B. K Emerson— The Deerfield Dyke and its Minerals. 197 



intact. The olivine is often the freshest looking mineral in the 

 slide except the magnetite. 



The rock at the new cutting and southward is very fine 

 grained, breaking with conchoidal fracture, dark gray and com- 

 pact at the base of the dyke, and there distinguished by the 

 abundance of the well-known feathery aggregations of the mag- 

 netite grains, while in the whole upper portion it is coarsely 

 amygdaloidal, the amygdules filled commonly 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 gran- 

 ular in texture, grayish and reddish varieties occur, sub-porphy- 

 ritic, and abounding with flattened steam cavities filled now 

 with diabantite which arranged in layers give the rock an indis- 

 tinct fluidal structure. These varieties continue northward and 

 are exposed in great force for nearly a mile of fresh cuttings, 

 where the road from Greenfield to Turner's Falls crosses the 

 dyke, 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 pivhnite 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 mid chalcedony which 

 abound farther south. The most interesting variety is a very 

 coarse one abundant on the Greenfield road, which contrasts 

 pleasantly with the somber gray of the prevailing types. 

 Broad white plates of the feldspar stand out upon a dark red 

 background of decomposed augite, the whole sprinkled with 

 the amygdules of prehnite and diabantite. That this coarse 

 variety is younger than the greenish gray sub-porphyritic 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 

 hitler is cut oil' immediately and -harply and without change, 

 while the former has a layer of deep red, very tine grained 

 rock 15 mm wide adjacent to the contact plane. It seemed to 

 me, however, to repi fterence in age and to 



be probably a ease of "schlieren " in the sense of K. Reyer.* 



An exceptional rock occurs abundantly in bowlders on the 

 south side of the Deerfield, but I have not met it on the north 

 or in place. It is a clear, light gray rock, with roundish 

 blotches of white, and it looks like a weathered leocitophyr. 

 Under the microscope the blotches are ^cen to be made up of 

 aggregated stout crystals of plagioclase, and the rest of the mass 

 between, of rod-like plagioclase and magnetite with almost no 

 *Tschermak, Min. Mitth. 



