and their Contact Zones. 



81 



secondary mineral products existed which could be referred to 

 the alteration of these species. The compact portions of rock 

 which were brought up from the mine, and came from the con- 

 tact, seemed to me to be indurated mudstones; these,under the 

 microscope, in thin slices, proved to consist of aggregates of 

 minute scales, suggestive of an orthorhombic mineral. A 

 qualitative examination pointed to kaolin. This dyke has 

 not yet been worked to such a depth as to have reached the 

 less altered stone. The gold is found in the quartz veins, 

 extending into the slate country, which is also, together with 

 the quartz, much impregnated by iron pyrites. 



a. Silurian slates (upper ?). 



b. Dyke. 



c. Auriferous quartz veins. 



In this reef, part of which is being mined by the well- 

 known Walhalla and Long Tunnel Companies, the auriferous 

 quartz lies, as a rule, more in the contact of the dyke and 

 of the silurian slates than is the case in the instance just 

 given. The dyke is a compact or fine-grained diorite of an 

 almost felsitic character,* which is highly mineralised by 

 arsenical and iron pyrites ; but, as a rule, is rather indurated 

 and mineralised than decomposed. Some slices prepared 

 from samples of this rock — for which I have to thank Mr. 

 Kosales, F.G.S., of Walhalla— showed me : — 



1. Cryptocrystalline ground mass, in which crystalline 

 grains of quartz were discernible. 



*Professor Ulrich calls this a "diorite- aphanite." See Descriptive Cata- 

 logue of the Specimens in the Industrial and Technological Museum, Met* 

 1875. 



