BY E. M. JOHNSTON, F.L.S. 23 



deposit formed from associated heated solutions. If, therefore, 

 this be the process — as Mr. Thureau avers it to be — " which 

 assimilates a great deal to what can be seen in its '■dead 

 state ' at our ' Iron Blow,' " it is Mr. Thureau himself who 

 overthrows his own argument, for it is not " volcanic 

 mud " which he likens to the baryta of the Iron Blow, but the 

 silica found as " lamina in the cups " which, without doubt, 

 by his own showing, was formed as a precipitation from 

 solution! Where, then, is Mr. Thureau's logic in finding 

 fault with me for preferring to believe the same thing in my 

 statement, quoted by him, viz., "It is probable that the four 

 principal elements — iron, bai-ytes, sulphur, and gold — were 

 oxx^vcaXi'^ precipitated together from solution V 



That there can be no mistake that the contents of the Iron 

 Blow were considered by him to be the analogues of the silica 

 precipitated from solution, and not the " greyish semi-liquid 

 mass," is proved by the following sentence : — " If baryta is 

 substituted for silica (as matrix?) in the latter case, the 

 question of origin as to both metalliferous deposits is not 

 only, in my opinion, very suggestive, but forms the only 

 possible true solution of the case." 



I am, of course, extremely gratified to find in this clear 

 expression of opinion that he thus agrees with me that 

 precipitation from solution is " the only possible true solution 

 of the case ;" for while it refutes his " volcanic mud " theory, 

 it more firmly establishes my opinion " that the four principal 

 elements — iron, barytes, sulphur, and gold — were originally 

 precipitated from solution." 



Besides this, there is no evidence at the Iron Blow to show 

 that the respective solutions were in anyway associated with a 

 " volcanic mud " corresponding to the " greyish semi-liquid 

 mass within the mouth of the fumaroles" of America, of 

 whose composition Mr. Thureau's description does not afford 

 us the slightest enlightenment. 



Strictly speaking, mud is a term more appropriately applied 

 to mechanical mixtures of various hydrous aluminous silicates, 

 and such mixtures are fundamentally different from the 

 definite chemical compounds, pyrites and barytes, which form the 

 characteristic contents of the lode at the Iron Blow. 



Causes which produced subsequent modification of 

 materials as originally precipitated. 

 This part of the subject does not concern me so much as 

 Mr. Ward, who is well able to defend his own views. I may, 

 however, be allowed to observe that Mr. Thureau's denial 

 that the soft and pulverulent combination of iron peroxide 

 and barium sulphate of a deep purplish colour, together with 

 the still more modified massive blocks forming the cap of this 



