142 Daly— The C oral-Reef Zone. 



reefs cannot be & priori assumed for the beginning of the 

 Glacial period. It is becoming increasingly clear that 

 each atoll or barrier owes its origin to a positive 

 (upward) shift of sea-level with respect to the founda- 

 tion of that reef. We know of no strong, general, posi- 

 tive movement of sea-level during the Tertiary, though 

 small shifts of level are practically inevitable in a geo- 

 logical era featured by important deformation of the 

 earth's crust. Local subsidence of the sea-bottom devel- 

 oped barrier reef or atoll, if the subsidence were slow 

 enough and all other conditions were favorable. Very 

 little is known as to the rates of crustal sinking ; this and 

 other difficulties stand in the way of inferring the late- 

 Tertiary development of numerous atolls and barrier 

 reefs because of subsidence. Since the existing barriers 

 and atolls can be well explained by a Recent general rise 

 of sea-level, logic forbids us to consider their present 

 abundance as indicating equal abundance at the dawn of 

 the Pleistocene. To make that an axiom would be to 

 "beg the question" concerning the validity of the Glacial- 

 control theory of reefs. 



Nor is it likely that many barriers and atolls would 

 have been planted on the stable, late-Tertiary shelves and 

 detached banks. At the edges of these flats, the water 

 averaged probably about 40 fathoms (73 m.) in depth; 

 for long distances inwards the average depths were 

 greater than 20 fathoms, or the lowest limit where reef 

 corals can thrive sufficiently to begin a permanent reef. 

 Farther inshore, where the depths on the shelves were 

 less than 20 fathoms, the rooting of corals must have 

 been largely or wholly prevented through the smothering 

 action of shelf muds and sands stirred up by major 

 storms. The same influence tended to injure even the 

 fringing reefs periodically, so that they had not been per- 

 mitted to grow to very great widths on the Tertiary 

 shelves. 8 



Tertiary coralliferous limestones are visible today 

 because of uplift, but not one, to the writer's knowledge, 

 lias been proved to represent an atoll or barrier reef. 9 



8 The rate of upgrowth of corals, as deduced from observation on growing 

 specimens, is relatively rapid. The rate of reef outgrowth, depending on 

 the preliminary formation of talus in water 40 to 1,000 or more fathoms 

 in depth, is presumably very much slower, and also merits specific study. 



9 Looseness of thought is inevitable without clear understanding of terms 

 employed. Unfortunately the standard dictionaries do not agree on the 

 definition of either "reef" or "coral reef." Probably a universally 



