THE DEERFIELD SHEET. 441 



Southward the trap rises higher and continues, with lofty, nearly 

 vertical walls on the east and west, between the river and tlie town of 

 Greenfield. President Hitchcock quotes "trap tutf" as constituting, a mile 

 east of Greenfield, "a large portion of the ledge of greenstone, which is in 

 places a hundred feet thick." This is the great pitchstone-breccia at the 

 base of the bed described above. 



Across the deep notch of the Deerfield River the sheet rises and 

 thickens in Deerfield Mountaui and looks down with vertical wall upon the 

 village of Deei-field at its foot. It shows just east of the village the finest 

 columns in the State, 2 to 3 feet in diameter, and in places distinctly curved.^ 



Farther south, just before crossing the river, the great sheet shows, 

 from below upward, four horizons of heavj- amygdaloids, indicating, doubt- 

 less, that it is a composite of as many great lava flows in this portion of 

 its extent. 



To the south, in Mount Toby, where it is thinner, it is amygdaloidal in 

 nearly its whole thickness, while at its north end it is compact at base and 

 heavily amygdaloidal in its upper portion. 



PETROGRAPHICAL DESCRIPTION. 



NORMAL DIABASE. 



The rock is a typical diabase, ranging from aphanitic varieties to those 

 where the white, flat feldspars are 2 to 4°"° square, and from compact to very 

 coarse amygdaloidal. The different veins are of very uniform texture and 

 always in an advanced stage of decomposition, though appearing quite 

 fresh; plagioclase, apparently of two sj^ecies, augite, magnetite, and oli%nne 

 are uniformly present. Apatite can not be detected. 



The common plagioclase, probably labradoiite, is always by far the 

 most abundant constituent, and the angle of extinction of its long rodlike 

 crystals is commonly 12°. Several varieties of the rock are subporphyritic 

 by the development of white spots, made up of groups of stout crystals of 

 a second triclinic feldspar, apparently distinct from the first, whose angle of 

 extinction is 21°. Both feldspars are thoroughly decomposed, commonly 

 from the center, and sometimes show only aggregate polarization. 



The augitic constituent has for the most part gone over into a mixture 

 of green and brown chloritic minerals, but here and there an exceptionally 

 arge crystal remains in whole or in part intact. 



'E. Hitchcock, Geol. Mass., 1841, p. 642. 



