CHEMICAL COMPOSITION 345 



Ti0 2 0.87 



P 2 5 0.27 



CI 0.10 



F 0.06 



S 0.01 



MnO 0.46 



C 0.80 



CaC0 3 0.34 



Total 100.10 



Selected rock from the center of one of the large lava balls, as free as 

 possible from calcite. E. W. Morley analyst. 

 The calculated norm of the rock would be 



Orthoclase 15.01 Olivine 12.64 



Albite 33.01 Magnetite 7.19 



Anorthite 18.90 Ilmenite 1.67 



Nepheline 1.42 Apatite 0.67 



Diopside 2.48 



Totaling 92.99 per cent, the residue consisting of water, carbon, and 

 calcium carbonate. 

 The rock classifies in 



Class 2, Dosalane; Rang 3, Andase ; 



Order 5, Germanare; Subrang 4, Andose. 



It is thus a member of a very large rock group of diabasic or basaltic 

 composition. It is quite near some of the Newark traps — that from 

 Medford, Massachusetts, analyzed by Merrill, for example — though it is 

 a more basic rock than the average Newark traps, which run from 52 to 

 53 per cent of silica. 



The carbon content was a wholly unexpected result. Gaseous com- 

 pounds of carbon are emitted by most cooling lavas, but a considerable 

 carbon content in the cold lava is most exceptional. Inquiry as to its 

 source is naturally suggested. 



The knob is surrounded by black shales and these at once suggesl 

 themselves as a possible source. There are do recognizable shale inclu- 

 sions in the lava, but it might be possible that the explosive ad ion of the 

 eruption should have mingled a certain content of comminuted shale with 

 it. The thin-sections, however, give no suggestion of any such admixture, 

 no shale particles being recognizable 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 he seen that the rock 

 at. the knob would of necessity contain from 15 io £0 per cent of shale, 

 in order to give the carbon percentage shown on analysis, provided it came 



