298 DEPARTMENT OF THE INTERIOR 



2 GEORGE V, A. 1912 



converted into greenish gray, medium-grained, hornfelsy rocks of quite differ- 

 ent habit and composition. (Plate 27, A.) 



Muscovite in foils running from 0-5 mm. to 2 mm. or more in diameter is 

 so prominent a constituent of these altered rocks as to give them a highly 

 lustrous and glittering look, quite similar to the mica schists in the Bayonne 

 granite aureole. Biotite in foils from 0-05 mm. to 0-5 mm. in diameter is a 

 second essential mica in most phases and quartz is invariably a third essential. 

 Along with these minerals, cordierite, cyanite, andalusite, and tourmaline 

 (Plate 31) are developed in varying amounts, giving the following principal 

 types of rock: musicovite-cyanite-quartz schist, cordierite-muscovite-biotite- 

 quartz schist, cordierite-andalusite-tourmaline-muscovite-biotite-quartz schist. 

 Cordierite is especially abundant and seldom fails from any of the thin sec- 

 tions. The optical properties of all these minerals are typical of them as des- 

 cribed in the standard text-books of petrography; their detailed description 

 need not burden this report and is omitted. An exceptional, non-micaceous 

 hornfels, composed of green hornblende, quartz, epidote, and zoisite with a 

 little feldspar, probably represents a greatly metamorphosed dolomitic quart- 

 zite. 



The gritty rocks of the Monk and Wolf formations have been but 

 little altered, though the once-argillaceous cement of the conglomerates in the 

 Dewdney formation has been completely recrystallized, with the generation of 

 much cordierite, sillimanite, muscovite, and biotite. Microperthite is present in 

 surprising amount and appears to have been in part introduced from the 

 magma. This mineral also occurs in the rocks of the Bayonne batholith 

 aureole. Its development in the phyllites at Ascutney mountain, Vermont, 

 where again it has been transferred from an alkaline magma, is another 

 example of the special ease with which this particular feldspathic substance 

 migrates into contact aureoles.* Two specimens of the Dewdney quartzite 

 taken from a point about 300 feet from the granite are very rich in micro- 

 perthite, soda-orthoclase, and a feldspar which is almost certainly anortho- 

 clase. In this case some feldspathization by the magma is probable but is not 

 so certain, since grains of microperthite occur in the unmetamorphosed quart- 

 zite. 



On the other hand, the composition of the intrusive has been affected by 

 the incorporation of material from the walls. The granite of the larger stock 

 is abundantly charged with fragments of quartzite, schist, and conglomerate. 

 In many cases these show no direct evidence of having lost substance by solu- 

 tion in the magma but the included blocks of conglomerate afford conclusive 

 proofs that, even in the magmatic period immediately preceding solidification, 

 the magma was able to absorb such material. 



*R. A. Daly, Bull. 209, U.S. Geol. Survey, 1903, p. 34. 



