442 GEOLOGY OF OLD HAMPSHIRE COUjSTTY, MASS. 



/A The rock at the new cutting south of the Deerfield River and south- 



ward is very fine-grained, breaking with eonchoidal fracture, dark-gray and 

 compact at the base of the dike, and there distinguished by an abundance 

 of the well-known feathery aggregations of magnetite grains,' while in the 

 Avhole upper portion it is coarsely amygdaloidal, the amygdules filled com- 

 monly with diabantite, calcite, or both — when one penetrates below the 

 deep layer of rusty scoriaceous rock from which all the secondary minerals 

 have been removed — and here the magnetite is never arranged in feathery 

 groups. At the old cutting on the other side of the Deerfield River, a few 

 rods north, the rock becomes more granular in texture, and grayish- and 

 reddish-white varieties occur, svibporphyritic and abounding- with flattened 

 steam cavities, filled now with diabantite. These colors are arranged in 

 layers, giving the rock an indistinct fliiidal structure. These varieties con- 

 tinue northward and are exposed in great force for nearly a mile of fresh 

 cuttings where the road from Greenfield to Turners Falls crosses the dike, 

 and from the Suspension bridge, at the end of this road, along the river 

 side for a mile north, to the mouth of Fall River and beyond. Through 

 all this area prehnite and the products of its decomposition occupy the 

 amygdaloidal cavities in very great quantity, accompanied everywhere by 

 traces of copper minerals in place of calcite and chalcedony, which abound 

 farther south. The masses of native copper found in the till must come 

 from here. 



The most interesting variety is a very coarse one, abundant on the 

 Greenfield road, which contrasts pleasantly with the somber gray of the 

 prevailing ty]3es. Broad white plates of the feldspar stand out upon 

 a dark-red background of decomposed augite, the whole sprinkled with 

 amygdules of prehnite and diabantite. That this coarse variety is younger 

 than the greenish-gray subporphyritic trap is clear from a large slab from 

 the middle of the slope on the Greenfield road, showing a contact of the two, 

 upon which the latter is cut off" immediately and sharply and without change, 

 while the former has a layer of deep-red, very fine-grained rock 1 5™™ wide 

 adjacent to the contact plane. It seems to me, however, to represent only 

 a slight difference in age and to be jjrobably a case of "schlieren," in the 

 sense of E. Reyer.^ 



An exceptional rock occurs abundantly in bowlders on the south side of 



1 Theoriitiache Geologie, 1888, p. 80. 



