20 AUSTRALASIA. 



everywhere a clay formed of foramiiiifera, radiolaria^ diatoms, and other remains 

 of minute organisms mixed with particles of pumice and various decomposed 

 products of volcanic origin. Neither gravel nor the bare rock has anywhere been 

 discovered on the deep bed of the Indian Ocean. 



The slight proportion of carbonate of lime in clays lying at great depths is 

 due to the carbonic acid present in the water. The countless calcareous organisms 

 falling as dust from the upper marine waters become completely dissolved before 

 reaching the bottom. But sharks' teeth and the skeletons of cetaceans occur 

 abundantly in the argillaceous deposits, from which the remains of extinct and 

 living animals are often fished up together. Nodules of iron of cosmic origin are 

 also found interspersed in the same clays. 



Atmospheric Currents. 



As attested by the very name of " Pacific," given to the great ocean by its first 

 discoverers, storms are less frequent in this basin than in the Atlantic, at least in 

 the tropical latitudes with low tides. This is due to the vast uniform surface 

 presented by an immense extent of the South Sea far from the neighbourhood of 

 continental seaboards, which owing to the great differences in their reliefs give 

 rise to abrupt changes in the climate and the course of the winds. The waters are 

 usually the least ruffled and navigation safest in the Eastern Pacific regions, 

 where vessels sail for thousands of miles without meeting a single island. Here 

 also the trade winds blow with the greatest uniformity. Those from the north- 

 east prevail with great constancy in the tropical zone some 7,000 miles broad 

 comprised between the Revilla-Gigedo and the Marianne groups. The south- 

 eastern trades have a less extensive range of about 8,000 miles between the 

 Galapagos and the Marquesas. 



But the course of the atmospheric currents is interrupted and frequently turned 

 backwards by the thousand independent centres of attraction formed by the insular 

 groups, some mountainous, others scarcely rising above the surface, which 

 are scattered over the West Pacific equatorial waters. The normal trades are here 

 often 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 most 

 regular ; but in summer their ascendency is contested by northern and north- 

 eastern breezes. Frequently also dtad calms set in, Avhile occasionally the con- 

 flicting currents 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 even 

 than the atmosphere itself. Between the hottest and coldest month on either side 

 of the equator within the tropics the mean temperature of 72° to 77° F. has an 

 extreme range limited to from four to eight degrees. Nevertheless, the oscilla- 

 tions for the whole year range from twenty-eight to thirty-six degrees according 

 to the position of the insular groups. 



The rainfall also shows discrepancies of as much as tenfold and upwards, accord- 



