96 B. K. EMERSON — PLUMOSE DIABASE AND PALAGONITE 



Resume of the structure of the abnormal area. — Down the middle of the 

 abnormal area runs a narrow band which contains rather sparingly frag- 

 ments and filaments of a light pearl-gray clayey limestone exactly like 

 much that is found in the upper surface of the sheet. The introduction 

 of this mud caused much of the magma to crystallize in a coarse gabbroid 

 diabase, greatly shattered the mass and seamed it with many quartz- 

 ankerite veins. The fine trap on either side of this band is full of 

 " schlieren " of the strange, coarse, gabbro-like varieties, which will be de- 

 scribed in detail. In them the pyroxene shoots* out into branching and 

 curving feathery plumes several inches long, with central suture and 

 transverse parting, which heightens the resemblance to a feather ; or the 

 feldspars occur in broad plates nearly an inch square, poikilitic with 

 abundant pyroxene and palagonite grains in beautiful plumose arrange- 

 ment ; or the whole is crowded full of clots of black palagonite, which 

 include spherulites of calcite and radiate blue quartz, and these glass clots 

 are surrounded by broad acid, almost purely albitic, areas. Finally, and 

 this seems of first importance for the explanation of the strange devel- 

 opment, the coarse schlieren, which taper to nothing in the fine trap, 

 have often a narrow central band of whitish aphanitic and very acid 

 diabase, containing little or no pyroxene, of the same type as that sur- 

 rounding t the glass. This type may be associated with the white diabase 

 which I have elsewhere described as " holyokeite.' 1 * 



Abstract of theory of formation of plumose forms and of palagonite. — We 

 can perhaps describe the multitude of exceptional and contradictory 

 forms and phenomena crowded together here more lucidly and not less 

 objectively if we arrange them in accord with the theory which seems 

 best to bind them together and which we may state baldly as follows : 



(1) Along a narrow band parallel with the front of the sheet a great 

 volume of the mud, like that found on the surface, was drawn down a 

 hundred feet or more into the interior or the molten trap sheet by irreg- 

 ular vortex motions of the flowing mass. 



(2) Under the high temperature and pressure the quartz and carbon- 

 ates of the mud were abundantly dissolved. Strong internal motions 

 carried sheets and filaments of the magma loaded with these solutions 

 out into the adjacent area as great schlieren. A central band contains 

 the remnants of mud which were not dissolved and carried out into the 

 magma by convection or diffusion, and here explosions of the muddy 

 water have shattered the newly solidified rock, and the calcite and 

 quartz solutions have cemented it into a central breccia. The normal 

 trap on either side is full of coarse schlieren, which represent the streaks 



* Holyokeite, a purely feldspathic diabase from the Trias ot Massachusetts. Jour. Geol., vol. x, 

 1902, p. 508. 



