466 



GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



beneath the trap (which has only sHghtly indurated it), and thus separates 

 it completely from the intrusive trap of the plug north of the brook. 



The removal of the mill and dam has improved 

 the outcrop greatly and disclosed a quite sharp anti- 

 cline of the sandstone beneath the trap in the south 

 bank of the brook, which, as the dip of the sandstone 

 I is very low to the east, and the slope of the hill is 

 I in the same direction, has the eflPect to very greatly 

 = increase the width of the exposure of the trap, 

 i although the sheet is only 35 to 40 feet thick here. 

 i The half of tliis anticline as formerl}- exposed 



I seemed to show the trap resting directly on the 

 } basset edges of the sandstone beds, and this, taken 

 I with the unexplained greater width, made it seem 

 I probable that the trap had broken through here.' 

 3 This anticline seems to cause the greater width of 

 ; the trap outcrop south nearly to Smiths Fen-y. 

 ^ From the burnt mill the outcrop of the upper 



I bed extends southward as a prominent ridge just 

 ; west of the river road and separated by a deep 

 I valley from the coiTesponding ridge of the Holyoke- 

 l Mount Tom bed to the west. In the steep west- 

 i ward declivity of tliis ridge the contact of the trap 

 I on the sandstone beneath can be found in many 

 I places, and the sandstone is indurated for a small 

 I distance downward and rarely sends up a steam 

 I hole into the trap above. The trap is covered by 

 \ the heavy tuff beds, which seem in the neighborhood 

 5 of the bunit mill to be l)lended with the trap itself, 

 as if it had fallen upon the latter while it was still 

 molten, so that it is hard to mark the true boimd- 

 ary, but no trace of such blending- could be detected 

 in slides cut for the pm'pose. 



The naiTOwing of the trap upon the map is 

 due to the westward advance of the tuff upon it, by the elevation of the 

 ridge, so that it outcrops in the steep westward-facing bluff. 



' E. Hitchcock, Geol. Mass., 1835, p. 429; 1841, p. 656. 



