328 iV. R. Junner : 



Mr. ( 'hapTiiaii ck'scrilu-s it as the remains of a })liyllncai'id re- 

 sembling ]Jit}ii/oc(iri.s praecox. Chapman, hut much smaller. A 

 similar fox'm has been found in the nielbotirnian at IVIerri Creek. 



Acid dj/hes. 



Three of these, namely, the Dry Creek dyke, the Diamond Creek 

 dyke, and the Warrandyte dyke occur in the area investigated, 

 although only the former paii- have been studied in detail. The 

 Diamond Creek dyke is most important economically, as it is 

 with this dyke that mining operations in late years have been 

 mainly concerned. Their exact age is not stratigraphically deter- 

 minate. They are later than the folding of the silurian, and are 

 overlaid in some cases by alluvium. In one case, in section 16, 

 allotment D, a basaltic dyke cuts across one of these dykes. The 

 age of the basic dyke is probably about middle tertiary, and so it 

 affords little value in the determination of the age of the acid 

 ones. Probably the intrusion of the acid dykes was connected with 

 the earth movements that caiised the folding of the silurian, and 

 these are probably devonian in age. 



Siih-lxtsalt ic river (/ravels, )^(nif/s, etc. 



Lithologically all stages are present in these between coai'se 

 gravels with pebbles up to six inches in diameter, and fine uncon- 

 solidated sands. Frequently the sands and gravels have been 

 cemented togetlier forming the following : — 



Silicified grits grading into quartzite. Ferruginous grits where 

 tlie cementing matei'ial is either limonite or hematite. Calcareous 

 grits, consisting of ([uartz grains, set in a paste of calcium car- 

 bonate. 



Where the grains of (piartz arc more angular, various ty})es of 

 bi-eccia are fornu'd. Thin bands of limestone occui- interbedded 

 with the sands in sevei'al 2:)laces. Silicification of the fine sands 

 frequently occurs with the forination of (piartzite. Professor Skeats 

 and Mr. Sumincrsl l:ave noted the intimate association of the 

 quartzites with the lava flows in the Macedon area. While this 

 is frequently the easi- near tin- basalts at Greensborough and 

 Kangaroo (Irounds, (juartzite <loes nevertheless occur associated 

 with the sands and gravels, where no basalt is present. In such 

 cases an origin such as that suggested by Prof. Gregory2 might be 



1 Professor Skeats and H. S. Summers, M.Sc. Bulletin Vict. Geol. Surv., No. 24, 1912. 



2 J. W. GreKor.v. The t;eogra|>hy of Victoriii, p. St4. 



