"The Nicobar Islands belong to an area of elevation which can be traced from the Bay of Bengal far into the southern seas,[111] and is characterised by two phenomena: first, the activity of the interior of the earth, showing itself in volcanic action; and secondly, the activity of the coralline animals, disclosing itself in the formation of that kind of coral reefs known as fringing or coast reefs. The islands occupy a gap without volcanoes between the volcanic ranges of Sumatra, and Barren and Narkondam Islands, and the occurrence of young volcanic rock in them is improbable. They are distinctly characterised as a portion of the chain of oceanic elevation which began in former geological periods and still continues, by the upheaved coral banks, and by the continuous formation of coral reefs. The synclinals and anticlinals in the geological structure of the islands are coincident with the direction of the great geological line of elevation which connects the northern part of Sumatra with the Andamans.
"Among the geological formations of the Nicobars, three are the most important:--(1) An eruptive serpentine, with gabbro formation. (2) Marine deposits, probably of a younger Tertiary age, consisting of sandstone, slates, clay marls, and plastic clay. (3) Recent coral-reef formations.
"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.