CONDITIONS OF DOLOMITIZATJON IN SEA 447 



of i:)riniary calcite and tloloinito was .so striking- as to raise the question 

 liow general it is. Limestone analyses are not as delicate a measure of 

 the FeO content of ealeite as stains, since some of the FeO may be in 

 shaly portions, bnt the}- confirm the results of staining very well. Seven- 

 teen analyses of limestones rc])orted in Bulletin 591, United States Geo- 

 logical Survey, averaged .18 per cent acid soluble FeO; 5 analyses of 

 dolomite from the same |)ublication averaged I.IU ]jer cent acid soluble 

 FeO. Only those analyses showing soluble FeO were taken. 



It is certain from this that the dolomites studied Avere formed under 

 reducing conditions, for ferrous oxide in aqueous solutions is very easily 

 oxidized. Since they all l^ear the marks of shallow-sea deposits, it is 

 almost certain that they were not precipitated directly from the upper 

 strata of sea-water. Wave action must have aerated the w^aters excepting 

 perhaps right near the bottom, where the air may have been excluded by 

 the gases of decay. One would infer that dolomite was formed at the 

 sea-bottom, where decaying flesh and plant material gave rise to reducing 

 conditions. 



'No definite comparison can be made between the salinity of dolomite 

 and limestone-depositing seas. Experiments show that high concentra- 

 tion, of the magnesium sul])hates and chlorides favors the .change from 

 calcite to dolomite. The association of dolomites rather tlian limestones 

 with inclosed basin deposits, gypsum and salt beds also suggests that 

 salinity may be an important factor in dolomitization. It does not ap- 

 pear to be true, however, that the open seas in which dolomite formed 

 were very much more saline than those from wdiich limestojies Avere de- 

 posited. The difference in salinity Avas not sufficient to cause a diiference 

 in the kind and development of fossils m the tAvo, according to Weller. 

 If a granular marine-deposited limestone bed free from impervious ma- 

 rine structures could be found to grade into granular marine-formed 

 dolomite deposited at exactly tlie same time, ojie would l)e forced to the 

 vicAV that the composition of tlie sea-A\-ater was not always a critical factor 

 in dolomitization. One would look to either temperature, pressure, or 

 organic processes as more decisi\e. Such a bed does not appear to be 

 known, and the opinion that salinity is an importaiit factor in dolomiti- 

 zation therefore seems to Ijc sujjijorted both by field and experimental 

 "facts. 



SuAlMAliV 

 MARINE OR POtiT-MARlNK (MUG IS OF DOLOMITE 



Most dolomites Avere deposited in the sea. A minority of dolomites 

 AA-ere foi-med h\ tlie replacement of limestones b}- undei-ground waters. 



