Mr. Sorby has shown that inferences may be drawn from the examination of these current-marks as to the velocity and direction of the currents which caused them, and that we may thus reason back to some conclusions regarding the physical geography of the district at the time they were formed.

Although, as above stated, ripple-marked beds are not usually such as were formed between tide-levels, yet surfaces are occasionally found which exhibit not only ripplemarks, but also sun-cracks, rain-prints, worm-tracks, and the footmarks of the various creatures which travelled over the moist, exposed surface of the bed. Such surfaces have doubtless been formed on flat shores, where large tracts were exposed between the lines reached by neap and spring tides, so that the surface beyond high-water of neap tides, being exposed to the action of the sun, became cracked and hardened, and the next spring tide only spread over it another layer of sand or mud.

CHAPTER VI.

JOINTS.

IN N the last chapter we described some of the structures imparted to stratified rocks at the time of their deposition; but all rocks, whether stratified or unstratified, exhibit other features which have been produced during the process of their consolidation.

There are few exposures of stratified rocks which do not exhibit certain other planes of division besides those of lamination or stratification. These are the joints, or long cracks which cut across the bedding planes and separate each bed into blocks of various shapes. It is obvious that in stratified rocks there must be at least two sets of joint planes besides the planes of stratification, in order to cut up the beds into blocks. In igneous or unstratified rocks it is equally obvious that in order to form squarish blocks we must have at least three sets of joints crossing each other, each set more or less nearly at right angles to the other two.

A good-sized clean lump of coal taken from the coalscuttle forms an excellent model for the exhibition of lamination and joint-planes. If the student will place such a lump on the table before him, he will perceive that the mass is divided by three regular and distinct sets of division planes. "The coal splits most readily along the lines of lamination, and the surfaces thus exposed along the tops and bottoms of the lumps are generally dull and earthy, and readily soil the fingers. At right angles to these surfaces others may be observed, which are generally bright and shining, and if the coal be freshly broken, these surfaces will soil the fingers much less than those on the top or bottom of the lump. He will see that there is one set of smooth, vertical surfaces (or joint planes), along which there occur the cleanest, largest, and most even sides to the block, the vertical surfaces at right angles to that

set being shorter, rougher, and more irregular. The first large, smooth, vertical surfaces are known by the name of the face, the slyne, or the cleat, of the coal in different districts, the more interrupted set being spoken of sometimes as the end of the coal."1

Cause of Joint Structure.-Everyone who has examined the muddy bottom of a dried-up pond or lake is aware that as the water evaporates, and the mud dries, long and deep cracks are formed which traverse the dry mud or clay in various directions, and cut it into polygonal blocks. The width and depth of these cracks seem to depend upon the extent and thickness of the mud.

In the same way molten rock shrinks and cracks in cooling. Attempts have been made to utilize the slags from iron-furnaces by running them into moulds, but the quadrangular blocks so obtained proved to be useless for building purposes because they cracked and crumbled into small cuboidal fragments in consequence of the numerous joints developed during the process of cooling and solidification. Differences in the texture of rocks will naturally cause differences in their manner of cracking, and so different forms of jointing will be produced. The number of joint planes will also partly depend upon the thickness of the bed or mass. Some beds will shrink more than others, and some masses of igneous rock must have cooled more rapidly than others.

Mr. Jukes has remarked that it does not follow that all the joints in any mass of rock should be formed at any one time. "The consolidation of the mass may take place slowly and gradually, and successive sets of joints be produced in it at different times during that process. A rock, moreover, may be, at some subsequent period, placed under circumstances calculated to produce a greater degree of consolidation, and a fresh set of joints may be produced in it from that cause.

"The small or short joints confined to individual beds of stratified rocks may have been those first formed on the original consolidation of the one bed before the next was deposited on it, those joints being then, perhaps, quite im

1 Jukes' "Manual of Geology," second edition, p. 212.

perceptible divisional planes with no interspace between the blocks. Whole sets of beds may have subsequently been subject to one, two, or more actions of consolidation, which may have produced larger joints traversing the whole mass. Still more extensive joints may have been formed subsequently by the mechanical agency of the upheaving forces acting on the crust of the globe."

M. Daubrée has lately obtained some curious results by submitting plates of various substances to the effects of torsion, and finding that he could by this means produce sets of cross fractures similar to ordinary joints, he boldly ascribes all joints to the action of force similarly applied.2 Mr. J. G. Goodchild, however, has recently examined this question, and shows that joints are of various kinds, and are not likely to have all been formed in the same way and by the same force. Some are clearly shrinkage cracks, while the larger "rift-joints" he regards as produced by the unequal pressures and strains exerted upon rockmasses during movements of subsidence and upheaval.*

Cuboidal Jointing.-Any mass of stratified rock which is traversed by two sets of joints that are roughly at right angles to one another will fall into cuboidal or squarish blocks. This may be seen in almost any stonequarry, and fig. 67 may be taken to represent a part of such a quarry. The rock-faces, one set of which receive the light, while the other set are in shadow, are the surfaces formed by the two sets of joints cutting the planes of stratification at right angles, and also cutting each other at right angles, and thus making the square corners.

It sometimes happens that small irregular oblique joints more or less obscure the two rectangular sets, and the rock then falls naturally into blocks of various shapes.

In a series of stratified rocks each bed generally has its own system of joints, which are close and regular and do not penetrate the beds above and below. There are always others, however, which are common to several successive

1 Jukes' "Manual of Geology," second edition, p. 218.

Geologie Experimental," vol. i., p. 300 et seq.

3 46 Geol. Mag.," Dec. 2, vol. x., p. 397.

See also G. K. Gilbert, “ Amer. Journ. of Science," ser. 3, vol. xxiv., p. 50.

beds, and seem to have been formed simultaneously, though they often change their direction a little in passing from one bed to another. Lastly, there are some still larger and longer planes of division, which cut through many successive beds, and maintain the same direction both vertically and laterally for very long distances; these are called the masterjoints. The master-joints sometimes form a more conspicuous feature in the rock than the planes of stratification do, and are occasionally found to cut across whole mountains in regular parallel lines. See the Frontispiece," in which the joints traversing the "Drei Zinnen" in the Dolomite Mountains are well exhibited. These large joints

are often open fissures with a space of an inch or more between the walls, and in limestones this space has frequently been widened still more by the action of percolating water (see p. 80 and fig. 17).

Surface Exhibition of Joints.-Although the general characters and vertical course of joints may be readily observed in almost any stone-quarry, there are not such frequent opportunities of studying their horizontal extension or surface exhibition over large areas. Mr. Jukes found that the newly formed beds of stone which occur on

1 From Prof. Bonney's "Alpine Regions."