52 THE OCEAN WORLD. 



prevent snow from melting as it falls. On the approach of congela- 

 tion the surface solidifies, and seems as if covered with oil ; small 

 circles are formed, which press against each other, and are finally 

 soldered together until they form a vast field of ice, the thickness of 

 which increases from the lower surface. 



The water produced from melted ice is perfectly fresh the result 

 of a well-known physical cause. When a saline solution like sea- 

 water is congealed by cold, pure water alone passes into the solid 

 state, the saline solution becomes more concentrated, increases in 

 density, and sinks to the bottom. Blocks of ice, therefore, in the 

 Polar Seas, are always available for use. There are, however, salt 

 blocks of ice, which are distinguished from fresh-water ice by their 

 opaqueness and their dazzling white colour their saltness is due to 

 the sea-water retained in flieir interstices. 



The ice-fields, which are formed in high latitudes, are driven 

 towards the south both by winds and currents ; but sooner or later 

 the action of the waves breaks them up into fragments. The edges 

 of the broken icebergs are thus often rising and continually changing. 

 These asperities and protuberances are called hummocks by English 

 navigators ; they give to the polar ice an odd, irregular appearance. 

 Hummocks form themselves of the stray, broken icebergs which come 

 in contact with each other at their edges, and thus form vast rafts, 

 the pieces of which may exceed 100 yards in length. 



When these icebergs are separated by open spaces, through which 

 vessels can be navigated, the pack ice is said to be open ; but it often 

 happens that mountains of ice occur partly submerged, where one 

 edge is retained under the principal mass, while the other is above the 

 water. Scoresby once passed over a calf, as English mariners call 

 these icy mountains ; but he trembled while he did so, dreading lest 

 it should throw his vessel, himself, and crew into the air before he 

 could pass it. The aspect of the ice-fields varies in a thousand ways. 

 Here one forms an incoherent chaos resembling some volcanic dis- 

 trict, with crevices in all directions, bristling with unshapely blocks 

 piled up at random ; there it is a strongly-marked plain, an immense 

 mosaic formed of vast blocks of ice of every age and thickness, the 

 divisions of which are marked by long ridges of the most irregular 

 forms ; sometimes resembling walls composed of great rectangular 

 blocks, sometimes resembling chains of hills, with great rounded 

 summits. 



In the spring, when a thaw sets in, and the fields begin to break 

 up, the pieces of light ice which unite the great blocks into single 

 masses are the first to melt j the several blocks then separate, and 



