ABUNDANT EVIDENCE.

47

be guilty of a false assumption. The true state LECT. II. of the facts is the very contrary to what he supposes. We are acquainted certainly, I might almost say perfectly, with the character and succession of the deposited substances which, laid upon each other, compose the crust of our globe; and we know the totally different constitution of the materials which lie underneath. We see demonstrated, with satisfactory clearness, the distinct character and the opposite mode of production of these two classes of mineral formations. We have all the evidence that can reasonably be desired of the previous condition of those underlying rocks, their ancient, and at a depth not great their present, liquidity by heat, their boiling up, their extrusion both in the melted state and in different degrees of advancement towards being cooled and hardened, their being driven upwards through the overlying formations of deposited layers, their sometimes insinuating themselves between the previously contiguous surfaces of those deposits, their filling long furrows of outburst, and their being laid bare in many cases to open day-light. It is therefore no presumption to affirm that we do know, with the clearness of sensible evidence, the constituent formations of the crust of the earth, their modes of production, their relations to each other, and the fact of their enveloping a mass of materials, similar in composition to the lowest rocks, and which we have much

LECT. II. reason to think are, at certain depths, still in a state of constant fusion.

Interior of

the earth in a state of fusion.

Those who bring forward this objection are, perhaps, not aware of its bearing. Were it well founded, its effects would be to augment, by immeasurable degrees, the antiquity which must be attributed to the earth.

In replying to this objection, which is brought up at the very threshold of geological inquiry, I have been led into an anticipation of several positions, which must be stated more in regular detail.

II. There are good grounds for supposing that, beyond a certain thickness for the solid crust of the earth, which can hardly be estimated at so much as thirty miles, the next contiguous matter is in a state of fusion, at a temperature probably higher than any that man can produce by artificial means; or any natural heat that can exist on the surface. Whether, in like manner, the whole interior of our planet be composed of melted matter; or whether there be a solid nucleus; and whether such nucleus be close-grained, or more probably cavernous, the solid partitions being infusible and the disseminated vesicles filled with gaseous substances at a very high temperature; thus presenting an analogy to the appearance of ordinary boiling liquids;—are parts of the problem upon which eminent geologists are not agreed. But in this they are agreed, that they will not

ROCKS OF FUSION.

49

put conjecture, however probable, in the rank LECT. II. which is due only to decisive evidence; and that they will wait with patience till such evidence shall be attained. In the mean time the highest efforts of mathematical genius are on the stretch for the resolution of the problem. But that a large part of the interior matter of the earth, and that part in contact with the solid crust on which we dwell, is in the state of fusion by heat, appears to be a doctrine established by most satisfactory proofs. It should be considered, that the mean density of the earth is not quite five sevenths of that of iron, nor half that of silver, or one fourth of that of gold: facts utterly inconsistent with the supposition that the interior is a solid mass, or occupied by vast bodies of water, united or detached; or indeed any thing but a fluid or fluids maintained in that state by the action of heat as an antagonist power to gravitation. This mean density is rather more than double that of granite.

rocks.

III. The rocks which lie the lowest in the Pyrogenous descending order, and which of course are under all the stratified deposits, are in the state which has been produced by the prodigious heat that has been mentioned, acting under a pressure from above so great as incomparably to exceed any familiar weight or force that we could mention as a measure of comparison. Those rocks bear clear marks of having crystallized in cooling from a state of fusion. It has been objected,

E

LECT. II. that the component parts of those rocks melt at unequal degrees of heat; as in the constituents of granite, which are quartz, mica, and felspar, the last of these ingredients is fused at about half the temperature which the first requires.

But they who make this objection overlook the fact of the extreme pressure under which the power of heat was exerted; which would prevent the most fusible substance from being volatilized at the highest point that could exist: neither can they argue from the inequality of the points of fusion of the minerals when extricated, that the compound would not melt even in far less favourable circumstances; for most persons are acquainted with the ready fusion of metallic compounds, though at a point considerably different from that which each ingredient would require singly.

Stratified IV. The rocks which lie above these, though

rocks.

partial crystallization, generally aqueous but sometimes igneous, is found in them, are demonstrably of a different origin. They are all composed of earthy matter, that is, different mixtures of sand, clay, and lime, with minor proportions of some other interspersed minerals. These have been washed away from the previously elevated rocks, by the action, first, of the atmosphere and variations of temperature, disintegrating and loosening the surfaces; and then of dropping rain and running rills and streams, washing off the materials, in fine par

DISPOSITION OF THE BEDS.

51

ticles or coarser grain, through all degrees of LECT. II. attenuation; carrying them down into lower situations; and finally, after perhaps a very long succession of these transporting and sedimentary processes, depositing them on levels of rest, in the quiet bottoms or local depressions of lakes and seas. Each sediment or deposit is called a layer, or bed; for conveniency using the Latin word stratum. These stratified formations may be called about forty in number; in thickness, sometimes only a few feet or even inches, but usually many hundreds or several thousands of feet. Stratification must be distinguished from homogeneous lamination, which is a frequent character of single strata, presenting at their edges the appearance of leaves, like those of a book or a bundle of pasteboards. Taking some general resemblances of mineral composition as a principle of classification, the whole of the existing beds may be distributed into a small number of groups, in a measure according to the convenience of the geological observer, describer, or reasoner; though most acquiesce in making about twelve divisions, which, for the most part, have very distinct natural characters. Such a distribution is, at least, useful as an aid to the memory.

of strata.

V. These beds of deposited earthy substances Disposition are not to be conceived of as concentric spheres, spread universally over the earth, the outermost including lower ones, and thus embracing the