Prof. Sedgwick on the Structure of large Mineral Masses. 481 



age from a fluid or semi-fluid, into a solid state. Cleavage planes are, on the 

 contrary, the results of the ultimate chemical arrangement of the particles of 

 a rock, and appear in most cases to be unconnected with any direct mecha- 

 nical action. 



A slaty and jointed structure are, however, often exhibited together; and 

 cases may arise where it is almost impossible to decide whether a certain set 

 of fissures are to be called joints or cleavage planes. For example, a true 

 slate rock may have been acted on by mechanical forces, producing a set of 

 fissures parallel to the cleavage. In such a case the rock has mechanical 

 fissures and true crystalline cleavage planes coincident with each other. 

 Again, vvhen a cleavage is imperfect, it is sometimes only exhibited by pa- 

 rallel planes at definite distances, in which case it may be difficult to say 

 whether the phaenomena are to be classed with joints or cleavages. 



Difficulties of this kind are the exception, and not the rule ; and they de- 

 stroy not the reality of the distinctions I am attempting to draw : they only 

 prove how impossible it is for us to constrain the vast and complicated opera- 

 tions of nature by the fetters of a rigid definition. It is the business of a 

 geologist to consider both the resemblances and the differences of the things 

 he describes ; and after a broad view of nature's kingdoms, he learns to seize 

 upon those resemblances which are essential to his classification, and to cast 

 from his thoughts those differences which are unessential. In this way he calls 

 by a common name things with a hundred points of difference, and puts in 

 distinct classes, things with a hundred points of resemblance. But on this 

 account his advance is neither unphilosophical nor insecure. He that dwells 

 only on resemblances may be hurried into rash generalizations, which, how- 

 ever, experience will tend to correct : but he who continues to dwell only on 

 the difficulties of geology, and to haggle on mere exceptions and points of dif- 

 ference, should he attempt to advance at all, must stumble at every step. 



The jointed structure is best seen in unstratified rocks, such as basalt and 

 certain varieties of granite. The regularly jointed pillars of basalt, and the 

 rectangular prisms composing some parts of the Cornish granite, are good 

 examples of it. When the prisms are very symmetrical, and subdivided by a 

 series of parallel cross joints, the whole configuration is supposed by many to 

 have been produced by the interference of distinct spheroidal corrections. 

 This is a beautiful theory, founded on direct experiment, and quite capable 

 of explaining some cases of prismatic jointed structure ; but it has, I think, 

 been pushed too far. If it be true as applied to the granite Tors of Cornwall, 

 we ought, in examining the cuboidal blocks of which they are composed, to 

 see some traces of spheroidal structure in the arrangement of the component 



