through this conglomerate, but the limestones with which it is associated.

Joints in limestones are often of the most marked kind. In many cases there is no difficulty in distinguishing the bedding from the joints. In others, however, the observer will not find it so easy to determine between the two surfaces, without much care. It sometimes happens, that the joints have a much more marked appearance than the divisions of true bedding. As, for example, in the annexed sketch (fig. 250), wherein the joints are prominently shown, one in particular being somewhat opened at a, while the

true bedding, bb, is more obscure. In such cases, the observer has carefully to search for lines of organic remains, dissimilar beds, or partings of shale or other substances, in order to be sure of the true bedding.

The courses of joints, though often of a marked kind through various rocks in the same district, and in the same general directions for long distances, as if the power producing them had been brought into action under some great leading influence, affecting a great mass of mineral matter in that district, however modified in character its parts may be, appear not a little adjusted, as in cleavage, to the main position of the component beds, there being frequently a tendency in joint divisions to take courses at right angles, as a whole, to it. As in cleavage, also, divisions resembling jointing, so far as their distance from each other is concerned, appear to run through certain beds of a general accumulation more abundantly than in others. Of this kind, the divisions through parts of the shales of the lias near Lyme Regis may be taken as an example. Though joints are not there observed in the mass of the argillaceous limestones composing that deposit, in certain beds of shales, on the west of the town, divisions, perpen

dicular to the beds, may be seen to run like so many planks on a floor, stretching as far as the beds are exposed at low water.

As there appears little reason to doubt that joints, like cleavage, have been formed, under suitable conditions, at different geological times, and as these cleaved or jointed rocks may readily have been moved after they were divided in this manner, it would be expected that, sometimes, the position of the one and the other, as regards their direction with the horizon, is not that in which either the cleavage or jointing was effected. Cleaved and jointed rocks are sometimes found in positions to render such subsequent movements probable. For example, the old red sandstone series of southern Ireland reposes upon Silurian rocks probably cleaved, if they were not also jointed, prior to the accumulation of the former, and the same series is also traversed by similar divisions. Upon studying that portion of Ireland, the observer finds that the old red sandstone, with also the carboniferous or mountain limestone series resting upon it, has been also disturbed since its deposit; hence, the lower rocks having been again moved to permit the rolling and bending of the great mass of matter resting upon them, their original planes of cleavage, if not of joints also, can scarcely be in their original position. The probability of such movements may, therefore, somewhat interfere with first views as to the original position of cleavage and joints, and the geologist should bear in mind, that the movement of a body of rock, divided in this manner, into flexures, might be accompanied by the friction of some of the surfaces of the divisional planes upon each other, thus embarrassing his researches into the original condition of such surfaces. Movements of this kind may give an uncertainty to the slightly-inclined planes of joints which are sometimes found, though there is, as yet, no evidence to show that joints have originated in a manner to render divisions in the mineral matter improbable at these angles with the horizon. Such planes of joints require to be well distinFig. 251.

a

a

guished from those of true beds, which they often much resemble, as, for example, in the preceding section (fig. 251), where a mass

of argillaceous matter, originally a thick accumulation of clay or mud, though now consolidated into hard rock, shows joint lines, a a a, and sections of the planes of cleavage, bb, but does not exhibit a surface sufficiently large to show the planes of true bedding.

Occasionally, the division of an original deposit of clay or silt, by cleavage and joints, becomes most complicated, requiring no slight care on the part of an observer to arrive at the surfaces of the true beds, more especially when organic remains are absent, and the mineral matter is of a common character throughout. Of this kind of complication, the following sketch (fig. 252) of a quarry at Fig. 252.

Brewer's Hill, county Wicklow, may be useful as an illustration. The true bedding is a plane, facing the reader, while there are divisional planes ranging in the direction a a, in that of b b, and in that of c c.

CHAPTER XXXIII.

BENDING, CONTORTION, AND FRACTURE OF ROCKS.—EARTH'S RADIUS COMPARED WITH MOUNTAIN HEIGHTS. VOLUME OF THE EARTH WITH MOUNTAIN RANGES.-EFFECTS OF A GRADUALLY COOLING GLOBE.— MOUNTAIN RANGES VIEWED ON THE LARGE SCALE.-DIRECTIONS OF MOUNTAIN CHAINS. CONDITIONS EFFECTING THE OBLITERATION OR PRESERVATION OF MOUNTAIN CHAINS.-LATERAL PRESSURE ON BEDS OF ROCK AMID MOUNTAINS.-BENDING AND FOLDING OF DEPOSITS IN THE APPALACHIAN ZONE, NORTH AMERICA.-FLEXURES AND PLICATIONS OF ROCKS IN THE ALPS, AND OF THE RHINE DISTRICT.—IGNEOUS ROCKS AMID CONTORTED BEDS.-CONTORTED COAL MEASURES OF SOUTH WALES. -CONTORTION OF THE COMPONENT PARTS OF BEDS.

THOUGH it has been necessary to allude to the disturbance of various accumulations, as well igneous as those formed by means of water, while noticing rocks of different kinds which have been more or less moved after their deposit or intrusion, it may be desirable to call attention to this subject as one which may also be conveniently considered by itself. It will have been seen, when pointing out the intrusion of igneous rocks, that the disturbance of mineral matter accumulated at one geological period, while the deposits of another were comparatively unmoved, assisted in affording evidence of the relative time when the igneous rock may have been elevated in a molten state from beneath (p. 564); and also that the arrangement of conglomerates and sandstones against or around beds of prior-formed disturbed rocks was useful in showing the probability of ancient dry lands having occurred in particular situations, edged by beaches and coast cliffs, (p. 475).

Though mountains by no means present us with the only means of studying the bending, contortion, and fracture of rocks on the large scale, they become important from the masses of matter raised in them comparatively high into the atmosphere and sometimes continuous for considerable distances, from the frequent adjustments

of lower grounds to them, and the opportunities
afforded for obtaining illustrative sections in
various planes. A glance at any artificial
globe of fair dimensions will be sufficient to
show the ranges or chains, as they have been
termed, of those mountains which constitute
marked ridges upon the surface of the earth.
With such a globe before him, and bearing in
mind the heights of the various ranges or
chains of mountains as compared with the
diameter of our planet, an observer may, pro-
bably, be led to infer that, however elevated
and important these may be considered by
those wandering amid their depressions, or
striving to ascend their heights, viewed as
ridges on the surface of the earth, they con-
stitute very minor protrusions, interfering
little with the general form of the world. It
is somewhat important, in searching for facts
illustrative of the production of mountains,
that their relative proportion to the volume
and diameter of the earth should not be ne-
glected. If, in the annexed diagram (fig. 253),
a be, represent a section of a portion of our
planet, from its surface a, b, to its centre e;
the thick line, a, b, would be the elevation of
even the highest mountains as compared with
the radius of the earth. Hence it is not diffi-
cult to conceive that the rending of any portion
of consolidated or partly consolidated mineral
matter, distributed in various ways over the
surface, a, b, and the squeezing of the sides of
these rents or fissures against each other, (with
or without the propulsion upwards of any
molten substances amid interstices in the
squeezed masses of consolidated or partly-con-
solidated mineral matter,) would present ridges
of varied forms more or less corresponding with
the lines of the fissures.

It has been seen that igneous rocks have been ejected in various ways, that mineral matter worn from them by the action of the

Fig. 253.