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NEW YORK STATE MUSEUM 



rock segregates and concentrates the magnesia of the sea water more than 

 any other sedimentary material. We may note that the admixture of bitu- 

 minous matter in these Guelph dolomites is further indication of their coral 

 reef origin, or is at least in harmony with recent observations on living coral 

 reefs where petroleum has been found in process of formation as a result of 

 the transformation of the organic matter of the reef. The cavernous char- 

 acter of the dolomites may, according to the views of Walther and others, 

 be regarded not as the result of subsequent corrosion but as the remnants 

 of original cavities in the growing reef which have not been closed up with 

 coral sand. 



The chert concretions which are characteristic of the upper Guelph 

 horizon at Rochester and Shelby are doubtless a byproduct of the diagene- 

 sis which altered the coral limerock to a dolomite. These nodules contain 

 fossils with their exterior ornament finely retained, that is replaced in amor- 

 phous silica, while in the dolomite the shell substance has been removed and 

 never replaced. The source of the silica here, as in the like segregations 

 associated with limestone, is probably to be found in spicules of sponges, 

 organisms which contribute importantly to the comminution of shells and 

 coral skeletons, specially the boring forms like Cliona. 1 It is known that 

 sponges, both silicious and calcareous occur abundantly on the western edge 

 of the Florida bank. 2 



The suggestion, derived from its lithologic characters as to the reef 

 origin of the rocks, is fully borne out by the character of the fauna. The 

 dolomite everywhere contains fragments or traces of corals specially of 

 Stromatopora, Favosites, Syringopores and Cladopora. Here, however, as 

 in other cases of fossil coral reefs, the coral masses have been largely 

 destroyed or altered beyond recognition. 



We have noted the existence of extremes of size in the Guelph organ- 

 isms, the ponderous heavy shelled species, and the diminutive forms. One 

 reading the account given by Klunzinger 3 of the life inhabiting the reefs 



'Hancock. Ann. and Mag. Nat. Hist. ser. 2. 3:231. 

 2 Agassiz, A. Three Cruises of the Blake. 1888. 1:149. 

 ^Bilder aus Oberagypten. 1878. p. 334. 



