94 B. K. EMERSON — PLUMOSE DIABASE AND PALAGONITE 



This deposit on the upper surface is repeated on the under surface. 

 That which was deposited on the upper surface was carried forward and 

 downward at the front of the sheet by under-rolling and found its place 

 at last (inverted) beneath the sheet in contact with the subjacent beds. 

 If one compares the base of the trap at the water's edge north of Titans 

 pier, or along the roadway blasted up onto mount Nonotuck above 

 Mount Tom station, with the surface of the trap just south at the great 

 section at Dibbles Crossing on the Westfield-Holyoke railroad, on the 

 south line of Westfield, the identity of the two cases is clear. There 

 may be seen the same blending of the trap with the dove-colored lime- 

 stone which decreases upward. The mass is very scoriaceous at base, 

 and this decreases upward, with the bubbles inverted. One marked 

 structure appears both in the surface and the basal deposit which fur- 

 nishes conclusive proof of this inversion by under-rolling, namely, the 

 long tubular steamholes. So much water was carried into the trap that 

 the latter was not only made very scoriaceous, but the steam escaped up- 

 ward through the trap forming long parallel tubes and keeping them 

 open until the trap was congealed (see plate 25, figure 1, a). The same 

 long empty cavities appear inverted at the base of the trap as at Titans 

 piazza running up six inches from the base (see plate 25, figure 1, 6). 

 The sandstone beneath is soft and unbaked, so that the trap was solid 

 when it reached this position, and it does not seem possible, even if the 

 trap had been liquid, for steam generated in the sand on which it came 

 to rest to bore a great number of parallel holes up into the trap while 

 the layers of the soft sand directly beneath were quite undisturbed. 

 This brings out strongly another important point in which this case is 

 wholly unlike the former one — that is, the Meriden type — namely, the 

 limestone which crowds the lower 12 to 15 feet of the bed can not have 

 been driven up into the trap by explosions from below, because the rock 

 below is a coarse arkose wholly unlike the material in the trap, while in 

 the former case the material carried high up into the trap is of the same 

 kind as that below and is continuous with it, and the bottom scoriaceous 

 layer of the trap is shattered and its fragments are carried with the mud 

 high up into the compact trap. In this second case the lower scoriaceous 

 la} 7 er is continuous and grades regularly upward into the compact trap. 



The reason for assuming that the fine material was carried out over 

 the trap by rapid convection currents of water is that so large an amount 

 of this material was spread on the sheet during the short time of the 

 outflowing and underrolling of the lava and before it came to rest, while 

 the material deposited on it after it came to rest was much coarser. It 

 is perhaps possible that the fine mud was spread over the trap by the 

 ordinary process of sedimentation, because of an exceedingly slow ad- 



