316 Professor John W. Gregory [April 27,. 



This pin tonic water answers all requirements for a depositor of 

 ores. It is coming from the proximity of the metalliferous interior 

 of the earth : it is superheated water— so it is a ready solvent of 

 materials which are insoluble in a less heated water ; and it is usually 

 alkaline, and therefore is the readiest solvent of metallic sulphides — 

 the primary condition of the vast majority of ores. 



Changes in Plutonic Waters during their Ascent. 



From whichever source the water comes, it is obvious that 

 aqueously deposited ores must be limited to the depth at which water 

 exists and works ; and the range of ores of economic value depends 

 upon the conditions which govern the precipitation of metals from 

 underground solutions. Water, although its constituents may come 

 from vast depths within the interior, is limited to a depth of perhaps 

 only six or seven times the depth of existing mines. The lower limit 

 is due to the internal heat of the globe, which increases from the 

 surface at a rate which may be taken as 1° F. for every 55 feet of 

 descent. This is the traditionally accepted average ; and the con- 

 ditions are too uncertain to make it worth while at present to re- 

 average it. Now water cannot exist at a temperature higher than its 

 critical point, 687° F. At the assumed rate of increase, that tempera- 

 ture would occur at an average depth of about 87,000 feet ; the depth 

 is probably much less in the neighbourhood of igneous rocks and in 

 areas of recent volcanic action ; and it may be much lower in ancient 

 rocks in areas which have long remained geologically undisturbed, as 

 e.g. in the Transvaal. 



The figures that can be quoted are only rough approximations, 

 and they will vary in different fields according to the conditions of 

 increase of underground temperature. 



At depths below about 37,000 feet, the temperature would be above 

 the critical point of water, which therefore could not exist as such. 

 Its elements would be given forth as separate gases from the slowly 

 cooling magma ; the gases would rise, and having passed into a zone 

 with a temperature below the critical, would combine to form water. 

 This water would ])e at temperatures far al)ove the normal boiling 

 point ; l)ut it would be kept liquid by the immense pressure of the 

 overlying rocks. This water, owing to its tension, would tend to 

 force its way to the cooler areas through any lines of passage open to 

 it. They Avill be minute capillary passages, as nothing else could 

 exist at such depths. Nearer the surface the pressure is less, so the 

 spaces will gradually increase in size, and in them the water will 

 collect and continue its upward flow. At length, it would reach a 

 level, somewhere al^out the average depth of 1S,000 to 20,000 feet, 

 where the temperature would be about 400° F., at which water has 

 its maximum capacity for solution ; up to that level the water will 



