SKEATS : CHEMICAL COMPOSITION OF LIMESTONES. 87 
The massif at the southern half of the island is volcanic, and the highest 
ridge reaches 1000 feet. 
Chemical. — Four rocks were analyzed, and were found to contain very 
little magnesium carbonate, the amounts varying from 1.6 to 3.4 per cent. 
Calcium Magnesium Insoluble 
Height. Carbonate. Carbonate. Residue. 
Summit of Mount Makama 97.2 2.8 
South side of Apra 96.8 3.4 
400’ 96.5 1.6 1.94 
20’ 97.0 3.0 
Microscopical. Summit of Mount Makama.— The matrix of the rock 
consists of “mud,” part of which has been converted into calcite. Many 
small ellipsoidal fragments occur in the rock, and their shape and size 
suggest that they may have passed through the bodies of fish. 
2. InpDIAN OCEAN. 
Christmas Island. 
Dr. Andrews has described the structure of the island in detail in his 
monograph published in 1900 by the Trustees of the Natural History 
Museum. It will, therefore, not be necessary for me to do more than 
briefly summarize the chief points in his description. The island lies in 
the eastern part of the Indian Ocean, in latitude 10° 25’ S., longitude 
105° 42! E., 190 miles south of Java, and 550 miles northeast of Keeling 
Cocos atoll. The greatest length of the island is 12 miles, the greatest 
breadth 9 miles, and the total area about 43 square miles. The accom- 
panying map is drawn on a reduced scale from the one published in the 
monograph. Upon it are marked the localities and lines from which Dr. 
Andrews obtained the limestones which form part of the subject-matter 
of this inquiry. The specimens were collected in vertical sequence from 
the cliffs and slopes, and their heights were carefully noted either by 
measurement or by the use of an aneroid barometer. 
The basis of the island is almost certainly a volcanic peak, upon whose 
summit and flanks accumulations of tertiary limestones have been depos- 
ited. Eruptions, probably submarine, took place from time to time, and 
the products of eruption were interstratified with the limestones. The 
earlier eruptive rocks were first trachyte, then basalt, while the latest 
consisted of beds of palagonite tuff, upon which the great mass of the 
Miocene (Orbitoidal) limestone rests. . 
The greater part of the exposed surface of the island consists of a pla- 
teau at a general level of about 600-700 feet. Higher ground is found to 
