June, 1906. New Forms of Coxcretioxs — Nichols. 47 



between these limits are of very unusual occurrence, as are carbonate 

 rocks with magnesia much in excess of that in dolomite. Experi- 

 ments by various chemists and experimental geologists have amply 

 demonstrated that a co-precipitation of magnesia and lime car- 

 bonates under normal conditions of concentration, pressure, etc., 

 is impossible. The depth of 28 fathoms, however, corresponds to a 

 pressure of 100 pounds to the square inch, more or less, and under 

 this pressure and at ordinary temperatures experiments have not been 

 carried out. 



These considerations are not intended to prove that dolomites and 

 magnesian limestones are never formed by metasomatic processes or 

 by direct precipitation. The evidences of metasomatic origin for 

 some dolomitic limestones which have been summarized by Van Hise* 

 are convincing. It is, however, evident from the above considerations 

 that the conditions under which the Bermuda nodules grew are not 

 such as favor either of these processes of dolomite formation. Inas- 

 much as the nodules are evidently organic in origin, direct secretion 

 of magnesia by the organisms concerned seems a reasonable hypoth- 

 esis, especially as such an action would be to the advantage of the 

 organism by rendering its skeleton more insoluble. As some brach- 

 iopods and all vertebrates secrete phosphates, and some sponges, 

 diatoms, etc., silica, there seems to be no a priori reason why corals, 

 etc., should not secrete carbonate of magnesia together with carbonate 

 of lime. There appears to be an impression which is very wide 

 spread that all such calcareous skeletons are extremely pure carbonate 

 of lime, but a cursory examination of available literature discloses no 

 grounds for such a belief. Dana, Geikie and Prestwichf quoting 

 Dolter and Homes' work upon the dolomites of the Tyrols, note 

 that some organically deposited limestone is slightly magnesian at 

 the time of formation. Many writers refer briefly to the work of 

 Forchhammer discussed in the following pages, but either minimize 

 the importance of his results or fail to see their significance. 



To determine whether calcareous organisms ever become magne- 

 sian enough to account for the character of these nodules magnesia 

 was determined by the author in the Museum laboratory for twelve 

 skeletons of calcareous organisms of various types. With the results 

 of this work are tabulated twenty-one determinations by other 

 analysts. The determinations as given in the table are of the speci- 

 mens as prepared for exhibition. These naturally contain dried 



* U. S. G. S. Mon. XLVII, p. 802. 



tDana: Manual of Geology, p. 134; Geikie: Textbook of Geology, p. 321; Prestwich: Geol. 

 Vol. I, p. 113. 



