Vol. 54.] ON THE BALA & OTHER ROCKS OF LAMBAT. 141 



between Trinity Well and Raven's Rock, these beds appearing to 

 have been due to small outbursts which scattered fragments of 

 pumiceous and compact andesitic rocks over the top of the lava- 

 flows at various points. 



The Scotch Point ashes are of medium grain, and in a hand- 

 specimen show rounded and subangular fragments of andesitic 

 rocks, dark- and light-green in colour. A microscope-section shows 

 angular and rounded fragments of felspathic rocks in which all the 

 felspars are replaced by calcite, and many of the fragments are 

 exceedingly vesicular. In many places blocks of coarse porphyrite are 

 to be seen, often very amygdaloidal and probably of the nature 

 of volcanic bombs, while dykes of a similar coarse rock are seen 

 cutting the ashes here and there. 



The other large patch of ground in this corner of the island 

 occupied by pyroclastic rocks is between the Castle and Broad 

 Bay. It is separated from the Scotch Point ashes by a mass of coarse 

 porphyrite, a drift-covered region, and the outcrop of slates men- 

 tioned above (p. 136). Here the ash-bed rests on what is apparently 

 an andesite containing large rounded blocks of amygdaloidal coarse 

 porphyrite ; this, in its turn, is underlain by a band of coarse por- 

 phyrite, which can be traced for some 300 yards in a north-and-south 

 direction. But the microscope shows that the supposed andesite is 

 really andesitic ash, and at the northern end of the exposure 

 occur well-marked bands of fine ash, striking roughly south-west and 

 dipping at about 80° S.E. 



The occurrence in these ash-beds of bomb-like portions of coarse 

 porph)Tite points to the near presence during their formation of the 

 coarse rock in the liquid state, and we may well imagine a series 

 of explosions drilling a hole through the andesitic flows and forming 

 a passage for the uprise of the porphyrite : the ash-beds being 

 chiefly formed of fragments of the shattered andesites, but including 

 portions of the uprising rock which were blown out from it at 

 intervals. 



On the south-western coast of the island, between Talbot's Bay 

 and Carnoon Bay, is exposed a fairly coarse tuff or agglomerate, 

 numerous exposures of the same rock being found for some distance 

 inland. It ends off against much-crumpled slates in Talbot's Bay, 

 the actual junction being a faulted one, while in Carnoon Bay a 

 few finer tuffs and slates, dipping east at a high angle, intervene 

 between the coarse tuff and the crumpled slates. The coarse bed is 

 largely composed of rounded and angular blocks of slate and fine 

 andesite, but blocks of coarse porphyrite also occur, generally 

 rounded and very amygdaloidal. This mass of agglomerate may 

 very possibly mark the spot where a volcanic neck was drilled 

 through the sea-floor which was formed of the slates of Talbot's Bay 

 and Carnoon Bay. 



Owing to insufficient exposures, it is difficult to say decidedly 

 whether these three masses of ash and tuff are to be regarded as 

 marking the positions of old vents, or as being merely beds of ashes 

 formed by the scattering over the sea-floor of fragments which were 

 shot out from some vent in the vicinity. 



