THE OEANGE AND MONSON AREA. 59 



PETROGRAPHICAL DESCKIPTION. 



As the type of the Mouson gneiss the rock of Flynt's quany at 

 Monson is naturally taken for detailed description, and for this purpose 

 fine specimens of the two varieties which served for the analyses (juoted 

 on page 62 were kindly fui-nished me by the proprietor of the quarry, 

 Mr. W. N. Flynt, and form the material of the following description: 



(a) The darker variety is a rather dark pure-gray granitoid rock of 

 medium and very even grain. The quartz and orthoclase are almost 

 equally colorless and glassy, so that they are not easily distinguished; the 

 latter is disseminated in a certain small porphyritic way, so as to show many 

 shining facets upon freshly broken surfaces. The black constituent is pres- 

 ent in small quantity, but from the translucency of the other constituents 

 it gives a quite dark color to the mass. It is this translucency which causes 

 the marked distinctions between the dark polished and the white pounded 

 surfoce. The black constituent is a lustrous black biotite, greenish-brown 

 or bright emerald-green by transmitted light. It is arranged with very 

 imperfect parallelism, and yet is somewhat concentrated along certain 

 lamination planes, although not joined into membranes, and this alone gives 

 the rock its gneissoid texture. Seen edgewise, it resembles hornblende, 

 but I have not detected this mineral in the present variety. Wine-yellow 

 grains of titanite are associated with it, and rarely grains of a light-red 

 garnet. Plagioclase could not be detected. 



The biotite is often changed to a bright-green chlorite, and parallel there- 

 with is an abundant development of hematite in isolated, regular hexagonal 

 plates or elongated congeries. A few deep-green prisms of tourmaline 

 occur, short, stout, and hemimorphic. 



(b) The lighter variety is characterized by a somewdiat coarser grain. 

 The black constituent is almost equally biotite and magnetite. It is, how- 

 ever, much less abundant and so arranged in the cleavage planes that trans- 

 verse to these the rock is mottled with white elongate spots from which it 

 is absent. 



The important distinction between the two is in the fact that musco- 

 vite in thin, membranous patches is quite abundant upon the lamination 

 planes of this variety. This is an exception to the rule that muscovite is 

 absent from the Becket and Monson gneiss in the three river counties and 

 characteristic of the next formation above. 



