Metamorjphic Conglomerate in the Green Mountains. 271 



involved for decipherment. These phenomena are particularly 

 true of the conglomerate horizon and its many phases, and it is 

 in this rock that I wish to describe some of the evidences and 

 effects of metamorphism shown by the destruction of old 

 clastic minerals and in the production of new ones. 



Occurrence of oUr elite schist. — One of the most conspicuous 

 phases of the conglomerate is due to the development of ottre- 

 lite in great abundance so that it is not uncommon to find 

 fully twenty-five per cent of the rock made up of this mineral. 

 The ottrelite is commonly most abundantly developed where 

 the rock has now a well-marked schistose character that is due 

 either to an original fine-grained deposit, or is a result of the 

 shearing and crushing action of dynamic forces. It is often 

 found, however, occurring in the groundmass of the coarsest con- 

 glomerate or along planes of shearing in a blue, hyaline quartzite. 

 Still another phase is more nearly massive, fully forty per 

 cent being ottrelite, the rock at first sight simulating in appear- 

 ance some porphyritic hornblende dike. The rock is a very 

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

 most important stratigraphical horizons found in the more 

 crystalline areas of the Green Mountains. In lateral extension 

 it has been traced with unimportant breaks all the way across 

 the Green-mountain anticlinal axis, as mapped by Hitchcock,* 

 from Mendon, Vt., to North Sherburne, Vt. In vertical exten- 

 sion it has considerable thickness although accurate determina- 

 tion is very difficult owing to the obliteration of planes of bedding 

 in many 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 

 extending south from Mount Carmel, in the town of Chitten- 

 den, and the east and west crest forming the southern portion 

 of the mountain, somewhat inappropriately named " Old Aunt 

 Sal," in the town of Mendon. 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 microscopical characters. — In the hand speci- 

 men the ottrelite of the most massive occurrenjce of the ottre- 

 lite-bearing rock appears either as isolated areas generally with 

 rudely circular outlines, or groups of these in a background of 

 finegrained, pinkish-brown to dark purple quartz, in places 

 constituting nearly pure ottrelite. These areas possess an 

 approximate common diameter of about three-sixteenths of an 

 inch. In structure they are made up of tabular areas of 

 radiating imbricated plates generally arranged in essentially 

 one plane for any single area ; but the positions of the 



* See fig. 4, Section VI, Hitchcock's " Green Mountain gneiss," Geol. of Vt., vol. 

 ii, 1861. 



