444 1^. STEIDTMANN ORIGIN OF DOLOMITE 



ill volume of 13 per cent. Tn tlie oo/e tlic record of the cJuuige would 

 have become lost^ but not in the solid casts. 



B.elations of dolomUe to pervious marine ,s//'//r///re.s\— Attention is again 

 called to the more frequent dolomitization of the matrix than of tlie casts, 

 plate 22, figures 1 and 2, or plate 25; the frequent deveh)pment of dolo- 

 mite crystals in the liollows of tests, plate 22, figure 2. and the dolo- 

 mitization of limestones adjacent to worm l)orings, plates 26 aud 2^. 

 These facts seem to show tliat the structures Avhidi are most permeable 

 under marine conditions are most susce])tibJe to dolomitization. True, 

 tJiese structures are also permeahle under emergent conditions in t1ie sea 

 of underground Avater. If underground waters, however, had been the 

 dolomitizing agent, it would seem that the walls of fissures would be 

 affected even more tlian worm IjoJ'iugs. Tn the Platteville limestone at . 

 Escanaba and the mottled limestones of Winnipeg, however, it is the 

 pervious marine structures in Avlu)se proximity dolomitization has taken 

 place and not the joints. 



DOLOMITIZATION liY REGRYSTALLIZATIOy OF Mf/0-BEARING ARAGONITE 

 AND CALCITE SKELETONS 



No facts A\'ere gathered from the Paleozoic of Wisconsin which would 

 prove that dolomite was formed l:)y recrystallization of ^IgO-bearing 

 skeletons. Tlie suspicion tliat dolomite may develop in this way is 

 founded on the fact that the skeletons of many modern marine organisms, 

 Ijoth calcific and aragonitic, contain up to 14 per cent MgCOg without 

 showing dolomite. Normal dolomite could either develop from them by 

 the solutiou of tlie excess CaCOg or by addition of MgCO.. The latter 

 would invohe an improbable vohmie increase. No case seems to be 

 known where it is proA^en that dolomite resulted from the recrystalliza- 

 tion of such matei-ials. Blackwelder^' has shown that the very pure, com- 

 pact Bighorn dolomite of Wyoming lias the structure of algal secretions. 

 The fact that some modern alga- contain considerable MgCOo at least 

 suggests that the Bighorn dolomite may haNC resulted from the recrys- 

 tallization of MgO-bearing algal secretions. Unless they Avere unlike 

 modern algal secretions and had the MgO content of a normal dolomite, 

 the excess CaCOg must have been leached out. Since algal secretions are, 

 as a rule, strong and compact, the remo\al of C^aCO., from them Avould 

 have given, rise to considerable ])oT-e sj)ace. Of this the rock shows no 

 evidence. 



1'^ Eliot Blackwelder : Oi-)§in of Bighorn dolomite. Bull. Geol. Soc. Am., vol. 24. pp. 

 607-624, 



