312 I'UocKKDixfis or thl; acadkmv of [1S<)(). 



Bluii(|uilla. On the other hand, immediately west of the Islote de 

 Pajaros, and at various points southeast and south of Gallega, there 

 seems to have been a considerable amount of shoaling, some places 

 to the extent of 8 or 10 feet, or more, but in most parts I suspect 

 that this shoaling is due almost wholly to the displacement of shift- 

 ing sands, and not to coral encroachment. Apart from the testimony 

 of the depth of water which is still found over these shallows, this 

 condition seems to be largely indicated by the circumstance that 

 at a few points a greater depth was found than appears on the map, 

 ])robably the result of uncovering. It is not improbable that at a 

 few places south of the Gallega Reef, or between the reef and the 

 mainland, the shallowing that we observed is actually due to coral 

 up-growth ; at any rate, the discrepancy in the depths is here very 

 marked, as much as 12-15 feet (depth of water 21-27 feet), and the 

 waters cover a rich growth of brain-coral (Mceandrina, Diploria). 

 It is hardly possible that a layer of corals approaching so closely to 

 the surface should have completely escaped the early hydrographers, 

 or that these investigators could have been in error in their deter- 

 minations of the then existing depths. But then if neither prop- 

 osition is tenable, and we are compelled to fall back upon the as- 

 sumption of simple coral up-growth — which to me appears every 

 way plausible — we must assume a rate of growth much more rapid 

 than has until recently been allowed these lowly organisms. A 

 number of instances are, however, on record where individual spec- 

 imens of il/ceanc?rma are known to have increased in height from 

 a half-inch to an inch per annum, and of Madrepora where the in- 

 crease for the same period has been three inches.' An average 

 growth of an inch and a half per year would add in a period of 

 ninety years eleven feet. 



The reefs which have been described in the preceding pages 

 manifestly belong to that group which Darwin recognized as being 



1 Dana " Corals and Coral Islands," 2d ed., 1890, pp. 123-lii7 (quoting 

 Pourtal^s, Hunt, Verrill, etc.). On the other hand, the observations of Mr. H. T. 

 Woodman (communicated to Prof. Dana under date of Jan. 25th, 1890 — Op. cit., 

 appendix, pp. 418-419), conducted over a period of fourteen years, would seem 

 to indicate a very much slower growth. During the above-mentioned period the 

 following upward growths (among others) were registered : Orbicella annularis, 

 one and a quarter inches ; Diploria cerebriformis, nearly three-fourths of an inch ; 

 MtEandrina sinuosa, an inch and a quarter; and M(j:andrina labyrinthica, an inch 

 and seven-eighths. Specimens of Madrepora cervicornis, however, seem to have 

 encroached upon the channel under observation to an extent of 6-8 or even 10 feet. 



