Ch. XXXVI.] SLATY CLEAVAGE. Y49 



large portions of the earth's crust have, in the course of past ages, been 

 subjected again and again to very different degrees of heat and cold. 

 These alternations of temperature have probably contributed largely 

 to the production of joints in rocks. 



In some countries, as in Saxony, \ here masses of basalt rest on 

 sandstone, the aqueous rock has for the distance of several feet from 

 the point of junction assumed a columnar structure similar to that of 

 the trap. In like manner some hearthstones, after exposure to the 

 heat of a furnace without being melted, have become prismatic. 

 Certain crystals also acquire by the application of heat a new internal 

 arrangement, so as to break in a new direction, their external form 

 remaining unaltered. 



Professor Sedgwick, speaking of the planes of slaty cleavage, where 

 they are decidedly distinct from those of sedimentary deposition, 

 declared, in the essay before alluded to, his opinion that no retreat of 

 parts, no contraction in the dimensions of rocks in passing to a solid 

 state, can account for the phenomenon. He accordingly referred it 

 to crystalline or polar forces acting simultaneously, and somewhat 

 uniformly, in given directions, on large masses having a homogeneous 

 composition. 



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 themselves, or at least 

 on their own axes, some general law must then determine the position 

 in which these particles will rest on cooling. Probably that position 

 will have some relation to the direction 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 pre- 

 cipitated sulphate of barytes, 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." f 



Professor Phillips has remarked that in some slaty rocks the form 

 of the outline of fossil shells and trilobites has been much changed by 

 distortion, which has taken place in a longitudinal, transverse, or 

 oblique direction. This change, he adds, seems to be the result of 

 a " creeping movement " of the particles of the rock along the planes 

 of cleavage, its direction being always uniform over the same tract 

 of country, and its amount in space being sometimes measurable, and 



* Margaric acid is an oleaginous acid, formed from different animal and vegeta- 

 ble fatty substances. A margarate is a compound of this acid with soda, potash, 

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



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



