MA^'^EK OF MAKIXE DOLOMITIZATIOX' 441 



in limestones of similar age ; hence the in lerior nnml)er of fossils in dolo- 

 mite can not be regarded as due to sedimentary conditions nu suitable to 

 organisms, but points to the secondary desl ruction ol' fossils in dolomite. 

 That replacement bv dolomite destroys fossils was evident from many 

 samples. 



In pure dolomilc I)e(ls fossils usually consist of hollow casts surrounded 

 by perfect molds of dolomite (see plate 22, figure 1). Evidently the 

 shell in plate "^2 was buried in a dolomite paste and then dissohed out 

 after the ooze had set firm enougb to preserNe openings. Less commonly, 

 the casts themselves are dolomite. This was seen only in shaly dolomitic 

 facies of the Cincinnati shale and the Trenton limestone. The fossils iji 

 this case evidently were preserAcd because of the shaly molds. 



In beds of mixed primary calcite and dolomite the calcific areas are 

 generally charged with fossil casts, mostly microscopic in size. In the 

 dolomitic portioiis only the megascopic casts are seen, as a rule. These 

 show all gradations of destruction l)y the invasion of dolonrite crystals 

 from the outside. In most cases, however, dolomite grains surround shells 

 without piercing them, and appear in the holloAvs of shells as in plate '2'i, 

 figure 2. It looks as if the sliells themselves had stimulated the growtli 

 of dolomite. Perha])s ammonia or otber substances generated l)y tlic 

 decaying flesh caused dolomite to form. IS^ot infre(piently dolomite 

 grains follow rather grotesque outlines, as in plate 24, as if their distribu- 

 tion had been controlled by some organic structure. Plate 26 shows the 

 mottled limestone of Manitoba,^^ whose dark dolomitic ])atclies Wallace 

 has reservedly called algal iji a very interesthig paper. ^^ As to the cor- 

 rectness of tin's view, the Avriter is inca])a]jle ol; judging. The ])resence 

 of calcite casts (of sliell-beai-ing oi'ganisms) iu the dolomite aieas jun-ds 

 explanation under this hypothesis. It seems that the worm borings which 

 pierce all the dolomite patches and none of the limestone areas may lunc 

 beon an important factor in dolomitization. They admitted the sea-water 

 to tlie rock. In addition, the life processes and decay c>l' tbe Avorms mav 

 have favored dolomitization. ^lany oC tbe dolomite crvstals Mere found 

 to project into the borings aJid hence are younger. 



About 2 miles northeast of Sun Prairie, in Dane Couuty. Wisconsin, 

 is located a veiy interesting section suggesting that certain fossils are 

 more susceptible to dolomitization than others. Tbe base consists oC 

 about 20 feet of horizontal ( Jaleua-'ibvnton doloiuite beds. This is 

 capped by about 3 feet of mixed dolomite and linu'stone l)eds, and these 



'* Specimen obtaiued thi-onsh courtesy of It. C. WaUace. 



1^ n. C. WaUace : Pseiulohrccciation in Ordovician limestones in Manitoba. .Toui 

 Geo!., vol. 21, pp. 402-421. 



