MR GARDINER ON ATOLLS. 12 1 



stone, but generally it bears little or no resemblance to the limestone 

 of a coral reef such as we know it at the present day. It consists, 

 indeed, mainly of foraminifera, remains of sponges, echinoderms, 

 molluscs and other organisms, such as might be supposed now to 

 be building up mounds on any elevations of the sea-floor. 



One of the most characteristic points about the formation of 

 this limestone is its alternation with layers of the so-called " soap- 

 stone," a deposit, according to Brady', such as might be expected 

 to form "from 150 to 200 fathoms (rather more than less) in the 

 neighbourhood of any of the volcanic islands of the Pacific." In 

 my previous paper I feel that I did not lay sufficient stress on 

 Brady's immense experience and unique knowledge of foraminifera. 

 I would still, however, point out the general resemblances between 

 this rock and the muddy sand inside the barrier reefs of Viti Leva 

 and Vanua Levu. The greatest height, at which I found the lime- 

 stone, I calculated at about 900 feet, and, if it has been formed in 

 shallow water and Brady's view of the " soapstone " is correct, an 

 almost incomprehensible series of elevations and subsidences, each 

 of at least 900 feet, would be required. 



Hence it seems probable that the limestone was built up at 

 depths greater than 150 fathoms and alternated with layers of 

 soapstone, the whole being elevated by one continuous upheaval, 

 a view which is supported by the absence of conspicuous terraces 

 on its slopes. 



On a plateau within the bath}-metrical limit of the reef organ- 

 isms an atoll could be directly formed by their growth, which would 

 naturally be greater at its edges owing to the more frequent renewal 

 of the sea water. Such banks as the Tizard, Macclesfield, Coombe, 

 Waterwitch, Alexa, Great Chagos and others appear to be growing 

 up in this way. The irregularity of their slopes is absolutely 

 opposed to the view that they are being built up on the tops of 

 drowned atolls, which themselves were formed in the first place 

 by slow subsidences. Nearly all occur in areas of great volcanic 

 activity, and it is more probable that their foundations have been 

 formed as Wharton has suggested by the cutting down of volcanic 

 islands. Each separate bank should, however, be considered in 

 relation to the reefs and coral islands in its vicinity. 



I am myself rather concerned to show that mounds, such as 

 I have suggested above, may be the foundations of atolls. Such 

 mounds would generally be dome-shaped, and on their summits 

 I consider that limestone peaks can be built up by reef organisms 

 from a depth of 100 — 200 fathoms to the surface. Numerous peaks 

 such as these are found on the charts of the Red Sea, Torres Straits, 

 East Indies and other coral seas. By the breaking off of masses 

 of corals and nullipores, and by the heaping up of the remains of 

 various other organisms, such pinnacles then gradually approximate 



1 "Note on the so-called 'Soapstone of Fiji'," Qiiar. Jour. Geo. Soc, vol. xliv, 

 p. I (1888). 



