THE DEER FIELD SHEET. 



437 



It has specific gravity of 2.87, and melts easily to a black magnetic aud 

 frothy glass. 



Basic pitchstone from ^^ash bed'" northeast of Meriden. 



ORIGIN OF THE GLASS AND MINERALS. 



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

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

 so great a quantity. 



It might seem ^Jrobaljle that the introduction of so much quartz would 

 have permitted some solution, so that the glass, being more acid, would 

 more easily take the A-itreous form. The percentage of silica is, however, 

 somewhat less than in the average of the diabase, and a study of a gi'eat 

 numl^er of slides failed to show any trace of quartz or tridymite, except 

 in 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 3 per cent of water, and pitchstones 7 per cent, while 

 the corresponding porphyries average only IJ per cent. 



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



