220 PROCEEDINGS OF THE AMERICAN ACADEMY. 



matolytic agents are active. That such agents, particularly water 

 or water vapor, are of prime importance in its formation is also borne 

 out by its occurrence as a secondary product from the alteration of 

 the original hornl)lende as noted above. Indeed, the de^•elopment 

 of the very closely related, or perhaps almost identical, crocidolitic 

 amphibole found in the cavities in the pegmatite pipes and along 

 joint surfaces in the granite, indicate that its solution and recrystalli- 

 zation may be carried out under conditions considerably removed 

 from those prevailing during magmatic period. In this connection 

 it is perhaps worth suggesting that relatively small changes in the 

 hydration of the molecules making up the hornblendes, and particu- 

 larly in the amounts of ferro-ferri-silicate molecules brought about 

 by the oxidizing or reducing effects of the pneumatolitic agents may 

 cause disproportionally large changes in the colors of the various 

 rays. The chemical composition of the hornblendes will be taken up 

 when the chemical analyses of the rocks are considered. 



The mode of occurrence of the aegirite has been in part described 

 when speaking of its intergrowth with the hornblende. Its occur- 

 rence in the form of microliths in the feldspar has also been noted. 



The growths of aegirite are often so considerable that it predomi- 

 nates in amount over the hornblende. It is also found in the form of 

 distinct grains which are most closely associated with the quartz with 

 which there are occasional intergrowths. There was perhaps some 

 tendency on the part of a portion of the aegirite to develop its 

 prismatic zone toward quartz. Aegirite is, however, almost wholly 

 anhedral and the contacts with other minerals are in general char- 

 acterized by very irregular surfaces, projecting points, nubs, hooks, 

 and by the presence of isolated or but slightly attached small particles, 

 sometimes in considerable abundance. There is usually an elongation 

 in the direction of the vertical axis. In many instances larger grains 

 seem to have been broken apart and separated; again, a greatly 

 elongated crystal or series of crystals end to end, may be seen extend- 

 ing along between the feldspar and quartz, or between two feldspars, 

 or along fracture lines. Many aegirite patches, consist either of a 

 relatively large crystal which contains elongated, prismatic crystals 

 irregularly orientated and often with a subradial arrangement, or they 

 consist wholly of an aggregate of variously placed prismatic crystals. 

 It is also frequently noted that a number of small aegirites though not 

 attached, form nevertheless what might be termed a community of 

 grains. Zonal growths are not uncommon and indicate an earlier 

 stage of development which is evidently connected in many cases, and 



