17i THE MAKQUETTE litO^^-BEAKIXG DISTltlCT. 



mixscovite iu large, naiTOw flakes and groupings of flakes. The mosaic is 

 also traversed by bands in which the fragments are very much smaller 

 than elsewhere, as though the rock had slipped along certain planes and 

 had ground into powder the neighboring fragments. These bands run in 

 the same direction as do the stringers of chlorite and muscovite, and so help 

 to iujpress schistosity on the mosaic. They are microscopic shear zones. 



The structure of all these granites is that described by Tornebohm 

 under the name of "mortar-structure." Williams has already referred to 

 it as characteristic of the granites of this region, and has cited its existence 

 as evidence that the rocks in which it is found have been subjected to 

 severe dynamo-metamorphism. 



The gneissoid granites differ from the more massive phases of the 

 rocks simply in the possession of more marked foliation. The mortar- 

 structure is most beautifully exhibited in all the sections. The larger rem- 

 nants of the crushed original components are embedded iu the mosaic, 

 which surrounds them as the crystalline matrix surrounds the eves of an 

 " augen-gneiss," the combination of fragments and mosaic producing lentic- 

 ular areas, separated from C)ther like areas by narrow bands of verv fine 

 mosaic. 



It is not uncommon to see in a slide of the "ueissoid eranite a o-rain of 



o fr ?T^ 



orthoclase or of plagioclase broken into three or four pieces and the pieces 

 separated from one another by distances of a quarter millimeter. The 

 fissures between the fragments are filled with an aggregate of crystallized 

 quartz and microcline, or with a jjortion of tlie fragmental mosaic. 



The quartz grains have suffered crushing, Init their parts have not been 

 separated. Quartz areas now consist of nuclei peripheral! v granulated, or 

 of several grains differently orientated, the whole forming a leuticule. Each 

 component of the leuticule exhiliits undulatory extinction. 



THE MUSCOVITE-GRANITES. 



Nearly all of the granites of the Northern Complex are biotite-granites. 

 A very few of a different character are found whose relations to the com- 

 mon granite have not l)een determined. In the SW. ^ sec. 29, T. 48 N., 

 R. 28 W. (Atlas Sheet XVIII), for instance, is a mediumlv fine grained, 



