Granite, porphyry, sienite, greenstone and basalt, pass by insensible gradations into each other, and into rocks known to be volcanic: hence the probability of their having a similar origin is greatly increased. And if the internal fires that have acted successively on the surface of the globe, were of vast extent, as the remaining craters indicate, they may also in numerous instances have melted or softened pre-existing rocks and strata, and occasioned the bending and contortions of the strata, and other phenomena, on which the theory of Dr. Hutton was founded.. The defect of that theory consists, I conceive, in extending the operation of this cause, further than existing appearances will support.

Were we to admit that rocks were produced by successive igneous and aqueous eruptions, poured through craters and fissures of the surface, these, with subsequent elevations and subsidences of the surface, might be sufficient to explain all the va rious phenomena which the position, contortion, succession, and alternation of rocks and strata present to our notice. In some situations granite mountains are covered with a series of schistose rocks, to which succeeds transition limestone; and on this are laid the sandstones of the coal formation. In other instances these sandstones rest immediately on granite, without the intervention of schistose rocks. Here then we may suppose, that no eruptions of matter took place between the formation of the granite and the sandstone; while in other situations, a succession of formations had produced all the intermediate rocks. In some

countries, the eruption of matter which formed granite, after ceasing for ages, had again taken place; and thus sometimes we find granite covering rocks to which it is most frequently subjacent. To a like cause may we ascribe the occasional appear ance of beds similar to the lower rocks, alternating with or appearing in the upper strata. The sili ceous and calcareous solutions in a state of tranquillity, might also envelop the fragments and sand from pre-existing rocks, and form the various breccias and aggregated sandstones. Saline and bituminous matter may have been thrown up in detached lakes, and subsequently consolidated, as in the pitch-lake in the island of Trinidad. The local formation of beds of trap alternating with other rocks, has before been alluded to, and the gradation of basalt into clay, or sand, will be con, sistent with this mode of formation, Many of the solutions containing terrene matter might be erupt, ed at a boiling temperature, like the siliceous water thrown out of the hot springs in Iceland; and on cooling they might deposit their contents, the matter from each eruption forming a separate layer or

stratum*.

* To compare great things with small,—there is an analogous formation taking place every day, in the channels which receive the boiling waters from some of the steam-engines in the county of Durham. This water contains a large quantity of earthy matter, which is deposited every day, except Sunday, in regular layers that may be distinctly counted, with a marked line for the interval of repose on Sunday, between each week's forma. tion: hence the stone got out of these channels, has received from the country people the name of Sunday stone.

In some parts of the earth, the quantity of matter thrown out during one eruption, may have been sufficiently great to admit the crystallization of whole groups of mountains: in other instances, it may have been so widely diffused as to form very thin strata. And here it may be proper to remark, that different beds and strata are not arranged in nature in the order of their specific gravity; the lowest are not always the heaviest: neither are they arranged according to their more perfect crystallization; for, though generally the lower rocks are more crystalline than the upper, we not unfrequently find some of the upper strata, more perfectly crystalline than the subjacent rocks. Now if the matter of which the upper and lower rocks are formed, had been co-existent in the same fluid medium, one or other of the above effects must have taken place; but if each stratum were formed by a separate eruption and deposition, they might vary both in specific gravity and degrees of crystallization, without any regard to the order in which they were deposited.

In endeavouring to trace the causes of very complicated phenomena, those explanations are to be preferred, which apply to the greatest number of cases, and are consonant with existing or analogous facts. Now I conceive that the alternation of aqueous and igneous eruptions, offers a more satisfactory explanation of the formation of rocks, than any that has yet been proposed. At the same time it assigns an office to the immense craters and fractures which have once perforated or intersected the globe.

It is an acknowledged maxim, that Nature, or to speak more correctly its divine Author, does nothing in vain ;-and can we suppose that the interior part of the earth is constructed with less skill, than what is evinced in the organization of the simplest animal or vegetable? Or, when we contemplate our planet pursuing its trackless path through the heavens with unerring precision, can we believe, that its internal motions are not governed by determined laws, destined to answer the most important purposes in the economy of na

ture?

Though I am inclined to regard the explanation, here offered respecting rock formations, as consonant with existing facts, and as reconciling the phenomena of aqueous and igneous products alternating with or graduating into each other,-facts that appear so contradictory to the theories hitherto advanced, I would, however, willingly adopt any other explanation that may afford a more satisfactory solution. The Roman poet, after conducting his hero through the subterranean abodes, dismisses him through the Ivory Gate*: and should my readers infer from these speculations respecting the subterranean operations of nature, that I treat them in the same manner, it will not occasion disappointment. Embarked with them in a voyage of discovery, I shall gladly hail the signal for the appearance of solid ground, whoever the fortunate discoverer may be.

* En. lib. vi,

394

CHAPTER XVII.

ON THE REPOSITORIES OF METALLIC ORES.

Metallic Matter disseminated through Rocks.-Masses of Metallic Ore.-Metallic Beds.-Metallic Veins.-Rake Veins. -Flat Veins.-Accumulated Veins.-Cross Courses.-The remarkable Structure of the Botallack Mine worked under the Sea. On the Formation of Metallic Ores.-Remarkable Phenomena in Mines.-Stream Works.-Rocks in which certain Metallic Ores are found.

THE rocks and strata, and the mineral veins or dykes, described in the preceding chapters, are composed of earthy minerals, sometimes combined with a portion of metallic matter, chiefly iron. The mineral substances to be described in the present chapter, as forming beds or veins, or irregular masses, or grains imbedded in other rocks, consist of metallic matter either pure or in combination with sulphur, oxygen, or acids. Metallic beds and veins may be regarded as fixed constituent parts of the crust of the globe. The difference of external character between a pure metal and an earth is so great, that we find some difficulty at first in conceiving how metallic matter can form beds interstratified with earthy rocks; but the discoveries of modern chemistry have shown, that metallic and earthy minerals are closely allied. Nothing can appear more essentially different than a piece of polished iron and a piece of marble or limestone: yet if iron be exposed to the action of air and water,