LHE DWkRT AND GEOLOGIC FIME 735 
miles of continuous ice cliffs ending directly in water hundreds 
of fathoms deep, and have reason to believe that this is only a 
fraction of what exists, and yet icebergs are sometimes almost 
unknown in the southern seas for years at atime. Again there 
will be years in which they are abundant and extraordinary in 
size, but at no time is their quantity comparable to what ought 
to exist were the discharge anything like a free one along the 
barrier.‘ It can be reasonably assumed, therefore, from what we 
know at present that the movement of the southern ice-cap is 
also extremely slow, notwithstanding the favoring conditions 
of direct discharge into deep water. 
Yhe great Laurentide glacier, extending over four million 
square miles of surface, can also be safely assumed to have hada 
very slow motion as a whole, fully as slow as that of the Green- 
land or southern ice-cap. In fact the question arises, and is not 
at first sight readily answerable, how it had any motion at all. 
Gravitation certainly had less play than in Greenland, for instead 
of an area nowhere more than three hundred miles from the 
ocean * we have one eighteen hundred or two thousand miles in 
t** As has already been stated, there are years of very few or no icebergs, and then 
years when great numbers are reported. In the year 1832, the southern ocean was so 
covered with icebergs that a number of whaling vessels, bound round Cape Horn, 
encountering them, put back to Valparaiso to await a more favorable season, because 
it appeared too dangerous to undertake the voyage. Again in 1854 there was a great 
accumulation of icebergs, and now during the past few years, notably 1892 and 1893, 
there has been another notable output from the great berg factories of the Antarctic 
regions. During the intervals between these periods there have been very few bergs 
reported. What causes this occasional great accession of bergs? Some authorities 
offer as a probable explanation the breaking off of the ice margin by volcanic erup- 
tions, and others that earthquakes cause numerous pieces of the glacier to become 
detached and set adrift as icebergs, and others that unusual heavy annual snowfall 
is favorable for increase in number of bergs. The rapidity of glacier movement seems 
usually to regulate the number of bergs cast off. If the ice at the bottom of the glacier 
moves so slow that the melting of the margin on coming in contact with the salt water 
equals the advance, then we would have no icebergs, except perhaps those breaking 
off from the upper part of the outer margin, and these would be comparatively small.” 
W.T. Gray, M.S. U.S. Hydrographic Office, Pilot Chart, N. Pacific Ocean, Novy. 
1895. 
‘Prof. Chamberlin finds no evidence that the ice-sheet of Greenland ever very 
greatly exceeded its present limits. 
