encased in or covered by solid stalagmite, these relics have remained for ages. Most of our knowledge of the animals which inhabited Europe at the time when man appeared, is derived from the materials disinterred from these Bone-caves. Allusion. has also been made to the travertine formed by mineral-springs and to the facility with which leaves, shells, insects, and small birds, reptiles, or mammals may be enclosed and preserved in it (p. 58). Thus, while the plants and animals of the land for the most part die and decay into mere mould, there are here and there localities where their remains are covered up from decay and preserved as memorials of the life of the time.

(b) On the bottom of the Sea the conditions for the preservation of organic remains are more general and favourable than on land. Among the sands and gravels of the shore, some of the stronger shells that live in the shallower waters near land may be covered up and preserved, though often only in rolled fragments. It is below tide-mark, however, and more especially beneath the limit to which the disturbing action of breakers descends, that the remains of the denizens of the sea are most likely to be buried in sediment and to be preserved there as memorials of the life of the sea. It is evident that hard and therefore durable relics have the best chance of escaping destruction. Shells, corals, corallines, spicules of sponges, teeth, vertebræ, and ear-bones of fishes may be securely entombed in successive layers of silt or mud. But the vast crowds of marine creatures that have no hard parts must almost always perish without leaving any trace whatever of their existence. And even in the case of those which possess hard shells or skeletons, it will be easily understood that the great majority of them must be decomposed upon the sea-bottom, their component elements passing back again into the sea-water from which they were originally derived. It is only where sediment is deposited fast enough to cover them up and protect them before they have time to decay, that they may be expected to be preserved.

In the most favourable circumstances, therefore, only a very small proportion of the creatures living in the sea at any time leave a tangible record of their presence in the deposits of the seabottom. It is in the upper waters of the ocean, and especially in the neighbourhood of land, that life is most abundant. The same region also is that in which the sediment derived from the waste of the land is chiefly distributed. Hence it is in these marginal

parts of the ocean that the conditions for preserving memorials of the animals that inhabit the sea are best developed.

As we recede from the land, the rate of deposit of sediment on the sea-floor gradually diminishes, until in the central abysses it reaches that feeble stage so strikingly brought before us by the evidence of the manganese nodules (p. 78). The larger and thinner calcareous organisms are attacked by the sea-water and dissolved, apparently before they can sink to the bottom; at least their remains are comparatively rarely found there. It is such indestructible objects as sharks' teeth and vertebræ and ear-bones of whales that form the most conspicuous organic relics in these abysmal deposits.

Summary. Plants and animals leave their records in geological history, partly by forming distinct accumulations of their remains, partly by contributing their remains to be imbedded in different kinds of deposits both on land and in the sea. As examples of the first mode of chronicling their existence, we may take the growth of marsh-plants in peat-bogs, the spread of mangrove-swamps along tropical shores, and the deposition of infusorial earth on the bottom of lakes and of the sea; the accumulation of nullipore sand into solid stone, the formation of extensive shell-banks in many seas, the wide diffusion of organic ooze over the floor of the sea, and the growth of coral reefs. As illustrations of the second method, we may cite the manner in which the remains of terrestrial plants and animals are preserved in peat-bogs, in the deltas of rivers, in the stalagmite of caverns, and in the travertine of springs; and the way in which the hard parts of marine creatures are entombed in the sediments of the sea-floor, more especially along that belt fringing the continents and islands, where the chief deposit of sediment from the disintegration of the land takes place. Nevertheless, alike on land and sea, the proportion of organic remains thus sealed up and preserved is probably always but an insignificant part of the total population of plants and animals living at any given moment.

How the remains of plants and animals when once entombed in sediment are then hardened and petrified, so as to retain their minute structures, and to be capable of enduring for untold ages, will be treated of in Chapter XV.

CHAPTER IX

THE RECORDS LEFT BY VOLCANOES AND EARTHQUAKES

THE geological changes described in the foregoing chapters affect only the surface of the earth. A little reflection will convince us that they may all be referred to one common source of energy— the sun. It is chiefly to the daily influence of that great centre of heat and light that we must ascribe the ceaseless movements of the atmosphere, the phenomena of evaporation and condensation, the circulation of water over the land, the waves and currents of the sea, in short the whole complex system which constitutes what has been called the Life of the Earth. Could this influence be conceivably withdrawn, the planet would become cold, dark, silent, lifeless.

