372 PEOCEEDINGS OF SECTION C. 



alkaline types, and sodalite is abundant in one of them. Andesites of 

 an alkaline type are well represented, and there are trachydolerites and 

 basanites. One leucitophyre occurs sparingly. Basalts and dolerites 

 are in alternating lava flows with the phonolites, and are subsequent to 

 the trachytes. 



Banks' Peninsula is composed chiefly of andesites, especially of a 

 variety of olivine andesite, with very large porphyritic felspar and augite. 

 Basalts and dolerites occur chiefl}^ among the later lavas and in dykes. 

 In addition, there is a trachyte, with large crystals of tridymite near 

 Lyttelton. Two dykes of trachytoid phonolite only have been 

 recognised at present, in one of which Mr. Speight has recognised 

 arfvedsonite. 



In the North Island volcanic activity has been almost continuous 

 from early Cainozoic times to the present day. 



The earlier volcanic eruptions appear to have been confined to the 

 Coromandel Peninsula and the country to north of that area. Waita- 

 kerei. Great Barrier Island, Little Barrier Island, Whangerei Heads, 

 Whangaroa Heads and numerous other places, including Hen and 

 Chicken Islands, have rocks of similar nature and appear from their 

 stratigraphical position and extent of decomposition to be of similar 

 age. The commonest rock is hypersthene augite andesite. In the 

 Coromandel area the hypersthene has been greatly chloritised, especially 

 in those portions bordering on the quartz veins, and it has been supposed 

 that hydrothermal action has caused the change, and that one of the 

 results has been the removal of supposed metallic matter in the hyper- 

 sthene and a subsequent concentration of the metal in the quartz 

 veins. 



These andesites have been fully described by Professor Sollas, in 

 a work lately published by the New Zealand Government. Un- 

 fortunately, no attempt has been made in that work to define the area 

 over which each rock occurs. A chronological classification of the 

 rocks by Mr. McKay is given. Each division appears to include rhyol- 

 ites, with andesites, and the age varies from Eocene to Pliocene. No 

 attempt is made to indicate the areas of different rock types within 

 each group. Professor Park classed the andesites as the older rocks 

 of Eocene age, and this arrangement has been provisionally accepted, 

 though it is not improbable that the rocks are younger than this. 



Associated with the hypersthene andesites are other types, pro- 

 bably of the same age. Professor Sollas has recognised hornblende 

 andesite, dacite and various glassy types, while the structure of the 

 ground mass is widely different in different samples. Outside of the 

 Thames and Coromandel region Capt. Hutton recognised hypersthene 

 augite andesite at Whangarei Heads, while I have found a similar type 

 abundant at Waitakerei. At Little Barrier Island a hypersthene 

 andesite without augite appears to be the prevailing rock, judging from 

 specimens forwarded to me by Mr. T. Cheeseman. At the Hen and 

 Chickens Islands dacite and hornblende andesite is the characteristic 

 type, so far as I can judge from specimens given me by Mr. C. N. Boult. 



