346 H. P. CUSHlNG NORTHUMBERLAND VOLCANIC PLUG 



from this source; but neither the thin-section nor the chemical analysis 

 gives any suggestion of such an admixture. 



A trifling amount of carbon may have been obtained from dissolved 

 limestone, as this contains a small amount of organic matter, and the 

 abundant opaque particles developed in the glass at the limestone con- 

 tacts definitely suggests something of the sort; but there is no evidence 

 of a large amount of limestone corrosion, and the ruck is low in lime, 

 lather than high. It could not have obtained much carbon from this 

 source. 



There remain apparently two possible sources for the carbon. The 

 heat of the intrusion may have liberated hydrocarbon from the shales 

 below ground, which were then taken up by the lava, or the lava itself 

 may have contained more carbon than usual originally, more than could 

 be oxidized by the usual volcanic processes. Either process is unusual, 

 but then the occurrence is itself unusual. Graphite does occur in igneous 

 rocks; as, for example, in some of the Adirondack pegmatites. Its oc- 

 currence in meteorites is well known. It is quite possible, therefore, that 

 it is here one of the primary minerals of the lava ; but the large amount 

 present, together with the rarity of its occurrence in igneous rocks, en- 

 forces caution in attributing such a source to it. In so far as possible 

 derivation from the shale is concerned, it must be stated that the associ- 

 ated shales in the vicinity of the knob show no indication of any loss of 

 carbon due to action of the lava, or indeed of any change whatever, so 

 that if we look to them as a source we must assume that the process only 

 took place at greater depths. It would be a debatable question whether 

 hydrocarbons, liberated from the shales by the heat of the intrusion, 

 would migrate into the lava; whether they would not rather be driven 

 away from it. Even if taken up, why should they have been converted to 

 graphite? All, or nearly all, lavas contain carbon; but on cooling they 

 give it off in combination with oxygen or with hydrogen. Why an excep- 

 tion was made in this case we can not say. 5 



Is the Lava in Place ? 



Starks Knob is a small mass of igneous rock inclosed in shales which 

 are everywhere greatly folded and sheared, and which the evidence, both 



5 Dr. H. S. Washington has been so kind as to examine the thin-sections of the rock 

 of the knob, and the black, insoluble residues obtained chemically. His comments were 

 received too late to be inserted in the paper. Under date of May 25, 1913, he writes : 

 "The black material is certainly carbon, but is not graphite. The idea has occurred to 

 me that its presence is possibly due to reduction of CO* by FeO, the C0 2 supplied from 

 the limestone inclusions. This seems a rather wild hypothesis, but is suggested by the 

 appearance of the sections where the carbon is about the mafic minerals and not in the 

 feldspar. I have talked it over with Hostetter, who is working on the iron silicates, 

 and he says that it is possible, given proper temperature and pressure, for FeO to re- 

 duce C0 2 ; to what extent, of course, is uncertain, and dependent upon conditions." 



