468 GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



portion of the western wall of the plug, just grazed by the Mount Tom- 

 Holyoke fault, and from its point of contact with the sandstone on the 

 north the boundary of the plug seems to run first north and then about 

 east beneath the continuous area of trap. The southern half of its bound- 

 ary can be much more closely located. 



Continuing south from the south contact of the ti-ap which forms part 

 of the core and the sandstone in fig. 25 along the highest portion of the 

 bluff, and turning round the face of the high bluff at its south end where 

 it overlooks the reservoir, one finds the sandstone to be only a thin veneer- 

 ing on the face of the walls of the great throat, and one can dig at the 

 surface and see that the trap extends down behind the sandstone. The 

 surface boundary of trap and sandstone is, along this line, almost the 

 boundary of the core also, and erosion has spared little of the western half 

 of the overflow. This boundary skirts the eastern vertical face of the bluff 

 for a little way north, and as this bluff soon turns to face south, and runs 

 east less steep and elevated, the surface boundary of trap and sandstone, 

 turns and runs parallel with it, but not quite so near the edge of the bluff, 

 and becomes the south boundary of the sheet as the latter extends east from 

 the plug. 



The baking of the sandstone from the point where the plug was first 

 reached has been exceptionally marked, but along this wall it is more intense 

 than anywhere else in the valley and can be clearly perceived 12 to 16 feet 

 from the trap. Along the middle of this south wall, which continues east 

 from the plug, near the top, a foot-wide dike of trap is intruded between the 

 nearly horizontal layers of the sandstone beneath the sheet. It is unusually 

 decomposed, to a pistachio-green porous mass, with spheroidal structure. 



About 20 feet below this a great horizontal dike or sill starts, just at the 

 reentrant angle made by the southward projection of the high bluff — that is, 

 just where the plug ends and the wall of sandstone facing south begins and 

 seems to branch off from the main trap mass. It starts with a width of 2 

 feet and runs down east, widening soon to 12 feet, and continues with the 

 bottom concealed, and at its end it bends up suddenly, with the sandstone 

 on its back, into a vertical position. It is exposed about 150 feet and is very 

 fine-grained, black, and horizontally fissured for 2 feet at surface, and is an 

 exceptionally fresh, ringing, small-columnar rock in the center. It sends two 

 narrow dikes, an inch to a few inches wide, up into the overlying sandstone. 



