379 



The quartzite in the immediate neighbourhood of the granite is often 

 traversed by veins of all sizes down to microscopic dimensions. These 

 are sometimes discontinuous. As a result of this complicated veining 

 the sedimentary rock is impregnated with granitic material, and a felspathic 

 quartzite is produced. The felspars belong to orthoclase, microcline and 

 plagioclase (oligoclase). This impregnation of the quartzite with granitic 

 material has taken place to such an extent that included fragments in 

 the granite might be mistaken for gneiss were it not for the fact that a 

 precisely similar rock occurs in contact with the granite and stands in 

 visible relation with the ordinary quartzite. A similar impregnation of 

 contact-rocks with granitic material has been described by M. LEVY^ and 

 by Dr. LEHMANN/ 2 > 



M. BARROIS recognizes, therefore, three principal varieties of meta- 

 morphic rock ; (1) quartzites micace's (2) quartzites sillimanitise'es and (3) 

 quartzites felspathise's. The two former are derived from the original rock 

 by a recrystallization of its constituents ; the latter is due to a recrystal- 

 lization of the original constituents accompanied by the actual addition 

 of material from the granite. 



A very interesting case of contact metamorphism has been described 

 by Dr. HAWES.^ An offshoot from the main mass of Albany granite 

 intersects Mt. Willard in the form of a dyke about 300 feet wide. A 

 distinct variety of granite, known as the Conway granite, forms the hanging 

 wall, and an "argillitic mica-schist" the foot wall of this dyke. The 

 Albany granite is easily recognized by its conspicuous porphyritic crystals 

 of orthoclase, twinned on the Carlsbad plan. These crystals have somewhat 

 rounded outlines and lie in a granitic matrix. Under the microscope 

 the large felspars are seen to consist of interlaminations of orthoclase 

 and albite. The ground-mass is composed of hornblende, biotite, quartz, 

 felspar (including plagioclase), magnetite and apatite. The quartz is 

 without form, and may often be seen filling the angular interspaces 

 between the other constituents. Fluor and augite occur in some speci- 

 mens. Square prisms of zircon are present in all. The above description 

 applies to the normal Albany granite, and to the rock forming the central 

 portion of the dyke. As the rock is traced towards the margin of the dyke 

 important changes may be observed. At a distance of 100 feet from the 

 margin the ground-mass becomes finer in texture, the porphyritic crystals 

 remaining the same size. At a distance of 60 feet quartz begins to 

 show traces of crystalline form. At 15 feet the quartz occurs in well- 

 crystallized double-pyramids, and the ground-mass is no longer resolvable 

 by the naked eye or with a lens. At the contact the matrix is almost 

 black and flinty in aspect. Microscopic examination proves that, as the 

 junction is approached, hornblende disappears and biotite only is present. 

 The biotite-crystals become smaller and smaller, and the only minerals 



(1) Sur les schistes micaces de Saint-Leon (Allier) B.S.Q-.F. 3e serie. T. IX. (1881), 

 p. 181. 



(2) Die Entstehung der altkrystallinischen Schiefergesteine, Bonn 1881, p. 20. 



(3) The Albany Granite, New Hampshire, and its contact phenomena. A.J.S., 

 Vol. XXI. (1881). 



