[Illustration: Fig. 4.--Sequence of Beds.] 41. _Joints._--Besides _planes_ or _lines of bedding_, there are certain other division planes or _joints_ by which rocks are intersected. The former, as we have seen, are congenital; the latter are subsequent. Joints cut right across the bedding, and are often variously combined, one set of joint planes traversing the rock in one direction, and another set or sets intersecting these at various angles. Thus, in many cases the rocks are so divided as easily to separate into more or less irregular fragments of various sizes. Besides these confused joints there are usually other more regular division planes, which intersect the strata in some definite directions, and run parallel to each other, often over a wide area: these are called _master-joints_. Two sets of master-joints may intersect the same strata, and when such is the case, the rock may be quarried in cuboidal blocks, the size of which will vary, of course, according as the two sets of joints are near or wide apart. Joints may either gape or be quite close; so close, indeed, as in many cases to be invisible to the naked eye. Certain igneous rocks frequently shew division planes which meet each other in such a way as to form a series of polygonal prisms. The basalt of Staffa and Giants' Causeway are familiar examples of this structure. Jointing is due to the gradual consolidation of the strata, and hence, in a series of strata, we may find the separate beds, according to their composition, very variously affected, some being much more abundantly jointed than others. Master-joints which traverse a wide district in some definite direction probably owe their origin to tension, the strata having been subjected to some strain by the underground forces. [Illustration: Fig. 5.--Beds of Limestone (_a_), Sandstone (_b_), and Shale (_c_), divided into cuboidal masses by master-joints.] [Illustration: Fig. 6.--Columnar Structure.] [Illustration: Fig. 7.--Bedding, Joints, and Cleavage (after Murchison).] 42. _Cleavage._--Fine-grained rocks, more especially those which are argillaceous, occasionally shew another kind of structure, which is called _cleavage_. Common clay-slate is a type of the structure. This rock splits up into innumerable thin laminæ or plates, the surface of which may either be somewhat rough, or as smooth nearly as glass. The cleavage planes, however, need not be parallel with the planes of bedding; in most cases, indeed, they cut right across these, and continue parallel to each other often over a very wide region. The original bedding is sometimes entirely obliterated, and in most cases of well-defined cleavage is always more or less obscure.