344 H. P. CUSHlNG NORTHUMBERLAND VOLCANIC PLUG 



mainder and is also packed with the fine, opaque material, which is evi- 

 dently a product of the corrosion, as elsewhere both limestone and lava 

 are free from it. 



The pitchstone which occupies the spaces between the balls is found 

 massive, and uncrushed only when protected by limestone inclusions. It 

 consists of clear, green glass, so precisely like that formed in similar 

 situation in the balls as to suggest a like origin. The field evidence is 

 plain that the pitchstone has had much more powerful corrosive action 

 on the limestone of the inclusions than the lava of the balls had ; hence 

 the much greater quantity of this glass in the pitchstone. Except for 

 an occasional pyrite and frequent nests of abundant, tiny, opaque matter, 

 there is no sign of any crystallization. 



Chemical Composition 



Since the rock from the centers of the large lava balls seemed in quite 

 fresh and unaltered condition, except for the olivine, it was confidently 

 expected that a chemical analysis would definitely show the composition 

 of the lava and would be attended by no especial difficulty; but analysis 

 developed the presence of a considerable amount of carbon in the rock, 

 rendering determination of ferrous iron very difficult, and showed also 

 the presence of a large quantity of water. This also was difficult of 

 exact determination. But Professor Morley has labored indefatigably at 

 the problem. The ferrous iron Avas finally determined by bichromate 

 titration. Professor Morley states that the results can not be vouched 

 for to the single drop, as with permanganate, but that the uncertainty 

 is not great; that he can not hope that the accuracy of the three de- 

 terminations — Fe 2 3 , FeO, and H 2 0+ — is as great as that of the others, 

 but that he has every reason to believe in their reasonable accuracy. 

 Our indebtedness to him for the painstaking labor and the eventual very 

 satisfactory result is most emphatically expressed. 



The water is no doubt present as a constituent of the glass. The 

 interest attaching to the presence of carbon more than compensates foi 

 the slight uncertainty introduced into the analysis. 



Si0 2 47.55 



A1 2 3 16.65 



Fe 2 3 4.94 



FeO 5.12 



MgO 5.63 



CaO 4.77 



Na 2 4.20 



K 2 2.52 



H 2 0+ 5.57 



H 2 0- 0.24 



