490 GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



Following the vertical wall of the diabase north 335 feet from the 

 quaiTj — a wall which is the contact surface against which the sandstones 

 formerly rested — one comes upon a most interesting point, where this wall 

 is continued as sandstone, a fine contact being exposed, and the boundary 

 line of the diabase and sandstone goes into the hill at a right angle. Climb- 

 ing to the top of the bluff", one can follow this contact east, the sandstone 

 at a distance of 4 feet from the diabase being baked into a dark-blue, 

 hornstone-like rock. When the boundary bends round from east to north 

 the thin-fissile sandstones have the unusual position, strike N. 70° W., dip 

 40° E., being thus tlu-own off from the eruptive rock. Continuing, the 

 boundary returns westwardly,' and thus embraces a great projection of the 



Fig. 27. — Section of contact of Black Rock plug and the Mount Holjoke diabase bed. 



sandstone which extends far into the diabase, and then turns round to the 

 east, parallel to the direction of the older bed. 



For a long distance one can follow up the bed of Dry Brook running 

 on the back of the older diabase, while its left (south) bank is a vertical 

 wall of sandstone dipping southward and ending abruptly against the 

 diabase of the Black Rock dike, as indicated in fig. 27. 



At the point where the first outcrop of the sandstone on the brook 

 appears, about 590 feet from the contact in the vertical wall last described, 

 occurs a curious metamorphosed Hmestone-breccia, with garnet, near the base 

 of the sandstone. This nearness of the two diabase bodies continues, and 

 one goes east a long distance tlu'ough a valley with its right or north side 



