1914] Lawson: Ore Deposition by Meteoric Waters 



229 



become an argument either for the hypothesis of magmatic waters or 

 against the hypothesis of meteoric waters. The failure of "evidence 

 of fracture or paths which could have been followed by the water" 

 applies equally well to magmatic as to meteoric waters, and is of little 

 moment when we reflect that the water in either event was probably 

 in the form of superheated steam. 



Hydrot hernial Attack. — Professor Lindgren's next objection is 

 stated as follows: "Inevitably, such a hydrothermal circulation as is 

 suggested by Professor Lawson would result in a strong alteration of 

 the intrusive. In many instances there is no such evidence of replacing 

 solutions. Fresh granite rock often borders the ore." 



The obvious answer to this objection is of course that, in so far as 

 it is a fact, magmatic waters escaping from a magma constitute a 

 hydrothermal circulation no less than meteoric waters locally heated, 

 and that where they traverse the solidified intrusion, as is so often 

 assumed, they must also equally inevitably cause a strong alteration 

 of the rock. When this is found not to be the case, as Lindgren states, 

 there must be something wrong with the hypothesis of magmatic 

 waters. But, as a matter of fact, in those cases in which the waters 

 responsible for ore deposition are known to have traversed the igneous 

 rock, as for example, at Butte and at Clifton-Morenci, there has been 

 a notable amount of hydrothermal alteration ; and the whole question 

 at issue is whether this water is magmatic or meteoric in origin. 



Thermal and Reactionary Metamorphism. — Finally Professor Lind- 

 gren dissents from any attempt to discriminate between thermal 

 metamorphism and reactionary metamorphism at the contact of in- 

 trusive rocks. Thermal metamorphism I consider to have been proven 

 experimentally by the conversion of chalk into marble by heating it 

 in a sealed chamber 23 and also to have been proven inductively by 

 Rosenbush 's studies at Barr Andlau. 24 



Reactionary metamorphism may be freely recognized without 

 admitting that it is responsible for the deposition of ore bodies, par- 

 ticularly those occupying fissures in the irruptive mass which produces 

 the metamorphism. Emanations from magmas are not denied and 

 certain of these by reaction with the encasing rocks give rise to the 

 deposition of minerals which are rather characteristic and different 

 from those of the ore deposits in question. 



23 Sir James Hall, TraDS. Eoy. Soc. Edin., vol. 6, 1805, p. 107. 

 2* Die Steigerschiefer unci ihre Contactbildungen an den Granititen von Barr- 

 Andlau und Hohwald, Strassburg, 1876. 



