ROMANCE OF THE EARTH.

Lyttelton Times, Volume XCIX, Issue 11555, 16 April 1898, Page 2

ROMANCE OF THE EARTH.

FBy Pkopessor Bickeeton.] * IV. As tie temperature of the surface of the earth sinks, a new action comes into. play. The cooling and contraction of the crust become slower and slower until the internal part • is cooling by conduction almost at the same rate as the external by radiation, &o. The atmosphere has greatly decreased, most of the water is deposited, much of the carbonic acid has become fixed; solar radiation is, therefore, becoming more capable of penetrating to the surface of the globe. The sun has diminished exceedingly in size, while its temperature has proportionately increased ; for, remarkable as it may appear, the more heat a gaseous cosmical body gives out, the hotter it grows. As it shrinks, the resulting from the increased gravitation reduces the internal layers to smaller and smaller bulk, thus causing increasingly tremendous quantities of heat, A gaseous sun, in becoming compressed to one-half its volume, gives off enormous quantities of heat, yet is double the temperature wbenit has shrunk to one-half the diameter. Heat whose source is at a very high temperature can penetrate gases and vapours much more easily than heat from a comjaratively moderate source. Hence from these two causes much more heat reaches the crust of the earth, and retards the reduction of its tropical surface temperature. A time ensues comparatively free from volcanic and earthquake disturbances, and at this stage—during a glacial period—it is probable that the earth became cool enough in places to permit plant life to commence at one of the poles. How this may have occurred we will debate further on. At present we must be content to take a rapid survey of the physics of the earth’s crust. We have traced the molten earth in its process of cooling. We have taken an imaginary glance at surface solidification, seeing how, by solid rock sinking, this solidification may extend to great depths. We have noted that, when solid, the surface would tend to cool more quickly than the interior; and, shrinking and exercising enormous tension and pressure, it would split, and the interior would be forced out in a molten state; the heat producing this state being largely the energy of the pressure itself. We have traced- the oscillations of levels which water and -the deposit of sediment would produce, and then the gradual reduction of the temperature of the surface, until solar heat would tend to retard surface cooling, and external and internal contractions would proceed at almost equal rates, and a period of comparative quiet would ensue. Then, after a time, the gain of heat from the sun would balance the surface loss, and the crust would cease to cool and contract. Now a new order of events begins to operate. The hot interior loses heat by conduction through the crust, and continues to shrink, while the crust gets to he too big for the contracted contents of the globe; and. Just as the loss of water from a shrivelling apple causes the surface to wrinkle, so the shrinking interior of the earth must cause its surface to crush and wrinkle, and a second paroxysmal period ensues. During this time great earthquakes would cause the whole earth to shiver, as the rigid crust crumpled up in its efforts to fit the contracted interior. More and more slowly would the heat pass from the interior until the comparative quiet of the present period was reached. Thus, then, we have two great cosmic agencies to produce volcanic action. First, the cooling of the solid crust being more rapid than the internal cooling, the surface shrinks and splits; then, after a pause in the paroxysms, the crust ceases to cool and contract, whilst the continued cooling and contracting of the interior cause it to shrink away from its crust, and the crust begins to crush and crumple to fit its contracted interior. Probably it was during the interglacial periods occurring in the pause between the primitive volcanoes of surface tension and the volcanoes of surface crumpling that the enormous • forests of the carboniferous period clothed wide regions of the earth and provided our chief coal formations; storing up the solar energy of those far-gone ages to supply man in this present paroxysm of .fierce conflict and unrest, which is preparatory to his passing into his kingdom of peace and joy. Thus the earth has passed through its period of tight crust and splitting to a period of rest, and now we must consider in detail how the tight crust has to wrinkle and accommodate itself to its

