2. _Contraction joints or shrinkage cracks._—That many cracks in rocks are due to shrinkage, there can be no doubt. The shrinkage may result from the drying of sedimentary rocks; but more generally from the cooling of eruptive rocks. Every one has noticed in warm weather, the cracks in layers of mud or clay on the shore, or where pools of water have dried up; and we have already seen that these sun-cracks are often preserved in the hard rocks. They have certain characteristic features by which they may be distinguished from the joints of the first class. They divide the clay into irregular, angular blocks, which often show a tendency to be hexagonal instead of quadrangular. The cracks are continually uniting and dividing, but are not parallel, and rarely cross each other. Sun-cracks never affect more than a few feet in thickness of clay, and are an insignificant structural feature of sedimentary rocks. In eruptive rocks, on the other hand, the contraction joints have a very extensive, and, in some cases, a very perfect development, culminating in the prismatic or columnar jointing of the basaltic rocks. This remarkable structure has long excited the interest of geologists, and, although the basalt columns were once regarded as crystals, and later as a species of concretionary structure, it is now generally recognized as the normal result of slow cooling in a homogeneous, brittle mass. The columns are normally hexagonal, and perpendicular to the cooling surface, being vertical in horizontal sheets and lava flows, as in the classic examples of the Giant’s Causeway and Fingal’s Cave, and horizontal in vertical dikes (Fig. 37). They begin to grow on the cooling surface of the mass and gradually extend toward the centre, so that dikes frequently show two independent sets of columns. 3. _The concentric joints of granitic rocks._—In quarries of granite and other massive crystalline rocks, it is often very noticeable that the rock is divided into more or less regular layers by cracks which are approximately parallel with the surface of the ground, some of the granite hills having thus a structure resembling that of an onion. The layers are thin near the surface, become thicker and less distinct downwards, and cannot usually be traced below a depth of fifty or sixty feet. These concentric cracks are of great assistance in quarrying, and are now regarded as due to the expansion of the superficial portions of the granite caused by the heat of the sun. In reference to this view of their origin these may be properly called _expansion joints_.