SO-CALLED PORPAYVRITIC GNEISS OF NEW HAMPSHIRE 711 
than at Centre Harbor, but the properties of its magmalike 
gneiss with the sprinkling of large porphyritic crystals of feld- 
spar are identical with those of the other locality. Asa rule, 
there is a sharp contact between the porphyritic granite and the 
invaded rocks, like that which in general characterizes plutonic 
bodies. These belts of transition have at first sight a puzzling 
appearance. That they are, in reality, eruptive contacts seems, 
however, to be unquestionable. Durocher long ago noted such 
an intimate union along the boundary of gneiss cut by stock- 
granite. He considered the temperature of the igneous rock in 
such instances sufficiently high to produce a melting up of the 
bf 
gneiss the “ particules”’ of which ‘‘ont dtii posséder une assez 
grande mobilité, et cristalliser a peu pres dans les mémes condi- 
tions que les molécules du magma granitiques.* In his ‘‘ Geog. 
Beschreibung Bayerns,’’* Gumbel speaks of there being numer- 
d 
ous transitions from ‘‘bunter gneiss’ to ‘“‘ bunter granit ’’ which 
cuts the former. Michel-Lévy has very clearly discussed the 
plucnomenon im general Ee says:3-> ““C’est ici le cas de 
remarquer que lorsque deux grandes masses de roches acides se 
touchent, le plus souvent elles se trouvent réunies par une zone 
de passage plus ou moins puissante, dans laquelle les caractéres 
pétrographiques des deux roches sont, pour ainsi dire, confondus 
et mélangés.t In the same paper from which this quotation has 
been taken, the author cites many examples of such transition, 
among which that from granite to gneiss’ and that from 
granite to ‘‘micro-granulite”’ may be especially mentioned. He 
explains them as due to an impregnation of the older rock by 
‘‘les éléments fluides en voie de dégagement ”’ from the igneous 
rock. When the temperature and pressure are suitable, a part 
*Mém, de la Soc. Géol. de France, 2¢ sér. t. VI, p. 47. 
? Abtheil. II, p. 272. 
3 Bull. de la Soc. Géol. de France, 3° sér., t. VII, 1878-9, p. 852. 
4Cf. Ch. Vélain, Conférences de Petrographie. Paris, 1889, p. 6. 
5 Cf. LEHMANN, Untersuch. iiber die Ent. der alt. kryst. Schiefergesteine, p. 76. 
GREGORY, Q. J. Geol. Soc., 1894, p. 260 ff. BARROW has noted a complete amalga- 
mation at the contact of granitite and diorite, of which the former is the intrusive 
member, Q. J. Geol. Soc., 1892, p. 121. 
