43G E. STEIDTMANX ORlGiN OF DOLOMITE 



1. Stains show that sediments cuiiipusetl ul' niixiures of ealeite and dolo- 

 mite are subordinate in abundance to pure or nearly pure limestones and 

 dolomites. Analyses sliow this also (see diagram, iigure 1). Primary 

 ealeite is rare in dolomites; conversely, dolomite crystals are niicommoii 

 in limestones. Primary ealeite, as distinguished from vein or secondary 

 ealeite, is cJiaracterized by gray color, numerous minute inclusions, and 

 low degree of transparency hi tliin-sections. In all samples studied by 

 the writer it contained no FeO. Xo pi'ijnary ealeite was i'ound in the 

 Niagara, Lower Magnesian, and the Galena-Trenton formations excepting 

 tJie transition beds between the latter in western Wisconsin. The snudl 

 excess of CaCOo over dolomite requirements was in tlie form of clear Acin 

 ealeite, in all formations lacking primary ealeite. Complete gradations 

 from limestone to dolomite ought to l)e common if dolomitization A\ere 

 caused chiefly by underground waters. 



2. Xot uncommonly calcitic casts are eml^edded in dolomite, and hollow 

 casts are often inclosed by perfect molds of dolomite (see plate 23, figure 

 1). Either the casts were embedded in a matrix of dolomite and then in 

 some cases dissolved out after the rock was rigid enough to prescu've open- 

 ings, or else the matrix only was dolomitized by underground water ti fter 

 consolidation. The latter seems less probable. After the matrix had set 

 it seems that it would have been replaced no more readily than the casts. 



;>. The compact glasslike, fine-grained limestone beds at the base ol' the 

 Galena formation of southwestern Wisconsin: inclose many minute dolo- 

 mite rhoinbohedra (see plates 24 and 25). They appear to have l)een 

 embedded in the ealeite before consolidation. The rock at present seems 

 nearly impervious. If formed by underground waters when the rock A\as 

 in its present state, their development was effected without the slightest 

 sign of volume change. In the Ceeur d'Alene district very active ore- 

 bearing waters have replaced quartzite by siderite without volume change, 

 but these replacements are related to fissures. The dolomite gi'ains in 

 (piestion are not so related; hence it is quite certain that they formed in 

 the sea before the rock solidified. 



Published fads indicating tJie marine origin of (Ite inajorHij of dolo- 

 luites. — 1. The distribution of most dolomites is related to stratigraphic 

 planes and not to secondary structures such as faults and fissures. Altei-- 

 nations of limestone and dolomite beds and formations are not uncommon. 

 ^Fore, however, ought to be known on the distribution of dolomite forma- 

 tions with respect to stratigraphic horizons. It is still im])ossible to 

 decide in many cases whether a dolomite in one region grades into a con- 

 temporary limestone in another. Unsettled coi-relations and the fact that 

 many geologic reports call dolomites limestones handicap study along this 

 line. 



