126 B. K. EMERSON — PLUMOSE DIABASE AND PALAGONITE 



reached. He thought that the coarse outgrowths of augite showed 

 extra rapid crystalline growth, produced by extra rapid cooling of the 

 diabase by the inclosure. To me they seemed probably the result o^ 

 extra slow growth, in accordance with the usual rule, " the slower the 

 cooling the coarser the grain." As I conceive it, when inclosures were 

 thus surrounded they cooled the surrounding lava sooner, not down to 

 solidification, but down to the point where augite could begin to form, 

 and the surrounding lava remained in the range of temperature of augite 

 formation' longer, only passing below when the rest of the flow there- 

 about cooled below the temperature of augite formation and finished 

 solidification. 



I do not mean to say, however, that chemical relations and the intro- 

 duction of augite molecules may not also have been an important factor. 



There are a number of other cases of similar phenomena that I have 

 seen and would explain in a similar way — " plumose " growths of augite 

 along the contacts of gabbros and associated red rocks, and on the north 

 side of Nahant and elsewhere contact lines between successive gushes or 

 jets of igneous rock. 



Mr Lane has been so kind as to send me a copy of the above remarks, 

 which were called out by my explanation of the long-bladed pyroxene 

 as a case of rapid crystallization caused by the cooling effect of the 

 intruded mud. 



This was in a brief preliminary description of the occurrence, of which 

 I have preserved no record, and I can not now remember how far I tried 

 to enumerate all the factors concerned in the explanation of the peculiar 

 long-bladed type. I certainly gave more weight to the cooling effect 

 than I should now do, since the thin schlieren containing the long 

 crystals have traveled so far from the central mud breccia and contain 

 therefrom so little material in solution (H 2 0, CoCo 3 , Si0 2 ) that the tem- 

 perature differences seem to me to have been equalized long before they 

 reached their present relation and approached crystallization. 



None of these peculiar effects occur in the immediate vicinity of the 

 mud. There is here a somewhat coarser grain, for which I welcome the 

 ingenious suggestion given in the last sentences of the above remarks ; 

 but for the narrow schlieren carried out from this central area, where the 

 pyroxenes are a hundred times as long as in the adjacent trap, the prin- 

 cipal cause for the difference in size seems to have been the chemical 

 differences in the two magmas. The extensive development of skeleton 

 crystals and micropegmatitic structures and the simultaneous crystal- 

 lization of quartz and calcite, all in the midst of glass clots, speak for 

 rapid crystallization. 



