2^0 HISTORY OF GEOLOGY AND PALEONTOLOGY. 



converted into a barrier reef, and finally, by continued subsi- 

 dence of the floor, passed into the form of an atoll. The 

 essential feature is a certain reciprocity between the secular 

 movement of subsidence and the vertical or horizontal growth 

 of the reef. Darwin brings the movements of the area of 

 subsidence in the Pacific Ocean into correlation with the 

 volcanic phenomena so widely extended in that ocean. Where 

 fringing reefs still occur, he supposes that instead of subsidence, 

 local elevation is taking place. The presence of barrier reefs 

 and atolls, on the contrary, indicates a submergence of islands 

 and a subsidence of the sea-floor. 



The distinguished American geologist and zoologist, Dana, 

 had abundant opportunity during the Wilkes Expedition 

 (1839-41) of investigating coral reefs, and he accepted Darwin's 

 theory on all the essential points. The apparent naturalness 

 of Darwin's theory recommended it to all, and in 1860 it 

 seemed to find striking confirmation from the geological side. 

 In that year Ferdinand von Richthofen published his account 

 of the geology of Predazzo, St. Cassian, and adjacent localities 

 in the South Tyrol Dolomites. He described the limited 

 local occurrence of dolomite or dolomite limestone cliffs, in 

 many places 2000-3000 feet thick, -and the varying age of the 

 sedimentary deposits at the base of the cliffs. These were 

 sometimes the tufaceous Wengen strata, sometimes richly 

 fossiliferous Cassian marls, sometimes the older dolomite rocks 

 (Mendola Dolomite), sometimes volcanic lavas. Von Richt- 

 hofen suggested that the variation in the age of the deposits at 

 the base of the calcareous or dolomite cliffs, as well as the 

 great inequality in the dimensions of the cliffs, might be 

 explained in the sense of Darwin's theory on the supposition 

 that the cliffs represented coral reefs whose growth had in- 

 creased during a prolonged epoch of subsidence of. the sea- 

 floor, and had spread over deposits of different ages at the 

 base. Mojsisovics, in conjunction with other members of the 

 Austrian Survey, afterwards examined the area in greater 

 detail, and in 1879 published his work, The Dolomite Reefs of 

 South Tyrol, in which he confirmed Richthofen's suggestion 

 that the cliffs were fossil coral reefs, but declared the growth of 

 the reefs to have been contemporaneous with the sedimentation 

 of the earthy and volcanic rocks in the neighbourhood. 



Giimbel, however, proved the frequent occurrence of species 

 ofgyroporella, or sea-algas, in the dolomite rocks of South Tyrol, 



