AS OBSERVED IN THE WEDDELL SEA. 
821 
ice, and of this amount the bottom half was discoloured by diatoms. It was 
separated by a sharp line from 65 cm. of fibrous ice. From this specimen the 
following conclusions were drawn : — 
(1) The sharp line indicated a break in the freezing process during the summer 
1914-15, and separated ice formed in 1915 from that formed the previous 
year. 
(2) The presence of diatoms showed that the ice was afloat in summer 1914-15. 
(3) It also showed that the ice must have been very loose to allow of these 
minute plants penetrating 40 cm. into the ice. 
(4) This in turn pointed to a temperature at or about the freezing point — 
just what was required to produce the structural change from fibrous 
to spotted. 
The block examined on March 19, 1916, had been tilted up into a hummock 
a couple of days earlier. Its upper 2-3 feet consisted of spotted ice, in its lower 
portion discoloured by diatoms. The variation in thickness may have been partly 
due to summer melting in 1914-15 ; it resulted, at any rate, in an uneven boundary 
between the upper portion and the lower 1-3 feet of ice ; the latter was also 
slightly spotted, but with the fibrous structure not yet entirely obliterated ; in turn 
its lower layers also were strongly discoloured by diatoms. This was an important 
find, for it showed the effects by March of the summer heat of 1915-16 ; the change 
had already begun, but was not so far advanced as to obliterate entirely the fibrous 
structure. The arrival of diatoms seems to be the sign that the change is 
beginning. 
The earliest date on which the structural change was definitely known to 
have commenced was on December 30, 1915. On that date a visit was made 
to one of the numerous areas of brownish-looking flat ice (the brown appearance 
was connected with a very thin snow-covering) and an attempt made to bore 
through the floe. Merely by working with the point of an ice-axe, it was soon 
possible, despite the water oozing up in the first few inches, to reach down 2 feet 
6 inches. At one time it would have been impossible to have reached even a depth 
of 6 inches without excavating with a pick and shovel. The ice now was soft 
right through and through ; and the whole ice-thickness must, therefore, have 
been practically at the melting-point. 
On the same date, however, it should be noted that a floe with a 3-foot snow- 
covering was not so far advanced ; for in this case it was only possible to work down 
a few inches with the point of the ice-axe. The snow-covering was now retarding 
rise of temperature, just as once it retarded freezing ; this was evidence, therefore, 
of how the almost universal thick snow-covering on Weddell Sea floes worked 
towards prolonging their life. 
Physical Changes.— The fracture and hardness of spotted ice are very different 
