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 pi'essure, 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) andli'. — 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 

 bods 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 



