SOURCE OF THE METALS. 1031 



and in this monograph I have emphasized the point that if at any time the 

 temperature of the solutions exceed 365° C. they are necessarily gaseous. 

 I have also held that there is every gradation from ore deposits produced 

 by pure magmatic differentiation through those deposited by gaseous 

 solutions to ores produced by the work of underground water at ordinary 

 temperature. a 



But it has been explained (see pp. 1014-1016) that igneous rocks 

 influence the segregation of ores in other important ways than by contrib- 

 uting the metals, viz, by heating the solutions of the surrounding rocks 

 and furnishing hot solutions to them, by producing channels for circula- 

 tion, and by furnishing impervious formations so as to converge solutions 

 into trunk channels. 



Le Conte pointed out many years ago that the undoubted frequent 

 occurrence of workable ore deposits in regions of volcanism may be 

 explained by the heat furnished by the igneous rocks, thus promoting- the 

 work of underground solutions. 1 That the heat furnished by the igneous 

 rocks is a very important factor in the production of the ore deposits, I have 

 no doubt. Since it is often difficult to prove that the metallic content of 

 an igneous rock is original, it is impossible to make any general statement 

 as to whether the metallic content or the heat furnished by the igneous 

 rocks is the more important in the production of the ore deposits. It seems 

 clear to me that both are very important ; and equally clear that in many 

 cases both work together. That is to say, an igneous rock may furnish all 

 or a part of the metal which appears in an ore deposit, and the heat of the 

 same igneous rock may greatly assist its concentration by underground 

 waters of meteoric origin. 



While the later igneous rocks are the undoubted source of some, 

 perhaps many, ore deposits, it is also equally certain that another large 

 part is derived from the sedimentary rocks and the metamorphosed, or 

 partly metamorphosed, igneous and sedimentary rocks. As a case clearly- 

 illustrating the importance of igneous rocks in increasing the activity of 

 solutions and enabling them to derive a large amount of material from the 

 adjacent sedimentary rocks, there may be mentioned the Mammoth Hot 



« Van Hise, C. R., Some principles controlling the deposition of ores: Am. Inst. Min. Eng., vol. 

 30, 1901, pp. 174-175. 



''Le Conte, Jos., On the genesis of metalliferous veins:. Am. Jour. Sci., 3d series, vol.26, 1883, 

 p. 10. 



