78 
HINTS TO TRAVELLERS. 
found in such beds, for instance echinoderms (sea-urchins or star-fishes) 
or sharia teeth, should be carefully preserved with some of the matrix. 
Deep-sea deposits have recently been discovered in several parts of the 
world, for instance, the West Indies, the Solomon Islands, the islands of 
Torres Straits and Southern Australia, as well as in Europe. 
Atolls or Coral-Islands .—Each of the remarkable coral islands of the 
Pacific and Indian oceans consists usually of an irregular ring, part or the 
whole of which is a few feet above the sea, and which encircles an inner 
lagoon of no great depth. The outer margin of the reef around each 
island slopes rapidly, sometimes precipitately, to a depth of, usually, 
several hundred fathoms. Darwin, taking these facts into consideration, 
together with the circumstance that no coral-reefs are known to be 
formed at a greater depth than about 15 to 20 fathoms (90 to 120 feet), 
showed that all the facts of the case could be explained by the theory 
that coral-islands were formed in areas of subsidence. This view was 
generally accepted until Prof. A. Agassiz, Sir John Murray and other 
writers brought forward evidence in favour of coral-islands being founded 
on shoals that may be areas of elevation. 
Much light has been thrown on this subject by recent exploration. 
Two instances in especial may be mentioned. The examination of 
Christmas Island in the Indian Ocean, South of Sumatra, by Mr. Andrews, 
has shown it to be a raised atoll, founded on a volcanic base, whilst, 
on the other hand, borings on the atoll of Funafuti, one of the Marshall 
Islands in the Western Pacific, carried to a depth of over 1000 feet on 
the ring itself, and to 245 feet below sea-level in the middle of the 
lagoon, have yielded results which, in the opinion of the geologists 
engaged, Prof. Sollas and Prof. Edgeworth David, completely confirm 
Darwin's theory. Further and perhaps conclusive evidence on this 
question is expected to be furnished by the examination of the cores 
obtained at Funafuti by boring. 
It is probable that atolls originate in more than one way, some being 
formed in rising or stationary tracts, others in areas of depression. The 
important question, from a geographical point of view, is not so much 
how isolated atolls were formed as whether the great tracts in the 
Pacific and Indian oceans in which no islands occur except atolls, for 
instance, the Marshall, Gilbert and Low archipelagoes in the former, 
and the Laccadives and Maldives in the latter, have been areas of 
