558 THE DISTRIBUTION, ETC., OF ALKALINE ROCKS, 



The rocks of the Port Cygnet group, which in many respects 

 bear close resemblance to Australian alkaline rocks, are con- 

 sidered, without very much evidence, to be of lower Mesozoic 

 age. They are definitely known to be later than the Permo- 

 Carboniferous and to antedate the Pliocene, but direct evidence 

 to fix their age more closely appears to be wanting. 



The alkaline basic rocks found in cones near the Shannon River 

 are definitely known to be of Middle Tertiary age, the same as 

 similar Australian occurrences. It seems strange that the inter- 

 mediate rocks of the foyaitic magma should be so far removed in 

 age from the more basic ones, and future research may possibly 

 prove the intermediate rocks to be later than hitherto supposed. 



The alkaline rocks described by Dr. Paul (loc. cit.) comprise — 



(a) At Regatta Point, Port Cygnet : — Augite syenite poor in 

 quartz, nepheline syenite, essexite, jacupiraugite facies of nepheline 

 syenite, melanite-hauyne syenite porphyry, garnet-bearing mica 

 solvsbergite, tinguaite, garnet tinguaite, porphyry, nephelinite, 

 etc. 



(b) At Shannon Tier : — Melilite-nepheline basalt, eudialite- 

 nepheline basalt. 



(c) Near Hobart : — Nepheline basalt and kongadiabase. 

 Although very complete petrological descriptions of these rocks 



have been given, their chemical characters and field-relations are 

 not sufficiently dealt with, or perhaps not investigated, hence 

 little can be said of the differentiation which has taken place. 

 Further, the volcanic or plutonic sequence is not worked out. 



The existence of such minerals as melanite and garnet in some 

 of these rocks is suggestive of an assimilation of sedimentary 

 rocks having taken place. The kelyphite borders round the 

 garnet crystals are significant in this connection. 



Both New Zealand and Tasmania are looked upon as the 

 faulted remnants of the Antarctica land-mass over which Meso- 

 zoic sedimentation has been but slight, and tangential movements 

 producing folding have been very feeble since the Carboniferous. 

 Fracturing, on the other hand, has taken place on a gigantic 

 scale. 



