Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/311

Rh JOINTS.-j II. J OINTS. All rocks are traversed more or less distinctly by vertical or highly inclined divisional planes termed Joints. Soft rocks indeed, such as loose sand and uncompacted clay, do not show these lines ; but wherever a mass of clay has been subjected to somepressure and consolidation, it will usually be found to have acquired them. It is by means of the intersection of joints that rocks can be removed in blocks; the art of quarrying consists in taking advantage of these natural planes of division. Joints differ in character accord- ing to the nature of the material which they traverse; those in sedimentary rocks are usually distinct from those in crystalline masses. 1. In Se¢lz'1nentm'3/ It’or/cs. Joints vary in sharpness of deﬁnition, in the regularity of their perpendicular and horizontal course, in their lateral persistence, in number, and in the directions of intersection. As a r11le, they are most sharply deﬁned in proportion to the ﬁneness of grain of the rock. In limestones and close-grained shales, for example, they often occur so clean—cut as to be invisible until revealed by fracture or by the slow disintegrating effects of the weather. The rock splits up along these con- cealed lines of division whether the agent of demolition be the hammer or frost. In coarse-textured rocks, on the other hand, joints are apt to show themselves as irregular rents along which the rock has been shattered, so that they present a11 uneven sinuous course, branching off in different directions. In many rocks they descend vertically in straight lines at not very unequal distances, so that the spaces between them are thus marked off into so many wall- like masses. But this symmetry often gives place to a more or less tortuous course with lateral joints in various random directions, more especially where the different st)‘-ata vary considerably in lithological characters. A single joint may be traced sometimes for many yards, or even for several miles, more particularly when the rock is ﬁne- grained, as in limestone. But where the texture is coarse and unequal, the joints, though abundant, run into each other in such a way that no one in particular can be identi- fied for so great a distance. The number of joints in a mass of stratiﬁed rock varies within wide limits. Among strata. which have undergone little disturbance the joints may be separated from each other by intervals of several yards. But in other cases where the terrestrial movement appears to have been considerable, the rocks are so jointed as to have acquired therefrom a ﬁssile character that has nearly or wholly obliterated their tendency to split along the lines of bedding. An important feature in the joints of stratiﬁed rocks is the direction in which they intersect each other. As the result of observation we learn that they possess two domi- nant trends, one coincident in a general way with the direction in which the strata are inclined to the horizon, and the other running transversely at a right angle or nearly so. The former set is known as (fip;}'0i-)u‘s, because they run with the dip or inclination of the rocks, the latter is termed stri/.-c;jo2'2zt.-, inasmuch as they conform to the general st:-[Ice or mean outcrop. It is owing to the existence of this double series of joints that ordinary quarrying opera- tions can be carried on. Large quadrangular blocks can be wedged off, which would be shattered if exposed to the risk of blasting. A quarry is usually worked to the dip of a rock, hence the strike-joints form clean-cut faces in front if the workinen as ‘they advance. These are known as back-S: {Did the d1p~jomts which traverse them as “cutters.” The way in which this double set of joints occurs in a quarry may be seen 111 ﬁg. 15, where the parallel lines which traverse the shaded and unshaded faces mark the successive strata. The broad white spaces running along the length GEOLOGY 297 of the quarry behind the seated ﬁgure are strike-joints or “backs,” traversed by some highly inclined lines which mark the position of dip—joints or “ cutters.” The shaded "'11, .' ,../_
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/:C r/r;,.$« 4*‘ -J // -F '. _,L//4;; V F IG. 15.—Joints in limestone quarry near Mallow, co. Cork. (G. V. Du Noyer.) ends looking towards the spectator are “cutters” from which the rock has been quarried away on one side. In some conglomerates the joints may be seen traversing the enclosed pebbles as well as the surrounding matrix. Large blocks of hard quartz are cut through by them as sharply as if they had been sliced in a lapidary’s machine, and the same joints can be traced continuously through many yards of the rock. Such facts show that the agency to which the jointing of rocks was due must have operated with considerable force.1 Further indication of movement is often supplied by the rubbed and striated surfaces of joints. These surfaces, termed slit-A-ezzsizles, have evidently been ground against each other. They are often coated with haematite, calcite, chlorite, or other mineral, which has taken a cast of the striae and then seems itself to be striated. Joints form natural lines for the passage downward and upward of subterranean water. They likewise furnish an effective lodgment for surface water which, frozen by a lowering of temperature, expands into ice, and wedges off blocks of rock in the manner already described. As they serve, in conjunction with bedding, to divide stratiﬁed rocks into large quadrangular blocks, their effect on cliffs and other exposed masses of rock is seen in the apparently splintered, dislocated aspect so familiar in mountain scenery. Occasionally a prismatic or columnar form of joints may be observed among stratified rocks. When this occurs among unaltered strata it is usually among those which have been chemically formed, as in gypsum, where, as observed by Mr J ukes in the Paris Basin, some beds are divided from top to bottom by vertical hexagonal prisms. A columnar structure has often been superinduced upon stratiﬁcd rocks by contact with intrusive igneous masses. Sanrlstones, shale, and coal may be observed in this condi- tion. The columns diverge perpendicularly to the surface of the injected and altering substance, so that when the later is vertical the columns are horizontal, or when it undulates the columns follow its curvatures. Beautiful examples of this character occur among the coal-seams of Ayrshire. 2. In Cr;2/stal/z'ne (Igneous) Ia°o«:K‘s.—lVl1ile in stratiﬁed rocks the divisional planes consist of lines of bedding and of joint, cutting each other usually at a high if not a right angle, in massive igneous rocks they include joints only ; and as these do not as a rule present the same parallelism as lines of bedding, unstratiﬁed rocks, even though as full of joints, have not the same regularity of arrangement as in the stratiﬁed formations. Granite, for example, is traversed by two sets of chief or “ master—joints,” cutting each other somewhat obliquely. Their effect is to divide the rock into long quadrangular, rhomboidal, or even polygonal columns... 1 See an interesting series of experiments by ll. Daiibrée (C0”’1'5'-'3 Ilcndus, lxxxvi., 1878) on the production of fault{ and _]OInt8S. - . — 3
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