/» THE MINERALS OF TASMANIA. 



numerous, appear generally to be true fissure veins, with a 

 prevailing strike west of north. The argentiferous material 

 may be crudely divided into three vertical zones, but with- 

 out defined bands of demarcation, the upper or surface por- 

 tion mainly consisting of a lithomargic or clay-like body, 

 usually much seamed with iron oxide. This is known to 

 be of very limited extent, but occasionally contains high 

 values in silver, which is mostly in the form of choro- 

 bromide (embolite) or filiform masses of the native metal. 

 The second, or middle, zone is apparently enriched to 

 some degree, and extends to an undefined depth, from the 

 surface, of several hundreds of feet. This portion of the 

 metalliferous lode comprises the highly argentiferous 

 galena for which the field in question is celebrated. At 

 still greater depth much of the sulphide ore appears to give 

 place to sdderite, which is the prevailing gangue mineral 

 of the district. In portions of the Zeehan field a light- 

 coloured melaphyre, locally known by the miners as " white 

 rock," occurs, which is at times distinctly interstratified 

 with the ordinary slate rock. It is stated, by those having 

 an extended experience of this mining locality, that when 

 the lodes occur in connection with this igneous rock the 

 ore has a much higher silver content. This interbedded, 

 and perhaps occasionally intrusive, melaphyre has been 

 described in detail by Mr. W. H. Twelvetrees {vide Pro. 

 Roy. Soc. Tas., 1896). It is an altered vesicular basalt, or 

 otherwise a basic lava. It is in a varied state of altera- 

 tion, and is embedded with the slates of the district, which 

 have been allocated to the base of the Middle Silurian 

 epoch. The melaphyre apparently follows the stratifica- 

 tion of the slates. In colour the igneous rock is a light- 

 grey, somewhat mottled with a darker shade. It contains 

 numerous macroscopic and microscopic steam pores or 

 cavities, now filled or partly filled in numerous instances 

 with delessite and calcite, or sometimes with both. Sider- 

 ite is also nlentiful throughout the base, and calcite also 

 occurs in the form of thin veins. These minerals are pre- 

 .sumably an impregnation after consolidation, and are 

 responsible for the pale colour of the rock. 



The complex sulphide ores of the Mt. Read and Rosebery 

 districts in their g^eological, and to some extent their 

 tnineralogical, features present a marked contrast to those 

 which occur at the adjacent mining fields of Zeehan on the 

 one hand and Mt. Farrell on the other, and although 

 differing in their metal and economic production, the 

 salient points generally have a marked similarity, and 



