president's address SECTION c. 175 



Geological Relations (See Plate I., Figs. 1 and 2). 



At Heathcote the diabase oiitci'ops between the Sihirian rocks 

 which form high ground to the east, and the series generally regarded 

 as Lower Ordovician, which forms undulating ground to the west. 

 Black chertiS are developed at intervals near the contact with the 

 diabase. They generally occur to the W. of the diabase, but in one 

 or two localities come between the diabase and the Silurian rocks. 



The age and stratigraphical relations of the rocks of Heathcote 

 have been the subject of considerable controversy. Mr. Dunn (13), 

 described the igneous rocks as in the main lavas and tuft's, and stated 

 that they, with the black bedded cherts, formed a Pre-Silurian (Pre- 

 Ordovician) series. Later (7) he refers to them as Cambrian. 



The late Dr. Howitt (19) claimed the bulk of the igneous rocks 

 as altered intrusions, maintained that the black cherts were Ordo- 

 vician sediments alteied by the intrusion of the diabase, and stated 

 that the diabase Avas also intrusive into the Silurian rocks along their 

 junction witli the Ordovician series. 



According to Howitt the cherts are altered Ordovician rocks, 

 while the diabase lie regaixled as probably of Devonian age. Lidgey 

 (16), who mapped the bulk of the area, at first agreed with Dunn's 

 interpretation, but aftei'wards sided with Howitt's view. Professor 

 Gregory (2-4) agreed with Howitt that the bulk of the igneous rocks 

 are intrusive into the cherts, as to whose origin he expressed no 

 opinion. In respect to the relations between the Ordovician and the 

 cherty series he claimed that the evidence of the geological mapping 

 of Lidgey and Whitelaw was inconsistent with Howitt's view that the 

 cherts were Ordovician rocks, and really showed that both cherts and 

 diabase consist of an older series upon which the Ordovician was laid 

 down unconforinably. The absence of diabase dykes in the Ordovician 

 he regards as evidence that the diabase is Pre-Ordovician. Lidgey had 

 pointed out that chert fragments occur in the Silurian conglomerates, 

 and Gregory stated that diabase fragments were to be found in the 

 Silurian sandstones, thus demonstrating that both diabase as well as 

 cherts are of Pre-Silurian Age and not Devonian as maintained by 

 Howitt. Later (9) Prof. Gregory describes the '" Heathcotian " series 

 as of Pre-Cambrian Age (p. 412), and also as U. ArchiBan (p. 596), 

 hut gives no reasons for stating that they are older than 

 Cambrian. The fact that such competent obseiTers as the above 

 have come to such dift'erent conclusions as to the age of the series 

 is no doubt partly due to the comparative poverty of exposures, 

 especially near the contacts of different rocks, partly to the 

 alteration which the rocks have undergone, but it is also a tribute 

 to the inherent complexity of the problem. It will be noted 

 that Howitt described the cherts as Ordovician, and the diabase 

 as Devonian. Dunn describes both as Pre-Ordovician, while Gregory 

 first states that both series are Pre-Ordovician, and later more pre- 

 cisely defines them as Pre-Cambrian and Upper Archaean. 



Recently (36) I have been led by evidence in the field and 

 laboratory to the conclusion that both cherts and diabase are probably 

 basal members of the Ordovician series. The reasons for this view are 



