78 B. K. EMERSON — DIABASE PITCHSTONE AND MUD ENCLOSURES. 



FeO 4.67 



NmO trace. 



BaO 03 



SrO. trace. 



CaO 9.42 



MgO 7.69 



K 2 2.02 



Na 2 1.85 



Li 2 trace. 



H f at 110° 1. 29 



2 I above 110° 3.43 



99.92 



Origin op the Glasses and Minerals. 



It remains to consider the cause of so extensive a development of glass 

 in the midst of the trap as a result of the introduction of water and sand 

 in so great a quantity. 



It would seem probable that the introduction of so much quartz may 

 have permitted some solution, and thus that the glass being more acid 

 was more apt for the vitreous form. The percentage of silica is, however, 

 somewhat less than in the average of the diabase, and a study of a great 

 number of slides failed to show any trace of quartz or tridymite, except 

 as a late vein filled with coarse calcite and analcite. Slides boiled with 

 concentrated HKO failed to show any change. 



It is more probable that water has been absorbed in such quantity as 

 to have contributed to the observed result. While obsidians are water- 

 free, pearlstones average .03 per cent of water, and pitchstones .07 per 

 cent, while the corresponding porphyries average only .01 J per cent. 



It is remarkable, considering the quantity of water which must have 

 been carried into the mass with the mud, that there is almost no trace 

 of amygdaloidal development. Only one fragment of a trap enclosed 

 with others in a breccia contained small steam holes as described above. 



The collapsed cavities with wrinkled interiors and the absence of the 

 common steam holes are explained by the absorption of the water by 

 the glass, and this absorption explains the unusually large development 

 of basic glass in connection with this exceptional occurrence. Above 

 the compact and columnar trap which rests on this hydrated glass is the 

 usual coarsely amygdaloidal surface layer of the trap, whose moisture 

 seems to have no connection with this development at the base of the bed, 

 and yet it is in this surface amygdaloid in the Deerfield bed that I found 

 perfect secondary albite crystals resting on delessite in the amygdules. 



The great abundance of calcite and its intimate admixture with the 

 other constituents is remarkable. I have elsewhere given reasons for 



