ALKALI ROCKS OF DUNDAS, VIC. I.S9 



The Age of the Alkali Rocks. — As stated earlier in the paper, 

 the relationship of the alkali rocks to one another and to the other 

 rocks of the area is very obscure, owing to the mantle of Later 

 Tertiary and alluvial deposits. 



Mr. Dennant^ has recorded the occurrence of otozamites in a 

 " felspathic tufa " from near the summit of Mr. Koroite, but makes 

 no deduction from this, except that in a later paper he states that 

 lie doubts if the trachytic rocks are younger than the Mesozoic. Mr. 

 Hogg,2 in referring to this record, says : — " This bed of tufa lying at 

 a higher level than the railway cutting through Mt. Koroite is 

 possibly connected with the trachytic rocks which occur in the 

 neighbourhood of Coleraine ; but I am inchned to regard them of 

 later origin myself." A careful search on Mt. Koroite failed to 

 show the excavation from which Mr. Dennant obtained 

 his specimen, but a microscopic section taken from the original 

 material containing the Otozamites showed an extremely altered 

 material, which might just as well form part of the Mesozoic sedi- 

 ments, which are in the main composed of igneous material, as a 

 tuff belonging to the alkali series. As Mr. Dennant advances no 

 evidence in favour of his identification of the rocks as a felspathic 

 tufa, the balance of evidence is in favour of its forming part of the 

 Jurassic series which undoubtedly occur lower down the slopes of 

 Mt. Koro'te, and if so it has no connection whatever with the alkali 

 series. But for this record there would have been no hesitation on 

 anyone's part to place the alkali rocks as Post- Jurassic. At Mt. 

 Koroite the basaltic type associated with the trachytes distinctly 

 overUes the slightly tilted Jurassic sediments, which are exposed in 

 the railway cutting, though unfortunately no junction between the 

 two has been noted. At Koolomert a certain amount of evidence is 

 forthcoming, but it is rather indefinite. As stated before, an alkali 

 basalt has broken through a bed of sand and produced columnar 

 jointing in the same. The absence of bedding in this sandstone 

 and the perfect columnar structure are most easily explained as 

 the result of the drying out of moisture from a loose, incoherent 

 sand by the heat of the intrusion. The field evidence points to the 

 basic rock occurring as a plug through the sandstone rather than as 

 part of a superficial lava flow. Quartzites, similar in every respect 

 to the Tertiary quartzites so common in Victoria, also occur just 

 underlying part of the igneous rock at Koolomert. Within half-a- 

 mile from the columnar sandstones and the quartzites fossiliferous 

 beds of Barwonian (?) age occur at about the same elevation as the 

 sub-basaltic material, so that they possibly belong to the same 

 horizon. In this case the alkah rocks would be Post-Barwonian in 

 age. This evidence is given for what it is worth, as at present it is 

 the best that is available. Mr. Hogg has called attention to the 

 youthful appearance of the hillocks Adam and Eve, and certainly 

 the form of these hills and that of the Giant Rock suggest a tertiary 

 age for these rocks. 



1 op. cit., p. 395. 

 2. op. cit., p. 360. 



