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 in the Blue Mountains of New South Wales, showing the characteristic block faulting seen along this part of the Australian coast.

There are, however, parts of the Great Barrier Reef region which are afflicted periodically by hurricanes and the reefs in these regions consist of large coral heads quite far apart instead of great numbers of small ones close together. Moreover, the pure ocean water of the Murray Islands is fairly free from silt, whereas that of Thursday Island bears a vast amount of mud in suspension and this, also, is fatal to corals and buries the small heads more readily than the large, and thus the reefs of Thursday Island are not rich, but the coral heads are of large size and are widely separated.

It is interesting to see how the 40 or more species of corals that grow