288 C. A. SUSSMILCH AND T. W. E. DAVID. 



attention has been paid to the occurrence and distribution 

 of minor accessories such as apatite and zircon which are 

 found in certain of the rocks. 



Taken as a whole the series, so far as is shown by the 

 mineralogical characteristics, includes a fairly wide range 

 of types. What the field relations and the magmatic rela- 

 tions of all these may be is not known with any great 

 degree of accuracy. The sequence of eruption, as is pointed 

 out elsewhere in this paper, appears on the whole to have 

 been one of decreasing basicity. The cycle of events is 

 usually initiated by the eruption of a hornblende andesite, 

 followed by pyroxene andesites of greater basicity, and 

 these in turn are succeeded by dacites, toscanites and 

 rhyolites. 



The Eelah section does not conform to this order of suc- 

 cession, and this and other apparent exceptions, it is 

 suggested, may be due to the overlapping of flows from 

 neighbouring vents, to an actual repetition of the cycle, or 

 perhaps even to certain of the rocks being intrusive, and 

 therefore liable to appear in any stratigraphical relation 

 towards the other rocks of the series. 



Only by the very closest of field examination and map- 

 ping will these matters possibly be cleared up. 



Rhyolites. — For the most part the rhyolitic rocks are of 

 sodic type, some of them, however, containing orthoclase, 

 and passing over by increase of this mineral into more 

 normal sodi-potassic types. The rocks are porphyritic 

 megascopically in quartz and felspar, with biotite in greater 

 or less abundance. The groundmass is usually stony or 

 felsitic in appearance, without visible flow-structure and 

 of buff, pink or pinkish-grey colour. Indeed there is a great 

 similarity between these rhyolites and many of the dacites 

 and toscanites, so that in hand specimens it is often impos- 

 sible to differentiate them. 



