j?kesident's addkess. 103 



Arctic Circle. Like the Eadiolarian Ooze it is a siliceous de- 

 posit, but is composed of the frustules of Diatomacese, which 

 minute vegetable organisms live on the surface, and sink to the 

 bottom after death. The deposit occurs in depths ranging from 

 1,260 to 1,950 fathoms, and, as observed by the * Challenger,'' is 

 best developed between the north edge of the Antarctic icefields 

 and lat. 60° S. The extraordinary abundance of Diatomaceae in 

 the Southern Sea has long been known, and Dr. J. D. Hooker 

 found them everywhere staining the ice and snow at the water's 

 edge a pale ochreous brown. "A deposit of mud, chiefly con- 

 sisting of the siliceous loricse of Diatomacese, not less than 400 

 miles long and 120 miles broad, was found at a depth of between 

 200 and 400 feet, on the flanks of Yictoria Land, and in 70° S. 

 latitude. Of the thickness of this deposit no conjecture could 

 be formed; but that it must be continually increasing is evident, 

 the silex of which it is in a great measure composed being inde- 

 structible."^' Similar deposits in a fossilized state are known in 

 the neighbourhood of the Mediterranean, also at Eichmond in 

 Virginia, and in Bermuda. 



The conditions of life, as they affect animals, at depths ranging 

 from 500 to 5,000 fathoms, are not so dissimilar as at flrst 

 might be imagined. Por while it is true that the pressure at the 

 former depth would be haK a ton to each square inch of surface, 

 and at a depth of five miles it would amount to no less than five 

 tons, yet the compressibility of water is so slight, that its density 

 even under such an enormous superincumbent weight is only 

 very slightly increased ; and thus animals permeated thi-oughout 

 with water are not oppressed with the burden which they have 

 to bear, as we might at first suppose. Again, there is a remark- 

 able uniformity of temperature throughout the great mass of 

 ocean waters when once the first few hundred fathoms have been 

 passed. Temperature at great depths is found to be limited in 

 range to the six or eight degrees above freezing point. Towards 

 the Poles, both JN'orth and South, this isothermal line rises to- 

 wards the surface ; as a consequence, invertebrata usually living 

 in abyssal regions are here found at much less depths. The 



* Carpenter, The Microscope, 5tli edition, p. S-tO. 



