224 THE POPULAR SCIENCE MONTHLY 



flat ranged through 21°; the hottest being 93° and the coolest 72° F. 

 Thus the reef-flat corals must be able to withstand a considerable range 

 in temperature. 



In the Pacific, the lithothamnion ridge always grows along the sea- 

 ward face of reefs, provided the reef is exposed to the breakers, for the 

 lithothamnion can thrive only in strongly agitated water. This ridge at 

 Maer Island is only about six inches above low tide, for the breakers are 

 not very high, the force of the rollers being partially spent upon the reefs 

 of the Great Barrier six miles to the eastward of the Murray Islands. 



We saw that at low tide the southeast reef flat of Maer Island is a 

 wide shallow lake dammed in by the lithothamnion ridge. Now as the 

 reef grew seaward this lithothamnion ridge always remained as a narrow 

 boundary wall upon its advancing edge. 



Thus in former times when the reefs began to grow outward, the 

 lithothamnion ridge must have been close to the shore, whereas now it 

 is from 1,800 to 2,200 feet out to sea. The lagoon behind this ridge is 

 about eighteen inches deep and it is evident that as the reef advanced out- 

 ward the shoreward edge of the lithothamnion ridge must have disin- 

 tegrated or dissolved and has thus disappeared. Indeed, the disinteg- 

 ration of the dead inner edge of the lithothamnion ridge is very apparent 

 in its ragged outline and the many loose blocks which are detached from 

 it and washed shoreward, the process of disintegration being accelerated 

 by the boring of numerous echinoderms; and it is evident that in 

 same manner, a thickness of about two feet of limestone has disap- 

 peared so that the bottom of the present lagoon is now about eighteen 

 inches below the crest of the lithothamnion ridge. 



Sir John Murray and Alexander Agassiz believed that limestone was 

 dissolved by sea water, but Dr. T. Wayland Vaughan collected samples of 

 water from the lagoon of Tortugas, Florida, for an entire month and 

 these were analyzed by Mr. E. B. Dole, who decided that they contained 

 no free carbon dioxide. Now, without carbonic acid, or some other free 

 acid, sea-water can not dissolve limestone. 



The Tortugas is peculiar in being surrounded by wide areas of chalky 

 mud and, moreover, the surface waters of the Florida-Bahama contain, 

 according to Drew and Kellerman, great numbers of bacilli which cause 

 a precipitation of calcium carbonate composed of such minute colloidal 

 particles that they float for some time before sinking to form the impal- 

 pable limestone ooze of the sea bottom, and, finally, to change into oolite 

 in the manner explained by Linck and by Vaughan, oolite being a rock 

 composed of small calcium carbonate balls causing it to resemble fish roe. 



One might expect therefore that the excessive amount of calcium 

 carbonate in the water would hold the carbon dioxide in chemical com- 

 bination, but Dr. Shiro Tashiro working with his marvelously sensitive 



