808 
MR J. M. WORDIE ON THE NATURAL HISTORY OF PACK-ICE 
If so, they belong to the third class of cracks, those due to heavy pressure. The 
detailed treatment of the latter, however, is best deferred until the phenomena of 
pressure have been discussed. 
As a yet further case of specialised strain cracks, tide-cracks should be at least 
mentioned. The drift of the Endurance was never near enough the coast to enable 
them to be studied, but from published descriptions tide-cracks appear to be a 
perfectly normal result of strain. 
Pressure . — When a moving ice-field became cut up by cracks running in 
different directions and many of them of considerable width, each floe as it was 
formed tended to travel at a different rate from that of the parent ice-field. A stretch 
of sea-ice was never a simple formation, but rather a great conglomeration of sheets 
va 
of young-ice and of floes which had survived the previous summer ; consequently, 
each of the new areas detached from it had its own peculiarities of thickness, depth 
of snow, extent, and so on. As was shown above, it was these very peculiarities 
which produced the strains ; and these in turn caused the cracks. They were also 
partly the cause of the different rates at which individual floes, formed • from the 
break-up of an ice-field, travelled under the never-ceasing impulse of wind and 
current. 
The main cause, however, of the different rates of travel was not so much the 
size and depth of the floes as the nature of their surface. Every hummock, in fact, 
acted as a sail, and the rate of movement therefore depended to a certain extent 
on the amount of hummocking in proportion to the area and weight of the floe. A 
legacy of previous pressure, these hummocks in turn became the cause of still 
further pressures. When two floes were moving at different rates, either the 
distance between them increased and a lead or lane passable for ships was formed, 
or the interval between floes decreased, one so to speak overtaking the other, and 
then the result, if there was sufficient momentum, was pressure in some form or other. 
