GEOLOGY OF THE HAYSTACK STOCK 197 



feet. The direction of schistosity agrees closely with the banding 

 of the gneiss, and both were apparently produced at the same time. 

 The dark schists consist essentially of biotite, feldspar, and horn- 

 blende. The contacts between the gneiss and the included schists are 

 not sharp and distinct, though a short distance away they appear to 

 be so, owing to the contrast in color between the light and dark rock. 

 The two merge into each other within a narrow zone, and the 

 bands of schist thin out and end within the gneiss. The forms assumed 

 by the bands of schist are extremely irregular; some of them are 

 curved lines ; sigmoidal forms are not uncommon ; rectilinear bands 

 do not occur for any considerable distance. Some of the curved 

 bands are fractured and broken by faults which have been completely 

 healed. 



Pegmatite. — The crystalline complex is cut by dikes of pegmatite 

 which is composed of feldspar, quartz, and mica. Such dikes are 

 especially well developed on Lake Plateau about two miles north 

 of the northeast corner of the area mapped (Fig. i) where red feldspars 

 occur in large crystals which inclose smaller bodies of quartz, most of 

 them . about two inches in longest dimension, and thick six-sided 

 plates of mica about half as large. The pegmatite is not mashed 

 and, therefore, is later than the metamorphism of the gneiss and schist, 

 but since it does not cut the Cambrian sediments it is probably of 

 pre-Cambrian age. 



CAMBRIAN 



Overlying the pre-Cambrian rocks unconformably are beds of 

 Cambrian quartzite, limestone, and shale. The basal member is 

 a buff, pink, or gray quartzite from 200' to 300' thick, and its 

 basal layers at some places contain small pebbles of the crystal- 

 line schists. The quartzite is thoroughly indurated, and under the 

 microscope shows characteristic secondary enlargements of the grains 

 of quartz. 



Above the quartzite and conformable with it in dip is blue or 

 gray limestone from 50' to 300' thick, near the base of which are a 

 few feet of shale. It is sometimes massive, more often thinly bedded, 

 and contains cherty layers. Some of the layers of the limestone are 

 limestone conglomerate, composed of flat limestone pebbles from 



