'^'Z Jntson and Chapman: Felsite Dyke, Mont Park. 



Conclusions. 



A dyke of felsite is considered to have intruded the Silurian 

 sediments by a forked fissure across the line of their strike. 



Associated with the dyke is a brecciated selvage, which we have 

 termed a felsitic breccia. We consider it piobable that this rock 

 originated by tlie dyke on intrusion, tearing off and enclosing frag- 

 ments of the adjoining rocks Anothei- aspect has been mentioned above. 



After the formation of the brecciated selvage, a strong earth 

 movement took place, by which the present coarse-giained fault oi- 

 friction breccia was formed. 



Later, further dynamic changes occurred along the same line of 

 fracture, as indicated by the features of the thin enamelled band of 

 rock, but as the origin of the latter is not clearly determined, 

 nothing very definite as to the displacement can be stated, except 

 that it was, in part at least, almost horizontal. 



Thus, including the original fissure, we have evidence of several 

 distinct earth movements along the same line of fracture, and so 

 strikingly confirming the general statement that lines of weakness 

 ■often suffer from repeated displacements. The rocks have probably 

 been altered by thermal waters as well as by dynamic agencies. 



Rgarding the age of the dyke and breccias, if our reading of the 

 phenomena be correct, they aie clearly post-Silurian. The Kaino- 

 •zoic grits at the northern end appear to belong to the series of 

 ■gravels and grits that cap the higher hills to the north-east of 

 Melbourne, such as Northcote, Studley Park and Preston, which are 

 regarded as Kalimnan. As these grits do not appear to have been 

 affected by the earth movements, all such movements, together with 

 the date of the intrusion of tlie dyke, must be regarded as pre- 

 Kalimnan. 



It is possible that the dyke is contemporaneous with the felsitic 

 ■dykes, which can be seen intruding the Silurian along the Alexandra 

 Avenue, Melbourne. 



EXPLANATION OF PLATE 



Platk VI. 



Fig. 1. — Fragmental felsitic rock from the enamelled band in 



section near southern end of outcrop, Sugarloaf Hill. 



Slide No. 1. X 28. 



Fig. 2. — The same, more highly magnified. Slide No. 1. x 184. 



Fig. 3. — Felsitic-breccia, with quartz-vein fragments and altered 



sandstone, southern outcrops. Slide No. 2. x ",9 

 Fig. 4. — Felsite. From the dyke, southern outcrop. Slide No. 3. 

 X 28. 



