MICROSCOPICAL DESCRIPTION. 119 



fection of outline indicates that it has not passed through the varied ex- 

 periences to which the original constituents of the rock may have been 

 subjected. It does not have the crystallographic relations with the augite 

 characteristic of paramorphic development. Titanite also appears here, 

 often in small rhombic cr3^stals. 



Specimen 155 (166) shows a pronounced plication of the bands. Along 

 with this the rock shows more or less schistosity in certain directions. 

 The direction of this cleavage, however, has no apparent relation to the 

 direction of the gneissoid banding. The augite streaks are less sharply 

 defined than in the specimen last described (156). 



Under the microscope there appears a uniformly very thin, fine grained 

 ground mass of feldspar and quartz, in which large grains of microcline 

 and numerous small grains of augite and hornblende arranged in bands 

 occasionally occur. It is somewhat more granular than the preceding, 

 but in other respects does not show any marked differences. 



Character and Relations of the Rocks. 



The relations of these rocks as observed in the field are thus seen to 

 be borne out by their petrographical character. Beginning with the 

 coarsely crystallized rock, in which the gneissic structure is but imper- 

 fectly developed, there is a gradation into finer grained and more gneissoid 

 forms, until we have in the last stage a well developed gneiss. In its 

 typical form it consists of a mixture of microcline and augite as essential 

 constituents, while quartz, titanite and pyrite play an accessory role. A 

 peculiar feature of this rock is the large size of the microcline and augite 

 grains and the segregation of both in lumps and patches. The plagioclase 

 feldspar appears in proportion to the extent of the granular areas and 

 constitutes an essential feature of these areas, though usually playing a 

 less important part than the microcline grains. It is usually much 

 fresher than the microcline, but in some cases the opposite is true. 



The augite, which occurs in large crystals and grains or segregated 

 masses in the coarse grained rock, also appears in smaller grains as the 

 rock becomes more granular and gneissoid, distributed along the middle 

 of the gneissoid bands. 



The constituents of these bands are strictly allotriomorphic in their 

 relations and often show a distinct micropoikilitic structure. These in- 

 dications of recrystallization become more pronounced in the more 

 gneissic forms. The indications of secondary enlargement of the micro- 

 cline, and sometimes of plagioclase grains, are also more apparent in 

 these latter specimens. In the coarse grained rock plagioclase feldspar 

 is generally present in small amount as a cement between the grains of 

 microcline. This cement-like arrangement is also apparent to a small 



XVil-BuLL. Geol. Soc. Am., Vol. 7, 1895. 



