when these beds were at or near the surface of the ground. In all probability it took place when they were buried deeply, probably many thousand feet under other similar strata; for they can be shown to have been certainly so buried once, and we can more easily understand this effect to have been the result of enormous internal force, overcoming immense superincumbent pressure, than of a force acting near the surface without much to resist it.

11. The next cut, fig. 5, is introduced in order to show both the method of accumulation of some rocks not yet spoken of, and to put us on our guard against always taking for granted, that highly-inclined beds have been upheaved by the forces just alluded to.

It is a sketch of a cliff on the shore of Killiney Bay near Dublin. The "rocks" here are loose incoherent sands and gravels and soft clays. These materials are very irregularly interstratified, the beds of sand and clay thickening or thinning rapidly, or often ending and setting in again. The gravels vary in coarseness, from the smallest pebbles to blocks the size of a man's head, and occasionally there is a boulder of granite some feet in diameter derived from the immediately adjacent hills. Many of the pebbles, however, and in

some beds the whole of them, are fragments of limestone, which must have been brought a distance of some miles at least, and some of those on the beach are chalk flints washed from the north of Ireland. In some places the stratification and the sorting of the materials, according to their several degrees of fineness, is pretty regularly carried out, the beds being for the most part horizontal; but in other parts the layers are suddenly inclined, curving upwards at an angle of 40° with the horizon over spaces of more than 100 yards in length, and through cliffs 50 or 60 feet in height. One such spot has been selected for the illustration. It is well marked by a thick bed of very fine sand, which is left white in the centre of the drawing.

This irregularity in the size and assortment of the materials, and the varied angle at which they have been originally deposited, is a sure proof that the deposit was accumulated in a comparatively shallow sea, where the currents were numerous and strong, and frequently shifting, and where the materials were from some cause very abundantly supplied. Banks of sand and pebbles having steep slopes were thus often formed, and the subsequent depositions were thrown down on the sides of these banks and conformed to their outline. Somewhat of the same effect, though to a slighter extent, may often be seen in all sandstones; it is then known to geologists by the term of "false bedding," or "oblique lami

nation."

12. Such are a few of the principal phenomena exhibited in the structure of the stratified or aqueous rocks. They teach us that all such rocks have been deposited under water, some very slowly, some more rapidly, at various intervals throughout a long succession of ages, and under a great variety of circumstances and conditions. They also show us that at different periods after their deposition, they have been subject to actions involving great disturbance, causing great elevations and depressions, great bending and breaking, and also great destruction and wearing away of the masses that had been thus deposited, and thus affected by disturbing agencies. We may learn too from what

has gone before, that there is not a pebble or a stone, or even a single grain of sand, or a particle of mud, that has not at one time or other formed part of a solid and extensive mass of rock.

LESSON IV.

IGNEOUS ROCKS.

"And set on fire the foundations of the mountains."

1. At the beginning of this chapter we entered slightly on the subject of igneous rocks, we must now briefly recur to the subject for the sake of showing that there are some not yet described, which, though igneous, are not, strictly speaking, volcanic.

We spoke there of lava and ashes being interstratified with one another, and of lava streams being porous and cindery above, while they are solid and compact below. The porosity and cindery aspect is the result of the easy escape of the pent-up gases from the parts near the surface, the gases blowing bubbles, as it were, in the melted stone. Pumice, we may look upon as the froth or scum of lava thus formed, just as much as the frothy head of a pot of porter is produced by the escape of the carbonic acid gas from the liquid. In the deep-seated parts of a lava stream, -those which have cooled slowly under the pressure of the superincumbent mass-all is heavy, solid stone, very often having a columnar structure, such as that described as belonging to basalt in a former chapter.

2. This, alone, would be sufficient to show us that basalt itself is probably an igneous rock, and this is proved to be the case in the following way :— Where a mass of basalt comes in contact with other rocks, such as clay, sandstone, chalk or coal, it alters the rocks it touches, just in the way in which great heat would alter them. Clay is hardened into jasper, sandstone into quartz rock, chalk into crystalline marble, and coal into a substance resembling coke. Moreover, wherever there is a large development of basalt, as in the celebrated district of the Giant's Causeway in County Antrim, there are almost always found one or two or more ranges of columns one above

another, interstratified with beds of volcanic sand or ashes.

Fig. 6 is a sketch of a spot called the Giant's Chimney, near the Giant's Causeway, where there are four ranges of columns interstratified with three beds of what is there called ochre, but which is nothing else than volcanic ash. These beds of ochre are of a bright yellowish-red, and are easily perceptible in the cliffs; they are left white in the sketch of the far headland.

3. Basalt, with its kindred rock greenstone, often sends great wall-like masses called dykes; and also great and small veins into the cracks, fissures, and crevices of the rocks it comes in contact with. These intrusive rocks must evidently have been forced into the cracks and crevices, or between the joints of the limestone beds, while the "trap" was in a liquid or molten state.

4. Granite, syenite, and other similar rocks, send similar intrusive masses into the rocks, with which they come in contact, which alone proves them to have a similar igneous origin with basalt and greenstone. They, moreover, produce exactly the same kind and even a still greater amount of alteration on the rocks near them, altering shales and sandstones into micaschist, and gneiss.

5. Another character, which is common to the granites

and the greenstones, and more or less to all igneous rocks, is that of crystallization; the minerals composing them being confusedly crystallized together. This crystallization shows the rock to have been once fluid. When these minerals are such as can be made fluid by water, like carbonate or sulphate of lime, or common salt, the rock itself may have been formed from such an aqueous solution. But where these

minerals are such as are rarely or not at all soluble in water, such as feldspar, hornblende, mica, and quartz, while they are, either separately or combined, fusible by heat, it is obvious that an igneous origin is the one we must assign to them.

6. Another character common to the igneous rocks, and distinguishing them at once from the aqueous, is their want of stratification. It is true that great beds of them sometimes occur of regular thickness over pretty large spaces, and regularly interstratified with other rocks, but these beds have no internal marks of stratification within themselves. They are not subdivided into beds or laminæ, showing a gradual and successive deposition of layer upon layer to have produced the whole mass: on the contrary, the whole bed has evidently been poured forth at once, and any subdivision into parts has been a subsequent and superinduced arrangement.

This rearrangement is either the spheroidal one, giving a globular structure to the mass, that is, a tendency to form an assemblage of balls; or the prismatic one, causing the mass to split into long prisms or cylinders; or else what might be called the cuboidal one, giving a tendency to divide more or less perfectly into solid squares or cubical blocks.

7. These latter divisional planes are called "joints;" they are common to all rocks, igneous and aqueous, seeming to be the result of the consolidation of the masses, in passing from a soft or fluid condition into a solid one. They are, however, more perfectly carried out in igneous rocks, and of these most of all, perh ps, in granite. In this rock the joints are often so regular, at such equal distances, and different sets are so parallel