222 



CARNEGIE INSTITUTION OF WASHINGTON. 



specimens representing each species at each station computed. A tabular 

 statement of the annual averages is presented on a subsequent page. 



As the object of the investigation should be made clear, it should here be 

 stated that stony corals are not suitable subjects for a critical study of the 

 laws of growth-rate. The proportion of living tissue to the stony skeleton is 

 relatively small, and as the skeleton after very young stages usually is not en- 

 tirely covered by the living soft parts, other organisms may attach themselves 

 to the previously formed skeleton and increase its weight, or boring organisms 

 may enter the skeleton, begin its destruction, and decrease its weight. As 

 many boring organisms have calcareous tests, they destroy a part of the 

 original skeleton and add the weight of their own. Minute algaj, as Duerden 

 has shown, bore into the skeleton and ramify through it almost or quite to the 

 boundary of the living soft parts. Weights obtained from specimens cemented 

 to disks are subject to all the sources of inaccuracy enumerated, and also to 

 the impossibility of restoring the disk to its initial condition after affixing 

 and planting a specimen, because of organisms attaching theniselves to its 

 surface. These remarks render it clear that the object of the investigation 

 is not to make a contribution to the laws governing growth-rate. However, 

 it will later be made evident that some of the principles of gro\vth-rate of some 

 species have been ascertained. The actual object of the investigation has been 

 to aid in understanding the relative amount of work stony corals may do as 

 constructional geologic agents, and especially in the formation of those calcium- 

 carbonate structures designated ''coral reefs." 



In order properly to evaluate corals as constructional agents, the subject 

 needs to be studied from at least five different view-points, viz: (1) In dealing 

 with sediments uplifted above the sea, the quantity of material contributed by 

 corals and that contributed by other agents must be estimated and the 

 respective proportions determined; (2) in coral reef areas, the proportion of 

 the area covered by corals to that not covered by them should be estimated; 

 (3) the relations of coral reefs to continuity and discontinuity of marginal 

 submarine platforms must be ascertained; (4) marine bottom deposits must 

 be analyzed according to the source of the material, and the percentage of the 

 calcium carbonate contributed by the different agents estimated; (5) the rate 

 of growth of corals needs to be known, especially for the light it may throw 

 on the rate of reef formation. 



That corals have been tremendously overevaluated is established. In 

 this connection, I introduce Murray and Renard's table giving the composi- 



Average composition of the "Challenger" samples of coral mud and of coral sand} 



Composition. 



Coral mud. 



Carbonate of lime: 



Pelagic foraminifera I 31 .27 



Bottom-living foraminifera j 14 . 64 



Other organisms 39 . 62 



Residue: | 



Siliceous organisms , 1 . 36 



Minerals ' 1 . 00 



Fine washings 12.11 



85.53 



14.47 



100.00 



Coral sand. 



36.25 

 20.00 

 30.59 



5.00 

 3.75 

 4.41 



i.84 



13.16 



100.00 



1" Challenger" Repts., Deep-Sea Deposits, p. 246, 1891. 



