MAXNKi: OI' MAKIXI^: DOLONT iTIZATTON 448 



Staiiis show that lu beds coiitaiiiiug both primary calcite and dolomite 

 the dolomite grains are always considerably larger than the grains of 

 calcite. They average al)oiit A or .5 mm. and ai'e several hundred times 

 the size of the calcite grahis. Tlie calcite casts are single nnits^ as a rnle. 

 Dolomite grains honnded by grains of their own kind are anhedral, but 

 v\dierever they touch calcite^ even where they pierce the casts of shells, 

 they liaTe a strong rhombohedral habit. All the calcite grains areanhe- 

 dral. The boundary Ijetween calcite and dolomite is always sharp, and 

 the grains of calcite near the dolomite rhombs are no different in appear- 

 ance than those farther away. The dolomite grains were never seen to 

 include anhedral grains of prijnary calcite, and in fact only one inclusion 

 of calcite in dolomite was observed. In this unique case a rhomb of 

 calcite was inclosed by a dolomite rliomlj. 



The relations of dolomite grains to primary calcite can not all be ex- 

 plained by assuming that tJie dolomite crystallized out in the ooze where 

 it had room to develop its individuality, for it shows the same crystal 

 form where it invades com[)act calcific casts. 1'he fact that it is anhedral 

 and intei'lockiug Avhere it is in contact witii its own kind shows that the 

 roi-k had considerable rigidity when crystallization occurred. Otherwise 

 the cr\'stals could not ha\ c crowded each other out of shape. 



That replacement of calcite by dolomite has occurred is evident from 

 the relations of the dolomite both to the matrix and to calcite fossils. 

 There is no evidence to Ah)\v tluit the dolomite grains got either MgO or 

 FeO from calcite. TJie calcite contains no FeO. The growth of the 

 dolomite grains can not be compared to certain secondary garnets in 

 schists. Such garnets seem to have derived their nutriment from the 

 schists by process of assimilation. Often undigested particles of schist 

 materials appear within the garnet. The dolomite grains show no unas- 

 similated calcite residuals. It looks as it the dolomite grains had started 

 to form from a center by substituting for the calcite an exactly equal 

 volume of dolomite. The l)()undaries of calcite and dolomite show that 

 the volume of the dolouiite is neither greater nor less than that of the 

 (;alcite wiiich it replaced. A part ot the original calcite, about 71.2 per 

 cent, may have beeu, retained in the dolomite, but 28.8 per cent, calculated 

 on the basis of constant ^•olume, must have gone back into solution. If 

 all the calcite had remained, a volume increase of 181 per cent would 

 have taken place when enough MgO and FeO was added to it to make 

 dolomite. The idea, sometimes stated, that dolomitization takes place by 

 the substitution of one molecule of OaCO., by one of MgCOo has not been 

 checked so far ])y observations. Such a chajige would involve a decrease 



