314 Ernest W. Skeats : 



post Ordovician, and proves that the Ordovicians are resting un- 

 conformably upon the cherts and diabases.. In, seiveral of the 

 cases mentioned by Prof. Gregory he did not verify the map- 

 ping. From among a number two cases may be cited. From 

 the northern boundary of tlie Heathcote parish diabase is shown 

 in contact with the Ordovician for two and a-half miles to the 

 west of the northern end of the township, and along the whole 

 of this line no metamorphic rocks are shown between the Ordo- 

 vician and diabase series. 



In S. Heathcote, again, a patch of m©tam.orphic rooks is shown 

 in allotments Nos. 15 and 16, where a bay represeoited on the 

 map as unaltered Ordovician runs up between the diabase on the 

 north and an intrusion of Felspar porphyrite on the south-east. 



It nmst be admitted that the simplest explanation of the map- 

 ping at the latter point is, as Prof. Gregory stated, that the 

 Ordovician rests unconformably upon the cherts and diabases. 

 It seemed to me to be a case where the field evidence should be 

 closely examined. All the other cases mentioned by Prof. 

 Gregory seem to me to be susceptible of an alternative, if less 

 simple, explanation. It is admitted that in the areas mapped as 

 diabase there occur intrusive rocks, lava flows and I'ragmental 

 rocks (agglomerates, etc.). Apart from the question of the 

 mapping accurately representing the relations of the rocks, we 

 have then the possibility that the apparently capricious distribu- 

 tion of cherty rocks might stand in relation to the proximity of 

 intrusive rocks, and that the appearances claimed by Prof. 

 Gregory of the unaltered Ordovician resting unconformably on 

 the diabase might be explained by the overlapping of lava, flows 

 of diaba.se or diabase tuft's over different members of the Ordo- 

 vician series. In this wa.y it would be possible to explain tho 

 absence of alteration in certain parts, as recorded on the map. 

 Mr. Howitt's evidence, and especially the variability of silicifica- 

 tion of the Ordovicians with the distance from the diaba>:e, was in 

 apparent conflict with that adduced by Prof. Gregory. Much 

 of the latter is, as he notes., of a negative character, and the posi- 

 tive evidence is possibly susceptible of another interpretation. 



I felt that neither view was so firmly based but that further 

 evidence, especially of a positive character, if available, would 

 serve to strengthen one or nther of the rival views. I therefore 



