698 SUBANTARCTIC ISLANDS OF NEW ZEALAND. [Geology. 



MUDSTONE. 



On the eastern side of the head of Perseverance Harbour, in the locality where 

 Filhol records schist, there is now exposed a lamellar mudstone. In section it is 

 seen to contain abundant angular grains of quartz, imbedded in extremely fine- 

 grained dusty matter, in which occasional flakes of muscovite are distinct. 



QUAETZ. 



Pebbles of this mineral obtained from the conglomerate beneath the limestone 

 consist entirely of interlocked grains of the same appearance that is usually pos- 

 sessed by the quartz of schists. It frequently shows undulose extinction. 



Marble. 



At North-west Bay the limestone is represented by a crystalline rock which 

 proved to be a fine-grained marble. The recrystallisation was not quite complete, 

 for traces of the tests of Foraminifera still remain. 



Flint. 



Abundant in some of the limestone. Many flints are exposed on the shore at 

 the head of Perseverance Harbour and along the middle part of the coast of North- 

 west Bay. 



From the foregoing statements the sequence of volcanic rocks will be found 

 to be as follows : — 



The first effect of volcanic action was to form a great thickness of tuffs. 



The earliest lava-flows consisted of trachytic rock. Two distinct flows of this 

 rock can be seen on the flanks of Mounts Dumas, Lyall, Eboule, and Azimuth, and 

 it is possible that further work would reveal their presence on the other hills. On 

 Mount Dumas the lowest flow is nearly 100 ft. thick. The trachyte lavas lie at 

 rather a higher level on Mount Dumas and Mount Azimuth than on Mount Honey 

 and Mount Lyall. Rapid fluctuations of the barometer prevented any accurate 

 observations being made as to exact elevations. 



The trachytes are succeeded on the greater heights by coarsely porphyritic 

 basalts or dolerites. This lava decomposes rather easily, and does not present a 

 steep scarp, so its thickness could not be estimated, but it is relatively thin. 



On Mount Lyall the porphyritic basalts are followed by melilite basalt, and on 

 Mount Dumas and Mount Honey by extremely fine-grained basalts which closely 

 resemble the melilite rock, though the feldspar represents the melilite. 



Ordinary basalts do not appear in this succession, but the separate mass of 

 Mount Beeman is formed of this rock, and it is also found in numerous dykes and 

 in rather a coarse form at Terror Point. 



The remarkable agreement in the succession of lavas in the different hills is in 

 itself almost sufficient to prove that they have all issued from the same point of 

 eruption. The slight slope in the level of the trachyte-flow to the east may be suffi- 

 cient to prove that the main point of eruption was to the west of the present land. 



