the nature of the limestones under consideration, in their 

 unaltered condition, and when metamorphosed by subsequent 

 chemical changes. Since corals decay more readily than such 

 tissue as that of encrinites, I conclude that the greater part of 

 the organic clay has been derived from them, and that the De- 

 vonian limestones were formed chiefly from the decay of corals, 

 next to which come encrinites ; whilst the proportion of other 

 organisms is only small. The deposits have afterwards been 

 indurated by crystallization, and the infiltration of calcareous 

 spar; and in some cases metamorphosed by other chemical 

 changes. Then, on elevation and exposure to the oxygen of the 

 atmosphere, another set of changes took place, chiefly the conver- 

 sion of pyrites, and the protoxides of iron and manganese, into 

 higher oxides ; a process not yet completed. 



The actual constitution of the rocks being now described, I 

 proceed to consider the phfenomena of their slaty cleavage. In 

 this communication I must forbear to enter into the facts on a 

 large scale, seen in the field, for that alone would be a long 

 subject. However, I must state that I am convinced that there 

 is the most complete proof of rocks possessing cleavage having 

 been so acted on by mechanical forces that they have been very 

 considerably compressed in a direction perpendicular to the 

 cleavage, and elongated to a certain extent in the line of the 

 dip ; as proved by the change in the thickness of the same bed 

 when bent into contortions, and by various other facts described 

 by ]\Ir. Daniel Sharpe and myself (Quart. Journ. of Geol. Soc. 

 vol. ii. p. 74, and vol. v. p. Ill ; Edinb. Phil. Journ. for 1853, 

 vol. Iv. p. 137). I shall therefore consider this to be an esta- 

 blished fact, as seen on a large scale, and confine myself to 

 showing that it has so altered the ultimate constitution of the 

 rock as to produce the structure on which cleavage depends. 



