TI^rE OF DOLOMITTZATION 4B9 



3. 'Nov do dolomites show the relation to time that would have resulted 

 if average imdergronnd waters had effected their formation in the conrse 

 of ages. The Precamhrian of IS'orth America contains vast thicknesses 

 of nearly pnre dolomite in the 4:9th parallel section and in the Lake Sn- 

 perior region, bnt the thick Grcnville limestone^ of sontheastern Ontario 

 are remarkably low in magnesia. Dolomites predominate in the Cam- 

 brian and Lower Ordovician of the Appalachian province and in the ]\ris- 

 sissippi Yalley, but the Canil)rian of tlie Western States seems to have 

 mainly limestones. Limestones are more important than dolomites in 

 the Middle and Tapper (Ordovician of ihd IJjiited States excepting in the 

 Upper Mississippi Yalley and in the Western States. Tn the eastern part 

 of the Ignited States tlie Oi'dovician is an important sonrce of Portland 

 cement materials, and prol)ably includes the most extensive deposits of 

 high calcinm limestones of the Ignited States. The Silurian has some 

 limestones, bnt dolomites dominate in the Appalachians and in the Mis- 

 sissippi Yalley. In the Salina beds dolomites occur with salt and gypsum. 

 Limestones are more abimdant than dolomites in the Devonian. The 

 l\rississippian, too, has very little dolomite, but has great limestone beds 

 in the Mississippi A^alley. Mississippian dolomites, however, are asso- 

 ciated with salt and gypsum in southeni Michigan. The Pennsylvanian 

 has many limestone, but almost no dolomite, beds in the Eastern States, 

 the Mississippi Yalley, and in Texas. The western undivided Carbonif- 

 erous has both limestones and dolomite. In Montana early Carboniferous 

 dolomite beds show evidences of having been laid, down under arid con- 

 ditions. In the Permian dolomite beds are common with gypsum and 

 red beds, chiefly non-marine deposits, or at least without distinctly marine 

 features. The Permian of the open seas has limestones, but little dolo- 

 mite. The writer has not made a detailed study for Mesozoic and Ter- 

 tiary beds, but they seem to have more limestones than dolomite. Many 

 Tertiary coral reefs of the southern Pacific, however, are either partially 

 or wholly dolomitized. It may he seen from this necessarily l)rief sum- 

 mary that, althougli the carbonates of more recent times seem to be lime- 

 stones largeh^, the older ones have considerable thicknesses of both lime- 

 stones and dolomites. Pure limestones are abundant even in the Pre- 

 camhrian. These facts are more easily correlated with oscillations in 

 marine conditions than with the metasomatic action of underground 

 waters. 



4. The replacement of aragonite aiid calcite by dolomite has been 

 effected experimentally at ordinary temperatures by solutions comparable 

 to sea-water, but not by carbonate solutions similar to most underground 



