Volume 14 Australasia - dana ward's homepage

20 AUSTRALASIA.everywhere a clay formed of foraminifera, radiolaria, diatoms, and other remainsof minute organisms mixed with particles of pumice and various decomposedproducts of volcanic origin. Neither gravel nor the bare rock has anywhere beendiscovered on the deep bed of the Indian Ocean.The slight proportion of carbonate of lime in clays lying at great depths isdue to the carbonic acid present in the water. The countless calcareous organismsfalling as dust from the upper marine waters become completely dissolved beforereaching the bottom. But sharks' teeth and the skeletons of cetaceans occurabundantly in the argillaceous deposits, from which the remains of extinct andliving animals are often fished up together. Nodules of iron of cosmic origin arcalso found interspersed in the same clays.AtmosphericCurrents.As attested by the very name of " Pacific," given to the great ocean by its firstdiscoverers, storms are less frequent in this basin than in the Atlantic, at least inthe tropical latitudes with low tides. This is due to the vast uniform surfacepresented by an immense extent of the South Sea far from the neighbourhood ofcontinental seaboards, which owing to the great differences in their reliefs giverise to abrupt changes in the climate and the course of the winds.The waters areusually the least ruffled and navigation safest in the Eastern Pacific regions,where vessels sail for thousands of miles without meeting a single island. Herealso the trade winds blow with the greatest uniformity. Those from the northeastprevail with great constancy in the^troiDical zone some 7,000 miles broadcomprised between the Revilla-Gigedo and the Marianne groups. The southeasterntrades have a less extensive range of about 3,000 miles between theGalapagos and the Marquesas.But the course of the atmospheric currents is interrupted and frequently turnedbackwards by the thousand independent centres of attraction formed by the insulargroups, some mountainous, others scarcely rising above the surface, whichare scattered over the West Pacific equatorial waters. The normal trades are hereoften replaced by the alternating winds, which follow in the track of the sun.During the winter of the southern hemisphere the south-east trades are mostregular ; but in summer their ascendency is contested by northern and northeasternbreezes. Frequently also dead calms set in, while occasionally the conflictingcurrents give rise to cyclonic movements.A remarkably mild temperature usually prevails in the oceanic archipelagoes,surrounded by waters which are subject to less vicissitudes of heat and cold eventhan the atmosphere itself.Between the hottest and coldest month on either sideof the equator within the tropics the mean temperature of 72° to 77° F. has anextreme range limited to from four to eight degrees. Nevertheless, the oscillationsfor the whole year range from twenty-eight to thirty-six degrees accordingto the position of the insular groups.The rainfall also shows discrepancies of as much as tenfold and upwards, accord-