But besides the continual transformations of its surface due to solar energy, our globe possesses distinct energy of its own. Its movements of rotation and revolution, for example, provide a vast store of force, whereby many of the most important geological processes are initiated or modified, as in the phenomena of day and night and the seasons, with the innumerable meteorological and other effects that flow therefrom. These movements, though

slowly growing feebler, bear witness to the wonderful vigour of the earlier phases of the earth's existence. Inside the globe too lies a vast magazine of planetary energy in the form of an interior of intensely hot material. The cool outer shell is but an insignificant part of the total bulk of the globe. To this cool part the name of "crust was given at a time when the earth was believed to consist of an inner molten nucleus enclosed within an outer solid shell or crust. The term is now used merely to denote the cool solid external part of the globe, without implying any theory as to the nature of the interior.

Condition of the Earth's Interior.—It is obvious that we are

not likely ever to learn by direct observation what may be the condition of the interior of our planet. The cool solid outer shell is far too thick to be pierced through by human efforts; but by various kinds of observations, more or less probable conclusions may be drawn with regard to this problem. In the first place, it has been ascertained that all over the world, wherever borings are made for water or in mining operations, the temperature increases in proportion to the depth pierced, and that the average rate of increase amounts to about one degree Fahrenheit for every 64 feet of descent. If the rise of temperature continues inward at this rate, or at any rate at all approaching it, then at a distance from the surface, which in proportion to the bulk of the whole globe is comparatively trifling, the heat must be as great as that at which the ordinary materials of the crust would melt at the surface. In the second place, thermal springs in all quarters of the globe, rising sometimes with the temperature of boiling water, and occasionally even still hotter, prove that the interior of the planet must be very much warmer than its exterior. In the third place, volcanoes widely distributed over the earth's surface throw out steam and heated vapours, red-hot stones, and streams of molten rock.

It is quite certain therefore that the interior of the globe must be intensely hot; but whether it is actually molten or solid has been the subject of prolonged discussion. Three opinions have found stout defenders. (1) The older geologists maintained that the phenomena of volcanoes and earthquakes could not be explained, except on the supposition of a crust only a few miles thick, enclosing a vast central ocean of molten material. (2) This view has been opposed by physicists who have shown that the globe, if this were actually its structure, could not resist the attraction of sun and moon, but would be drawn out of shape, as the ocean is in the phenomenon of the tides, and that the absence of any appreciable tidal deformation in the crust shows that the earth must be practically solid and as rigid as a ball of glass, or of steel. (3) A third opinion has been advanced by geologists who, while admitting that the earth behaves on the whole as a solid rigid body, yet believe that many geological phenomena can only be explained by the existence of some liquid mass beneath the crust. Accordingly they suppose that while the nucleus is retained in the solid state by the enormous superincumbent pressure under which it lies, and the crust has become solid by cooling, there is an intermediate liquid or viscous layer which has

not yet cooled sufficiently to pass into the solid crust above, and does not lie under sufficient pressure to form part of the solid nucleus below. At present, the balance of evidence and argument seems to be in favour of the practical rigidity and solidity of the globe as a whole. But the materials of its interior must possess temperatures far higher than those at which they would melt at the surface. They are no doubt kept solid by the vast overlying pressure, and any change which could relieve them of this pressure would allow them to pass into the liquid form. This subject will be again alluded to in Chapter XVI. Meanwhile, let us consider how the intensely hot nucleus of the planet reacts upon its surface.

Rocks are bad conductors of heat. So slowly is the heat of the interior conducted upwards by them that the temperature of the surface of the crust is not appreciably affected by that of the intensely hot nucleus. But the fact that the surface is not warmed from this source shows that the heat of the interior must pass off into space as fast as it arrives at the surface, and proves that our planet is gradually cooling. For many millions of years the earth has been radiating heat into space, and has consequently been losing energy. Its present store of planetary vitality therefore must be regarded as greatly less than it once was.

VOLCANOES.

Of all the manifestations of this planetary vitality, by far the most impressive are those furnished by volcanoes. The general characters of these vents of communication between the hot interior and cool surface of the planet are doubtless already familiar to the reader of these chapters—the volcano itself, a conical hill or mountain, formed mainly or entirely of materials ejected from below, having on its truncated summit the basin-shaped crater, at the bottom of which lies the vent or funnel from which, as well as from rents on the flanks of the cone, hot vapours, cinders, ashes, and streams of molten lava are discharged, till they gradually pile up the volcanic cone round the vent whence they escape.

A volcanic cone, so long as it remains, bears eloquent testimony to the nature of the causes that produced it. Even many centuries after it has ceased to be active, when no vapours rise from any part of its cold, silent, and motionless surface, its conical form, its cup-shaped crater, its slopes of loose ashes, and its black bristling lava-currents remain as unimpeachable witnesses that the volcanic