GEOLOGY. 



GEOLOGY. 



970 



first new phenomenon, to be immediately followed by condensation 

 through pressure about the centre ; and thus two solid masses would 

 be produced and continually augmented a spherical nucleus, and a 

 spherical shell while between them -would remain a large but 

 diminishing zone of viscous matter, subject to some changes of 

 temperature through the conversion of its surfaces from a liquid to a 

 solid state. 



If, on the other hand, the effect of pressure to the centre became 

 superior to the expanding agency of heat, before the circulation of 

 liquid matter had ceased in the superficial parts, the centre would 

 solidify first; and the induration might proceed through a large 

 part of the globe, so as even to approach the surface before that 

 could be consolidated. If these conditions were reversed, consolida- 

 tion might proceed from the surface downwards, and would ultimately 

 reach the centre, and the whole mass be a stony globe. 



It is important to remark that upon neither of these suppositions 

 is it required to admit the continual augmentation of heat to the 

 centre; to which M. Poisson objected, and instead of which he 

 proposed to account for the phenomena of the earth's interior 

 temperature, by supposing that the solar system had once passed 

 through other ethereal spaces than those which it now occupies, and 

 there experienced much higher temperature at the surfaces of the 

 planets. This hypothesis may be perhaps not very different in its 

 development from the more general theory of the nebulous origin of 

 the planets ; but it appears unnecessary to discuss the speculation 

 after what has been said of the cooling of the earth. 



We may now proceed to examine the modern causes of changes on 

 the surface of the earth. The never-ceasing activity of the powers of 

 nature may be viewed as an inextinguishable and unavailing effort to 

 restore an equilibrium which is incessantly disturbed. The protean 

 changes of the atmosphere ; the varying effects which its chemical 

 and mechanical energies occasion among the masses of dead matter 

 and the forms of life ; the flowing of the ocean ; the subterranean 

 fire and wide wasting of the earthquake, are all efforts to obtain rest 

 consequent on a succession of perturbations. In this sense, not the 

 earth only, but all the solar system, and perhaps all the extent of the 

 heavenly spaces, conceivable rather than visible by man, is in the 

 condition of instability described in the Pythagorean Philosophy, 

 " Nihil est toto quod perstet in orbe." 



These changes on the surface of the earth affect the geographical 

 boundaries of land and water, the relative level of land and sea, and 

 the forms, proportions, and distribution of animal and vegetable life. 

 In a popular sense they may be classed by their proximate agencies, 

 as depending on chemical and mechanical powers originating from 

 atmospheric action, rains, springs, rivers, &c. ; as depending on similar 

 powers residing in the ocean ; and as affected by volcanic forces. We 

 may also venture to contrast the effects of the watery agencies, 

 whether of atmospheric or oceanic origin, with the products of 

 volcanic fires. For the general effect of the watery agencies is to 

 abate the high and to raise the low, to equalise the level of land and 

 sea by abrading the former and filling the latter; but volcanic 

 effects are directly the reverse. They augment the original inequality 

 of the surface ; in some parts they raise matter from within the 

 earth, and form new hills to bear the ravages of the atmosphere ; 

 and elsewhere cause tremendous depressions of land, and sink in 

 deeper hollows the original basins of the ocean. 



The external influences, thus contrasted with the interior powers 

 of the globe, are far more various in their aspect and more general in 

 their visible operation ; yet they may all be reduced to one or two 

 variable forces, independent of the terraqueous system. It is to the 

 unequal accession of heat from the sun, upon a globe whose distance 

 varies, whose parts are variously presented to the radiating beams, 

 and to the unequal abstraction of heat by the cold ethereal spaces 

 in which the earth circulates, that we may refer all the variations of 

 corpuscular and mechanical phenomena on the globe ; while in the 

 varying diffusion of light we recognise the prime element of change 

 in the animal and vegetable world. 



Minute as is their momentary impression, the sum of their effects 

 in a long time is prodigiously great ; heat and moisture by alternate 

 influence weaken ; frost bursts ; carbonic acid eats with cankering 

 tooth ; rains, swallowed up by the fissured rocks, abstract parts of 

 their substance ; land-slips, avalanches, and glaciers heap the valleys 

 with detritus, till swollen rivers or bursting lakes sweep away the 

 burden towards lower ground, or convey it even to the sea. Thus 

 chemically dissolved, mechanically suspended, or roughly rolled along, 

 the substance of all the rocks and mountains yields to a slow but 

 sure destruction, and those who, adopting the notion that ' time costs 

 nature nothing,' take as much of this as pleases them, may easily see, 

 in the effect of these operations, the total disintegration of the 

 exi.iting continents and islands, which is so conspicuous a feature in 

 Dr. Mutton's hypothesis of the decaying and renewing earth. 



Nor is the sea less a theatre of change than the land. For, inde- 

 pendent of its receiving the spoils of the land, and distributing them 

 on it* bed, the untiring agitation of its waves undermines the cliffs 

 which are above its level, grinds away the rocks which are covered 

 and uncovered by the tides, and distributes the materials in various 

 ways, here making dangerous sandbanks, there adding to the low 

 bores a valuable heritage. 



