Parr VII.§ 2] LOCAL METAMORPHISM. 581 
An important paper upon the contact phenomena of the granite of 
Albany, New Hampshire, has been published by Mr.G. W. Hawes.! His 
analyses indicate a systematic and progressive series of changes in the 
schists as they approach the granite. The rocks are dehydrated, boric 
and silicic acids have been added to them, and there appears to have 
been also an infusion of alkali directly on the contact. He regards the 
schists as having been impregnated by very hot vapours and solutions 
emanating from the granite. 
In Brittany Lower Silurian slaty rocks, where they approach masses 
of eruptive granite, assume a schistose character and contain large 
erystals of chiastolite, among which, in the same pieces of stone, 
specimens of brachiopods and trilobites may be seen.” 
Summary of facts.—The foregoing examples of the alteration 
superinduced upon stratified rocks in proximity to granite or other 
eruptive masses might be largely increased; but they may suffice 
to establish the following deductions in regard to contact meta- 
morphism. 
1. Groups of ordinary sedimentary strata (sandstones, shales, 
limestones, &¢.), where they have been pierced by granite or other 
lutonic rock, have undergone an internal change, whereby their 
usual lithological characters have been partially or wholly oblite- 
rated. : 
2. The distance to which this change extends varies within wide 
limits, being in some cases scarcely traceable for a hundred yards, in 
others continuing for two miles or more. ‘The subterranean surface 
of the plutonic rock, however, being unknown, it may frequently lie 
nearer the surface of the ground than might be supposed. Detached 
minor areas of metamorphism may thus be connected with eruptive 
bosses which have not yet been laid bare by denudation. 
3. As the alteration increases in intensity with greater proximity 
to the plutonic rock, it must be regarded as a result of the protrusion 
of that rock. But there occur exceptional areas or bands which 
have undergone a minor degree of change even in the midst of 
highly altered portions. 
4. The character of the metamorphism depends fundamentally 
upon the composition and texture of the rock on which it has been 
effected. Sandstones have been changed into quartzite; siliceous 
schists into hornstone, Lydian-stone, &c.; clay-slates into spotted 
schists, chiastolite-schists, mica-schists, &c.; argillaceous greywacke 
and greywacke-slate into “ knotenschiefer,” mica-slate, and gneiss. 
Alternations of distinct kinds of sedimentary strata, such as slate 
and sandstone, are represented by distinct alternating metamorphic 
bands, such as quartzite and mica-schist. 
5. In some eases the transformation of a thoroughly clastic rock 
(clay-slate, greywacke, greywacke-slate or flagstone) into a com- 
pletely crystalline one (andalusite-schist, mica-schist, gneiss) has 
1 Amer. Journ. Sci. xxi. (January 1881), p. 21. 
2 Boblaye, Comptes rendus, 1838, p. 186; Bull. Soc. Géol. France, x. p. 227. 
