COHESIVE ATTRACTION. 627 



II. COHESIVE AND CAPILLARY ATTRACTION; GRAV- 

 ITATION. 



1. Cohesion; Crystallization. — The power of cohesion acting 

 in solidification, and that in crystallization, appear to be identical. 

 Steel, bar-iron, marble, granite, are crystallized in their intimate struc- 

 ture ; and they show it in the angular grains which make up the mass, 

 and which may be observed on a surface of fracture. 



Crystallization is exhibited (1) in the angular solids it produces, 

 called crystals, and (2) in cleavage. Some of the forms of crystals are 

 illustrated in the early pages of this work (pp. 53-59), and cleavage 

 also has been explained (p. 53). 



(1.) Texture of Rocks. — From cleavage in minerals, comes the an- 

 gular form of the grains in a crystalline rock. The grains are coarser 

 the slower the crystallization, or, in other words, the slower the rate 

 of cooling during the crystallization, as already explained (p. 64) ; and 

 with rapid cooling they sometimes disappear altogether, and the ma- 

 terial comes out glass instead of stone. 



Perfect crystals are not generally found in the rocky mass because 

 the grains crowd on one another. The mineral in the mass that so- 

 lidifies first, or takes regular crystalline forms most readily, may make 

 crystals in the midst of the rest, and so a porphyry may result, or a 

 schist full of garnets, or a granular limestone full of tremolite. In por- 

 phyritic rocks, the feldspar crystals are sometimes a less fusible species 

 than the feldspar of the base, but sometimes they are the same species 

 of feldspar. Crystals that are perfect in their terminations are most 

 common in the cavities of rocks, where they have space to expand 

 without interference. 



To produce, or alter in texture a crystalline-granular rock, heat 

 short of fusion is generally sufficient. In tempering steel, sudden 

 cooling produces a fine grain, and slow, a coarse, the crystalline tex- 

 ture of the solid bar undergoing a complete change, and one in which 

 every molecule participates. 



(2.) Fissile and Massive Structure. — The eminently easy cleavage 

 of mica generally gives a very fissile structure to the rocks consisting 

 largely of it. This is uniformly true where the other mineral present is 

 mainly quartz in grains, for quartz has no cleavage of its own to con- 

 test with that of the mica ; yet it sometimes fails when the quartz is 

 greatly in excess. Hornblende is another cleavable mineral character- 

 izing fissile rocks. But when crystallizing freely at a high temperature, 

 its grains become interlocked in all kinds of positions, and hence horn- 

 blende rocks derived from igneous fusion, and a large part of those that 