shrinking interior. Apparently inextricable sets of agencies are put in operation by the crumpling of the earth’s surface. We will try to disentangle some of them. Owing to the inequality,in thickness strength of the crust, the chief crumpling would take place in the weakest parts; often, probably, the margins of continents. Sometimes the earth's surface would buckle up so much as to form mountain ranges; then, along the tops of these new ridges immense splits would open that would become wider and deeper as the crumpling continued; rain would fall upon the rents and torrents rush down them; so the fissures would become valleys. Along the far-dissevered sides of some of these mountain valleys the rock strata may be traced —in our preseut-day times—for miles. - Sometimes the efforts of the crust to fit itself to its shrinking contents would produce such tremendous pressure as to heat and fuse the rock; the molten substance would find vent and volcanoes he produced; the hot erupted matter would melt the sides of the outlets and form, in time, circular, craters, overflowing and forming the plopes of great volcanic cones. As each outrush was - exhausted the matter in the vent would solidify and the crater become a reservoir to catch and hold water, which would boil off and so rob the glowing rock of its heat. By-and-by, the pressure would renew itself, again bursting through all obstacles in tremendous eruption ; but as the upward pressure exploded through the bottom of the crater, the whole of the original walls of the volcano would be split by fissures radiating from* the vent. Into these fissures the fresh molten matter would flow, forming volcanic “dykes.” The district around Lyttelton may be cited in illustration. Quail Island is the last vent of that region, and “ dykes’— containing stone suitable for quarrying purposes—point towards the island from almost every direction. 1 Many of our mountain chains have been produced by the crumpling and bulging of v strata; but, in other cases, the stratified rock material has probably been thrust by lateral pressure up the mountain ridges; produced ages before by the great splits of the primitive volcanic period of tension. - GLACIAL EPOCHS. But of the complications of cosmic agencies there appears to be no end. In * addition to the two volcanic periods of tension and pressure and the erosive action of boiling torrents, there came into force—as soon as snow could fall upon the earth another tremendous factor in modifying surface conditions, i.e., glaciation, or the results of ice. Snow would settle on- the poles and accumulate there ,• it would cap all the higher mountains and gradually spread downwards, advancing from the'poles toward, the polar circles. An astronomic influence of surpassing potency must here be considered. The orbit of the earth is -an ellipse—an ellipse may be seen in a hoop leaning a ■ little away. Generally the earth’s orbit is nearly a circle, but sometimes, owing to attractions of the other planets, it becomes a long ellipse, the sun being at one of the foci, that is, near, one of the smaller curves of the orbit. In extreme cases the earth may be 13,000,000 miles nearer the sun at one part of its annual revolution than at another; at present the difference is only 2,000,000 miles. Wow, summer is not due to proximity to the sun, for we may be nearest at midwinter. The seasons are caused by the leaning of the axis of the earth. In the summer hemisphere the axis leans toward the' sun, and the directness of the rays causes the high temperature; ‘ Winter is caused by the axis leaning away from the sun. It is clear that if during, say, the south polar winter, the earth happens to be 13,000,000 miles nearer the sun than it was six months previously, the southern winter will be very mild; but six months later, when the earth has reached 13,000,000 miles from the sun, the winter of the northern’ hemisphere will be very cold indeed. In that cold winter most of; the water will fall as snow instead of rain, and the snow will pile up on the pole,- while the summer sun—instead of warming the surface of the soil—will be engaged in melting the snow. The heat may not suffice to do this completely, and next winter will increase the mass of snow. If this accumulative process lasts —as it may— for 13,000 years, the polar snow may creep down into temperate regions, and such a vast cap of ice be produced as to alter the centre of gravity of the earth, so that one hemisphere may have nearly all the water as ice and sea, and the other may have an almost entirely land surface. One polar hemisphere, the oceanic and icy one, will be nearly all frigid, while the other, the continental, will be very temperate; the seasons being almost equable, the summers cool and the winters without frosts quite to polar regions. Thus, organic life, both animal and vegetable, * will increase prodigiously. (To be continued.)