Van Tuyl — New Points on the Origin of Dolomite. 251 



cat.es its possible chemical deposition on a larger scale in 

 nature. Fournet (25) regarded the dolomite beds interstratitied 

 with limestone in the Tyrol as original precipitates. His 

 studies showed that the volcanic theory of Von Buch was no 

 longer tenable. Others who have advocated the primary pre- 

 cipitation theory in one form or another are Loretz, Furch- 

 hammer, Hunt, Vogt, Daly, Linck, and Sness. 



As to the nature and cause of the reactions which have been 

 supposed to give rise to the chemical precipitation of dolomite, 

 there have been differences of opinion. Forchhammer (26) 

 attributed the reaction to the action of calcium carbonate of 

 spring water on the magnesium salts of the sea, while Hunt, 

 (27) basing his views on experimental evidence, regarded dolo- 

 mite as the product of the action of sodium bicarbonate on 

 the magnesium chloride and magnesium sulphate of the sea. 

 Linck (28) and Daly (29), on the other hand, emphasize the 

 importance of ammonium carbonate furnished by decaying 

 organisms on the sea bottom as the precipitating agent. 



Still another primary theory is that introduced by Lesler (30) 

 to account for certain dolomitic layers in the u Calciferous " 

 limestone near Harrisburg, Pa. These he believed to repre- 

 sent ordinary mechanical sediments which were deposited at 

 the time the limestone was laid down. The clastic theory has 

 been adopted more recently by Phillipi (31), who regards cer- 

 tain impure dolomites of the Muschelkalk of Germany as 

 mechanical deposits possibly derived from the residuum of 

 limestones low in magnesia. Grabau (32) has concluded that 

 certain impure dolomitic limestones and waterlimes of the 

 Salinan and Monroan series have had a similar origin. 



An entirely different theory of origin is that which was in- 

 troduced by Grandjean (33) in 1844, to explain the production 

 of the dolomites of the Lahn district. He assumed that by the 

 atmospheric leaching of the lime from an original limestone of 

 low magnesia content, a true dolomite might in time result. 

 Both Bischof and Hardman later demonstrated the plausibil- 

 ity of this theory experimentally, and Hardman (34) immedi- 

 ately accepted it to explain the origin of the Carboniferous 

 dolomites of Ireland. In 1895 Hall and Sardeson (35) applied 

 the same theory in interpreting the history of the Lower Mag- 

 nesian series of the Upper Mississippi Valley. 



Hogboin (36) on the other hand, regards surface leaching as 

 of minor importance and emphasizes the effect of marine 

 leaching. He has proven the reality of this process, on a small 

 scale at least, in the modern seas and concludes that some dolo- 

 mites of former periods may have been formed in this manner. 

 Judd (37) is of the opinion that the weakly dolomitic portions 

 of the atoll of Funafuti may be explained upon the basis of 



Am. Jour. Sci. — Fourth Series, Vol. XLII, No. 249. — September, 1916. 



17 



