The Andamans are of Tertiary formation, and of similar geological structure to the Arakan Yoma Range, the line of elevation connecting them with which is represented by the Alguada Reef and Preparis Island. (The Cocos are an integral part of the Andamans.)
Two sedimentary formations have been distinguished up to the present, the one older, the other newer than the serpentine which intrudes in various localities. They have been called the Port Blair and Archipelago series respectively.[89]
The former, occurring in the south, consists of fine grey sandstone—the characteristic rock, generally non-calcareous—and beds of conglomerate and limestone as subsidiary members; red and green jasper also occur; possibly, however, of an older series than the sandstone in which they crop out. This series seems to be of early Tertiary or later age.
The second series,—Miocene, or even newer—of which the whole of the islands of the Archipelago are formed, consist typically of soft limestone, of coral and shell-sand, soft calcareous sandstones, and soft white clay, with occasionally a band of conglomerate, the pebbles of which seem to have been coral.
The intrusive rocks of the Andamans—similar to those of Manipur and Burma in the north, and the Nicobars to the south, and of later date than the Port Blair series—are of serpentine, often passing into crystalline diorite and gabbro.
The Archipelago series seem to cover a large area of the Andamans, while the Port Blair formation is restricted to the south. The greater part of Rutland Island is formed of serpentine, in which small layers of brown opal have been met with, and which throughout the group seems disseminated with minute crystals of chromium. The Cinques consist of intrusive rocks of serpentine, associated with metamorphosed, indurated, and sedimentary series, mostly calcareous. The rocks of Little Andaman are chiefly lime and sandstone, with a good deal of actual coral rock on the east and south coasts; while occasionally occur outcrops of an igneous nature. At Entry Island and Port Meadows, beds of volcanic origin exist.
In view of the connection with them, the apparent barrenness of the Arakan Hills goes to show that little may be expected from the Andamans in the way of mineral products. Amongst others, however, discoveries have been made of lignite, and the ores of chromium, copper, iron, and sulphur,[90] although not in quantities that would pay commercial development.
It is indubitable, as Kurz has shown,[91] that the Andamans are now undergoing subsidence; but there is ample evidence, in the raised coral beaches that fringe the shores, to show that in the immediate past this has been exceeded by elevation.
The islands have been subjected to earthquakes from time to time. The first recorded took place in August 1868, the next in February 1880, and several slight shocks occurred until December 1881, when a severe earthquake visited the group, made itself felt over a large area of the Bengal Sea and surrounding countries, did much damage to masonry at Port Blair, and raised waves 3 feet high, following each other at fifteen-minute intervals for a period of twenty-one hours. Another slight shock was experienced in February 1882.