118 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
stance of the coral, and crystals were deposited on the walls and on the 
surface of the “ mud floors.” Finally, many of the remaining spaces in 
the coral were filled with clear, slightly yellow crystals of calcite. On 
treating the section with Lemberg’s solution, the secondary calcite is 
stained pink, while the mud in the cavities does not stain. Apparently 
during dolomitization, the calcite mud was quietly replaced by dolomite 
without any sign of recrystallization having taken place in the rock. 
Two points of interest which arise in some dolomitic limestones re- 
main to be recorded. It is well established that many dolomitized 
limestones consist entirely of rhombohedra of dolomite with dark, muddy- 
looking centers. As analyses of such rocks rarely show more than 
40% of magnesium carbonate, it has been surmised that the muddy 
material in the centers of the crystals consisted of calcite. The author 
has demonstrated this to be the case in some sections of these rocks 
by treatment with Lemberg’s solution when the muddy centers of the 
crystals become clearly stained by the reagent. 
Finally it has been shown by the use of Lemberg’s reagent on some 
of the rocks from Mango, that many quite colorless rhombohedra which 
appeared to be dolomite really consisted of alternate zones of calcite 
and dolomite apparently in optical continuity with each other. The 
author is not aware that this feature has been described before, and it 
appears to throw some light on a possible mode of formation of dolomite. 
This zoning may be explained in two ways. 
It is possible that each crystal consisted originally entirely of calcite, 
and that alternate zones formed during the growth of the crystal pos- 
sessed slightly different physical structures. When the rock was sub- 
jected to the conditions producing dolomitization, certain zones were 
converted into dolomite, while the rest of the crystal remained as calcite. 
The alternative to this view is that the crystals originated in much 
the same way as artificially zoned crystals of alum are made. The crys- 
tal may have been originally dolomite or calcite, but from time to time 
the character of the solution in which the crystal formed changed in 
such a way as to determine the deposition alternately of dolomite and 
calcite directly from solution. The author believes this view to be more 
probable, and it seems to receive support both from the examination of 
the dolomite crystals found lining cavities in sections of some of the 
dolomitized limestones, and also from a macroscopic study of hand speci- 
mens which occasionally contain large cavities lined by concentric layers 
of incrusting dolomite crystals. 
