"The serpentine and gabbro formation is characteristically of an eruptive nature. The Tertiary sandstones, slates, and clay marls appear forcibly broken through; their strata is partly inclined, partly bent in flat, parallel, wave-like undulations. These rocks are accompanied by coarser and finer breccias, composed of angular fragments of these same rocks, and they can partly be regarded as friction breccias, partly as sedimentary tufas, in which beds of an argillaceous marl are interstratified. The eruption of these plutonic masses appears therefore to fall in a time when the formation of the marine deposits was partially completed, partially still in progress. They broke through on lines of fracture, of which the principal strike from S.S.E. to N.N.W. agrees with the longitudinal extension of the islands. On the middle islands the serpentine and gabbro attain their greatest development: on Tilanchong, Teressa, Bompoka, Kamorta, and Nankauri, they form bare hill ranges of 200-500 feet, and their configuration often marvellously resembles that of younger volcanic formations. The elevatory power has, however, acted most strongly on the southern islands, and has here upheaved sandstones and slates to heights of 1500-2000 feet above sea-level; on the northern islands the same power was, on the contrary, weakest.
"The clay marls of the northern and central islands (Kar Nicobar, Teressa, Bompoka, Kamorta, Trinkat, and Nankauri) and the sandstones and slates of the southern (Kachal, Little and Great Nicobar) appear to be only petrologically different products of one and the same period of deposition. There are, at the same time, very few materials from which the age of the marine formation could be determined, as the only fossil remains which have been found in their strata are fragments of driftwood changed to brown coal, plants resembling Fucoids, Foraminifera, and Polycistinæ. All these indicate more or less distinctly a young Tertiary age.
"We find a repetition of the geological condition of the Nicobars on the southern coast of Java and the south-west coast of Sumatra.
"The third principal formation of the Nicobars are coral formations belonging to the most recent or the present period. Coral banks of great thickness are found on Kar Nicobar, Bompoka, Trinkat, and other islands. They consist partly of compact coral limestone, partly of a coral and shell conglomerate upheaved 30 or 40 feet above the present level of the sea. On all the islands the original area is to be observed enlarged by coral land which is only separated by the higher sand-dunes along the shores from the still continuing formation of the coral reefs surrounding all the islands in the character of fringing reefs. Although these raised coral banks are decided evidence of the long-continued upheaval of the islands—that, in connection with the eruption of the serpentines and gabbros—the formation of the flat coral lands elevated a few feet only above the sea, can, on the other hand, be explained by the accumulation of coral fragments, of sand and shells, by the waves and breakers on the shallow surface of the fringing reefs."[112]
Coal of a brown variety has been found in Little Nicobar, Treis, Milo, and Kondul, but everywhere in isolated masses and single fragments, showing traces of rolling, met with here and there without order, in sandstone and slate, and evidently derived from driftwood.
The only traces of minerals discovered have been ores of copper and iron pyrites, finely disseminated through dioritic and serpentine rocks. The possibility of the occurrence of copper ores in the eruptive formation cannot be denied, but no discovery has yet been made which would indicate it. On the other hand, the islands are rich in useful building materials. The sandstone of the southern islands must give excellent working stones; the plastic clays of the north could, doubtless, be worked into bricks or pottery; the natives of Chaura largely employ it in their earthenware manufactures.
Although the islands are generally beyond the sphere of cyclonic disturbances, they have more than once experienced the effect of earthquakes. One of the most remarkable of these is said to have occurred from October 31 to December 5, 1847, when fire is reported to have been seen on one of the mountains of Great Nicobar. Part of the northern coast of the latter, especially in the vicinity of Ganges Harbour, sank beneath the sea, and for long the locality was deserted by the aborigines.[113]
On December 1881, an earthquake, felt also at the Andamans and throughout the Bengal Sea generally, caused extensive damage in Kar Nicobar to the coconut groves and huts of the natives. Vents were opened in the sandy soil; inland, trees were overthrown; sea-waves broke on the island, and at the village of Mūs, water rose into the houses of the Burmese traders, which stood on platforms 2½ feet high.
There was another earthquake at Kar Nicobar in November 1899, when strong, but not alarming, shocks, lasting ten minutes, were experienced. The last occurred on September 18, 1900, when two heavy and severe shocks, each lasting five minutes, were felt throughout the island, but caused no damage.
The climate of the Nicobars is more uniform than that of the Andamans, for it is less diversified by wet and dry seasons, heat and cold, and in this respect resembles that of the Malay Peninsula at the same latitude. The prevalence of malaria renders the group unhealthy alike for foreigners and, in certain localities, for the natives, and all the attempts at settlement have resulted in great mortality from this cause, although where jungle exists an improvement is said to have taken place when the land has been cleared.