52 NATURAL SCIENCE. ^'t^"- 



not at all improbable that the Maldives and Laccadives occur along 

 a submerged ridge that runs parallel to the line of the Western 

 Ghauts, and the southernmost peak of which is marked by Diego 

 Garcia. Hence the selection of this atoll might easily mean the 

 failure of the experiment, for if the reef in this case should be found to be 

 only a thin cap, it would simpl}' prove the existence of this submerged 

 chain, and not that the subsidence theory is unnecessary for the 

 atolls of the Pacific. The Central Pacific is the typical coral reef 

 area, and it is to be hoped that it may be possible to select this region 

 for the experiment. Its proximity to the Australian mining fields 

 is also in its favour, as machinery might easily be obtained thence. 



The mechanical difficulties in the way of a deep boring are, 

 however, especially serious on the small low atolls. The upper 

 crust of the coral rock is sure to be more or less rotten and perforated 

 b}' irregular cavities and channels. Hence, as soon as the work has 

 been well started, the weight of the machinery and the vibration of 

 the boring cylinder are not unlikely to cause a slight subsidence and 

 the breaking of the tool. The attempt made by the Tuscaroa failed 

 in this way. But the difficulties, though serious, do not appear 

 to be insuperable, and if the site be selected with sufficient care, and 

 a proper foundation be erected, there seems no reason why a 500 

 or a 1,000 foot core should not be obtained. 



Finally, in regard to the nature of the core : to judge by many of 

 the remarks made upon this question, some writers seem to expect 

 that, if the subsidence theory be true, the whole core should be of 

 solid coral. But an examination of the structure of raised reefs clearly 

 shows that this cannot be expected. The extensive excavations in 

 the Barbados reefs, made by the Barbados Water Co., has thrown 

 much valuable light upon the varying composition of coral rocks. A 

 large collection of these rocks has been presented to the British 

 Museum by Mr. E. Easton, and this has been supplemented by 

 donations from Professor Harrison and Mr. Jukes-Browne. Micro- 

 scopic examination of the specimens in tliis considerable collection 

 shows that the reefs consist mainly of coral-sand and debris, with beds 

 of limestone formed by nullipores, foraminifera, mollusca, &c., as well 

 as by corals. In many cases, too, there are masses of rock without 

 any trace of organic structure, though they have been unquestionably 

 formed from corals. The rock of which the atolls are composed is 

 likely to show similar variations ; and as the whole foundation of 

 the reef is probably saturated with sea water, it is quite possible that, 

 below a very limited depth, the rock will consist only of a jointed, 

 crystalline limestone. But even if this be the case, microscopic 

 examination will, no doubt, be able conclusively to demonstrate 

 whether the rock was formed at the depth where it was found, or 

 whether it was a shallow water deposit that has since subsided. 



J. W. Gregory. 



