138 Daly— The C oral-Reef Zone. 



of the subject have not been hitherto emphasized. The 

 chief novelty is the conclusion that the smothering (kill- 

 ing) of corals and other reef organisms by stirred sedi- 

 ment must have been specially extensive during the 

 Glacial period — with an important consequence to the 

 theory of the living reefs. Another enlargement of the 

 subject is found in the effort to describe factors con- 

 nected with the interglacial stages of the Pleistocene 

 period. For the sake of brevity and clearness the writer 

 has adopted a certain positiveness of statement, which is 

 not accompanied by full discussion of many points in the 

 history of the tropical belt. 



Duration and Subdivisions of the Glacial Period. 



The Glacial period was multiple. Its stages, as recog- 

 nized in North America, are given by Chamberlin and 

 Salisbury : 2 



First glaciation — Sub- Af toman or Jerseyan (preceded 

 by the Pliocene period). 



First interglacial interval. 

 Second glaciation — Kansan. 



Second interglacial interval. 

 Third glaciation — Illinoian. 



Third interglacial interval. 

 Fourth glaciation — Iowan. 



Fourth interglacial interval. 

 Fifth glaciation — Earlier Wisconsin. 



Fifth interglacial interval. 

 Sixth glaciation — Later Wisconsin. 



Post- Wisconsin deglaciation (followed by the 

 Eecent period). 



" These stages were by no means equal, the earlier being 

 markedly longer than the later. There was something 

 like a geometrical gradation from the earliest and longest 

 to the latest and shortest." These authors place the 

 climax of the Kansan from 300,000 to 1,020,000 years 

 ago; the climax of the Sub-Aftonian was much further 

 back still. Barrell has estimated the total length of the 

 Glacial period as from 1,000,000 to 1,500,000 years. 3 



The total length of the Glacial period in Europe was 

 estimated by Penck at from 500,000 to 1,000,000 years, 



2 T. C. Chamberlin and E. D. Salisbury, Geology, New York, vol. 3, p. 383, 

 1906. 



3 J. Barrell, Bull. Geol. Soc. Amer., vol. 28, p. 884, 1917. 



