296 



Transactions. 



district. Little is known 

 the pre-Notocene rocks. 



A.> ill 





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o 

 _o 



c 



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o 



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6 



of the age and the conditions of deposition of 

 They are clearly earlier than Clarentian (middle 

 Cretaceous), certainly Mesozoic in part, and 

 quite possibly wholly Mesozoic. They are 

 composed chiefly of greywackes and argillites 

 in endless alternations, both coarse and fine ; 

 marine fossils (Inoceramvs sp.) have been found 

 only in two places, in what are probably the 

 uppermost beds, while sandstones with obscure 

 plant-remains are reported by McKay in a 

 number of places. The rocks were, therefore, 

 probably deposited in shallow water, and with 

 considerable rapidity, in conditions under which 

 marine benthic life did not flourish. Cotton 

 (1918, pp. 56-57) considers that they might be 

 interpreted as the topset beds of a continental 

 shelf undergoing subsidence, but supplied with 

 a considerable but fluctuating supply of waste. 

 These conditions exist during the first stages 

 of sea-advance following a period of mountain- 

 building, so that we may see in the nature of 

 these sediments the evidence of an earlier epoch 

 of major diastrophism, which has, however, left 

 no other recognizable effects in the area under 

 consideration. Probably the mountains then 

 formed lay outside this area. The rocks stand 

 now for the most part in steep attitudes, 

 often showing in section closely folded syn- 

 clines and anticlines or contorted bedding, 

 with numerous small faults. Their lines of 

 strike vary rapidly from place to place. The 

 lowest Notocene rocks rest upon them with 

 strongly marked imconformity, the surface of 

 unconformity being a practically plane erosion- 

 surface truncating the pre-Notocene rocks at 

 angles approaching a right angle. It is obvious, 

 therefore, that the pre-Notocene rocks ex- 

 perienced a sharp folding to the degree of 

 mountain-building, and were subject to con- 

 siderable erosion before the deposition of the 

 lowest preserved Notocene rocks began. The 

 epoch of major diastrophism thus disclosed has 

 been termed by me (1917) the post-Hokonui 

 orogenic movement, from the fact that rocks 

 of Hokonuian age are involved in the present 

 area, and that in other parts of New Zealand 

 the youngest Hokonuian rocks • — viz., the 

 Wealden plant-beds of Waikato Heads — are 

 similarly involved. In the Clarence and 

 Awatere districts the lowest Notocene rocks 

 (Clarentian) are of middle Cretaceous age, so 

 that the date of cessation of the post-Hokonui 

 movement in this area must be early Cre- 

 taceous at the latest. 



