THE MUSCOVITE-GKANrrES. 175 



light gravish-piiik n»ck, forniing- a small ledge between the coarser biotite- 

 gi'anites south and the quartzite north of it. The rock inav be a dike in 

 the coarser granite, and pn)bably is, though no ol)servations on tliis point 

 are recorded in the note-books. The rock is so badh' shattered that it is 

 difficult to determine its original composition. The thin section shows no 

 more e^^dence of schistosity than does the hand specimen. It does exhiljit, 

 however, a crushed mass of plagioclase, orthoclase, (piartz, and nuisco\ite, 

 cemented by finer-grained debris of the same minerals and niicro<-line. 

 Between the finer grains there is sometimes quartz and sometimes nuisco- 

 vite, but usualh' the grains interlock with one another. All of the large 

 grains are sharply angular, and many of them are cracked across in ^•arious 

 directions. Others that were single fragments have been broken into many 

 small ones that are now separated from one another. p]nlargements of 

 quartz and plagioclase fragments are noted. The muscovite is in large 

 colorless flakes that, like the other components, are shattered. The cracks 

 are filled with fine shreds of the same mineral, and the edges of the plates 

 are frayed out into smaller shreds, wliich form a matted mass of tiin^ nnis- 

 covite fibers, in which the larger plates lie. These fibei's a})pear to be the 

 broken portions of the larger plates that ha\e been sjtlit Ijy the force that 

 crushed the quartz and feldspar. The matted aggregate of fillers is thicker 

 where a large plate has been fractured into four or five fragments than 

 where it is halved. Muscovite is also in minute laminre between the crushed 

 portions of the other constituents, where it is no doubt a ])roduct of the 

 alteration of orthoclase. The rock is evidently a muscovite-granite that 

 has been crushed but has not been rendered schistose. 



ORlCilN OF THE GEAMTES. 



As to the origin of the granites and their gneissoid pliases there can 

 be little question. The rocks appear like eruptives in the fiehl. The clastic 

 grains discoverable in their thin sections are evidently of d\namic origin. 

 All are sharply angular. None have the rounded outlines of waterworn 

 grains. The structtu-e of the rocks is verv similar to that of schists else- 

 where that have been shown to be mashed eruptives; hence there is no 

 reason to believe the granites and gneisses of the Northern Complex to be 



