JUNE, 1906. New Forms or CoNcCRETIONS—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 priort 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 Prestwich} quoting 
Délter and Hornes’ 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. 
+ Dana: Manualof Geology, p.134; Geikie: Textbook of Geology, p. 321; Prestwich: Geol. 
Vole DT ‘pein3- : 
