228 W. M. Davis — Shaler Memorial Study of Coral Reefs. 



ing this subsidence, although no direct statement to that effect 

 is made ; that an elevation of similar amount then occurred, 

 during which the valleys of the harbors were eroded ; and still 

 later a subsidence of from 40 to 70 feet, when the valleys were 

 drowned and the harbors were formed.* 



A somewhat different explanation of the Cuban harbors was 

 given nearly twenty years earlier by "W". O. Crosby, who 

 wrote: — "Every harbor is at the mouth of one or more rivers, 

 and their inlets, as I conceive, are the work, not of the sea, but of 

 rivers at a time when the land was higher than now .... The 

 main body of the harbor, in each case, is simply the broader 

 and older portion of the river valley behind the barrier reef, 

 which has been invaded by the rising sea. . . When the reef 

 was finally raised to something above its present level each 

 river . . . cut a narrow channel through the reef itself," and 

 then subsidence drowned the reef channel. f In both these 

 explanations, it is tacitly assumed that the reefs were continu- 

 ous when formed. A simpler explanation of the Cuban har- 

 bors, and one that does not include this improbable assumption, 

 has been suggested by R. T. Hill, to the effect that " the nar- 

 row outlets through the reef rock . . . are channels represent- 

 ing originally areas of non-coralline growth, such as are now 

 known to exist in submerged [non-elevated] reefs . . . and 

 such as biological laws tell us should exist opposite the mouths 

 of rivers.":]; Without additional field study it is impossible to 

 say which one of these views is correct ; but the features of 

 the Pacific reefs that I have seen support Hill's explanation. 



Value of Physiographic -Evidence. — To zoologists who are 

 not familiar with the evolution of shorelines or with the 

 inferences that can be drawn from them as to recent or subre- 

 cent changes of level, it may seem presumptuous to attach so 

 much importance to physiographic evidence as the second 

 preceding section gives it, in a problem that has usually 

 been treated from its zoological side; but to physiographers 

 who are acquainted with the modern progress of their science 

 the value of this evidence will be more manifest. It may 

 nevertheless seem even to them over-bold thus at a stroke to 

 sweep away all the stand-still theories, and so indeed it would 

 be if the postulate of a still-stand were an unessential element 

 of these theories ; but as a matter of fact it is a fundamentally 

 essential element, gratuitous as is the supposition of an ocean 

 bottom that can rise but cannot sink; for the stand-still theo- 



* C. W. Hayes, T. W. Vaughan and A. C. Spencer. Eeport on a Geological 

 Reconnoissance of Cuba . . . [Washington ?] 1901 ; see pp. 17, 23, 34. 



fOn the elevated reefs of Cuba, Proc. Boston, Soc. Nat. Hist., xxii, 1884, 

 124-130; see pp. 128, 129. 



X Notes on the Geology of the Island of Cuba. Bull. Mus. Comp. Zool. 

 xvi, 1895, 243-288 : see p. 279. 



