SKEATS : CHEMICAL COMPOSITION OF LIMESTONES. 73 
which occurs in some of the upper limestones of Christmas Island, and 
to which a Miocene age has been assigned. | 
Polytrema and Lithothamnion also occur in this rock, which has a 
matrix consisting largely of ‘“ mud,” while some of the cavities are lined 
with scalenohedral crystals of calcite. 
300’. A fragmental rock, which, by the thorough dolomitization of 
matrix and organisms alike, has become clear and crystalline. Litho- 
thamnion partly resists change, and one simple coral is represented by a 
cast in “mud.” No other forms are recognizable ; even the “ dirt lines” 
round the boundaries of formerly existing organisms have been recrystal- 
lized. Cavities and cracks in the rock are more or less filled with large 
plates of secondary calcite. 
298’. This limestone was probably a beach deposit. It consists of 
rounded fragments of limestone and dolomite, with other fragments of 
volcanic origin, probably basalt. These rounded grains are seen to touch 
one another in places, and are invested with a concentric coating of 
fibrous calcite, while the remaining space is filled with a mosaic of calcite 
crystals. 
290'. Figure IX. A thoroughly dolomitic cavernous rock, with 
Lithothamnion and meandrine structures, probably “ ghosts” of corals. 
Secondary calcite fills many of the cavities. 
280’. A dolomitic limestone, largely composed of Lithothamnion and 
Carpenteria. The former is usually unaltered, but the latter has recrys- 
tallized in large crystals of clear calcite, and would be difficult to recog- 
nize but for the numerous tubules running into the calcite. These 
tubules have been filled with a “mud,” which in places can be seen to 
have altered to little rhombs of dolomite. 
250’. A dolomitic limestone in which meandrine forms, possibly 
corals, are the ouly remaining organisms, while many cavities are filled 
with secondary calcite. 
195’. A comparatively fresh limestone with a fine-grained cement of 
calcite. Carpenteria and Polytrema are the most abundant organisms, 
but Lithothamnion and echinid spines are also noticed. 
105’. A rock in which all the matrix and most of the organisms 
have been dolomitized. Some specimens of Lithothamnion, however, 
show that the invasion of dolomite has been only partially effected, as 
the central parts are still calcite. Meandrine fragments of other organ- 
isms also remain as calcite, and a secondary deposit of this mineral fills 
some of the larger cavities in the rock. Many of the dolomite crystals, 
especially those formed round the meandrine organisms, have centers of 
