This study of the reefs of Cuba and the Bahamas naturally led him to renew his observations in Florida and to visit the Bermudas. He saw the Bermuda Islands in March, 1894, and in December of the same year he chartered a tug and steamed along the Barrier Reef of Florida.
He found that in common with the Bahamas the Bermudas consist of æolian limestone. In places the interior of these islands were dissolved away by the action of rain-water rendered acid by decomposing vegetable matter, and thus depressions were formed in the central parts of the islands. Then when the islands sank the sea broke through the rims and filled the lagoons, afterwards deepening them by its scouring action.
Thus the Bermudas have assumed an atoll-like shape, but their contour is not due to corals. Indeed, there are but few corals at Bermuda, and these form a mere veneer over the sunken æolian ledges. The so-called miniature atolls are mere pot-hole basins which have been scooped out by wave action in the æolian rock, and their rims are never more than eighteen inches high, and consist of a wall of æolian rock covered by a coating of serpulæ, algæ and corallines which enable them to withstand the wearing action of the sea. Thus Darwin's theory of coral reefs can not explain the conditions seen in the Bahamas and Bermudas.
The results of his study of the Florida Reef were finally published in 1896 in cooperation with Dr. Leon S. Griswold. Agassiz concludes that the Marquesas, of Florida, are not an atoll, but enclose a sound that has not been formed by subsidence, but by the solvent and mechanical action of the sea. Thus the Marquesas are similar in their geological history to other sounds back of the line of the Florida Keys.
He found an elevated reef extending along the seaward face of the Florida Keys from Lower Matacumbe to Soldier's Key. We now know, however, that the elevated reef actually extends from the southern end of Big Pine Key to Soldier's Key. Agassiz believed that the oolite limestone back of the elevated reef and along the mainland shore of Key Biscayne Bay was æolian rock; but Griswold decided that it was only a mud-flat which had been formed beneath the water, and afterwards elevated. Later studies have shown that Griswold was right.
In 1895 he instituted a study of the underground temperature of the rock walls of the Calumet and Hecla mine, and found that the increase is only 1° F. for every 223.7 feet as we descend. His deepest temperature observation was 4,580 feet beneath the surface of the ground.
He had now seen all of the coral reefs of the Atlantic and turned his attention to the exploration of the Pacific. In April and May, 1896, he cruised along the Great Barrier Reef of Australia in the little steamer Croydon, which he chartered from the Australian United Steam Navigation Company, Captain W. C. Thomson being in command.
