32 GEOGNOSY. [Boox If. 
‘he comparatively small but by no means unimportant proportions 
of these minor components of the atmosphere are much more liable 
than the more essential gases to variations. Chloride of sodium, 
for instance, is, as might be expected, particularly abundant in the 
air bordering the sea. Nitric acid, ammonia, and sulphuric acid 
appear in the air of towns most conspicuously. ‘The organic 
substances present in the air are sometimes living germs, such as 
probably often lead to the propagation of disease, and sometimes 
mere fine particles of dust derived from the bodies of living or dead 
organisms." 
As a geological agent the atmosphere effects changes by the 
chemical reactions of its constituent gases and vapours, by its 
varying temperature, and by its motions. Its functions in these 
respects are described in Book III. Part I. Section 1. 
= 2. The Oceans.—About three-fourths of the surface of the globe 
(or about 144,712,000 square miles) are covered by the irregular 
sheet of water known as the Sea. Within the last ten years much 
new light has been thrown upon the depths, temperatures, and 
biological conditions of the ocean-basins, more particularly by the 
Lightning, Porcupine, and Challenger, expeditions fitted out by 
the British Government. It has been ascertained that few parts 
of the Atlantic Ocean exceed 3000 fathoms, the deepest sounding 
obtained there being one taken about 100 miles north from the 
island of St. Thomas, which gave 3875 fathoms, or rather less than 43 
miles. ‘The Atlantic appears to have an average depth in its more 
open parts of from 2000 to 8000 fathoms, or from about 2 to 33 miles. 
In the Pacific Ocean the Challenger got soundings of 3950 and 
4475 fathoms, or about 45 and rather more than 5 miles. But 
these appear to mark exceptionally abysmal depressions, the average 
depth being, as in the Atlantic, between 2000 and 3000 fathoms. 
We may therefore assume, as probably not far from the truth, that 
the average depth of the ocean is about 2,500 fathoms, or nearly 3 
miles. Its total cubic contents will thus be about 400 millions of 
cubic miles. 
With regard also to the form of the great ocean bottoms, much 
additional information has recently been obtained. Over vast areas 
in the central regions of the sea, the floor appears to form great plains 
with comparatively few inequalities, but with lines of submarine. 
ridges comparable to chains of hills or mountains on the land. The — 
crests of some of these ridges rise above the sea-level, as in the — 
remarkable line of islands in the south-western region of the Pacific 
Ocean. It is significant that the islands which thus appear far from 
' 'The air of towns is peculiarly rich in impurities, especially in manufacturing districts, 
where much coal is used. ‘These impurities, however, though of serious consequence to 
the towns in a sanitary point of view, do not sensibly affect the general atmosphere, 
secing that they are probably in great measure taken out of the air by rain, even in the 
districts which produce them. They possess, however, a special geological significance, 
and in this respect, too, have important economic bearings. See on this whole subject 
Dr. Angus Smith’s Air and Iain. : 




