. Elements of geology : a text-book for colleges and for the general reader. Geology. CLEAVAGE STRUCTURE. 183 Again, it has been compared to crystalline cleavage, on a huge scale. It has been supposed that electricity traversing the earth in certain di- rections, while certain rocks were in a semi-fluid or plastic state through heat, arranged the particles of such rocks in a definite way, giving rise to easy splitting in definite directions. In support of this view it was urged that cleaved slates are most common in metamorphic regions; and metamorphism, as we shall see hereafter (p. 221 et se
. Elements of geology : a text-book for colleges and for the general reader. Geology. CLEAVAGE STRUCTURE. 183 Again, it has been compared to crystalline cleavage, on a huge scale. It has been supposed that electricity traversing the earth in certain di- rections, while certain rocks were in a semi-fluid or plastic state through heat, arranged the particles of such rocks in a definite way, giving rise to easy splitting in definite directions. In support of this view it was urged that cleaved slates are most common in metamorphic regions; and metamorphism, as we shall see hereafter (p. 221 et seq.), indicates the previous plastic state of rocks, which is a necessary condition of the rearrangement of the particles by electricity. The great objections to this theory are—1. That the cleavage is not like crystalline cleavage, between ultimate molecules, and therefore perfectly smooth, but be- tween discrete and quite visible granules; and, 2. That although the phenomenon is indeed most common in metamorphic rocks, yet meta- morphism is by no means a necessary condition ; on the contrary, when the real necessary conditions are present, the less the metamorphism the more perfect the cleavage. It is evident, therefore, that slaty cleavage is not due to any of the causes spoken of above. It is not flag-stone cleavage, nor crystalline cleavage, and of course can not be organic cleavage. Sharpe's Mechanical Theory.—The first decided step in the right direction was made by Sharpe. According to him, slaty cleavage is al- ways elite to powerful pressure at right angles to the planes of cleavage, by which the pressed mass has been compressed in the direction of press- ure and extended in the direction of the dip of the cleavage-planes. This theory may be now regarded as completely established, by the labors of Sharpe, Sorby, Haughton, Tyndall, and others. We will give a few of the most important observations which establish its truth. (a.) Distorted Shells.—Many cleaved slate
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