f.O 
Records of the Geological Survey of India. 
[vol. xx. 
In the southern extra-tropical latitudes, where coralline life does not exist on that large scale, 
the volcanic action is the only marked one, and equally so in tropical latitudes to the north of 
the equator,—where that action is locally wanting,—the peculiar formation of coral reefs must 
be considered as the principal argument for the continuation of this line of elevation. This 
is the case at the Nicobar islands. 
These islands occupy a gap without volcanoes, between the volcanic range of Sumatra, 
and the Barren and Narcondam islands, which lie to the east of the Andamans. 
Whatever may be hidden in the interior of the Nicobar islands, covered with perfectly 
impenetrable primeval forests and grassy plains, the occurrence of younger volcanic rocks is 
Ibe least probable. Although I have found on the north side of Car-Nicobar, the northern¬ 
most of the islands, two pieces of a porous basaltic rock, the size of a man's hand, in a 
coarse gravel in the forest near the village Mus, and a, larger angular fragment in the coral 
sand on the strand near the village Saui, still there is more reason to believe that these 
fragments were transported to the coasl of Car-Nieohar in the roots of stranded trees,* 
or even that they were remains from the travelling hags of the Danish naturalists of the 
Corvette Go la that ,—who in 1846, shortly before they landed on Car-Nicobar, visited the 
volcanic Barren island,—than that they came from the interior of the island. I have in 
vain searched for similar pieces in the stream-and river-gravels of Car-Nicobar, and I 
have not met with thorn on any of the other islands on which we landed. 
On the other hand, the Nicobar islands are distinctly characterized as a portion of the 
chain of oceanic elevations, which began in former geological periods and still continues, by 
the upheaved coral banks and by the continuous formation of coral reefs, which slowly, but 
in the course of hundreds and thousands of years perceptibly, enlarge the territory of the 
islands. 
The Austral-Asiatic area or elevation, above indicated in its entire extent, has in the 
Nicobars a mean direction to norfcli-'20°-west or from south-south-east to north-north-west, 
possessing a length of 148 Engl. (=37 Ger. geogr.) miles, and a width of 16 Engl. (=4 geogr.) 
miles. This direction indicates at the same time the strike of the strata on all the islands, 
while the dip is either towards cast or west. The synclinals and anticlinals in. the geological 
structure of the islands are thus coincident will) the direction of the great geological line of 
elevation which connects the northern point of Sumatra with (lie group of the Andaman 
islands. 
The total area of all the islands is calculated to he 33 to 34 German (geogr.) square 
miles (equal to about 628—644 Engl.). 
1 .—Geological Formations. 
To render properly intelligible the results which will he given in the following pages, I 
may he permitted to make a few preliminary remarks. 
It is at present extremely difficult to make any detailed geological observations on 
the Nicobar islands. One is limited to the sea coast, as impenetrable forests and grassy 
plains make the interior of llio islands perfectly inaccessible and hide the rocks. On 
the northern smaller island, this circumstance is of less importance, because the extent of 
the rocks through the whole island can easily be ascertained, as soon as it is possible to 
observe them on two opposite sides of the coast in, the same stratigraphicnl relation. The 
case is different with the southern larger islands. Samhelong or Great Nicobar has an 
area of 17i geographical square miles, and is larger than all the other islands put together; 
it offers hi the mountain ranges (rising up to 2,000 feet), and deep valleys, such a variety 
in the configuration of the ground, that it is impossible to suppose that what is to be 
seen on one or the other point at the coast should he characteristic for the whole island. 
The, mouths of rivers being generally occupied by mangrove swamps, it is even impossible 
to conic to any conclusions from gravels as to the rook which is to be found in the interior. 
But even on the coast there are great obstacles to geological investigation. Wherever the 
inquiring eye of the geologist observes promising cliffs, there breakers make it generally 
impossible to land, and where landing can be effected, we usually meet only a flat coast. 
* Chamisso mentions the transport of stones in the roots of stranded trees on the ltadek group, and Darwin gives 
a similar example from the Killing islands (Darwin’s natural history travels, part 11, p- 2414)* 
