176 president's address — section c. 



briefly as follows : — The great bulk of the diabasic rocks near Heath- 

 cote consist of two types. The most abundant is a somewhat platy 

 or foliated rock which, both in the field and under the microscope, is 

 fragmental and consists of an altered diabase tuff. Agglomerates 

 occur in one or two localities, and north of Heathcote amygdaloidal 

 altered lavas are abundant. Only minor bosses among the tuffs and 

 lavas were recognised by me as intrusive, and the diabase, in the 

 main, I regard as a series contemporaneous v\^ith the adjoining sedi- 

 ments to the West. While the relations of the cherts to the diabase 

 were not very clear, in several places there appeared to be a passage 

 from on© series to the other. The cherts are, in the main, finely 

 bedded, and under the microscope several sections showed strong 

 evidence that they are silicified tuffs. The presence of Radiolaria in 

 the cherts was suggested. Later observations, confirmed by Mr. 

 Chapman, have demonstrated their presence, so that I regard the 

 cherts as silicified submarine tuffs, practically contemporaneous with 

 the diabase, which I believe to have been partly subaerial, partly 

 submarine in origin. The important question of the relation of the 

 cherts to the normal Ordovician sediments I studied closely in view of 

 their uncomfoi-mable relations suggested by Prof. Gregory, largely on 

 the evidence of the geological maps. I have elsewhere given reasons 

 for regarding the mapping as misleading in part, for silicified fine- 

 giained diabase has been mapped as normal Ordovician. The 

 relations are, I maintain, such as one would expect to find if a basal 

 Ordovician series of lavas, agglomerates, and tuffs of variable thick- 

 ness gradually passed by cessation of volcanic activity into normal 

 marine sediments, and were subsequently folded and denuded. It is 

 significant that the Ordovician shales near the diabase contain 

 nodules of magnesite indicating probably an admixture with diabasic 

 material. If the cherts and diabase were Pre-Ordovician one would 

 expect the cherts to differ from the Ordovician in dip and strike, and 

 one would expect to find a marked conglomerate at the junction 

 between the cherts or diabase and Ordovician, such as one finds to the 

 east at the junction of the Heathcotian and the Silurian. Close 

 examination in the field led me to the conclusion that no such con- 

 glomerate existed, and that while in going from S. to N. changes in 

 strike of both series were noticeable, at any one point the strikes 

 of the chei'ty series and the Ordovician sediments were in substantial 

 agreement. Furthermore in several places as one passed from the 

 black cherts towards the normal Ordovician sliales less and less cherty 

 rocks were met with. In places where black cherts do not occur at 

 the junction of the diabase with the Ordovician the .sediments are 

 silicified, but to a lesser extent. The evidence in the field convinced 

 me that the cherts as maintained by Dr. Howitt are simply highly 

 altered Ordovician bedded rocks. A visit to the similar area east of 

 Lancefield showed in that locality that this is the true explanation. 

 West of the diabase occur dense black bedded cherts, and in a quarry 

 a section is exposed showing the interbedding of the cherts with less 

 silicified shales, from which the Lower Ordovician graptolites typical 

 of the Lancefield horizon as defined by Dr. Hall have been obtained. 



The precise age of the shales and cherts which junction with the 

 diabase at Heathcote is still a matter of some doubt. Mr. Ferguson 



