72 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
clear dolomite. The matrix consists entirely of silt, having a semi- 
transparent appearance, due to its consisting largely of minute rhom- 
bohedra of dolomite. The obliteration or solution of some of the corals 
and Foraminifera renders the rock somewhat cavernous. 
70’. Figure IV. <A “coral sand” or limestone, made up of small 
fragments of Carpenteria, Gypsina, Lithothamnion, and echinid spines. 
Some organic fragments are entirely recrystallized in clear calcite, the 
former boundary of the organism being marked by a “dirt line,” while 
the stereoplasm of the corals has been replaced by calcite, and the spaces 
between the septa filled with ‘‘mud.” The matrix consists partly of mud 
and partly of its alteration product calcite. 
350!. Interest in this slide centers in the fact that the dolomite 
rhombohedra consist of three parts, a central rhomb of dolomite, a 
middle layer of calcite, and an outer one of dolomite. All three layers 
have quite sharp rhombohedral angles and all are in optical continuity. 
335'!. The whole of the matrix is dolomitized, as are all the organ- 
isms except the central parts of the Lithothamnion. Clear crystals 
encrusting the fragments of some of the organisms have an outer layer 
of slightly higher refractive index. This outer layer does not stain, 
while the central crystal which is slightly rounded, after treatment 
with Lemberg’s solution for half an hour is colored a light pink. 
This, in conjunction with the evidence of refractive index, seems to 
suggest that the central layer may be calcite, containing some magne- 
sium, while the peripheral part consists of a purer dolomite. 
320'. Figure X. Reference to the figure shows that this slide con- 
sists of a longitudinal section of a coral, in which the organism has been 
converted into dolomite without the complete destruction of its structure. 
This is shown by the fact that the original borings made in the coral by 
algee, and subsequently filled with “mud,” still remain in the same posi- 
tion in the coral, with the mud apparently unaltered. Dirt lines mark 
the original extent of the walls of the septa, and these are now incrusted 
with clear dolomite crystals. Many cavities are filled with “mud” up 
to a certain level, and the surfaces of these “dirt floors” are roughly 
parallel to one another. Other cavities have been filled with large, 
slightly yellow plates of calcite. Lithothamnion occurs apparently unal- 
tered, but under the high power its tubules are seen to be filled with 
small dolomite crystals. 
310’, This limestone is interesting on account of the occurrence in it 
of many examples of the foraminifer Orbitoides. Mr. Frederick Chapman 
has kindly identified it for me as the form Orbitoides sumatrensis, a form 
