250 Van Tuyl — Neio Points on the Origin of Dolomite. 



Van Hise(lO) also attaches much importance to dolomitization 

 after the limestones emerge from the sea. 



In 18i() Green (11) offered the suggestion that some dolo- 

 mitic limestones might be formed by the decomposition of 

 olivine sand incorporated in the original limestone and the 

 recombination of the magnesia with the lime. He calls atten- 

 tion to the fact that olivine sand, derived from the action of the 

 waves on lava, constitutes an important constituent of the 

 coral-reef rock about the borders of the Hawaiian Islands and 

 regards this as significant. 



Dana (12) in 184:3, attempting to account for the dolomite of 

 the coral island of Metia, supposed that it had been formed i by 

 the action of magnesium salts of heated sea water on limestone. 

 Twenty-nine years (13) later he expressed the view that the 

 same dolomite had been formed in sea water at ordinary tem- 

 peratures but perhaps in a contracting lagoon where magnesium 

 and other salts were in a concentrated state. Sorby (14) like- 

 wise favored the theory of marine alteration and the same 

 origin has been urged, either for dolomites in general or in 

 special instances, by Yon Richthofen, Doelter and Hoernes, 

 Hoppe-Seyler, Mojsisovics, Murray, Skeats, and F. W. Pfaff. 

 In support of this theory are also the observations of 

 Weller(15) who, from a faunal study of the Galena and 

 Niagara dolomites of the Upper Mississippi Valley, concludes 

 that they were deposited originally as limestones and later 

 metamorphosed. More recently, Blackwelder (16) has also 

 advocated the replacement theory for the origin of the Big- 

 horn dolomite of Wyoming, but owing to the very slight por- 

 osity of this rock he is led to suggest that the alteration 

 proceeded contemporaneously with its deposition rather than 

 subsequent to its consolidation. 



F. W. Pfaff (17) believes that the alteration takes place at 

 considerable depth and in concentrated seas, but Phillipi (18) 

 vigorously controverts this view since he has good evidence 

 that dolomitization may proceed in the open sea and at shallow 

 depths. Skeats' (19) studies of the coral reefs of the Southern 

 Pacific also seem to show that concentration and pressure are 

 not important factors. On the other hand, both Nadson and 

 Walther (20) have suggested that bacteria may play an impor- 

 tant part in the alteration. 



Still other geologists have supported the theory that dolo- 

 mite represents a direct chemical precipitate from the ocean. 

 Boue (21) as early as 1831 advocated this method of origin. 

 Bertrand-Geslin (22) and Cormand (23) were also early sup- 

 porters of this view. That dolomite can be formed as a chem- 

 ical precipitate is pointed out by Zirkel(24r) who shows that 

 the occurrence of crystals of dolomite in veins and druses indi- 



