BY E. M. JOHNSTON, F.L.S. 25 



l^efore him proving that it actually contained 17 per cent, of 

 "baryta, thus :— 



Iron Pyrites, 

 (Section : 2 chains wide.) 



Per cent. 

 Iron bi-sulphide (pyrites) ... ... 83'0 



Barium sulphate (barytes) ... ... 170 



100-0 



The only difference of composition between the pyrites and 

 the purple rock is due to oxidation of pyrites, thus : 



Per cent. 

 Iron peroxide ... ... ... 77"75 



Barytes 19-85 



Water, etc. ... 2-40 



100-00 



It will be seen, therefore, that the derivation of the one from 

 ihe other is not such an inconceivable matter as Mr. Thureau 

 was led to imagine from his inaccurate interpretation of the 

 data at his command. 



Mr. Thureau again makes a curious reference to the baryta 

 of this purplish rock, in his expression — "Novritis Sb fact thsit 

 baryta is the 'matrix' of that purple rock." How baryta 

 can be the "matrix" of the larger constituent iron peroxide 

 (the latter being nearly four parts iron peroxide to one part 

 baryta) is a puzzle to me. 



The woi-d matrix is usually employed by geologists to 

 -designate the rock or main substance in which a crystal 

 mineral or fossil is embedded. According to this meaning of 

 the word, Mr. Thureau is far from correct in stating that 

 " it is a fact that baryta is the matrix of that purple rock." 



MTJD VOLCANOES. 



As regards mud volcanoes, there are two well-known kinds, 

 l)oth of which differ widely in characteristics from the phe- 

 nomena associated with the deposits of the Linda Iron Blow. 



The first kind is not volcanic in the proper sense of the 

 term, although variously named mttd volcanoes, salses, air 

 'volcanoes, and macalubas. Geikie describes these as forming 

 groups of conical hills formed by the accumulation of fine 

 and usually saline mud. They are distinguished from true 

 mud volcanoes in having their chief source of movement in 

 the escape of gases due to underlying chemical changes, usually 

 -carbon dioxide, carburetted hydrogen, sulphuretted hydrogen, 

 and nitrogen. The mud is usually cold. 



