1054 A TREATISE ON METAMOEPHISM. 



sequence of the normal increase of temperature with depth. But probably 

 ore deposits belonging here are moi'e commonly produced in connection 

 with eruptive rocks or orogenic movements, or both; that is to say, the 

 eruptive rocks and earth movements are important factors in raising the 

 water above its critical temperature. It has already been seen that ores 

 may grade from those deposited by igneous agencies to those deposited by 

 aqueous agencies. At one end the ores are the result of mag*matic 

 processes; at the other end the ores are deposited by water. At an 

 intermediate stage it is to be supposed that ores may be deposited from 

 gaseous solutions above the critical temperature of water, viz, 365°. 



Lindgren says: "We may assume with great confidence that at the 

 contacts of intrusive rocks with a sedimentary series the temperature usually 

 exceeded 365° C, and the pressure 200 atmospheres.'" 1 Theoretically one 

 would expect that rather frequently in connection with great intrusive 

 masses, and especially in connection with batholitic masses, the water in 

 the surrounding rocks would be raised above its critical temperature, and 

 therefore that its action is pneumatolytic. But the practical question is con- 

 cerning the importance of gaseous solutions in connection with the ores. 

 What are the ores which were thus deposited? What are the criteria by 

 which they may be identified! The only metallic deposits which Vogt 

 regards as produced by eruptive after-action in the sense of pneumatolytic 

 action, are cassiterite veins.* Beck agrees in regarding the tin deposits of 

 Zinnwald, in Saxony, as due to pneumatolytic action." 



While it may be possible that some tin deposits may be produced by 

 magmatic segregation or pneumatolytic action, it by no means follows that 

 all of the tin or even a greater part of it is of this origin. Recently Penrose 

 has visited and studied the deposits of the Malay Peninsula/ Here, as is 

 well known, the greater portion of the tin is actually obtained from the 

 alluvium; but Penrose's studies have shown that the tin of the alluvium has 

 its original source in veins in the granite axis of the peninsula and in the 

 limestones flanking this axis. The ore in these rocks is associated with 

 sulphides, such as bornite and chalcocite, and occurs in veins which have 



« Lindgren, Character and genesis of certain contact deposits, Trans. Am. Inst. Min. Eng., vol. 31, 

 p. 238. 



6 Vogt, tit., pp. 132-137. 



i' The origin of ore deposits (discussion by R. Beck): Trans. Am. Inst. Min. Eng., vol. 31, 1902, 

 p. 945. 



d Penrose, jr., E. A. F., The tin deposits of the Malay Peninsula, with special reference to those 

 of the Kinta district: Jour. Geol., vol. 11, 1903, pp. 135-154. 



