fracture of _secondary
limestone_.[1134]

The island of Pulo Nias, off Sumatra (see Map, fig. 39. p. 351), which
is about 3000 feet high, is described by Dr. Jack as being overspread by
coral and large shells of the _Chama (Tridacna) gigas_, which rest on
quartzose and arenaceous rocks, at various levels from the sea-coast to
the summit of the highest hills.

The cliffs of the island of Timor in the Indian Ocean are composed, says
Mr. Jukes, of a raised coral reef abounding in _Astraea_, _Meandrina_,
and _Porites_, with shells of _Strombus_, _Conus_, _Nerita_, _Arca_,
_Pecten_, _Venus_, and _Lucina_. On a ledge about 150 feet above the
sea, a Tridacna (or large clam shell), two feet across, was found bedded
in the rock with closed valves, just as they are often seen in barrier
reefs. This formation in the islands of Sandlewood, Sumbawa, Madura, and
Java, where it is exposed in sea cliffs, was found to be from 200 to 300
feet thick, and is believed to ascend to much greater heights in the
interior. It has usually the form of a "chalk-like" rock, white when
broken, but in the weathered surface turning nearly black.[1135]

It appears, therefore, premature to assert that there are no recent
coral formations uplifted to great heights, for we are only beginning to
be acquainted with the geological structure of the rocks of equatorial
regions. Some of the upraised islands, such as Elizabeth and Queen
Charlotte, in the Pacific, although placed in regions of atolls, are
described by Captain Beechey and others as flat-topped, and exhibiting
no traces of lagoons. In explanation of the fact, we may presume that
after they had been sinking for ages, the descending movement was
relaxed; and while it was in the course of being converted into an
ascending one, the ground remained for a long season almost stationary,
in which case the corals within the lagoon would build up to the
surface, and reach the level already attained by those on the margin of
the reef. In this manner the lagoon would be effaced, and the island
acquire a flat summit.

It may, however, be thought strange that many examples have not been
noticed of fringing reefs uplifted above the level of the sea. Mr.
Darwin, indeed, cites one instance where the reef preserved, on dry land
in the Mauritius, its peculiar moat-like structure; but they ought, he
says, to be of rare occurrence, for in the case of atolls or of barrier
or fringing reefs, the characteristic outlin