86 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
Microscopical. 200'. Third Terrace. —¥igure 4. The slide shows 
a. transverse section of a compound coral whose stereoplasm has been 
changed to granular crystals of calcite and whose inner and outer walls 
are now marked by dirt lines. Some of these calcite crystals are clear, 
but many have an inner kernel of opaque material often roughly parallel 
to the external boundary of the crystal. This apparent zoning is quite 
an uncommon feature of calcite in rock sections. Cavities between the 
septa of the coral and between adjacent organisms have been filled with 
“mud,” which has subsequently crystallized in the form of a muddy 
mosaic of calcite. 
150'. Basin. — The rock is largely made up of unaltered Polytrema 
and other fragments, which have been changed to clear calcite. The ma- 
trix of the rock consists of ‘ mud,” in which cavities occur lined with 
scalenohedral crystals of calcite (Figure 5). In one or two of these 
cavities the scalenohedra are seen to be zoned. 
Niav. — Niau or Greig Island lies in latitude 16° 10/8., longitude 
146° 20/ W., and is the only atoll in the Paumotus, visited by Mr. Agas- 
siz, in which the lagoon is entirely shut off from the sea. The ledge of 
older limestone forms a rim surrounding the circular lagoon, and is } mile 
wide and 15-20 feet high. 
Chemical. — Two specimens were analyzed, both from a height of 20 
feet, and were found to be limestones containing 5.0 and 2.7 per cent 
respectively of magnesium carbonate. 
Calcium Magnesium 
Height. Carbonate. Carbonate. 
20’ 95.0 5.0 
20’ 97.3 2.7 
Microscopical. 20'.— The specimen was collected halfway across the 
rim of the island, and consists of a section of a large coral with smaller 
coral fragments. The coral is very fresh, being brown in color, and in 
places show the centers of calcification very clearly. Some spaces in the 
coral are filled with ‘‘mud” to a certain level, forming horizontal “ mud 
floors,” and subsequently scalenohedral crystals of calcite were deposited 
from solution upon the surfaces of the “‘mud floors” and also lining the 
coral walls. 
E. The Ladrones. 
Guam. — This island is situated in latitude 13° 30/S., longitude 145° 
E. It is mainly volcanic in origin, but the northern half consists of ele- 
vated coralliferous limestone with vertical cliffs 100-300 feet in height. 
1 American Journal of Science, 1900, pp. 369-374. 
