GEOLOGICAL FORMATIONS 203 



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: — (i) 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, how- 

 ever, 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 



