DESCRIPTION OF THE ISLANDS: SOUTHERN THULE GROUP i8i 



ferent; they decrease from top to bottom, and though they fall quickly between 82 and 

 164 fathoms, no trace of the usual layering for these latitudes is to be seen. This is due 

 to the fact that the basin is closed. The water that reaches it must flow over either the 

 northern or southern edges, and since the depths here do not exceed 20 fathoms, only 

 that from the homogeneous uppermost layer can find access. Since there is a southerly 

 set of about i knot in Douglas Strait the inflowing water will come from the north. 



It will be noticed that the bottom temperatures in the basin are extremely low, much 

 below any of those in the comparative series. We believe this to be the effect of winter 

 conditions. In winter, and indeed for the greater part of the year, the islands are sur- 

 rounded by pack-ice, which will also invade the strait. Air temperatures will be very 

 low and the pack will remain as a solid frozen mass for long periods. Convection cur- 

 rents will be set up beneath the ice-covering and the whole of the water will become 

 chilled. When the ice disappears circulation will be restored; but it will be limited to 

 the upper layers, the bottom water imprisoned beneath it permanently retaining its 

 winter temperature. The temperatures afl^ord proof — if proof were needed — that no 

 trace of warmth persists in the crater. 



The figures for salinity indicate by their uniformity in the upper layers that active 

 circulation is in progress down to a depth of 40 fathoms, and in conformity with the 

 temperature readings they also show an abrupt change between 82 and 164 fathoms 



(Fig. 17)- 



The results suggest that the circulation in the strait, when not closed by ice, is on the 

 lines indicated in Fig. 18. The current running at the surface from north to south will 

 set up a flow in the same direction in the water immediately beneath it, with the return 

 current lying at a depth of 40 fathoms. The latter, by the friction it exerts, will induce 

 a much slower streaming movement in the water below ; this will rotate in the reverse 

 direction to that above, and its lower level will be at about 130 fathoms, where both 

 temperature and salinity show that a discontinuity layer exists. The bottom of the basin 

 from 130 to 400 fathoms must be filled with water which is almost or quite motionless. 



The plankton collections show that in Douglas Strait life exists in considerable 

 quantity down to at least 273 fathoms; for a net hauled from this depth to 137 fathoms 

 (500-250 metres), and then closed, contained numerous copepods and other organisms. 

 In all the nets used below 55 fathoms specimens of a very large red and white amphipod, 

 Eiisirus antarcticus, were obtained, and a larger net hauled obliquely from 80 fathoms to 

 the surface caught great quantities of Euphausia siiperba, the crustacean which in the 

 Antarctic forms the food of Blue and Fin whales. At the bottom itself life is deficient: 

 a dredge brought up a quantity of evil-smelling mud, containing practically nothing 

 beyond three star-fish and a number of Polychaete worms living in black tubes. 



Few instances of volcanic craters inundated by the sea have been recorded : we know 

 of two only — Deception Island in the South Shetland Group and St Paul Island in the 

 southern Indian Ocean. Both of these have only one opening and both are much 

 shallower than Douglas Strait. At Deception the depth of the water in the crater is 93 

 fathoms and at St Paul 38 fathoms. Cook and Thule Islands, whose steep cliffs border 



