122 BULLETIN OF THE 
contact with shale or sandstone on the eastern slope of the ridge, but it 
is generally very vesicular, and resembles in all particulars the upper 
surfaces of all the well determined extrusions in the valley. 
If the abnormal scoriaceousness and broken character of the under 
surface of the trap be rightly interpreted as a result of the flowing be- 
neath water, then its anomalous character, as compared with the lower 
contacts of numerous other extrusives in the valleys, remains to be ex- 
plained. We have little direct evidence on this point, but conclude, as 
sufficient heat and moisture to form a scoriaceous texture at the bottom 
of the flows were present in all cases, that some other factor must deter- 
mine the variation between the considerable disturbance manifested 
here and the lack of disturbance at the contact of sand beds and the 
base of flows in other localities. The most available additional factor is 
a variation of pressure, and this would be a minimum at the base of a 
thin flow in shallow water. The Hartford sheet is probably not over 
forty feet in thickness. Emerson has described a similar disturbance 
and brecciation at the base of a rather thin flow in Massachusetts. It 
may therefore be the case that thin lava flows in shallow waters develop 
an unusually scoriaceous structure at their base as they advance. 
Saltonstall Mountain. Localities 14 (Fig. 2) and 14'.—The curved 
outline of this ridge seems to be the result of a gentle folding after 
the sheet had taken its place in the bedded series, rather than a conse- 
quence of conditions attending the time of eruption; the same may be 
said of the larger and somewhat more irregular curve of ‘Totoket Moun- 
tain, next to the north. There is an almost intuitive hesitation before 
the suggestion that anything so massive as a lava sheet could be folded, 
but this must disappear on recalling the strong folds of the heavy sand- 
stones of Pennsylvania, or the stupendous contortions of the gneissic 
rocks on which the Triassic formation rests. If the sheet were intrusive, 
it might, to be sure, have wedged its way in between the sedimentary 
beds after they had been tilted and gently folded, thus accepting their 
guidance as to the form its outcrop should present ; and this has been 
currently believed, both here and in the case of the similar but larger 
curves of the trap ridges in New Jersey. It is therefore of more than 
local importance to determine whether the Saltonstall sheet is an intru- 
sion or an extrusion ; for if the latter, it surely cannot have originally 
taken its present form, but must have passively suffered deformation 
from an initial horizontal attitude. 
The small opportunity for observation of the contacts of this sheet 
