130 NEW YORK STATE MUSEUM 



The carbon^ content was a wholly unexpected result. Gaseous 

 compounds of carbon are emitted by most cooling lavas, but a con- 

 siderable carbon content in the cold lava is most exceptional. 

 Inquiry as to its source is naturally suggested. 



The knob is surrounded by black shales, which at once suggest 

 themselves as a possible source. There are no recognizable shale 

 inclusions in the lava, but it might be possible that the explosive 

 action of the eruption should have mingled a certain content of 

 comminuted shale with it. The thin sections however give no sug- 

 gestion of any such admixture, no shale particles being recogni- 

 zable in them. And when it is recalled that, in such black shales, 

 the entire content of carbonaceous matter does not usually exceed 

 5 per cent of the rock, it will be seen that the rock at the knob 

 would of necessity contain from 15 to 20 per cent of shale, in 

 order to give the carbon percentage shown on analysis, provided 

 it came from this source. But neither the thin section, nor the 

 chemical analysis give any suggestion of such an admixture. 



A trifling amount of carbon may have been obtained from dis- 

 solved limestone, as this contains a small amount of organic matter ; 

 and the abundant opaque particles developed in the glass at the 

 limestone contacts, definitely suggests something of the sort. But 

 there is no evidence of a large amount of limestone corrosion, and 

 the rock is low in lime, rather than high. It could not have ob- 

 tained much carbon from this source. ^j 



There remain apparently two possible sources for the carbon. 

 The heat of the intrusion may have liberated hydrocarbons from 

 the shales below ground, which were then taken up by the lava, 

 or the lava itself may have contained more carbon than usual origi- 

 nally, more than could be oxydized 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 occurrence in meteorites is well 



i 

 I 



1 Because of the interest attaching to the presence of carbon in an igneous \ 

 rock, Dr H. S. Washington kindly volunteered to look over the thin sections, jfl I 

 and to examine the insoluble black residues of the rock. In a letter received ' 

 from him too late for incorporation in this report he states that the material \ 

 is carbon, and is not graphite. He also makes the follov^ring suggestion to 

 account for its presence, that it is not impossible that, under proper condi- ; 

 tions of pressure and temperature, FeO will reduce Co^ He calls attention 

 to the limestone inclusions as the source of the C0^ and notes that the 

 carbon in the thin sections is in the glass, and not in the feldspars, hence 

 associated with the iron-bearing portion of the rock. 



