OEIGIN AND DEVELOPMENT OF OBE-DEPOSITS. 59 



On the whole, and excluding the culm series of North Devon 

 and East Cornwall, which lies beyond what is generally under- 

 stood as the West of England mineral district, it must be 

 admitted that there is a remarkable absence of carbon and its 

 hydrogen compounds from the district under review. 



Sec. 14. On the primary sources of the ore-material. 



1. In the foregoing discussion the expression " deep- 

 seated " has been constantly used. It is easy to refer the origin 

 of certain phenomena to the depths, but not so easy to define 

 what is meant. Nevertheless, the attempt must be made. 



By the use of the term " deep-seated," we evidently imply 

 sources which lie considerably below our present deepest work- 

 ings, and even below the greatest depths we are ever likely to 

 attain in our mining operations, which can hardly extend to more 

 than 4,000 feet. It will, perhaps, be thought reasonable to 

 assume for the upper limit of the deep-seated sources that depth 

 which has a normal temperature of 212*^ F, and this so far as we 

 know at present must be from 12,000 to 20,000 feet below the 

 present surface. 



Another reasonable datum is indicated by Mr. Sorby's well- 

 known observations and calculations.* His conclusion is that the 

 granitic rocks of widely separated districts must have solidified 

 from a state of pseudo-fusion at temperatures approximating to 

 680° F., and while subject to pressure equal to from 32,000 up 

 to 78,000 feet of rock. More basic and therefore more fusible 

 rocks must of course have solidified at much less depths, or under 

 much lower pressures, while it may well be in some instances that 

 the lower limit of solidification of acidic rocks lies still deeper. 

 Still, it would seem that a depth of 130,000 feet, or say 26 miles, 

 can hardly fail to include all that can be called solid in any 

 proper sense, and this is the limit of our circulation-zone pro- 

 perly so-called. Beneath this all the geological, and much of the 

 physical evidence indicates that there is a zone of rock-substance 

 existing in a state of pseudo-fusion from the abundant presence 

 of highly-heated aqueous solutions charged with all kinds of 

 metallic and non-metallic substances, all of which are in the 



* Structure of Crystals, Quart. Jour. 6eol, 8oc. , 1857. 



