MUSEUM OF COMPARATIVE ZOOLOGY. 129 



presence of augite, the lower sheet would be classed as a glassy form 

 of angite-porphyrite. 



The texture of the rock steadily grows finer, and the cavities due to 

 gas expansion more numerous, as we approach the surface of junction 

 with the upper sheet ; and there is at the same time a marked increase 

 in the amount of glass forming the base. At five feet below the contact 

 the vesicles occupy nearly one fourth of the space, and in slide 140 of 

 our collection they are seen to be elongated parallel to the surface of 

 contact, as if indicating flowing action. The origin of the vesicles bj 

 gas expansion is beautifully shown in this slide by the well marked 

 tangential arrangement of the feldspar crystals about the elongated and 

 tortuous amygdaloidal cavities, conforming even to their minor irregu- 

 larities. Sections from the red seam, just under the junction, show 

 this portion of the trap to have been blown almost to shreds by the 

 escaping gases. The scoriaceous character here cannot be doubted ; 

 fully two thirds of the rock is made up of secondarily deposited calcite 

 and quartz, filling the irregular cavities and vesicles of the porous mass. 

 The inter-vesicular areas consist of a greenish glass, thickly sprinkled 

 with hair-like microliths of feldspar and an occasional porphyritic crystal 

 of the same. The red color of the seam is due to the formation of iron 

 sesquioxide. Hawes noted that the oxidation of iron-bearing minerals 

 exposed to surface weathering is from the protoxide to the sesquioxide 

 state, while the change is from one protoxide to another when not thus 

 exposed, as is true of the eastern ranges.^ It therefore seems likely that 

 in this instance the red color of the surface of the lower sheet indicates 

 surface weathering before the upper sheet was erupted, thus confirming 

 the suggestion already made, tha*- the thickness of the flow was great 

 enough to raise its surface above water. It is rare that this red color is 

 seen in the traps of the Triassic area. 



The scoriaceous character of the sheet at its upper surface is much 

 more strongly marked than in others thus far examined in the Connecti- 

 cut valley ; this is also thought to be connected with the appearance of 

 the surface of the sheet above the surface of the body of water into 

 which it flowed. Cooling under the air must have taken place much 

 less rapidly, and under much less pressure, than when below the water 

 surface, thus permitting a more complete expansion of the occluded 

 gases and the production of a highly pumiceous surface layer. 



The trap of the upper sheet just above the red seam appears in the 

 hand specimen much less altered than that from below. Even -at the 

 1 Amer. Journ. Science, IX., 1875, p. 190. 



VOL. XVI. — NO. 6 9 



