FeLsUe Btjl,r, Mont Park: 6» 



Tlie Silurian riH-ks are not seen in the actual section, Imt are 

 iiisi-rted to show the )elatiojis. 



The (Ivkc at vliis section is about 12 feet ^vide ; the hrecciated 

 selvage Avould averatre about 2 J, inclies, but reaches 5 inches; and 

 the enamelled rock is about | inch in width. The width of the- 

 fault-breccia has not l)een defined, but about 4 feet of it can be seen. 

 The eastern face of the enamelled rock is smoothed, polished and 

 marked by faint, api>roximately horizontal striations, evidently indi- 

 cating considerable rock movement. The petrology of this rock and 

 of the dyke and selvage are described later. 



The fault-breccia is composed of angular fragments of apparently 

 fine-grained sedimentary rocks (no doubt of Silui-ian age), of the 

 dyke rock and of tlie selvage, the size of the fragments varying from 

 about !(• inclies tv \ inch. 



The selvage, on its western side, passes into the dyke by an 

 irregular, indistinct line, the rocks having the appearance of being 

 fused together. The junctions of the enamelled rock with the selvage 

 and the fault-breccia respectively are sharp, practically vertical, 

 and run approximately parallel to the course of the dyke ; they are 

 regarded by ns as fault planes. The actual junctions of the dyke 

 and the fault-breccia respectively with the Silurian cannot be seen. 



At another small excavation (noAv filled in), and elsewhere on the 

 hill, generally similar relations between the main rocks occur. 

 Points of difference are that the selvage is nearly three feet wide in 

 places, and is soinetijnes cavernous, and that well down on the soutli- 

 eastem slope of the hill, a coarse-grained breccia occurs, but the- 

 dyke ajijiears to be absent. 



On the northern slope of the hill, and a few feet below its summit, 

 tliere is a small outui-op of hard silicified coarse grits. These rocks 

 evidently belong to the series of Kainozoic gravels and grits capping 

 the hills at Preston. Xorthcote. Heidelberg, Studley Park, and other 

 places. They do not appear to have been disturbed l)v any earth 

 movement. 



Ptt ruUxji/ . 



The following is a (lescrif)tion of the three principal types of rock. 



The dyke rock is compact, of a chalky-white appearance, obscurely 

 laminated near its edge, and in places stained with iron. Travers- 

 ing the rock are numerous small, irregular planes, which may be 

 due partly to shrinkage on cooling, and partly to the severe pressure 

 the rock has sustained during the various earth movements, which 

 have occurred subsequent to its intrusion. Along certain planes, 

 bands of the brecciated selvage occur, showing clearly that the 

 brecciation of the dyke was not confined to its eastern edge. 



