Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/237

Rh THE SOLID GLoBE.] rocks being due to some of its combinations. The sixteen elements mentioned in the foregoing lists form about ninety-nine parts of the earth’s crust; the other elements constitute only about a hundredth part, though they include gold, silver, copper, tin, lead, and the other useful metals, iron excepted. It is clear then that, so far as accessible to our observa- tion, the outer portion of our planet consists mainly of metalloids, and its metallic constituents have in great part entered into combination with oxygen, so that the atmosphere contains the residue of that gas which has not yet united itself to terrestrial compounds. In a broad view of the arrangement of the chemical elements in the external crust, the suggestive speculation of Durocher deserves attention.‘ He regarded all rocks as referable to two layers or magmas co-existing in the earth’s crust the one beneath the other, according to their speciﬁc gravities. The upper or outer layer, which he termed the acid or siliceous magma, contains an excess of silica, and has a mean density of 265. The lower or inner layer, which he called the basic magma, has from six to eight times more of the earthy bases and iron oxides, with a mean density of 2'96. To the former he assigned the early plutonic rocks, granite, felsite, &c., with the more recent trachytes; to the latter he relegated all the heavy lavas, basalts, diorites, &c. The ratio of silica is 7 in the acid magma to 5 in the basic. Though the proportion of this acid or of the earthy and metallic bases cannot be regarded as any cer- tain evidence of the geological date of rocks, nor of their probable depth of origin, it is nevertheless a fact that (with many important exceptions) the eruptive rocks of the older geological periods are very generally super-silicated a11d of lower speciﬁc gravity, while those of later time are very frequently poor in silica but rich in the earthy bases and in iron and manganese, with a consequent higher speciﬁc gravity. The latter, according to Durocher, have been forced up from a lower zone through the lighter siliceous crust. 3. The Interior or N-ucleus.——Thougl1 we cannot hope ever to have direct acquaintance with more than the mere out- side skin of our planet, we may be led to infer the irregular distribution of materials within the crust from-the present distribution of land and water, and the observed differences in the amount of rleﬂexion of the plumb-line near the sea and near mountain-chains. The fact that the southern hemisphere is almost wholly covered with water appears explicable only on the assumption of an excess of density in the mass of that portion of the planet. The existence of sucl1 a vast sheet of water as that of the Paciﬁc Ocean is to be accounted for, says Archdeacon Pratt, by the presence of “some excess of matter in the solid parts of the earth between the Paciﬁc Ocean and the earth’s centre, which retains the water in its place, otherwise the ocean would flow away to the other parts of the earth.”2 The same writer points out that a deflexion of the plumb-line towards the sea, which has in a number of cases been observed, indicates that “the density of the crust beneath the moun- tains must be less than that below the plains, and still less than that l3tl)V the ocean—bed.”3 Apart therefore from the depressions of the earth’s surface in which the oceans lie, we must regard the internal density, whether of crust or nucleus, to be somewhat irregularly arranged,—there being an excess of heavy materials in the water hemisphere and beneath the ocean-beds as compared with the continental masses. In our ignorance regarding the chemical constitution of the nucleus of our planet, an argument has sometimes been 1 Translated by Haughton in his Jlamual of Geology, 1866, p. 16. '-’ Figure of the Earth, 4th edit., p. 236. ' 3 Op: cit., p._ 200. See also Herschel, Plrys, Geo!/,; and 0_ Fisher, (u.mbrzdge Plul. Trams, xii., part ii. GEOLOGY 223 based upon the known fact that the speciﬁc gravity of that nucleus is about double that of the crust. This has been held by some writers to prove that the interior must con- sist of much heavier material, and is therefore probably metallic. But in so reasoning they forget that the effect of pressure ought to make the density of the nucleus much higher, even if the interior consisted of matter no heavier than the crust. In fact, we might argue for the probable comparative lightness of the substance composing the nucleus. That the total density of the planet does not greatly exceed its observed amount seems only explicable on the supposition that some antagonistic force counteracts the effects of pressure. The only force we can suppose capable of so acting is heat. But how and to what extent this counterbalancing takes place is still unknown. If we regard the question from another point of view, however, the idea of a metallic nucleus seems not improbable. When the materials of the globe existed in a ﬂuid condition, as they are usually supposed to have done, they would doubtless arrange themselves in accordance with their relative speciﬁc gravities. The denser elements would sink towards the centre, the lighter would remain outside. That this distribution l1as certainly taken place to some extent is evident from the structure of the envelopes a11d crust. It is what might be expected if the constitution of the globe resembles on a small scale the larger planetary system of which it forms a part. The existence of a metallic interior has always been inferred from the metalliferous veins which traverse the crust, and which are commonly supposed to have been ﬁlled from below. Admitting the possibility or even probability of a metallic nucleus, in spite of the comparatively low density of the globe as a whole, we might speculate further as to the arrangement of the denser internal materials. The late Mr David Forbes suggested that the planet might be sup- posed to consist of three layers of uniform densities, enclosed one within the other, the density increasing towards the centre in arithmetical progression. Allowing 2'5 as the speciﬁc gravity of the crust or outer layer, he assigned 120 or thereabouts as that of the middle layer, and supposed that the inner nucleus might possess one averaging 20'O.4 Materials do not yet exist for any satisfactory conclusions on this subject. In the evidence obtainable as to the former history of the earth, no fact is of more importance than the existence of a high temperature beneath the crust, which has now been placed beyond all doubt. This feature of the planet’s organization is made clear by the following proofs :— (1.) Volcanoes.—In many regions of the earth’s surface openings exist from which steam and hot vapours, ashes and streams of molten rock are from time to time emitted. The abundance of these openings seems inexplicable by any mere local causes, but must be regarded as indicative of a very high internal temperature. If to the still active vents of eruption we add those which have formerly been the channels of communication between the interior and the surface, there are probably few large regions of the globe where proofs of volcanic action cannot be found. Every- where we meet with masses of molten rock which have risen from below as if from some general reservoir. (2.) II ot .S’p7-z'2zgs.—Where volcanic eruptions have ceased, evidence of a high internal temperature is still often to be found in springs of hot water which continue for centuries to maintain their heat. Thermal springs, however, are not conﬁned to volcanic districts. They sometimes rise even in regions many hundreds of miles distant from any active volcanic vent. The hot springs of Bath (temp. 120° Fahr.) and Buxton (temp. 82° F ahr.) in England are '5 Popular Science Reriew, April 1869.