352 bulletin: museum of comparative zoology. 



If the above arguments have vahdity, and I am firmly convinced 

 that they do, it follows that> little weight can be attached to coral 

 reef faunas in attempting correlation. 



In the channels between modern reefs varied conditions obtain, 

 and Dana describes the deposits as variable to a high degree, at one 

 place coral sand, at another coral mud, at others clay mud with little 

 coralline material and he states that " The facts show that the rocks 

 formed in such channels may be of all the kinds that occur in reef 

 regions — coral and shell conglomerate, compact impalpable limestone, 

 limestone full of Orbitolites, or containing, as well, remains of other 

 species of the seas, and also rocks made of clay, mud, sand or pebbles 

 of the movmtains or high lands adjoining."^ 



As illustrative of the data given in the above quotation Dana might 

 have cited the reefs of Gotland, as every word is strictly applicable. 

 There, on the same level, are fine-grained limestones made from lime 

 muds, limestone conglomerates, lime sandstones, clays, shell l:)reccias, 

 etc. In the passages between modern reefs the " tidal currents often 

 have great strength, and are much modified and increased in certain 

 places, or diminished in others, by tlie position of the reef with refer- 

 ence to the lands" ^ and there is little doubt that local erosion replaces 

 deposition to a considerable extent as the growth and erosion of the 

 reefs change the direction of the currents, thus developing local uncon- 

 formities. In this way the many discordances of the Gotland section 

 are readily explained. 



These varied conditions upon, within, and about a coral reef will 

 be and are reflected in the favmas of the bottom, each species selecting 

 that bottom and those conditions on and under which it best thrives, 

 and animals of the same species might be found on the top of the reef 

 and at its foot, as, for example, might have occurred (and where it is 

 believed it did occur) at Hoburgen where about seventy-five feet sepa- 

 rate the existing summit from the base. 



The sediments which are deposited on the flanks of a coral reef, 

 or any similarly elevated mass, are inclined away therefrom. The 

 compacting of the strata would increase the inclination while local 

 slumping would intensify the undulatory structure. As a consequence 

 anticlinal and synclinal structures would be developed. Some of the 

 undulations of the Gotland section may, and appear to have been 

 developed by subsequent movement, but it is certain that many are 

 contemporaneous and are directly referable to the influence of the 



' Dana. Loc. cit., p. 152. 

 2 Dana. Loc. cit., p. 151. 



