86 E. 0. Teak: 



a feAv inches wide between beds of jasper. There has clearly*, been a 

 selective action in the process of silicification, probably controlledi 

 mainly by i^rimary differences in conijjosftion existing in the 

 original strata. Thin sections, however, of the slates and cherts^ 

 have sq far not revealed what these differences were. 



The alteration is not attributable to the direct contact action of 

 igneous intrusion, but would appear to be explained by a selective- 

 metasomatic replacement due to aqueous solutions permeating the 

 rock. 



Instances of this type of chertification are found throughout the- 

 Palaeozoic rocks. It is natural, therefore, to find cherts of very 

 different ages resembling each other somewhat closely, and this has 

 led to considerable confusion on account of some geologists having 

 correlated numerous cherty outcrops with the " Heathcotian " on 

 purely lithological grounds. 



Professor Skeats (18") has discussed the origin and occurrence of 

 the cherts of the Heathcote area, and has shown that they are 

 largely due to metasomatic replacement of dialiase and diabase 

 tuifs. Tlie age is left an open question, but their inclusion in^ 

 the basal portion of the Ordovician was favoured on the then 

 known palaeontological evidence. 



Dr. Summers (21) has described cherts and associated rocks at 

 Tatong, where they are interbedded with fairly normal sediments,, 

 which he regards as Upper Ordovician. 



In the Wellington area and in the Tara Range, near Buchan, I 

 have found Upper Ordovician graptolites actually in the cherts. It 

 is therefore clear, as Dr. Summers points out, that the occurrence 

 of cherts as characteristic of the Heathcotian Series loses its 

 significance. 



Up to the present no radiolaria have been noted in thin sections- 

 of the Wellington cherts which have been examined. 



Reviewing the important structural points brought out by a 

 study of the Lower Palaeozoic geology of the Wellington area, it is 

 clear that the axis of this comjDlex inlier marks an important struc- 

 tural line, the direction of which is parallel with the main Palaeozoic 

 trend lines, along which a succession of important tectonic move- 

 ments have ]>een renewed many times. We therefore find the zonal 

 arrangement of successive formations l>earing in a prevailing 

 N.N.W. to N.W. direction very marked, exposing in this case 

 within a remarkably small area, narrow, parallel but unconformable 

 belts from Cambrian to Silurian. 



With this in mind, it is natural to look expectantly along the- 

 continuation of the strike of this belt. 



