114 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
per cent of the skeleton of the organism. When the amount of mag- 
nesium carbonate in a limestone rises above about 15 per cent, crystals 
of dolomite begin to make their appearance, at first in the matrix. As 
the amount of magnesium carbonate increases, the crystals become more 
plentiful, until a point is reached at which most of the matrix has 
been changed from calcite to dolomite. Even at this stage it often 
happens that no alteration has taken place in the structure of the 
organisms. The next change noticed is seen in Figure 7, a dolomitic 
limestone from Phosphate Hill, Christmas Island. 
The matrix of the rock is almost entirely dolomitic, as are the clear 
crystals filling a cavity in the rock; while many of the organisms are 
incrusted with scalenohedral crystals. Crystallization has started to 
invade the organisms, and rhombs of dolomite are penetrating Litho- 
thamnion from without inwards. It seems possible that the organism 
has already quietly absorbed magnesium carbonate without loss of 
structure, and that the present change is one of recrystallization. Cer- 
tainly in some cases Lithothamnion which under the low power appeared 
to be almost unaltered, was found on examination with the high power 
of the microscope to have its cells filled with minute rhombohedra of 
dolomite. In the figure it is seen that dolomite crystals have filled 
and disintegrated the walls of the tubules of a Halimeda, that many of 
the dolomite crystals have regular zones of dirt, but that an echinid spine 
and an Amphistegina as yet show no sign of dolomitization. The order 
of disappearance of the organisms under dolomitization is not an in- 
variable one, but it is usually noticed that Halimeda, if not already 
disintegrated, is one of the first organisms to disappear, while Forami- 
nifera such as Orbitolites and Carpenteria and the alcyonarian spicules 
often resist dolomitization much longer. Lithothamnion is somewhat 
capricious. J. Walther’ quotes a deposit in the Bay of Naples, in 
which Lithothamnion has disintegrated and formed a structureless lime- 
stone. He thinks the necessary condition for this to occur is that the 
organism should be subjected to percolating water. The decomposition 
of the animal matter within the organism gives rise to carbon dioxide, 
which, dissolved in the water, attacks the skeleton of the organism 
and dissolves it partly away. There is no doubt that in these coral 
limestones sometimes Lithothamnion does lose its structure compara- 
tively easily, but, as a rule, the conditions have been favorable to its 
preservation, and it is found to be one of the most persistent forms. 
Other forms which seem to be at least equally persistent are such 
1 Zeitsch. deutsch. Geol. Ges., 1885, Vol. XXXVIL., p. 329. 
