602 JOINTED STRUCTUEE AND CLEAVAGE. [Ch. XXXVL 



Fig. 707. 



D D 



Stratification, joints, and cleavage. 

 (From Murcbison's Silurian System, p. 245.) 



separate volcanic and plutonic rocks into cuboidal and prismatic masses. 

 On a small scale we see clay and starch, when dry, split into similar 

 shapes ; this is often caused by simple contraction, whether the shrinking 

 be due to the evaporation of water, or to a change of temperature. 

 It is well known that many sandstones and other rocks expand by the 

 application of moderate degrees of heat, and then contract again on 

 cooling ; and there can be no doubt that 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 temper- 

 ature have probably contributed largely to the production of joints in 

 rocks. 



In some countries, as in Saxony, where masses of basalt rest on sand- 

 stone, 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 crystalli- 

 zation, — 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 



