DISCUSSION OP THE CHEMICAL ANALYSES 115 



COMPOSITION OF THE GLASS AND INCLOSING LA VA 



The difficult separation of the glass was effected with great skill by Mr 

 George Steiger. He found for the glass the composition given in analy- 

 sis I, page 112. He remarks that as the mineral was dried at the water 

 bath after making the " Thoulet" separation some of the water coming 

 off below 100 degrees was probably lost before the analysis was started. 

 This would increase a little the close agreement with palagonite, which 

 has a content of water varying from 12.79 to 20.67 per cent. The ma- 

 terial dissolved easily and completely in hot HC1, leaving a sandy residue 

 of Si0 2 . He found that the glass gave off H 2 at 100 degrees = 8.57 ; at 

 150 degrees, 2.09; at 200 degrees, 3.12; at 250 degrees, 1.44; at 300 de- 

 grees, .71 ; by blast lamp, 1.15 = 17.02. I place beside these the analysis 

 of the palagonite of Palagonia (analysis II) and of Seljadalr (analysis 

 III) to show that they are as closely allied chemically as in all their 

 physical characters, and also by way of contrast that of the diabase 

 pitchstone or tachylite from the base of the diabase sheet at Meriden 

 (analysis VI). It is noteworthy that if this latter were made anhydrous 

 it is practically identical with the normal diabase (analysis V). There 

 has been no differentiation. Analysis IV, by Mr Steiger, is made from 

 a large sample of the glass-bearing trap and gives fully the average of 

 the rock. For comparison with this analysis that of Mr G. W. Hawes 

 (analysis V), of the trap of mount Holyoke, from the same sheet, a little 

 farther north, is given. The recalculation of analysis IV, given under 

 VIII, shows that the sample contained an unusual quantity of Ti0 2 , 

 which explains in part the large amount of FeO. It represents a mix- 

 ture of holyokeite, normal diabase, and some glass. The H 2 should 

 have been somewhat greater. Attention is directed to analyses X and 

 XI and the explanations given beneath them. The holyokeite is a fresh 

 looking network of albite crystals with many steamholes filled with 

 calcite. As this can not have come from the albite it may have been 

 dissolved calcite as at the reservoir locality. The practical identity of 

 these rocks in fresh condition with albite is remarkable. The identity 

 of the small holyokeite dikes and the lithoidal base surrounding the 

 glass is also made clear by figures 3 and 4 of plate 30. 



Thus at many separate localities in the whole extent of the trap sheet 

 this albitic leucocratic differentiate appears, generally in small amount 

 and generally giving indication that water has been the efficient agent 

 in its production. Only in this instance has the other differentiate, the 

 highly basic melanocratic residuum, been preserved, unless some of the 

 fibrous ferruginous material filling or lining cavities which we have called 

 delessite may be this residuum in a devitriried condition. 



XVI— Bull. Geol. Soc. Am., Vol. 16, 1904 



