340 EDWARD STEIDTMANN 



this process accounts for the stratigraphic separation of limestones, 

 magnesian limestones, and dolomites, or for any large proportion 

 of the dolomite in nature. 



Origin of dolomite by secondary replacement of limestone. — 

 Secondary dolomitization along fissures had been described by 

 Prestwich, 1 Geikie, 2 Bain, 3 Pfaff, 4 Spurr, 5 and others. The altera- 

 tion of limestones by hot, magnesian spring waters at Aspen, 

 Colorado, described by Spurr is significant, but obviously, second- 

 ary replacement by underground water seems to be only local. 

 A brief consideration of the chemistry of ground water and of the 

 ocean is suggestive in showing the probabilities for the relative 

 replacing power of sea water and underground water. Sea water 5 

 contains calcium and magnesium in the ratio of i to 3 . 1 1 . The 

 salinity is 3.301 to 3.737, of which 1.197 per cent is calcium and 

 3.725 per cent magnesium. The constitution of underground 

 water is variable and depends upon the local composition of the 

 rocks from which it is taken. Underground water of terrestrial 

 origin is nearly free from mineral matter at the start but contains 

 solvent gases from the atmosphere. Obviously, if a unit volume 

 of unmineralized underground water comes into contact with a 

 unit weight of carbonate rock, solution follows until equilibrium 

 is attained between the solid and liquid phases which are in contact. 

 If the temperature, pressure, and the volume of water remain 

 unchanged nothing further results. It is this history of the source 

 of the salts in terrestrial underground waters which is unfavorable 

 to replacement. 



In humid regions, underground waters are essentially carbonate 

 waters in which the principal positive ion is calcium. The follow- 

 ing analysis may be taken as illustrative : 



1 Prestwich, Geology — Chemical, Physical and Stratigraphical (Oxford, 1886), 

 I, I33-44- 



3 Gsikie, Textbook of Geology (3d ed.; MacMillan, London, 1893), 321. 



3 Bain, "Preliminary Report on the Lead and Zinc Dsposits of the Ozark Regions," 

 Twenty-second Ann. Kept. U.S. Geol. Survey, Part II (1901), 208-10. 



4 Pfaff, op. cit., XXIII (1907), 529-80. 



5 Spurr, "Geology of the Aspen Mining Dist., Colorado," Mon. U.S. Geol. Sur- 

 vey, XXXI (1898), 210-91. 



5 Mean of 77 analyses of ocean water from many localities collected by the Chal- 

 lenger expedition. Quoted from Bull. 330, U.S. Geol. Survey, 94. 



