DYNAMICAL GEOLOGY. 25! 



and for this and other reasons he regarded them as in the 

 main algal accumulations. Lepsius also thought there were 

 no sufficient stratigraphical grounds for regarding the dolomite 

 rocks of South Tyrol as other than a marine deposit. But the 

 coral-reef theory of origin had very numerous adherents, and 

 became a popular explanation for isolated limestone occur- 

 rences; for example, Oswald Heer wrote of fossil atolls and 

 barrier reefs in the Swiss Jura mountains, and Dupont described 

 fossil atolls in Belgium preserved in Devonian rocks. 



Recent researches in the Dolomites represent the occur- 

 rence of coral reefs only in insignificant thicknesses seldom 

 exceeding 150 feet, sometimes intercalated in the marly volcanic 

 rocks, and sometimes in the calcareo-dolomitic rocks. 



Several zoologists contested Darwin's theory Wilkes in 

 1849, Ross in 1855, the German geologist Semper in 1863, 

 upon the evidence of his exploration of the Pelew or Palaos 

 Islands. He found there all the varieties of reef-growth in 

 immediate proximity to one another, and older coral rocks 

 were present upon the dry land. Hence an explanation based 

 upon subsidence seemed inapplicable. Semper formed the 

 opinion that the tidal conditions, the breakers, and ocean- 

 currents were the chief influences which determined the 

 particular mode of growth of a coral reef. 



Similarly, Louis Agassiz (1851) and a number of American 

 geologists had studied the coral formations of Florida and 

 Tortuga, and could find no evidence of subsidence of the sea 

 bottom on which the reefs were growing. These reefs have 

 now undergone thorough investigation by Professor Alexander 

 Agassiz, the son of the famous glacialist and geologist, and the 

 conclusion arrived at by him is that the reefs are growing upon 

 a submarine plateau formed by the accumulation of mud, 

 sand, and organic remains. The prevailing winds and marine 

 currents constantly bring new material towards the plateau, and 

 as the latter continues to increase the corals are enabled to 

 keep within reach of fresh food-supplies. The whole thickness 

 of the Florida reefs, including both the coral limestone and the 

 submarine shelf of deposit, was determined by borings to 

 be about 50 feet. Agassiz is of opinion that the reefs of 

 Cuba, Bermuda, and Bahama, and also the Great Barrier Reef 

 of North Australia, may be explained in the same way as the 

 Florida reefs. 



Rein published in 1870 the result of observations made on 



