76 THE WONDERFUL CENTURY. 



translucent, contain abundance of very finely divided 

 matter, organic or inorganic, which, as in the atmos- 

 phere, reflects the blue rays in such quantity as to over- 

 power the white or colored light reflected from the fewer 

 and more rapidly sinking particles of larger size. The 

 oceanic dust is derived from many sources. Minute 

 organisms are constantly dying near the surface, and 

 their skeletons, or fragments of them, fall slowly to the 

 bottom. The mud brought down by rivers, though it 

 cannot be traced on the ocean floor more than about 150 

 miles from land, yet no doubt furnishes many particles 

 of organic matter which are carried by surface currents 

 to enormous distances and are ultimately dissolved be- 

 fore they reach the bottom. A more important source 

 of finely divided matter is to be found in volcanic dust 

 which, as in the case of Krakatoa, may remain for years 

 in the atmosphere, but which must ultimately fall upon 

 the surface of the earth and ocean. This can be traced 

 in all the deep-sea oozes. Finally there is meteoric dust, 

 which is continually falling to the surface of the earth, 

 but in such minute quantities and in such a finely- 

 divided state that it can only be detected in the oozes of 

 the deepest oceans, where both inorganic and organic 

 debris is almost absent. 



The blue of the ocean varies in different parts from a 

 pure blue somewhat lighter than that of the sky, as seen 

 about the northern tropic in the Atlantic, to a deep 

 indigo tint, as seen in the north temperate portions of 

 the same ocean: due, probably, to differences in the 

 nature, quantity, and distribution of the solid matter 

 which causes the color. The Mediterranean, and the 

 deeper Swiss lakes are also blue of various tints, due 



