142 LYELL'S ELEMENTS OF GEOLOGY. 



Slaty Cleavage. 



nected with sedimentary deposition, Professor Sedgwick is of 

 opinion that no retreat of parts, no contraction in dimensions, in 

 passing to a solid state, can account for the phenomenon. It 

 must be referred to crystalline or polar forces acting simultane- 

 ously and somewhat uniformly, in given directions, on large 

 masses having a homogeneous composition. 



A fact recorded by Mr. Darwin affords confirmation to this 

 theory. The ore of the gold mines of Yaquil, in Chili, is ground 

 in a mill into an impalpable powder. After this powder has Been 

 washed, and nearly all the metal separated, the mud which passes 

 from the mills is collected into pools, where it subsides, and is 

 cleared out and thrown into a common heap. A great deal of 

 chemical action then commences, salts of various kinds effloresce 

 on the surface, and the mass becomes hard, and divides into con- 

 cretionary fragments. These fragments were observed to pos- 

 sess an even and well defined slaty structure ; but the laminae 

 were not inclined at any uniform angle.* 



Mr. R. W. Fox lately submitted a mass of moist clay, worked 

 up with acidulated water, to weak voltaic action for some months, 

 and it was found when dry to be rudely laminated, the planes of 

 the slightly undulating laminae being at right angles to the direc- 

 tion of the electrical forces.f 



Sir John Herschel, in allusion to slaty cleavage, has suggested, 

 " that if rocks have been so heated as to allow a commencement 

 of crystallization ; that is to say, if they have been heated to a 

 point at which the particles can begin to move amongst them- 

 selves, or at least on their own axes, some general law must then 

 determine the position in which these particles will rest on cool- 

 ing. Probably, that position will have some relation to the di- 

 rection in which the heat escapes. Now, when all, or a majority 

 of particles of the same nature have a general tendency to one 

 position, that must of course determine a cleavage plane. Thus 

 we see the infinitesimal crystals of fresh precipitated sulphate of 

 baryte, and some other such bodies, arrange themselves alike in 

 the fluid in which they float ; so as, when stirred, all to glance 

 with one light, and give the appearance of silky filaments. Some 

 sorts of soap, in which insoluble margarates:}: exist, exhibit the 

 same phenomenon when mixed with water ; and what occurs in 

 our experiments on a minute scale may occur in nature on a 

 great one." 



* Journal, p. 324. 



t Although the lamination in the specimen shown to me was very imperfect, 

 it was sufficiently evident to encourage farther experiments. 



t Margaric acid is an oleaginous acid, formed from different animal and vege- 

 table fatty substances. A margarate is a compound of this acid with soda, pot- 

 ash, or some other base, and is so named from its pearly lustre. 



$ Letter to the author, dated Cape of Good Hope, Feb. 20, 1836. 



