SKEATS: CHEMICAL COMPOSITION OF LIMESTONES. U7 
the rock. These have been subsequently filled with secondary calcite, 
forming broad slightly yellow crystals showing the characteristic cleav-— 
age, and ramifying into the cracks and cavities in the limestone. In this 
way the bulk analysis of a dolomitic limestone may be profoundly modi- 
fied so that a dolomitic rock in which all organisms have been destroyed 
may, as a result of the infiltration of calcite, have its percentage of mag- 
nesium carbonate lowered from over forty to twenty or even under. 
Manco, 320 feet. (After staining with Lemberg’s reagent.) A longitudinal 
section of a dolomitized coral. ‘“ Mud-floors” are seen partly filling cavities. Sub- 
sequently an incrusting layer of dolomite crystals was deposited, and finally many 
remaining cavities have been filled with secondary calcite which is now stained 
red.  X 30. 
Figure 10, from Mango, at a height of 320 feet, shows how this change 
affects the appearance of a dolomitized coral. Some time before dolomiti- 
zation started many of the cavities in the coral were filled up to a par- 
ticular level by calcite mud slowly settling down from muddy water. 
The horizontality and general parallelism of these ‘‘ mud floors ” 
stitutes a marked feature in some longitudinal sections of reef-forming 
corals. It will be noticed that the “mud” extends quite up to the coral 
wall, showing that it reached its present position before the deposition of 
dolomite crystals began. Dolomitization subsequently invaded the sub- 
con- 
