470 THE POPULAR SCIENCE MONTHLY. 



depths ranging from 500 to 2,800 fatlioms of water in equatorial 

 and temperate latitudes. The reason that it is not found in arctic 

 seas may be that the cold surface waters of these regions do not 

 bear such an abundant fauna of foraminifera. This is supported 

 by the fact that it extends ten degrees farther north than south in 

 the Atlantic, the warm water of the Gulf Stream bearing a richer 

 fauna than the waters of a corresponding degree of latitude in the 

 southern sea. 



The pteropod ooze has only twenty-five per cent of carbonate 

 of lime. It contains numerous shells of various pteropods, hete- 

 ropods, and foraminifera, but nearly fifty per cent of its substance 

 is composed of the siliceous skeletons of radiolaria and the frus- 

 tules of diatoms. According to Murray, it is found in tropical and 

 subtropical seas at depths of less than 1,500 fathoms. 



The radiolarian ooze is found only in the deepest waters of the 

 central and western Pacific Ocean. In some of the typical ex- 

 amples not a trace of carbonate of lime was to be found, but in 

 somewhat shallower waters a few small fragments occurred. A 

 diatom r-jize, mainly composed of the skeletons of diatoms, has also 

 been found in deep water near the Antarctic Circle, but it has not 

 apparently a very wide range. 



Of all the deep-sea deposits, however, the so-called " red mud " 

 has by far the widest distribution. It is supposed to extend over 

 one third of the earth's surface. It is essentially a deep-sea de- 

 posit, and one that is found in its typical condition at some con- 

 siderable distance from continental land. Like the globigerina 

 ooze it is never found in inclosed seas. To the touch it is plastic 

 and greasy when fresh, but it soon hardens into solid masses. 

 When examined with the microscope it is seen to be composed of 

 extremely minute fragments, rarely exceeding 0'05 millimetre in 

 diameter. It contains a large amount of free silica that is prob- 

 ably formed by the destruction of numerous siliceous skeletons, 

 and a small proportion of silicate of alumina. It usually contains 

 the remains of diatoms, radiolaria, and sponge spicules, and occa- 

 sionally lumps of pumice stone, meteoric nodules, and, in colder 

 regions, stones and other materials dropped by passing icebergs. 



In the great oceans, then, we find in the deepest places red 

 mud, or, where there is an abundant radiolarian surface fauna, 

 radiolarian ooze ; in water that is not deeper than about 2,000 

 fathoms, we find the globigerina ooze ; in shallower waters and in 

 some localities only pteropod ooze. It must not be supposed that 

 sharp limits can anywhere be drawn between these different kinds 

 of deposits, for they pass gradually into one another and present 

 many intermediate forms. 



It is probable that the sea water, by virtue of the free carbonic 

 acid it contains in solution, is able to exert a solvent action upon 



