1846.] SHARPE ON SLATY CLEAVAGE. 75 



through the writings of Professor Sedgwick, Sir R. I. Murchison, 

 Professor Phillips, Sir H. T. De la Beche and Mr. Austen, in addition 

 to which Mr. Darwin has just given us a large collection of observa- 

 tions upon cleavage made in various parts of South America, with 

 some important generalizations to which the extended field of his 

 researches gives great weight. The most valuable of his remarks 

 appear to be those which connect together the cleavage of slates and 

 the foliation of gneiss, mica slate, &c. 



Still we had no knowledge of the general laws which governed 

 the formation of the cleavage and regulated its direction, and without 

 previously discovering these it was idle to speculate upon the causes 

 which had produced it. I have now endeavoured to supply part of 

 this deficiency and to establish some general laws relating to the 

 subject, which have been deduced from new observations and from 

 combining insulated facts recorded by others. The appearance of 

 symmetry which has resulted from putting these together, gives me 

 great confidence in the soundness of the results, and should those 

 which relate to the connection between the elevation and the cleavage 

 of rocks be confirmed by other observers, they promise when fol- 

 lowed out to throw great light on the theory of elevation. 



Connection between the distortion of the Organic Remains found in 

 Slaty Rocks and the cleavage of the beds. 



In determining the species of the fossils of the older formations, 

 great difficulties arise from the distortion of form which they have 

 frequently undergone ; which sometimes makes it impossible to as- 

 certain the species or even the genus to which they belong. This 

 distortion varies very much in specimens from the same locality, and 

 still more so in those from different districts. 



While examining the fossil shells from the Ludlow rocks of West- 

 moreland, most of which are slightly distorted, it struck me that if 

 we could find out that the changes of form in the shells followed any 

 certain law we might make allowance for them, and thus discover 

 the original form of the shell. Following up this idea, I found that 

 whenever the impressions of several shells occurred on one slab of 

 stone, they were all distorted in the same direction ; the change 

 having no reference to the original figure oC the shells, but to their 

 position on the stone : it appeared as if every specimen had been 

 contracted in the same direction. This remark enabled me to throw 

 together many shells which I was before disposed to consider distinct 

 species. 



Examining other specimens to see how far this observation could 

 be extended, I found that it held generally true, and also that there 

 was a connection between the direction of the cleavage planes and 

 what may be called the axis of distortion. Instances were met with 

 showing the greatest variety in the amount and nature of the changes 

 of form in fossil shells, from a slight alteration in the outline to the 

 most extravagant and complicated distortion in which the proportions 

 of the parts were completely changed ; but whenever the rock showed 



