SNOW AND ICE. 
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metres which is compressed to 1000 and further on 
to 500 metres. 
The pressure acts on the ice in two ways 
Firstly, in the same manner as it produces lamination 
in rocks; and secondly, by partially liquefying the 
ice, thus facilitating the escape of the air-bubbles, 
which causes its whitish appearance. 
Liquid Disks. 
The Solar beams also form innumerable liquid 
disks. As the water occupies less space than the 
ice each disk is accompanied by a small vacuum, 
which shines like silver, and is often taken for an 
air-bubble. 
Dirtbands. 
If we look down on the Mer de Glace we see 
(Fig. 30) a series of grey, curved, or bent bands, 
which follow each other in succession from Trelaporte 
downwards. 
These “dirtbands” have their origin at the ice 
cascade upon the Glacier du Geant. The glacier is 
broken at the summit of the ice-fall (Fig. 28), and 
descends the declivity in a series of transverse ridges. 
Dust, etc., gradually accumulates in the hollows, and 
though the ridges are by degrees melted away and 
