CLEAVAGE, or SLATY CLEAVAGE, is a condition of rocks in which they split easily into thin plates. The direction of these lamina: may be in the plane of stratification, but it much more frequently differs from it. C. is the result of an operation which is subsequent to, and entirely independent of, the original stratification of the rocks. It is impossible to determine what is the producing cause of this phenomenon. By some it has been considered to be due to crystalline agency, while others maintain that it arises from the pressure of mechanical forces at right angles to the planes of C., and yet others seek an explanation in a combination of these two agencies. Prof. Sedgwick, who has carefully examined the phenomena of C., has arrived at the following general results: 1. That the strike of the C. planes, when they were well developed, and passed through well-defined mountain-ridges, was nearly coincident with the strike of the beds; 2. That the dip of these planes (whether in quantity or direction) was not regulated by the dip of the beds. inasmuch as the C. planes would often remain unchanged while
they passed through beds that changed their prevailing dip, or were contorted; 3. That where the features of the country or the strike of the beds was ill defined, the state of the C. became also ill defined, so as sometimes to be inclined to the strike of the beds at a considerable angle; 4. Lastly, that in all cases where the C. planes were well developed among the finer slate-rocks, they had produced a new arrangement of the minutest par ticles of the beds through which they pass.
C., though generally confined to clay-slate, yet occasionally occurs in lime and sand stone; but in proportion as the rocks are coarse, the C. planes become fainter andwider apart. In the fine-grained clay-slate, on the other hand, the laminae are thin, smooth, and parallel; and as C. is always accompanied with more or less induration in the rock where it exists, clay-slate, thus altered, is of great economic value for roofing.