GABBROID DIABASE IN WESTFIELD 581 



material by the water. This is quite contrary to our knowledge 

 of the effect of the rate of cooling upon the grain of igneous rocks. 

 The writer after some study has reached the conclusion that the 

 coarseness of crystallization of the gabbroid diabase is due to 

 absorption of confined water by the still molten portion of the 

 flow, with the formation of miniature pegmatite chambers. Quite 

 certainly here as elsewhere the lava of this flow encountered moist 

 mud flats and ponds of water. The coarse gabbro occurs for the 

 most part near but not at the top of the flow. If we may con- 

 ceive the top of the flow as forming here a resistant crust due to 

 early solidification, the water from below heated to a high tempera- 

 ture and ascending through the still molten lava must come to rest 

 beneath the previously solidified crust and might readily be con- 

 ceived to mix with the still molten material to form a magma 

 rich in volatile constituents and thus capable of remaining fluid 

 for longer periods and at lower temperatures than when in its 

 original dry state. This would permit the formation of coarse- 

 grained pegmatitic textures like those here described. Emerson 

 has shown in the paper cited that water did ascend through the 

 flow and that such water in some instances caused explosions rup- 

 turing the upper crust. It is easy to understand how pressures 

 could obtain in a pegmatitic chamber of the sort indicated sufficient 

 to rupture the roof of solid diabase after the formation of large 

 plumose crystals of augite was well advanced. The remaining 

 molten material would be chilled following the explosion and yield 

 perlitic glasses associated with plumose diabases exactly in the 

 manner described by Emerson. 



