The Geology of Arran 257 



has afforded clear evidence of a depression of 460 feet or, 

 so far as it was carried. It is true signs of slight elevation 

 are shown in the Bay of Bengal. There are none of great 

 depression on the Arakan coast, but about midway, between 

 the mouths of the Mahanadi and the Koladyni rivers, is 

 the ' Swatch of no Ground,' a channel 1800 feet deep in the 

 sea-bed, which on either side is only about 100 feet below 

 the surface of the water. This could not be due to submarine 

 erosion, and, if a result of subaerial, it must have been 

 formed when the land was at least 2000 feet above its 

 present level, which accords with the distribution of the 

 garial. 



Sir A. Geikie mentioned an interesting discovery made 

 by the Geological Survey in the Isle of Arran. Here the 

 granite and other eruptive rocks have been for some time 

 past considered to be referable to the great period of Tertiary 

 volcanic activity, when so much igneous material was 

 extruded in the north-western portion of the British Isles, 

 but no actual proof of their age had been obtained before 

 last summer, when a tract, about two miles in diameter, 

 was found on the west side of the southern half of Arran, 

 which obviously marks the site of a volcanic crater, for it 

 is occupied by coarse agglomerates, with abundant bosses 

 and dykes of various intrusive rocks. In these agglomerates 

 are blocks of sedimentary strata, containing fossils, which 

 show some of them to be Rhaetic, others Lower Lias, 

 together with large masses of white limestone, indistinguish- 

 able from the hard chalk of Antrim and full of Cretaceous 

 foraminifera and sponges. This shows that the southern 

 part of Arran, when the eruptions occurred, must have been 

 covered with sedimentary rocks resembling those preserved 

 under the basalt of Antrim. These probably extended into 

 the south of Scotland, from which they have since been 

 removed by denudation. This justifies the inference that 

 the main topographical features of that region have been, to 

 at least a great extent, carved out since the time of the 

 chalk. 



p.c. 



