ORIGIN OF THE DOLOMITES. 101 



"That some cluMnical ivi>UuHMnont did occwv atlmits of no doul)t ; hut it miiiht 

 be p:oin,«r farther tluin the evidence warrants to oonehide that the wiiole rock was 

 entirely aUered l)y true rephiceinent without any direct chemical i)recipitation of 

 magnesia." 



From the foregoiiiij: brieT tabulation — it cannot be assigned tlie dignity 

 of a sunnnary — of the leading literature of one hundred years on the 

 dolomites it will be seen that some diversity of o[)ini()n has obtained; 

 yet in considering it as a whole the consensus of search lias been in the 

 field of chemical geology. Occasionally an investigator would step into 

 another field, ))ut it was probably rather for tlie purpose of presenting a 

 new view to his contem[)oraries than with any strong convictions that 

 the results of his search would prove correct. The admirable summary 

 of views presented by Naumann* and the later one by Zirkelf sho>v a 

 wider range of opinions than the writers can here give. Little has been 

 done in the investigation of the carbonate rocks of this class since the 

 paper of Sorby cited above beyond the examination of a few somewhat 

 limited fields. 



SedLnieat-building. — The ])henomena of sediment-building in modern 

 seas are much better understood now than formerly. The organic sedi- 

 ments are chiefly calcium carbonate, yet magnesian rocks, chloride of 

 sodium, etcetera, occur. The quantity of calcium carbonate in the ocean 

 receives constant additions from the land, the shells of moUusks, and 

 other shell-secreting forms, and from disintegrating limestone cliffs along 

 the seashore. Keeping pace with this accumulation is the constant witli- 

 drawal of material througli the agency of organisms and, possildy, the 

 formation of salts in which the components of calcium carbonate play a 

 l)art ; j'et all the limestones formed from the calcium carbonate of the 

 sea contain Imt a small proportion of other constituents. 



Contents of Scaw(Uer. — In normal seawater there is a preponderance of 

 magnesium salts over the salts of calcium. The cliemists who investi- 

 gated the material gathered Ijy the Challenr/er found the following compo 

 sition of tlie soHds from seawater per 100 grammes of total salts. It is a 

 mean of 85 samples : 



CI 42.917 (after basic O is deducted). 



SO, 0.415 



CaO l.r)92 



MkO f;.214 



K/) 1.333 



Na,0 41.433 



100.004 



• Lehrbiicli dor Geognosie, seoon<l odition, vol. i, pp. 7t'>.1 ft .se<i. 

 fLehrbuch der Polrographie, Bonn, 1806, vol. i, pp. '234-'ii.Vj. 



XXVII-BcM.. Gbol. Hoc. Am.. Vol. fi, 1894. . 