Nor even below the deep water of the middle ocean is all at rest. 

 There multitudes of sea animals, the Infusorial Animalcules, the 

 Zoophytes, and Mollusca, by their mere exuviae tend to fill up the 

 depths ; and certain tribes (the lamelliferous corals in particular), by 

 their peculiar growth and mutual adherence form calcareous islands 

 and reefs, similar in some important particulars to the ancient lime- 

 stone rocks. These coralligenous rocks are however not reared from 

 the extreme depths of the sea, but based on the summits of submarine 

 hills, or the crests of volcanic cones, and thus, in a general expression, 

 we may say that in modern nature most of the deposits of solid 

 matter in the sea are joined to the shores or shallows of the 

 previously formed land. 



The sediments transported by rivers, and gathered by the wasting 

 of the elevated coasts, being for the most part deposited along the 

 sea-shores, and almost wholly below the level of high water, it is 

 obvious that from this cause alone the bed of the sea is filling up, 

 and its depth diminishing toward the shores ; but as the quantity of 

 water on the globe must be supposed sensibly constant, it follows 

 that the oceanic area must expand, or its surface rise a little. But 

 since the land is wasted by the waves, as we may suppose the aug- 

 mentation of area which results from this cause sufficient to balance 

 the elevating tendency of the littoral deposits of sediment, and that 

 upon the whole the effect of the watery agencies on the globe is 

 insensible in altering the level of the surface of the sea, as compared 

 to the deeper parts of its bed ; it follows, as a strict consequence, 

 that the area of the ocean is enlarging. This appears also probable 

 from observation ; for the small addition of marsh-land on particular 

 shores, by the influence of rivers, winds, and storms, in raising 

 littoral sediments above the reach of all but the extremely high tide, 

 is not enough to balance the continual waste of land along many 

 thousand miles of perishing cliffs. By the mechanical agency of 

 water considered alone, the land is certainly losing in area, continually. 

 The accumulation of marine exuvije on the bed of the sea acts in the 

 same direction, and the growth of coral principally concurs in the 

 same result. Left to watery agency alone then the land may be 

 imagined to be continually diminishing, as Dr. Button and Sir Charles 

 Lyell suppose. If the shores of the sea did not waste away, the 

 annual additions of sediment brought from the uplands would every- 

 where cause the water to rise in level ; if the land were supposed to 

 overhang its base at a certain angle depending on the diameter of 

 the earth, the area of the ocean would remain invariable ; but as 

 neither of these conditions applies, it is certain that the area of the 

 ocean is extending, and probable that its level does not materially 

 change. 



Volcanic phenomena, the earthquake, and the ignivomous mountain, 

 are to be viewed as cases of critical action. Whether the heat of the 

 interior of the globe be the residual portion of its original tempe- 

 rature (chaleur d'origine of Arago), or generated by the access of 

 water, or other bodies containing oxygen, to certain chemical 

 substances, it is to the disturbance of its equilibrium that the violence 

 and the tumult of volcanic excitement are owing. But there are 

 other and more gradual effects of the distribution of heat in and 

 upon the globe which require notice. The most important of these 

 is the gradual change of level of certain parts of the land, as coin- 

 pared with the general level of the ocean, one instance of which is 

 supposed to occur on the shores of the Baltic, where certain tracts 

 appear to be slowly rising above the sea. (Lyell, in ' Philosophical 

 Transactions,' 1835.) 



Concerning this ' secular inequality ' (as it may be termed), of level 

 of land and sea, it is unfortunate that nothing at all important is 

 known towards determining the important question whether the 

 elevation of one tract of diy land or sea-bed is balanced or over- 

 balanced by the depression of another. Lyell assumes that the 

 depression of land from this cause exceeds the elevation, but it is 

 difficult to find sufficient evidence for this important postulate ; and 

 to adopt it merely as a consequence of another unproved assumption 

 of a continual compensation of the agencies of nature is altogether 

 inadmissible. 



If there be in the earth a pervading high temperature, which 

 diminishes from the interior toward the surface, in consequence of 

 the radiation from the surface, it appears from Sir John Herschel's 

 reasoning (given in Mr. Babbage's ' Ninth Bridgewater Treatise ') that 

 along the shores of the sea the isothermal lines of the interior of the 

 globe should rise, because of the continual deposition of imperfectly 

 conducting sediments there. For thus the radiation of heat along 

 these lines would be diminished until the interior heat had come 

 nearer to the surface. By the consequent expansion of the subjacent 

 earthy substances the sea-shore should rise, and thus the addition of 

 sediment from watery action, and the effect of the effort to restore 

 equilibrium in the disposition of the interior temperature would, 

 upon the whole, coincide in minutely raising the surface of the sea. 



It is chiefly near the sea-coast, on the land or in the ocean, that 

 volcanic phenomena are at this day seen in activity, and this appa- 

 rently because the admission of water to some depth below the 

 surface is necessary to the excitement of the imprisoned forces of 

 heat. The elevated cones and large areas of melted rock, or accumu- 

 lations of scoriae and ashes, mark one of the prevalent effects of the 

 volcanic forces to be the withdrawal of matter from the interior to 



