Appearance and Disappearance of a Mud Island 187 
sland above water appeared somewhat diminished and levelled 
down to not more than half the height observed on the Ist, but 
under water was about the same. The soundings had not altered 
nor had the smell disappeared. 
No further visits were made until the 7th of June, when it was 
found that the island had entirely disappeared. Careful soundings 
were again taken over the spot. The bottom had assumed its original 
depth, and no sign of the island was to be found. The water was 
dirty and covered with bubbles on the surface as before, and the 
smell of sulphuretted hydrogen was still perceptible in the air. 
Nothing happened on the coast about the time of the upheaval of 
the island to indicate that anything unusual was taking place. A 
airly high tide occurred a night or two previous to the appearance 
of the island, and a report came from Swakopmund that a very 
heavy sea had swept away 30 metres of a breakwater in course of 
construction there by the German Government ; but as similar heavy 
seas are frequent at this time of year on this coast, so exposed to 
the full force of the South Atlantic, no great importance can be 
attached to the occurrence. 
From the first appearance of the island on the Ist of June a 
constant look-out was kept, but there was no reappearance of it up 
to the time of my departure from Walfish Bay on the 10th of July. 
The following facts are interesting to consider in connection with 
the phenomena, viz. :— 
The remarkable soundings mentioned in the South African Pulot, 
referred to above : 
The existence of sulphur holes in various spots on the peninsula 
and at the head of the Bay: 
The frequent evolution of sulphuretted hydrogen gas during the 
winter months; the dirty appearance of the water with the occur- 
rence of bubbles on the surface similar to those seen over the spot 
where the island disappeared; also the fact of fishes becoming 
suffocated during these times and floating on the surface of the. 
water. 
As a probable cause of the presence of gases in the mud of 
Walfish Bay, large quantities of animal matter must from time to 
time be collected in the still water of the Bay from the myriads of 
seabirds there, and from the flamingoes, pelicans, and other birds 
that exist in the lagoons at the head of the Bay. The Bay also 
abounds with fish of various kinds, and remains of large numbers of 
whales are strewed along the coast for many miles, 
