128 Transactions. 



Art. XV. — A Conglomerate at Onerahi, near Whangarei, Auckland, 



New Zealand. 



By J. A. Bartrum, Auckland University College. 



{Read before the New Zealand Science Congress, Palmerston North, 27th January, 1921: 

 received by Editor, 21st February, 1921 ; issued separately, 27th June, 1921.] 



Plate XXVIII. 



Introduction. 



There are certain conglomerates intercalated in Tertiary and earlier strata 

 in various parts of the North Island of New Zealand, amongst the con- 

 stituent pebbles and boulders of many of which there is material showing 

 evidence of having been subjected to far more intense pressures than have 

 any of the rocks constituting the basement of that Island, with the 

 exception of some at Whangaroa (Bell and Clarke, 1909, p. 44). At this 

 place schistosity is locally developed upon rocks described by Bell and 

 Clarke as altered igneous types, but the cause is ascribed by them to 

 shearing-stresses along a fault-zone. 



In a recent paper the writer summarized the recorded occurrences in 

 the North Island of conglomerates containing granitic or dioritic material 

 showing acute pressure-effects, and described a variety of rocks from an 

 occurrence near Albany, in the vicinity of Auckland (Bartrum, 1920). 

 In a second paper he described granitic and granulitic boulders from a 

 conglomerate in the basement rocks of Great Barrier Island.* 



The present note, published here by permission of the Director of the 

 New Zealand Geological Survey, describes interesting pressure -affected and 

 other rock-types from a conglomerate at Onerahi, near Whangarei, which 

 furnish additional evidence of the widespread nature of this pre-Mesozoic land. 

 It is believed that it was not wholly destroyed until after mid-Tertiary times, 

 for the boulders of the Tertiary conglomerates seem too free from decom- 

 position to be merely a re wash of Mesozoic conglomerates. These latter, 

 in any case, are scarce in relation to the wide extent of basement rocks 

 now uncovered for examination. 



In a paper read before the Melbourne meeting of the Australasian 

 Association for the Advancement of Science in January, 1921, the present 

 writer adduced arguments in favour of the correlation of the rocks of the 

 pre-Mesozoic land-mass of the North Island with those of the Aorere 

 system, so well developed in the South Island. 



General and Petrographic Description of Onerahi Conglomerate. 



In December, 1919, in company with Mr. H. T. Ferrar, of the New 

 Zealand Geological Survey, the writer discovered a conglomerate intercalated 

 in greensands exposed between tide-marks a short distance east of the 

 wharf at Onerahi, near Whangarei. The series of beds of which it is part is 

 probably Tertiary in age, and is considerably disturbed by folding. The 

 beds below the conglomerate are not visible, but above it come about 20 ft. 

 of greensands, and then a gradual passage to an argillaceous limestone 



* See this volume, pp. 119-20. 



