Rocks })ear Heatlicote. 311 



first visit to Heathcote for various reasons was deferred till 

 Februain', 1908, but durintr the last three years I have made 

 myself familiar with the most important papers on the district, 

 and their perusal left me in doubt as to the balance of evidence 

 adduced in favour of the pre^Ordovician and post-Ordovician 

 hypotheses respectively of the ajre of the cherts, diabases and 

 granite rocks of the district. At the same time various points 

 of criticism suggested themselves as to which special examination 

 in the field and with the microscope might be expected to be 

 fruitful in results. 



The age of the older sediments has been the subject of debate, 

 as Mr. Etheridge described the Dinesus as a Cambrian form, 

 while Prof. Gregorj' has shown reasons for including the beds con- 

 taining the trilobites with the Ordovician as its basal member. 

 The age of the normal older sediments is recorded as Ordovician 

 on lithological grounds, as no fossils had been found in them. 

 No real objection caxi be taken to this view, because at various 

 points in the field they are recorded as succeeding the Ditiesus 

 beds, show similar dips and strikes, and are only separated from 

 them by a naiTow brecciated conglomerate a few inches in thick- 

 ness. 



The great stratigraphical problem is, of course, the relations of 

 the cherts, jasperoids, diabases, and granitic rocks to one another 

 and to the normal Ordovician sediments, and apart from the 

 geological mapping the most important contributions are the 

 paper by Mr. Howitt, in which the post-Ordovician age of the 

 diabases and granitic rocks is asserted, and those of Mr. Dunn 

 and Professor Gregory, in which a pre-Ordovician age is claimed 

 for the igneous rocks and cherts. Mr. Dunn's first paper in 1888 

 ia purely descriptive, and does not deal with the relative ages of 

 the rock.-;. In his letter to the Mines Department, in 1891, how- 

 ever, he definitely states the pre-Silurian (pre-Ordovician) age of 

 the cherty and igneous series. It is difficult to discuss Mr. 

 Dunn's commvmication, because he does not present the evidence 

 on which he based his claim for the pre-Ordovician age of the 

 rocks, but his view was evidently based on considerable work 

 in the district, since he states (Rec. Geol, Surv. Victoria, Vol. II., 

 Part II., 1907, p. 81), that he had mapped the rocks of the area 

 and sent the map to the Mines Department in 1892, but that 

 the map had been lost. 



