THE ICE. 153 



a thin ice stratum (in Greenland it is "5 to 7 inches thick) 

 must be crushed upon it, and thus be brought into close 

 contact with the finely-granulated crystals of this firn- 

 snow or ice, be absorbed by it, and thus added to increase 

 its volume. The same process must be repeated on the 

 lower surface of the stratum of ice, as this also is pressed 

 upon the deeper lying snow beneath it. Thus, these 

 sheets of ice grow in thickness by increasing their volume, 

 but the layers of snow diminish by reduction of their mass 

 and volume, and gradually change into hard, air-enclosing 

 ice ; how this air is expelled is still unknown, but the 

 final result of the whole process of transformation is the 

 brilliant pure blue ice of the lowest strata of the whole 

 mass. 



In some cases the formation of this pure blue ice will 

 not be completed, not indeed in consequence of some 

 change in the process of transformation, but because of 

 the entry of foreign substances. Mention has been made 

 in former places of ddbris-bzax'mv icebergs, and it was 

 pointed out that sometimes the imbedded masses are 

 also distinctly stratified. In some regions, especially in 

 the more remote neighbourhood of periodically active 

 volcanoes, the ashes are strewn over the snow at every 

 period of eruption, and these are then at the periods of 

 rest covered by snow masses more or less thick. Upon 

 a transverse section such as these icebergs exhibit, these 

 strata are seen as fine brown or blackish stripes. Such 

 kind of bergs, whose upper surface also is occasionally 

 coloured brown by solid substances, have been seen by 

 Bruce and by the members of the Challenger expedition. 

 Future investigations may possibly show that the green 

 colour of icebergs occasionally noticed is due to inorganic 

 substances distributed through the whole mass of the 

 berg. In many cases the rubble imbedded in the ice 

 is not stratified, being derived from moraines, which have 

 been uninterruptedly received. Such bergs Wilkes has 



