594 KEPORT — 1902. 



produce chemical and mineralogical changes at the very liearta of minerals, 

 we must fully realise how completely porous rocks and minerals are, to the 

 heated liquids which carry these reagents with them in solution. Heat, as before 

 stated, not only increases chemical energy, but destroys more or less completely 

 the cohesion between molecules, and increases the amplitude of the vibrations, 

 or other motions of the molecules, and consequently facilitates the entrance of 

 liquids and gases into the pores of minerals, and their complete permeation by 

 these powerful agents of change. Thus far we have been chiefly concerned with 

 some of the principles underlying the branch of our subject embraced by the 

 term contact-metamorphism, which implies operations conducted at considerable 

 depths below the surface of the ground, under conditions of heat and pressure. 



We must DOW consider very briefly changes produced at or near the surface 

 by the agency of water, or, as Bischof in his well-linown work termed it, meta- 

 morphism in the ' wet way.' 



No hard and fast line, however, can be drawn between the two classes of 

 operations, as the one gradually shades by fine gradations into the other. At one 

 end of the scale we have high pressure and hiirh temperature, and a fluid igneous 

 magma holding water in solution, above a red heat, and giving up heated water 

 or vapour charged with salts to the rocks in contact with it. 



Passing to the other end of the scale through diminishing temperatures and 

 pressures, we reach a condition in which the water circulating through the rocks 

 at ordinary pressure and temperature is more abundant in amount, and holds 

 acids and salts in solution, capable of setting up important chemical reactions in 

 the rocks and minerals to which it gains access. 



In the case of surface operations, moreover, the metamorphic agents— water, 

 acids, salts — are being constantly renewed. Conditions ditfering as widely as the 

 conditions at tlie extreme ends of our scale do not yield, however, precisely the 

 same results. In both metamorphic change goes on with more or less briskness, 

 but the products are diiferent. Some minerals require great heat and great 

 pressure for their production, and such minerals are never formed by any surface 

 process of weathering. For instance, the temperature reached determiiips 

 ■whether titanium dioxide crystallises as rutile, or in one of its other two forms, 

 rutile requiring a temperature of over 1000° C, and being the only form of 

 titanium dioxide ' stable at a high temperature.' 



Temperature also seems to determine whether the silicate of alumina crystal- 

 lises as andalusite, kvanite, or sillimanite, the two former being transformed into 

 the latter, at a temperature of 1320° C. to 1380" C. 



On the other hand some minerals require little heat for their formation, and 

 are readily produced by metamorphic changes in the 'wet way.' 



There seems to be some correspondence between the melting point of minerals 

 and their density ; thus in tlie case of eleven minerals produced by contact- 

 metamorphism, whose average specific gravity ranges from 8-06 to 4'03, I find 

 that their melting-point ranges from 954° to above 1770° C, high temperature 

 and high pressure (a concomitant of plutonic conditions) appearing to be factors 

 in the production of high specific gravity in minerals. 



The genesis of individual species of minerals is a fascinating study, but the 

 subject is too large to enter upon here. 



Water gains access to rocks in several ways. It falls as rain ; it rises from 



. hidden depths ; it leaks from the sea into horizontal beds or into strata dipping 



away from it ; and it penetrates through faults and fissures. Rain in its descent 



takes up from the air oxygen, nitrogen, carbonic acid, and in some cases small 



amounts of nitric acid. 



It is thus in itself a powerful solvent and potent agent in producing chemical 

 change. 



In its passage through the surface soil it dissolves humic and other organic 

 acids, the products of vegetable decay, which add greatly to its solvent power and 

 enable it to break up many silicates and to dissolve even silica. 



By the time the rain-water reaches the solid roclis below the surface soil, it has 

 become a very active agent in producing chemical change in them. It is by such 



