70 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
tively. At a height of 280 feet a rock is found in which magnesium 
carbonate is represented to the extent of 27.3 per cent. An interesting 
point is that most of the analyses yield from 38 to 42 per cent of magne- 
sium carbonate, but none of them approach nearer to the composition of 
a true dolomite. Calcium phosphate is found in most of the rocks, but 
usually only in traces, as .29 per cent was the maximum amount obtained. 
Insoluble residue is present in exceedingly small quantities, except 
where the limestones are associated with volcanic rocks. This associa- 
tion will explain the unusually high value of 4.03 per cent obtained from 
the limestone at 420 feet. The adjoined tabular statement gives the 
details of the analyses, and the accompanying diagram represents the 
proportion of calcium to magnesium carbonate, expressed graphically in 
the case of three rocks which occur in vertical succession in one cliff face. 
The shaded area represents the percentage of magnesium carbonate, the 
unshaded area representing calcium carbonate. The dotted line is drawn 
at a point representing 46 per cent of magnesium carbonate, the propor- 
tion represented in a true dolomite. The curve is obtained by joining 
all points at which specimens were collected in vertical succession from 
a cliff face. The remaining specimens were collected at the stated heights 
from isolated spots in various parts of the island, but are interesting as 
showing the occurrence of dolomitic and non-dolomitic limestones at 
different heights in the island. 
These diagrams, therefore, do not represent completely the chemical 
composition of all the rocks between the highest and lowest specimens, 
but are used as a convenient means of presenting at a glance the detailed 
results of the analyses at stated heights, 
Microscopical. 510!.— A thoroughly dolomitized rock in which the 
organisms are nearly obliterated. Some of the cavities are lined by a 
deposit resembling agate. It consists of 6 layers of sharply defined crys- 
tals alternately dolomite and calcite, as shown by staining; the central 
layer filling the remainder of each cavity consists of broad platy crystals 
of calcite. Some of the dolomite rhombohedra have dark opaque irregu- 
lar centers, which by staining with Lemberg’s solution are proved to 
consist of calcite. 
400'.— A fine-grained dolomitic limestone, in which organisms such 
as Lithothamnion and Carpenteria are found, only slightly altered, while 
others, such as Amphistegina (?) are quite removed, and their former 
presence can only be suggested by the similarity of the shape of the 
cavities to the external boundary of the organisms. Many easts of coral 
occur, in which the septa are represented by a gray “silt” lined with 
