258 LEADING AMERICAN MEN OF SCIENCE 



infer even from the last edition of the Manual. Dana recognizes 

 indeed the occurrence of oscillations in the progressive emergence 

 of the continent, but he seems not to appreciate adequately their 

 magnitude and importance. In earliest Cambrian time, for in- 

 stance, the area of dry land was far greater than in later Cam- 

 brian or Ordovician time. In the earliest Cambrian (Georgian 

 period), the Mississippian Sea was only a sound or strait, the 

 greater part of the area of the Mississippian Sea of later Cambrian 

 and Ordovician time being then dry land. While the progressive 

 deepening of the oceans and emergence of the continents is unques- 

 tionably a great truth, the oscillations were so considerable that 

 an alternation of marine and terrestrial conditions over vast areas 

 must be equally recognized. But we do not refuse the honor due 

 to Copernicus, though he made the planetary orbits circular, 

 instead of elliptical; and to Dana belongs no less the credit of the 

 great conception of continental evolution, though he made the 

 curve too simple. 



Dana's first introduction to the problem of coral islands was at 

 the Paumotu Islands in 1839. The coral animals, whose skeletons, 

 broken or comminuted by the waves, furnish materials for the 

 reefs, live only in shallow water, seldom ranging much below a 

 depth of one hundred feet. It is accordingly readily intelligible 

 that the debris of these skeletons may accumulate to form fringing 

 reefs, closely bordering the shore of a continent or island. But 

 a more difficult problem is presented by the barrier reefs and 

 atolls. The barrier reefs may be separated from the shore by a 

 channel ten or fifty miles in breadth or even more, and hundreds 

 of feet in depth. Still more startling are the atolls rings of coral 

 reef, which may be crowned with scattered islets or with a more 

 or less complete crest of dry land, inclosing a comparatively 

 shallow lagoon, and surrounded by water deepening rapidly to 

 thousands of feet, and far from any other land. The supposition 

 that the reef has actually been built up from a depth of a thousand 

 feet or more, is obviously inconsistent with the fact that the animals 

 live only in shallow water. When Dana arrived at Sydney in the 

 latter part of the year 1839, his mind was full of the problem, 



