Page:Popular Science Monthly Volume 85.djvu/228

224 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 seaward face of reefs, provided the reef is exposed to the breakers, for the lithothamnion can thrive only in strongly agitated water. This ridge at Maër 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 Maër 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 outward the shoreward edge of the lithothamnion ridge must have disintegrated or dissolved and has thus disappeared. Indeed, the disintegration 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 disappeared 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. R. 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 impalpable 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 combination, but Dr. Shiro Tashiro working with his marvelously sensitive