Page:A Treatise on Geology, volume 1.djvu/83

 CHAP. IV. limestone of the highlands, with the calcareous rocks of later production, this dependence of the frequency and regularity of joints, on the age of the rocks, will clearly appear.

Among the argillaceous slate rocks, a further peculiarity of internal structure takes place, which is deserving of special attention, since it appears to be the case of divisional planes carried to extreme in number and symmetry.

This structure, commonly called cleavage, is really distinct from joints and stratification, and may be, perhaps, understood in its relation to them by the accompanying sketch.



In this drawing, S is a plane of stratification dipping in the direction Δ; c c are the edges of planes of cleavage, which in the plane S continue in lines c′ c′. These planes are continuous, and very numerous in the fine