26 THE "iron blow" at the LINDA GOLDFIELD. 



The true mud volcano occurs in volcanic regions proper, 

 and " is due to the escape of hot water and steam through 

 beds of tuff or some other friable kind of rock. The mud is 

 kept in ebulition by the rise of steam through it. As it 

 becomes more pasty the steam meets with greater resistance; 

 large bubbles are formed which burst, and the more liquid 

 mud below oozes out from the vent." 



These true mud volcanoes, in my opinion, neither in their 

 mode of appearance, nor in their characteristic contents, show 

 the slightest correspondence with the metalliferous fissure 

 lodes of the Linda district. 



I may mention that although my examination of the 

 various lodes in this district was necessarily limited, they- 

 occupied my close attention for the better part of three days, 

 at a time when they were well exposed by working opera- 

 tions j 



Discussion. 



Mr. W. p. "Ward, G-overnment Analyst, said : — 



The point under discussion is the origin of the "formation"" 



known as the " Iron Blow," the oxidised portion of which was 



described by Mr. Thureau as " volcanic mud or ash." Mr. 



Johnston, however, from close examination on the spot, and I 



myself, from the " internal evidence " yielded by specimens, 



etc., attribute to this a non- volcanic origin. 



The materials of this formation are (1) barytes, sulphate of 



barium, or heavy spar, (2) iron pyrites, or disulphide of iron, 



(3) haematite, or sesquioxide or peroxide of iron, 



I will glance briefly at the usual modes of occurrence of each, 



as showing in the first place that they are not usually 



" volcanic products." 



1. " Heavy spar" occurs commonly in connection with bed& 

 or veins of metallic ore as part of the " gangue " of the ore. 



It is found crystallised in the Cumberland haematite mines 

 in the carboniferous limestone, and as much as 14 per cent, of 

 sulphate of barium has been found disseminated in hsematite 

 from another district. 



2. " Iron pyrites " is very widely distributed and abundant 

 in rocks of all ages. By the decomposition (by the action of 

 water and air) on the large scale of masses of pyrites, deposits 

 of brown iron ore maybe produced, sulphur being lost and 

 oxygen and water taken up by the iron, and a very moderate 

 heat suflBces to convert this hydrated brown oxide into the red 

 oxide or hsematite by driving out the combined water. 



3. *' Hsematite " occurs in many forms differing in texture 

 and state of aggregation as : (o) crystallised, forming 



