Joints are divided by their characteristics and modes of origin into three classes as follows:— [Illustration: Fig. 36.—Quarry showing two systems of parallel joints.] 1. _The parallel and intersecting joints._—This is by far the most important class, and has its best development in stratified rocks, such as sandstone, slate, limestone, etc. These joints are straight and continuous cracks which may often be traced for considerable distances on the surface. They usually run in several definite directions, being arranged in sets or systems by their parallelism. Thus in Fig. 36 one set of joints is represented by the broad, flat surfaces in light, and a second set crossing the first nearly at right angles, by the narrower faces in shadow. By the intersections of the different sets of joints the rock is divided into angular blocks. Although many explanations of this class of joints have been proposed, it has long been the general opinion of geologists that they are due to the contraction of the rocks, _i.e._, that they are shrinkage cracks. We shall soon see, however, that they lack the most important characters of cracks known to be due to shrinkage; and the present writer has advanced the view that movements of the earth’s crust, and especially the swift, vibratory movements known as earthquakes, are a far more adequate and probable cause. It is well known that earthquakes break the rocks; and, if space permitted, it could be shown that the earthquake-fractures must possess all the essential features of parallel and intersecting joints. [Illustration: Fig. 37.—Columnar dike.]