80 Transactions. 



Generally tlie argillite has afforded planes of weakness, so that the fault- 

 lines have often a N.N.E. direction, following approximately the general 

 strike of the rocks. Only in the case of the Wellington fault is there good 

 physiographic evidence of faulting ; in other cases, however, the fault-lines 

 are readily recognized by their zones of crushed and slickensided rock. 



A secondary effect of the faulting has been the production of silicified 

 fault-zones. In the western part of the area these have been worked for 

 their gold-content. 



The "Red Eocks." 



Tuff-beds, cherts, and red and green slates (with associated quartz veins) 

 have been recorded together at various points, and referred to as the " Red 

 Rocks." They may conveniently be described under this heading. 



Tuff -beds. 



Thin beds of diabase tuff occur at two, and probably three, points, inter- 

 bedded with the argillite and greywacke. Two of these occurrences are 

 indicated on the map ; the third was noted by A. McKay as a dyke rock 

 encountered in the gold-mines of Terawhiti, the term " dyke " having been 

 used by that writer for rocks later called " tuff " (8, pp. 66-7). 



The exposure at Red Rock Point is associated with red and green 

 slates. The rock is chocolate-coloured, but this colour is largely masked 

 by green stainings of epidote and chlorite ; it shows also irregular veins 

 and nests of calcite. Under the microscope the rock is seen to be com- 

 posed largely of colourless needles arranged in sheaf-like bundles ; these 

 are embedded in an irresolvable brown paste. The needles polarize in 

 low colours — whitish-grey of the first order — and show negative elongation. 

 Calcite veins and smaller veins of haematite are present. Plate VII, 

 fig. 2, shows the microscopic appearance of this rock. Dr. J. A. Thomson 

 has kindly shown me sections of a rock from Western Australia, which he 

 terms " fine-grained greenstone " (15, p. 634). These are very similar to 

 sections of the diabase tuff. The greenstones, according to Dr. Thomson, are 

 altered diabase lavas or tuffs (15, p. 670) ; and the colourless needles, an 

 alteration from the secondary amphibolite, he considers to be hornblende 

 which has in some way had its birefringence lowered. 



Notwithstanding the evident alteration which the diabase tuff has under- 

 gone, its chemical composition still approximates to that of an average 

 diabase rock as given by R. A. Daly (16) — compare analyses 6 and 7. 

 In comparison the tuff shows a deficiency of magnesia, differs in the pro- 

 portions of ferrous and ferric oxide, and has a higher percentage of loss 

 on ignition. The formation of epidote and chlorite, as noted above, would 

 increase the hydrous content of the rock. The relative proportions of 

 ferrous and ferric iron have probably altered in a way similar to that of 

 the red and green slates. 



Grey Cherts. 



The rocks named " grey cherts " by A. McKay are abundant in the 

 area. They are well seen on the coast between Lyall Bay and Island 

 Bay, east of Red Rock Point, and at several places between there and 

 Oterongu Bay. They form most of the block west of Oterongu and Ohau 

 Bays (17, p. 3). The rock is quite evidently a greywacke altered by 

 secondary silicification ; small veins form a closely anastomosing mesh- 

 work in the rock, so it is not possible in parts to select even a small sample 

 free from veining. 



