200 B. K. Emerson — The Deerjield Dyke and its Minerals. 



of black-green diabantite with crystals of chalcopyrite, blende 

 and albite upon it. The interior is very often filled with a 

 dark olive-green, fine-granular mixture of crystals of diaban- 

 tite which can be shaken out of the cavity as a fine powder, 

 each grain of which appears, under the microscope, as beauti- 

 fully vermicular as the helminth of the older rocks. 



The paragenesis of the mineral is thus quite definitely fixed. 

 It was the first product of the decomposition of the diabase 

 and its formation ceased not very long after calcite and prehn- 

 ite began to be deposited in the cavities and fissures. As the 

 formation of the latter minerals was attended by quite ener- 

 getic decomposition of the trap, the formation of the diabantite 

 occurring still earlier ma}*- well have been promoted by the 

 increased chemical activity of the waters during the cooling of 

 the trap after its solidification. The mineral seems to me 

 plainly of secondary formation, and I can see no good ground 

 ing that it was formed and deposited where we find it 

 during the rise of the lava through the sandstones. The cav- 

 ities flattened and fluidally arranged in the lower part of the 

 mass with well terminated feldspars projecting into them, and 

 growing very abundant, and graduating into long vertically 

 placed tubes in the upper portions, have been certainly formed, 

 by steam, and cannot have been filled till after son 

 of the rock. This is much more certainly the case with the 

 broad fissures which extend across the whole dyke, which seem 

 for a time to have furnished a passage-way for boracic acid 

 springs, and in which the diabantite must have been formed 

 •ere the calcite and prehnite which it has impregnated. 

 The filling of the cavities also with several concentric and bot- 

 ryoidal layers sometimes separated by calcite would indicate 

 slow deposition from water. 



The great amount of protoxide of iron in the mineral does 



it is an original constituent of the rock. The waters brought 

 up in the lava, on becoming liquid, or the waters which reached 

 the bed after percolating through the bituminous sandstones of 

 the valley may have been deprived of oxygen and able to 

 exert strong solvent activity, without peroxidizing the iron of 

 the augite. In discussing a mineral, which seems to be iden- 

 tical with our diabantite and which forms the tirst coating of 

 the amygdulcs of the phillipsite-bearing feldspar- basalt of 

 Salesl, von Zepharovich* derives the same from spluerosiderite, 

 the radiated and concentric structure and the botryoidal surface 

 resembling in miniature that . m.mmi, in the carbonate. A 

 cellular structure and traces of rhombohedrai forms were also 

 observed. I have seen here no traces of any such crystal forms. 



