General C. A. McMahon's Address. 465 



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 now consider very briefly changes produced at or near 

 the surface by the agency of water, or, as Bischof in his well-known 

 work termed it, metamorphism 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 fiue gradations into the other. At 

 one end of the scale we have high pressure and high 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 rooks and minerals to which it gains access. 



In the case of surface operations, moreover, the metamorphio 

 agents — water, acids, salts — are being constantly renewed. Con- 

 ditions differing as widely as the conditions at the extreme ends 

 of our scale do not yield, however, precisely the same results. In 

 both metamorphio change goes on with more or less briskness, but 

 the products are different. 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 determines whether titanium dioxide crystallises 

 as rutile, or in one of its other two forms, rutile requiring a tem- 

 perature 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 crystallises as andalusite, kyanite, or sillimanite, the two 

 former being transformed into the latter, at a temperature of 1^20° 

 C. to 1380° C. On the other hand, some minerals require little heat 

 for their formation, and are readily produced by metamorphio 

 changes in the ' wet way.' 



There seems to be some correspondence between the melting-point 

 of minerals and their density ; thus, in the case of eleven minerals 

 produced by contact-metamorphism, whose average specific gravity 

 ranges from 3-06 to 4-03, I find that their melting-point ranges from 

 954:° to above 1770° C, high temperature and high pressure (a con- 

 comitant of plutonic conditions) appearing to be factors in the 

 production of high specific gravity in minerals. 



DECADE IV. VOL. IX. NO. X. 30 



