Nansen's Continental Oscillations — Spencer. 225 
There are two distinct layers on the bottom of the 
Polar seas ; — a brown clay resting on gray clay, or vice 
versa — the brown requiring the longer time for its accumu- 
lation. This difference may in part have arisen from more 
waste being carried down to the sea at one time than at 
another; or with equal probability, it may be due to the 
movement of the shore lines, causing the advancement or 
recession of the mouth of the river, but only a considerable 
change of level of land and sea would make a difference 
where the oscillating coast line was characterized by a 
steep descent. 
The Siberian continental shelf, northwest of Koteloi,. 
at 1,400 metres, showed lo-ii centimetres of gray clay rest- 
ing on the brown, as a distinct layer, with no Foraminifera 
in the gray, and only rarely in the brown. . The composi- 
tion of the two kinds is nearly the same. The brown color 
indicates a slower rate of deposition. The comparatively 
recent submergence of the Siberian tundra (100-150 
metres) would greatly reduce the basins of the rivers, 
causing the embouchures to recede so that the sedimenta- 
tion down the continental slope would permit the slow 
accumulation of the brown deposits. The rise of the land 
would favor the deposition of the gray. In Barentz sea, 
a very thin upper layer of green clay rests on the gray. 
In the basin of the Norwegian sea, which is separated 
from that of the Arctic, by a ridge between Spitzbergen 
and Greenland, the brown oxidized clay rests upon the 
gray, which contains no animal remains, or any great 
amount of carbonate of lime. On the continental slope, 
to about 900 metres, the layer of brown clay is very thin, 
but it increases in thickness on approaching the oceanic 
depths. So also the Biloculina, etc. increase as well as the 
percentage of carbonate of lime to 15-20 per cent in the 
true foraminiferal clay. The gray clay must have here 
been deposited in a former period, when there was a rapid 
deposition of terrigenous waste with very little animal 
matter. This can be explained by the Norwegian shore 
line extending seaward, when the land, compared with the 
sea-level stood much higher than now. At such a time the 
Greenjand-Iceland-Scotland ridge cut off the Atlantic 
