Pirsson and Washington — Geology of New Hampshire. 493 



Art. XLIII. — Contributions to the Geology of New Hamp- 

 shire : No. 11, Petrography of the Belknap Mountains ; 

 by L. V. Pirsson and H. S. Washington. 



[Continued from p. 457.] 



Gilfordal-camptonose. 



The rock mass forming the west lower slope of Locke's Hill 

 has been described in respect to its geologic relations in the 

 preceding paper on this region nnder the field term of diorite. 

 While the mass shows some variation in structure and in min- 

 eral composition in places and especially so near the southeast 

 border, there is a certain type which, because it strongly dom- 

 inates and has certain strikingly peculiar features, impresses 

 itself as the characteristic one of the area. The description of 

 this rock type is as follows : 



Megascopic. — Phanerocrystalline ; medium to coarse grain; 

 dark gray to black ; composed chiefly of roughly equidimen- 

 sional, anhedral, black hornblendes from l-2 cm in diameter, so 

 thickly crowded with short rough tables and anhedral masses 

 of dull white feldspar from 2-3 mm , which they poikilitically 

 enclose, that there generally appears almost as much feldspar 

 as hornblende ; interspaces between the nearly juxtaposing 

 hornblendes filled with granular masses of the same feldspars 

 and black ferromagnesian minerals of similar size of grain 

 2-3 mm . Contact facies similar but of finer grain, the large horn- 

 blendes from 3-6 mm in diameter ; contains rare specks of pyrite. 

 Fabric subporphyritic and poikilitic granular ; fracture tough. 



Microscopic. — Hornblende, augite and plagioclase essential ; 

 biotite, iron ore, titanite, pyrite, apatite accessory ; chlorite, 

 muscovite, epidote and gothite secondary ; scapolite local, 

 secondary. 



A ugite, short columnar prismoids, 010, 100 and 110, poorly 

 developed ; extinction angle greater than 40° ; pleochroic c 

 and h pale rose-red, a pale brownish-yellow ; varies in depth 

 of color, sometimes nearly colorless with pale greenish tone ; 

 contains inclusions of the brown hornblende mentioned below 

 in parallel position as original intergrowths. 



Hornblende in irregular masses, anhedral; includes all the 



other minerals, especially feldspar ; strongly pleochroic ; c 



umber-brown, 6 olive-brown and a pale brownish-yellow ; 



absorption marked, c > b > a; angle of c on c about 18° ; double 



refraction c-a about 0'020 ; it varies into a green variety which 



exists also in independent masses but is not so common and is 



then in small individual grains, c and b olive-green and a pale 



brownish-yellow. Cleavage good. 



Am. Jour. Sci.— Fourth Series, Vol. XXII, No. 132.— December, 1906. 

 34 



