348 Geology of 



Tocks ; but the whole is so bleached or altered in colour by gases and 

 rapour, that the different constituents are unrecognisable, so that their 

 original texture can only be guessed at. The whole is intersected by 

 a perfect network of dykes, all of a trachytic nature, of which the 

 greatest number point to two centres, both a little to the south-west of 

 Quail Island, although a few also come from the west. Before reaching 

 the north-eastern point the whole group disappears below a series of 

 basic lava-streams, which have all issued from a shallow crater now form- 

 ing the centre of the island The rock by which this small volcanic system, 

 and a few others of still smaller size situated in and near Charteris 

 and Ehodes' Bays, have been built up, consists of a fine-grained basalt 

 containing grains of magnetic iron ore, and grains and concretions of 

 olivine. It has either a columnar or tabular structure, or breaks in large 

 prismatic blocks. These basaltic lava-streams have been well exposed by 

 denudation on the north-eastern face of Quail Island. The vertical cliff, 

 about 200 feet high, consists here of four lava streams, of which the 

 uppermost and most important one is about 45 feet thick. This 

 lava- stream has the peculiar feature of possessing only irregular 

 prismatic jointings, where it issues from the vent with a fall of about 

 15 degrees. For the rest of its course, where it has only a slight 

 inclination towards the horizon, it is divided into two clearly defined 

 parts, of which the lower one lying directly upon a bed of rounded 

 conglomerate is divided into a series of regular vertical columns haying 

 four to six faces, whilst the upper one has only the irregular prismatic 

 joints, which the whole stream possesses close to the crater's mouth. 

 The lower lava-streams are also jointed in the same manner, the whole 

 series being separated by beds of conglomerate and ashes. The 

 conglomerate consists mostly of waterworn blocks of rock, sometimes 

 of large size. All the rocks of which Banks' Peninsula is built up 

 have contributed, but trachytic and domitic and bleached or altered 

 rocks are the most numerous, proving that a greater portion of the 

 older crater wall must have been existing when these last eruptions 

 took place. The newer basalts, of which the streams themselves are 

 composed, are, however, also well represented, whilst the older basic 

 rocks, of which the original caldera wall has been formed, are, if not 

 entirely wanting, at least of very rare occurrence. Many of the 

 boulders have the appearance of having been much decomposed before 

 they were deposited in their present position. They are interstratified, 

 covered or mixed up with beds of ashes and scoria?, from which we can 

 conclude that the lava-streams were preceded by a clearing out of the 

 vent, during which the ashes and scoriae were ejected. The presence 



