THE ORIGIN OF CORAL REEFS. Pe 
the island, in the course of time, loses its original form. Hence it 
is that, although the crescent is the primitive shape of each island, 
this structure is partly disguised in the case of some of the larger 
islands by the union of several of smaller size. The Admiralty 
chart gives but an imperfect idea of the true shape of the islands ; 
but, notwithstanding, its inspection will prove very instructive. 
In truth, Keeling Atoll exhibits, in an incomplete manner, the 
features of the large compound atoll of the Maldive Group. If it 
was considerably larger and possessed a less protected lagoon, so that 
open-sea conditions prevailed in its interior, it would have all the 
features of a compound Maldive atoll ; that is, an atoll consisting of 
a circle of small atolls or atollons. In its original condition, how- 
ever, it was an atoll consisting of a circle of crescentic islands. 
Such it is essentially now, but extensive changes have often partly 
disguised this feature. 
Before proceeding to explain the origin of the incompleted 
compound atoll of the Keeling Islands, it will be necessary 
to dwell on the exaggerated prevailing notion of an atoll. 
This kind of coral reef is usually described as a circular reef 
inclosing a deep basin or lagoon; but this description only 
applies to very small atolls less than a mile across. By drawing 
a section on a true scale of an atoll of average size, like Keeling 
Atoll, it will at once become apparent that such a description 
gives a very misleading idea of the real nature of this class of reef. 
A section of Keeling Atoll, drawn from the 1,000-fathom line on a 
true scale of an inch to the mile, and intended to illustrate a breadth 
of six miles, and a depth in the lagoon of 9 or 10 fathoms, would 
represent to the naked eye a flat-topped mountain, the depth of the 
so-called basin on the summit being merely represented by a slight 
central depression of about 1/100 of an inch. If the lagoon possessed 
a depth of 30 fathoms, the inclosed basin so-called would only be 
indicated in this section by a central depression of about 3/100 of 
an inch. So trifling a proportion does the depth of an atoll of 
ordinary size bear to the breadth, that such a reef can only be 
accurately described as possessing a broad level surface, with very 
slightly raised margin. A correct model of Keeling Atoll would at 
once convey a just idea of the true relative dimensions of a reef of 
this class. The lagoon would be there only represented by a film of 
water occupying a slight hollow in the level mountain-top. By thus 
grasping these facts, we at once perceive that by reason of our 
failing to view an atoll in relation to its surroundings, and 
