divisional plane

— All rocks are traversed by divisional planes of less or greater width and frequency, and thus divided into masses of definable shapes and proportions. These "joints," as they are often called, present themselves under a great variety of appearances, but almost always such as to be intelligible on the supposition of the mass of the rock having been contracted, so as to separate into prismatic and other forms, as clay, starch, &c. contract and split by drying.