TITLES AND ABSTIMCTS OF PAPERS 153 



ORIGIN OF DOLOMITE AS DISCLOSED BY STAINS AND OTHER METHODS 

 BY EDWARD STEIDTMANN 



(Al) Struct) 



That most dolomites were formed in the sea seems to be indicated by the fol- 

 lowing facts: (1) Dolomites and limestones are frequently interstratified. (2) 

 Dolomitization is often related to original structures, such as bedding, worm 

 borings, etcetera, but rarely to faults and joints and other secondary struc- 

 tures. (3) Chemical analyses of limestones and dolomites, as well as the dif- 

 ferentiation of limestones and dolomites by stains, shows that limestones free 

 or nearly free from dolomite, and dolomites nearly free from calcite are vastly 

 more common than beds composed of mixtures of limestones and dolomite. If 

 most dolomites had resulted from the action of underground waters, gradations 

 between limestones and dolomites ought to be common. (4) Calcite fossil 

 casts are often embedded in dolomite. Hollow casts are frequently enclosed 

 by perfect dolomite molds. In either case the calcific shells evidently were 

 deposited in a dolomite ooze. (5) Perfect dolomite rhombs are sometimes em- 

 bedded in compact hornlike calcific beds. It seems more probable that such 

 dolomitic crystals developed when the bed was still in the form of an ooze. 

 (6), Dolomites and limestones have about the same range of porosity. Had 

 dolomites developed dominantly by underground waters, one would expect them 

 to be, on an average, more porous than limestone, for two reasons: (a) the 

 porous limestones would be more likely to alter to dolomite because of their 

 permeability: (J)) from theoretical considerations, the change from calcite to 

 dolomite has been assumed to Involve a decrease in volume. (7) If dolomites 

 had resulted chiefly from the action of underground waters, they ought to be 

 most abundant in the ancient formations. Both limestones and dolomites, 

 however, are well represented in the pre-Cambrian, and the distribution of 

 dolomites in the rocks of later periods can not be consistently correlated with 

 the length of time to which they have been subjected to the action of under- 

 ground waters. 



The differentiation of calcite and dolomite by stains yields facts which show 

 that replacement is an important dolomitization process. In mixed beds of 

 dolomite and calcite, the dolomite has a bunchy, irregular, sometimes grotesque 

 distribution which disregards the bedding planes and the joints. Dolomitiza- 

 tion frequently is local being adjacent to or within pervious marine structures 

 such as worm borings, shell cavities, etcetera. That dolomite replaces calcite 

 without regard to the crystal boundaries of calcite is indicated by the fact that 

 dolomite grains in contact with calcite are rhombohedral ; the calcite grains, 

 however, are anhedral. Dolomite rhombs also invade calcific fossils, but only 

 in the more advanced stages of replacement. They replace calcite by a process 

 of complete reconstruction of the conquered territory. Undigested calcite 

 grains within dolomite rhombohedra or dolomite skeletons were not observed 

 by the writer. 



Fossils and the shallow water structures of most dolomites show that, like 

 most limestones, they were laid down in shallow, warm seas. Salinity seems 

 to have favored dolomitization, since the dolomites of certain periods are 

 chiefly in association with the salt beds and not with the contemporaneous 

 open sea beds. 



