Notes on the Area of Intrusive Rocks at Dargo. 131 



This rock is far more altered than the argillite, and 

 strongly resembles some samples of '' Grauwacke." The 

 argillaceous paste has been entirely converted into mica^ and 

 the rock, as a whole, greatly silicified. The silicification of 

 the sandstones in a greater degree than the argillaceous beds 

 flanking them I have frequently observed. In the meta- 

 morphism of the strata which converts their argillaceous 

 paste into definitely crystallised minerals, and especially 

 where mica is the result, silica is eliminated and again 

 redeposited in a crystallised condition, either in the form of 

 strings or veins, or else in the beds themselves as secondary 

 quartz surrounding original cr3^stalline grains in the beds. 

 Under these conditions, it seems to me that the silica has 

 been deposited in the quartzose beds rather than in those 

 which were purely argillaceous. But it must be distinguished 

 between such cases as these and the general silicification 

 which has, in East Gippsland, for example at Jingallala, 

 affected all the strata. I have figured in Plate II., fig. 4, an 

 instance of the silicification of certain quartzose beds, 

 which I have observed at Stringer's Creek, as illustrating my 

 above remarks. 



On proceeding from Waterford along the Dargo Road the 

 transition to hornfels rock is complete within about a mile's 

 distance. A similar change occurs in going southwards 

 towards Castleburn, at about the same distance, or a little 

 over. In following up the Mitchell Eiver from Waterford 

 the contact zone of rock is found at no great distance on 

 the west side, and it is therefore evident that the argillites at 

 Waterford are of local occurrence only, and are probably an 

 easterly tongue of the less altered Silurian rocks from the 

 extension of those rocks to the west. 



I collected a set of rocks in this district to illustrate 

 various degrees of alteration between the argillites and 

 hornfels. The best series can be found on the road leading 

 from Dargo to Grant. 



(1.) This sample represents the argillites of the district, 

 and was collected near Sippery Pinch, about three miles from 

 the contact. It is slightly slaty in structure, and of a 

 yellowish colour, due to infiltration of iron oxide. Under 

 the microscope it is seen to be composed mainly of some 

 chloritic mineral in small flakes, which are mostly twisted, 

 and which^ when seen on edge, have the appearance of stout 

 fibres. The whole mass is stained by iron ochre. In the 

 mass are a few flakes both of colourless alkali-mica and of 

 biotite, as well as clastic grains of quartz. 



K 2 



