Van Tuyl — New Points on the Origin of Dolomite. 253 



period of one month. It was noted that the carbonates came 

 down separately with the OaC0 3 much in advance of the 

 MgC0 3 . The precipitate then contained only the mixed car- 

 bonates — no dolomite was formed. Scheerer (40) previously 

 obtained the same results in a similar experiment. Negative 

 results were still obtained when a solution prepared as above 

 was inoculated with a crystal of dolomite and allowed to evapo- 

 rate. Nor could the double carbonate be prepared upon evapo- 

 rating spontaneously a solution of the two carbonates obtained 

 by the action of carbonated waters on normal dolomite even 

 when a dolomite crystal was introduced and a concentrated 

 solution of sodium chloride and magnesium salts was added. 

 The experimental evidence so far obtained, therefore, does 

 not suggest the circumstances under which large masses of 

 dolomite can be formed in nature under ordinary conditions 

 either by the alteration of limestone or by chemical precipita- 

 tion. It is to be regretted that a careful study of the process 

 of dolomitization where it is going on in the seas to-day has 

 never been made. Such a study would doubtless throw much 

 valuable light on the problem. It may well be that bacteria 

 play an important role in the production of dolomite as sug- 

 gested by Nad son. 



Field JEv idence. 



Realizing the importance of careful field studies of dolomitic 

 formations in interpreting the conditions of their origin, the 

 writer undertook a study of the dolomites of the Upper Missis- 

 sippi Valley under the auspices of the Iowa Geological Survey 

 during the field season of 1912. More recently a grant from 

 the Esther Herrman Research Fund of the New York Academy 

 of Sciences has made possible much more extensive field studies 

 in the eastern and central states. Dolomites ranging in age 

 from the Cambrian to the Mississippian have now been exam- 

 ined and many samples collected for detailed chemical and 

 petrographic study. It is possible to outline in this paper 

 only some of the more important results obtained. 



It should be pointed out that the term dolomite is used here 

 in the broad sense to include both normal dolomite and dolo- 

 mitic limestone. It is not necessary to differentiate between 

 these in a discussion of their origin. 



The field studies undertaken during the course of this inves- 

 tigation have alone furnished irrefutable evidence that most of 

 the dolomites examined, regardless of their age, are replace- 

 ment products. The following facts support this contention : 

 (1) the lateral gradation of beds of dolomite into limestone, 

 sometimes very abruptly; (2) the mottling of limestones by 



