THE ADIRONDACK GRAPHITE DEPOSITS 145 



ascension through graphitic sediments picked it up and deposited it 

 on cooling at the margins of its mass. 



Winchell 1 states that " graphite is practically insoluble in silicates 

 at ordinary temperatures of magmas ; it is upon this fact that its 

 value as a refractory material partly depends. How, then, can the 

 occurrence of graphite as a constituent of igneous rocks ... be 

 explained?" The temperature of liquefaction and vaporization of 

 graphite is in the neighborhood of 3000 Centigrade at normal 

 pressures. The effect of increased pressure in raising the melting 

 point is so small that we may neglect this factor. 2 " Therefore" the 

 suggestion sometimes made that sublimation from carbon vapor 

 [is responsible] seems highly improbable." 3 It seems equally clear 

 that carbon could not be in silicate solutions as a solid in an 

 undissolved form or in true solution. 



Volatile hydrocarbons are then offered as a source of graphite.' 1 

 That such gases do occur as constituents of igneous rocks, has long 

 been recognized."' Winchell has ably discussed the probabilities and 

 points out that " it seems improbable that they are the source of 

 graphite deposits, for several reasons. First, in the presence of 

 water they will apparently be wholly transformed into carbon mon- 

 oxide and hydrogen at 700 to 8oo° C. Second, in the absence of 

 water the decomposition of hydrocarbons occurs (only) at high 

 temperatures . . . ; any graphite that resulted from such a 

 process should be formed in the hottest part of the intrusive magma 

 rather than in the much cooler (margins) . . . where the 

 graphite is actually found ; and ... no reaction is known for 

 the decomposition of hydrocarbons below 500 C." 



In a similar way Winchell disposes of the theory that the role 

 played by carbides is responsible for the formation of graphite. 



The most plausible theory is that the ultimate source is either 

 carbon dioxide or carbon monoxide. Examination of the pegma- 

 tites from a great many of the contact deposits of the northern area 

 shows microscopic gas bubbles of the oxides of carbon in great 

 abundance. Lincoln/' Chamberlain 7 and Salisbury 8 point out that 



1 Winchell, A. N., " A theory for the Origin of Graphite," Econ. Geol. 191 1. 

 6:222. 



2 Harker. Alfred, " The Natural History of Igneous Rocks," 1909, p. 163. 



3 Winchell, ibid. 



4 See references given by Winchell. 



5 Lincoln, F. C, Econ. Geol. 1907, 2:257; Chamberlain, R. T., " Gases in 

 Rocks," Carnegie Inst. Pub. 106. 



6 Lincoln, F. C, Econ. Geol., 2:258. 



7 Chamberlain, T. C. & Salisbury, R. D., " Geology," 1 :6i8, 619. 



8 Chamberlain, T. C, Carnegie Inst. Pub. 106. 



