BY W. F. PETTERD, C.M.Z.S. 79 



afford a parallel to the occurrences at Mt. Lyell. More- 

 over, the same felsitic igneous intrusions are common to 

 both, these being a pronounced characteristic along the 

 line from Lyell on the south to the northern flanks of Mt. 

 Read, and fhence some distance further north. This felsite 

 has been described by Mr. W. H. Twelvetrees and the 

 writer (Pro. Roy. Soc. Tas., May, 1899). In this paper 

 reference is made to the relationship of this extensive 

 igneous mass. An endeavour is therein made to show that, 

 so far as known, the igneous intrusive does not contain 

 any appreciable metallic sulphides, and that as soon as 

 this rock is intersected in the various mining workings, the 

 characteristic ores of the locality at once disappear. 



The mixed sulphides consist, as a rule, of a fine granular 

 mixture of galena, sphalerite, and pyrite, with a small 

 infusion of chalcopyrite, the whole giving varying assay 

 returns for gold and silver. Occasionally fair-sized masses 

 of highly cubical galena are met with, as well as patches 

 of sphalerite. It is characteristic of the Hercules Mine 

 at Mt. Read that some so-called lodes, or lenticles, may 

 be in parts unusually rich in either lead, zinc, or copper, 

 and again in places the gold is much higher than the 

 normal occurrence. The same remarks apply in a modified 

 degree to the other occurrences on the Mt. Read field 

 as well as at Rosebery. These mixed sulphides form len- 

 ticular masses, often with an irregular outline, in the 

 argillaceous and other schists and phyllites usually parallel 

 with the planes of foliation. The lenses of ore have pro- 

 bably been produced by a molecular process of chemical 

 replacement, originating from circulating thermal solu- 

 tions, which followed lines of weakness throughout the 

 rock. The segregation and disposition of the metallic 

 sulphides were doubtless subsequent to the marked folia- 

 tion of the ore-bearing rocks. It has been shown by 

 mining operations that as the ends of the metallic lens 

 become too small to be remunerative a crosscut at the 

 extremity may often intersect the starting-point of another 

 mass. These lenses vary in size from a mere thread, in 

 many cases following the foldings of the encasing rock, 

 and imperceptibly dying out, through substantial seams 

 a few inches in thickness, thence to huge masses of great 

 size, which are practically solid mixed sulphides without 

 any noticeable included rock or gangue. These inter- 

 mixed masses of mixed ore are not peculiar to this island, 

 but are known to occur in many mining regions, notably 

 in France and other places in Europe, America, in Angle- 



