"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. "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.