This section is from the book "An Introduction To Geology", by William B. Scott. Also available from Amazon: An Introduction to Geology.
With the exception of loose, incoherent masses, such as soil, gravel, sand, etc., all rocks which are accessible to observation, are divided into blocks of greater or less size by systems of cracks and crevices, which are known as joints. These may be easily observed in any stone-quarry, where they are taken advantage of in getting out the stone.
In the igneous rocks all the division planes which separate the blocks are true joints, which vary greatly in their number and manner of intersection and in the consequent shape of the joint-blocks. Fine-grained basalts display a very general tendency to columnar jointing, forming more or less regularly prismatic columns, which are commonly hexagonal. Several modern lavas (see p. 76) display these hexagonal columns, as do the ancient basalts of very many regions. In certain cases, as in the famous Giant's Causeway of Ireland, the columns are divided transversely by concave joints, giving a ball-and-socket arrangement which has a curiously artificial appearance. Although the regular hexagonal columns are most frequent among the fine-grained basalts, they also occur in the coarser rocks of the gabbro family and in other families also. The acid glass of Obsidian Cliff (see Fig. 26) shows columnar jointing, and the phonolite of Mato Tepee in South Dakota is jointed in magnificent columns, and many other examples might be cited.
In many of the granites and other coarse-grained igneous rocks, the joints divide the mass into cubical blocks, or into long, rectangular prisms, or into broad, slab-like plates. In others, again, the blocks are of exceedingly irregular form and size. 2 b 369
Fig. 193. - Platy jointing in diabase; above, spheroidal weathering and transition to soil. Rocky Hill, N. J. (Photograph by Sinclair).
In sedimentary rocks the joints are ordinarily in only two planes, the third being given by the bedding-planes. In homogeneous, heavily bedded sediments, such as limestones and massive sandstones, the joints are apt to form cubical or rectangular-prismatic blocks, making a weathered cliff look like a gigantic wall of regular masonry. Other sedimentary rocks are, as a rule, more irregularly jointed.
Fig. 194. - Regular jointing in gneiss, near Washington. (U. S. G. S).
Fig. 195. - Irregular jointing in gneiss, Little Falls, N.Y. (Photograph by van Ingen).
Fig. 196. - Jointing in shale, Cayuga Lake, N.Y. (U. S. G. S).
Joints are of very different orders of importance: some, the master joints, traverse many strata and remain constant for long distances and considerable depths, while each layer usually has minor joints which are confined to that bed. One set of joints, the strike joints, run more or less parallel to the strike of the beds, while the second set, the dip joints, follow the dip; the former are usually the longer and more conspicuous. Oblique or diagonal joints intersect the other two systems, and many irregular cracks may occur. In general, the more disturbed the rocks have been, the more broken they are.