128 ORIGIN AND DEVELOPMENT OF ORE DEPOSITS. 



at South Petherwyn, and first described by Sharpe in 1846.* In 

 these rocks, shells originally little more than one inch long, are 

 found to be extended in the direction of the dip to four inches 

 or more, and at other times compressed to half an inch or less. 

 Similar phenomena are observable wherever fossils occur in 

 cleaved rocks. f An interesting example of the effect of pressure 

 in producing cleavage in fine-grained material, and crumpling 

 in material of coarser grain in one and the same set of beds, at 

 Ilfracombe, is given by Mr. H. C. Sorby. This is illustrated in 

 fig. 7, Plate IX, taken from his paper. 



The foregoing experiments, observations and reasonings all 

 go to shew that the causes of slaty cleavage are altogether 

 mechanical, and that crystallization properly so-called, has nothing 

 to do with it. In fact Mr. Sorby concludes his paper (already 

 cited) with the remark that "in place of crystallization producing 

 slaty cleavage, it has a contrary tendency." He adds, "can I 

 therefore hesitate to conclude that slaty cleavage is the result of 

 mechanical and not of crystalline forces. J 



All this is true using the word "crystallization" in its 

 ordinary technical and exact sense. Yet there may be a kind 

 of crystallizing force acting on a large scale as suggested by 

 Sedgwick, which is under certain circumstances competent to 

 produce cleavage. Fifty years ago and more, §Mr. K,. W. Fox 

 found that well-marked lamination could be produced in the most 

 thoroughly kneaded clays by the long-continued action of a weak 

 electrical current, just such a current as might be expected to 

 exist between one rock-mass and another of a different nature ; the 

 lamination being at right angles to the direction of the current. 

 Such weak earth-currents have been proved to exist in many 

 experiments by him and by other observers. The following is an 

 abstract of the description of one of these experiments as given 

 by Mr. R. W. Fox himself. || ' 'In this most interesting experiment 



*Quart. Journ. Geol. Soc, 111. 1846. 



fSee the Geology of the Rio-Tinto Mines, by J. H. Collins. Quart. Journ. 

 Geol. Soc, 1885, p. 247, (note). 



JPhil. Mag-. 1856, p. 37. 



§Rep. Roy. Corn. Polytech. Soc, 1837, p. 68. 



||Catalogue of the works of Robert Were Fox, F.R.S., by J. H. Collins, 

 F.G.S., Truro, 1878. 



