Concerning the geology of the islands, it is curious that in the valley of the Irrawadi hot springs and other evidence of volcanic action occur in the same relative position to the Arakan Hills as the two islands of Narkondam and Barren Island occupy in respect to the Andamans. There seems little doubt that these two islands--now respectively extinct and quiescent--belong to the great line of volcanic disturbance that appears in Lower Burma and extends right through Sumatra and Java and the further islands of the Malay Archipelago. Thus it would seem that the Andamans proper, possessed of no volcanoes themselves, lie just outside the line of activity, and, with the Nicobars, occupy the same position with regard to the volcanic track as do the chain of islands west of Sumatra. Possibly the land now constituting the Andamans first appeared above the sea as an extension of Cape Negrais in the latter part of the Tertiary epoch, at which time occurred the elevation of the Arakan Yoma Hills, and later became isolated by subsidence due to neighbouring volcanic action. As Mr A. R. Wallace points out,[88] the presence of active volcanoes produces subsidence of the surrounding area by the weight of every fresh deposit of materials ejected either in the sea or on land. "The subsidence such materials produce around them will, in time, make a sea if one does not already exist." 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.