B. K. Emerson— The Deerfield Dyke and its Minerals. 199 



It is the earliest product of the decomposition of the diabase, 

 and proceeded doubtless from the alteration of the augite. In 

 one case I found a mass having the shape of an augite crystal 

 b magnetite toward the outside and polarizing as a 

 liyidual but possessing the bright green color and the 

 strong dichroism of diabantite. It is disseminated in compar- 

 atively small amount through the mass of the rock between 

 rystals and thus in the place of the augit 



more abundantly in the steam cavities and shrinkage cracks 

 with which the rock abounds. It generally coated the empty 

 amygdaloidal cavities first with a quite thick (£-l mm ) foliated- 

 ;tyer with minute, delicate botryoidal surface. Several 



such layers sometimes followed each other, and then the cent*, 

 became filled with a confused granular mass of the same ma- 

 terial, the whole making a very pleasing effect under the micro- 

 scope with its bright green color and striking dichroism. Under 

 crossed Nicols this central granular portion often assumes a 

 deep chlorite green studded with bright colorless spots (calcite?) 

 and maintains this color through a whole revolution of the 

 object, the bright spots being alternately extinguished. Some- 

 times, in the gray mottled diabase, a layer of magnetite or coal 

 grains was interposed between the layers of diabantite, and 

 rarely large distinct crystals of magnetite appear wholly sur- 

 rounded (in section) by diabantite, and in one case a fine, large 

 feather of magnetite projected into the diabantite. The long 

 feldspar crystals, also, which border the cavity, often project 

 freely into it and are then perfectly and more complexly termi- 

 nated than when in the mass. The diabantite folds around 

 and does not penetrate them. Often the center of the cavity is 

 filled with calcite, impregnated with diabantite, so as to pro- 

 duce a pegmatitic appearance on cleavage faces, or with finely 

 fibrous prehnite and this also is for a greater or less distance 

 toward the center blackened by the abundance of the diaban- 

 tite which it has enclosed. 



On the other hand where over the botryoidal layer of dia- 

 bantite there appear quartz, datolite, natrolite, sphalerite or 

 other sulphurets, they are entirely free from this impregnation. 

 In the broad mineral- bearing fissures, the diabantite often so 

 impregnates layers of scaly or fibrous prehnite l-5 mw thick 

 over considerable surfaces that a black or blackish-green mass 

 results, often abundantly slickensided, which so resembles a 

 very fine grained scaly or fibrous schist that I supposed it to be 

 formed by the pulverizing of the trap by friction and the 

 cementing of the powder by prehnite, until the microscope made 

 known its true character. 



Farther north on the dyke opposite Turner's Falls the large 

 flattened cavities are lined with a botryoidal layer l mm thick, 



