LOCALITY AND CHARACTERISTICS. 149 



variable one, but, considered as a whole, it forms one of the most impor- 

 tant stratigraphie horizons found in the more crystalline areas of the 

 Green mountains. In lateral extension it has been traced, with unim- 

 portant breaks, all the way across the Green-mountain anticlinal axis, 

 as mapped by Hitchcock * from Mendon, Vermont, to North Sherburne, 

 Vermont. In vertical extension it has considerable thickness, although 

 accurate determination is very difficult, owing to the obliteration of planes 

 of bedding in most instances and the complexity of the flexures; but it 

 seems safe to assume a thickness of several hundred feet, at least in sev- 

 eral localities that have been most carefully studied, viz : a spur extend- 

 ing south from Mount Carmel, in the town of Chittenden, and the east 

 and west crest forming the southern portion of the mountain, somewhat 

 inappropriately named "Old Aunt Sal."f The latter mountain is situ- 

 ated in the town of Mendon, the next town south of Chittenden. The 

 phases studied thus far in the laboratory are from this latter locality 

 and from the western part of the " Rabbit ledge," just south of Mendon 

 " city." 



Physical and microscopic Characters. — In the hand specimen the ottrelite 

 of the most massive occurrence of the ottrelite-1 tearing rock appears either 

 as isolated areas, generally with rudely circular outlines, or as groups of 

 them in a background of fine-grained pinkish-brown to dark-purple 

 quartz, in places constituting nearly pure ottrelite. These areas possess 

 approximately a common diameter of about three-sixteenths of an inch. 

 In structure the}'' are made up of radiating imbricated plates generally 

 arranged in essentially one plane for any single area ; but the positions 

 of the different areas seem in the main to be accidental, although locally 

 they may be arranged parallel, as shown by a tendency in the rocks to 

 cleave into rude slabs, a tendency augmented by thin folia of serecite. 



A well marked spherocrystalline habit characterizes all the ottrelite 

 areas. In some this radiated growth seems to be perfect. Microscopic- 

 ally the radiated structure is much more evident. Composite and fan- 

 shaped areas, penetrating one another irrregularly, coexist with isolated 

 prisms and beautiful spherocrystalline aggregates, yielding imperfect 

 crosses in polarized light. Only sections cut parallel to the bundles of 

 plates (basal sections) show well the radiated structure ; all other sec- 

 tions show this character less and less, depending upon the plane of the 

 section, until it is transverse, when the mineral appears prismatic. The 

 areas of spherocrystals are not infrequently bounded by overlapping six- 

 sided plates, of which three are usually free; the others are intergrown 

 and confounded in thy central position of the aggregate. 



* Figure 4, section vi, Hitchcock's Green mountain gneiss, G-eoI. of Vt., vol. 4. 1861. 

 fThe name Blue Ridge is given this mountain on the Rutland topographic sheet completed in 

 1891, butl have decided to use here the most commonly adopted local name 



