39. When a bed is followed for any distance it is frequently found to thin away, and give place to another occupying the same plane or horizon. Thus a shale will be replaced by a sandstone, a sandstone by a conglomerate, and vice versâ. Sometimes also we may find a shale, as we trace it in some particular direction, gradually becoming charged with calcareous matter, so as by and by to pass, as it were, into limestone. Every bed must, of course, end somewhere, either by thus gradually passing into another, or by thinning out so as to allow beds which immediately overlie and underlie it to come together. Not unfrequently, however, a bed will stop abruptly, as in [fig. 3].

Fig. 3.—Sudden ending of Bed at ×.

40. Sequence of Beds.—It requires little reflection to see that the division plane between two beds may represent a very long period of time. Let the following diagram represent a section of strata, s being beds of grit, and a, b, c, beds of sandstone and shale. It is evident that the beds s must have been formed before the strata b were deposited above them. At ×, the beds a and b come together, and were attention to be confined to that part of the section, the observer might be led to infer that no great space of time elapsed between the deposition of these two beds. Yet we see that an interval sufficient to allow of the formation of the beds s must really have intervened. It is now well known that in many cases planes of bedding represent 'breaks in the succession' of strata—'breaks' which are often the equivalents of considerable thicknesses of strata. In one place, for example, we may have an apparently complete sequence of beds, as a, b, c, which a more extended knowledge of the same beds, as these are developed in some other locality, enables us to supplement, as a, s, b, c.

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.

Fig. 5.—Beds of Limestone (a), Sandstone (b), and Shale (c), divided into cuboidal masses by master-joints.
Fig. 6.—Columnar Structure.