VanTuyl — New Points on the Origin of Dolomite. 259 



which lend weight to this view are as follows: (1) The dolo- 

 mite areas of mottled limestones are believed to have'undergone 

 reei'3'stallization at the same time as the associated limestone 

 areas as suggested by the occasional development of zonal 

 growths of calcite and dolomite. (2) In imperfectly altered 

 limestones the dolomite is seen to follow original lines of weak- 

 ness rather than secondary structures such as joints or fractures. 

 (3) In most cases of mottling the dolomitization appears to 

 have progressed uniformly as we should expect it to in an 

 uncrystallized rock, rather than to have progressed !>y forming 

 veinlets and stringers in the early stages. (4) The existence of 

 perfect rhombs of dolomite in many imperfectly altered lime- 

 stones suggests that the latter had not yet solidified when the 

 dolomite rhombs were formed. (5) The widespread extent 

 and nearly uniform composition of many dolomites indicates 

 that they must have been formed by an agent capable of oper- 

 ating uniformly over wide areas. (6) An adequate source of 

 magnesia for transforming extensive limestone formations into 

 dolomite is found only in the sea which contains many times as 

 much of this constituent as ordinary ground water. (7) Many 

 dolomites are directly and regularly overlain by pure limestone 

 formations or by thick shale beds, proving that they must have 

 been formed before these overlying beds were deposited. 



Some dolomites of minor importance, such as those associ- 

 ated with ore deposits and probably most, if not all of those 

 related to fractures (vein dolomites), must have been formed 

 through the agency of ground water. But in general, ground 

 water is incapable of carrying dolomitization far. Study of 

 analyses of ground water, and of river water, shows these to 

 be uniformly low in magnesia, this constituent normally being 

 greatly exceeded in amount by lime. How, then, could such 

 waters dolomitize limestone when they already contain lime 

 far in excess of magnesia ? The law of mass action speaks 

 strongly against ordinary ground water being able to accom- 

 plish extensive dolomitization. In the case of mineral springs 

 and the mineralizing solutions which are related to ore 

 deposition, however, it is conceivable that magnesia might be 

 present in sufficient proportions to accomplish local dolomitiza- 

 tion and doubtless most vein dolomites have been so formed. 



University of Illinois, 

 Urbana, HI. 



References to the Literature. 



1. Von Buch, quoted by Zirkel, Lehrbucli der Pefcrograpbie, 2d ed., vol. 



iii, p. 505. 



2. Fiapolli, Bull. Soe. geol. France (2), vol. iv, p. 832, 1847. 



3. Durocher, Compt. Eend., vol. xxxiii, p. 64, 1851. 



4. Favre, ibid., vol. xxviii, p. 364, 1849. 



