SEPTEMBER 4, 1913] 
NATURE ; II 
exist, and because of this absence of extensive 
tracts of sea-bottom of moderate depth, fisheries 
on the scale of those of the North Atlantic enclosed 
seas are non-existent. Because of this relative 
unimportance of the sea-fisheries, the fauna of the 
Mediterranean is not nearly so well known as, 
for instance, that of the North Sea and Baltic; 
and the remainder of the reports of the Danish 
expeditions, dealing with the biological investiga- 
tions, promise to be of exceptional interest on this 
account. 
The sea-bottom deposits are of relatively little 
interest. Over by far the greater part of the 
Mediterranean the bottom is covered by ter- 
rigenous materials. These contain far less volcanic 
débris than might have been suspected. Siliceous 
materials are also relatively rare, and the chief 
calcareous deposits are to be found over relatively 
small areas, and contain Pteropod shells. 
The hydrographic conditions*in the Mediter- 
ranean depend mainly on the fact that this water 
area is one of high concentration. The amount 
of water received from the rainfall over the land 
area which it drains is far less (less than a quarter, 
it is said) than the amount of water removed by 
evaporation. The temperature of the superficial 
strata of water is relatively high: even at a depth 
of 1000 metres it is uniformly 13°C., while the 
salinity is also relatively high, being everywhere 
about 38 per mille in the bottom and intermediate 
strata. This excess of evaporation over precipita- 
tion would lead, of course, to a reduction of water- 
level, were it not compensated by the strong inflow 
from the Atlantic through the Straits of Gibraltar. 
But this inflow tends, of course, to raise the hydro- 
static pressure of the water in the sea, and there- 
fore a counter-current sets out from the Mediter- 
ranean into the open Atlantic Ocean. The inflow- 
ing current is superficial, has a velocity of from 
one to three knots, a temperature which is that 
of the Atlantic water in the Spanish Bay, and a 
salinity of about 36 per mille. The outflowing 
current is a deep one, its velocity varies from one- 
half to about five knots, its temperature is uni- 
formly about 13° C., and its salinity is about 
38 per mille. The variations in velocity are due 
to the tidal streams in the straits. 
The volume of relatively warm and dense water 
flowing out from the Mediterranean is very con- 
siderable. This water is so highly saline that it 
flows on as a bottom or intermediate current in 
spite of its high temperature. Although its direc- 
tion is nearly east to west as it emerges from the 
straits, it soon becomes deflected to the north and 
east as the result of the earth’s rotation, and it 
approaches the coasts of the British Islands. 
Normally it flows to the west of Ireland, and 
Dickson has shown that it may be present even 
so far north as the channel between Rockall and 
Scotland, but as a rule the current must flow 
along deep depressions of the sea-bottom. If, 
however, it is unusually strong it may enter the 
shallower sea-basins, and Bassett has recently 
suggested that unusually high salinities in such 
enclosed sea areas as the English Channel or Irish 
NO. 2288, VOL. 92] 
| Sea may be due, not to an unusually strong Gulf 
Stream drift, but more probably to the presence 
of this highly saline Mediterranean water. This 
indeed, appears to have been the case in the 
summer of 1912 in the Irish Sea and adjacent 
waters. 
Precisely the opposite conditions exist in re- 
| lation to the Black Sea and the Sea of Marmora. 
The latter basin has a depth of 1000 to 2000 
metres, and the Black Sea has a maximum depth 
of about 2200 metres. The Black Sea is an area 
of excess of precipitation over evaporation, so that 
the superficial strata of water are of low salinity. 
From the surface down to about 20 metres the 
salinity is about 17°5 per mille, and it is nearly 
constant at this limiting depth, increasing towards 
the bottom. The temperature appears to be nearly 
constant at about 80 metres depth, and also in- 
creases slightly towards the bottom. Because of 
the excess of precipitation over evaporation the 
water-level of the Black Sea tends to rise, but 
this is prevented, of course, by an outflow of 
relatively light water through the Bosporus into 
the Sea of Marmora, and from the latter basin 
through the Dardanelles into the Mediterranean. 
| But since this outflow reduces the hydrostatic 
pressure of the communicating water masses, a 
counter-current of relatively dense Mediterranean 
water enters the Sea of Marmora, and then the 
Black Sea through the Bosporus. The water 
flowing out from the Black Sea is a surface 
current, that flowing in a deep one. The depth 
of water at the entrance to the Black Sea is, how- 
ever, very small, and the existence of this “sill” 
prevents the complete renewal or ventilation of 
the deeper strata of water, a condition which also 
exists, on a much smaller scale, in some of the 
Norwegian fjords. The absence of renewal of 
water leads to the stagnation of most of the water 
of the Black Sea: not only is oxygen absent in 
the deeper layers, but its place is actually taken 
by sulphuretted hydrogen, and except for some 
forms of bacteria this water-mass is lifeless. 
The horizontal water circulation in the Mediter- 
ranean depends on the Atlantic inflow. This is 
at first west to east in direction, but, becoming 
deflected to the right in consequence of the rota- 
tion of the earth, it flows along the coast of Africa. 
The direction of flow of surface-water then follows 
the general scheme of that in the northern hemi- 
sphere. Two cyclonic circulations are set up in 
the western basin—one in the Balearic Sea to 
the west of Sardinia and Corsica, and another in 
the Tyrrhenian Sea. The main stream enters the 
eastern basin through the channel between Sicily 
and Tunis, and then becomes deflected, forming 
another cyclonic circulation. There is also an 
intermediate level water circulation which depends 
for its direction on a complex resultant of super- 
ficial horizontal circulation and vertical circula- 
tions due to concentration and cooling of super- 
ficial waters. This intermediate circulation is 
difficult to explain, and, indeed, is still imperfectly 
known. It is, of course, the origin of the westerly 
flowing deep current in the straits, and seems to 
