234 A NATURALIST ON THE " CHALLENGER." 



perpendicular, but long ledges run out from the base of the cliffs 

 below water, the immersed part being thus much larger in figure 

 than the exposed ; and, secondly, the exposed part is of lighter, 

 less compact ice, and often further lightened by excavation of 

 caves, and presence of crevasses. 



So large a proportion of the bergs being required to be 

 immersed in order that the bergs broken off from the parent 

 ice masses should float in stable equilibrium, with their surfaces 

 originally uppermost maintained still in that position, it is 

 necessary that the pieces thus breaking off, supposing their upper 

 surfaces to be square, should be at least as wide as they are 

 thick If this were not the case, if the density of the ice masses 

 were uniform, the bergs would necessarily topple immediately 

 they broke free, and this fact would be shown by their strati- 

 fication being vertical to their plane of flotation. This, however, 

 seems never, as far as I could judge from the bergs I saw, to 

 occur. Tilting only takes place after bergs have been long 

 weathered. The bergs seem nearly always to be of large area 

 in proportion to their thickness, and to maintain their original 

 balance for very long periods. No doubt the much greater 

 density of the ice composing the lower portions of the bergs 

 tends to keep them in their original position. 



The waves, partly no doubt because of the water at the very 

 surface being warmed by the sun, and partly no doubt by heat 

 resulting from their motion, cut a wash-line all round the bergs, 

 which appears as a concave groove-like channel with a polished 

 inner surface, just at the water-level. 



When bergs rise to a higher level, or tilt, these wash-lines 

 remain marked on the bergs, as straight polished streaks, visible 

 from a great distance (coloured plate, fig. 5), giving evidence 

 of the former lines of flotation of the bergs. Sometimes, several 

 ancient wash-lines are visible on one berg, and where the 

 cliff surfaces on which they are scored are protected at their 

 base from the waves by secondary cliffs or projections, they may 

 remain intact for very long periods. 



The wash-lines being: hollowed out at the bases of the cliffs, 

 these latter soon overhang, and large masses split off along 

 the lines of joint and cleavage, and fall. The masses evidently 



