MAGNESIAN LIMESTONE.

285

glomerate, from its abounding with those concretions we have already noticed, as being the usual accompaniment of a change of formation; and which you compared to the small articles so useful in restoring an even surface, when one is packing a trunk.

"The conglomerates of the magnesian limestone often exhibit fragments of the mountainlimestone, compounded sometimes of large pebbles of that rock, and sometimes of grains so small as to be scarcely distinguishable. Then again, these concretions appear in masses as large as a cannonball, or grouped together like chain-shot, or clustered like a bunch of grapes. They are dispersed through a sandy stratum of similar materials.1 think you will allow that the name conglomerate is well bestowed on such a rock as this. Why it is also called magnesian limestone I have already explained.

2

I

"Like the mountain-limestone, this rock presents us with hills and caverns. The principal range of the former extends from Sunderland, on the coast of Durham, to Nottingham. They are covered in many places by poor herbage, which is attributed to the magnesia contained in the rock

being unfavourable to vegetation of that kind. Some plants thrive luxuriantly in the shallow soil which covers it, especially sweet-scented violets, and yellow roses, with single and double flowers. It is said that animals are not fond of the grass which grows on magnesian limestone: it is, therefore, a good thing for the farmer that his land, when improved by lime, will produce corn.

"One of the coal-fields in the neighbourhood of Newcastle exhibits a striking instance of those rents or dislocations of the coal-strata which I have mentioned. When such rents or faults are inconsiderable, they are called troubles, or slips-when large, dykes. The displacement they occasion in the coal-strata is sometimes prodigious, the same bed of coal being found on the opposite sides of the fissure at very different levels. The fissures do not remain empty, but are filled with sand-stone and various other substances.

"The most celebrated of these fissures is called the Ninety-Fathom Dyke, because the beds on the northern side are ninety fathoms lower than those on the southern side. It seems to have been occasioned by the irruption of a vein of basalt, which is seen in the fissure, lying in detached masses, coated with yellow ochre. The strata of coal have

NINETY-FATHOM DYKE.

287

been displaced by the injected basalt, which occasions faults on both sides of the dyke; but the magnesian limestone lies evenly, close upon the top of the dyke and the displaced coal-beds, without being penetrated by the basalt, or disturbed by the derangement of the coal. Hence it is concluded that the limestone was not deposited till after the formation of the dyke,' which being itself a vein, must be of later date than the coal-measures it has penetrated."

Coal

Magnesian Limestone

Strata

Dyke

Copied from Conybeare.

"The diagram expresses this irruption of the basalt very well," said Harry. "Do you suppose it was melted by subterraneous heat, and thrown up like a granite vein ?"

"The

"I should think so," replied his mother. coal in the vicinity of some of these fissures exhibits traces of the effects of heat, having first become sooty, and at length assumed the appearance 1 Conybeare, p. 305-309. Ure, p. 161.

of coke, which is a kind of cinder, produced by burning common coal, with only a partial admission of air. This appearance in the coal-strata is said to be unknown, except in the vicinity of basaltic dykes, and perhaps affords as strong evidence as we can obtain, that the irruption of basalt has been the effect of subterraneous heat.1

"Above the magnesian limestone is found the great deposit of red marle, which I have already described. We have therefore completed the first stage of our journey through the secondary strata."

1 Conybeare, p. 376.

CHAPTER VIII.

Here the blue lias yields a monstrous brood,
The ancient tyrants of the wat'ry plain;

Swift borne on oary feet in quest of food,

They scann'd with eagle-eye the far-surrounding main.

Of feebler structure some, in ambush laid,

Stretch'd the long serpent neck with pliant sway;

And, as the finny tribes around them play'd,

They slily seiz'd the unsuspecting prey.

OLD FRAGMENT.

"THE interval between the red marle and the iron-sand," said Mrs. Beaufoy," is occupied by a series of strata, called oolites. The name is derived from two Greek words: o-on an egg; and lithos a stone. It was assigned to the rocks in question from a fancied resemblance between the little round masses of which many of them are composed, and the roe of a fish: indeed, formerly it was supposed really to consist of the roe of fishes, in a petrified state; and hence it has been called roe-stone.