PRINCIPLES UNDERLYING METAMORPHIC PROCESSES 493 
cooling is accompanied by a progressive change of the orthorhombic 
into the tetragonal form with a corresponding alteration of compo- 
sition of the mixtures in both modifications, until finally at about 
—20° the system is completely transformed into tetragonal mix- 
crystals of the composition g.5 mol. per cent HgBr., 90.5 mol. per 
cent HgI,. Thus at temperatures between +80° and —20° (under 
ordinary pressure) mix-crystals of the above composition repre- 
sent an equilibrium between two modifications of different com- 
position, either of which is completely stable in presence of the 
other. 
Now among the minerals of geologic importance there are many 
which are to be considered as solid solutions; from the foregoing, 
therefore, one must speak not of the inversion temperature of any 
such mineral which exists in more than one form, but only of the 
inversion interval. For instance, if the relation between augite and 
hornblende should prove to be enantiotropic (at present the nature 
of their relation is unknown), they could coexist in true equilibrium 
throughout a range of temperatures. It would therefore be incor- 
rect to speak of a definite temperature, or a definite pressure, of 
transformation of these important minerals. 
Furthermore, the modifications which coexist in equilibrium 
under any particular set of conditions will not have the same chemi- 
cal composition. As a matter of fact, such differences have been 
observed in gabbros from Katéchersky by L. Duparc,’ who gives 
the following analyses: 
Augite Amphibole 
SiO7 ase ae ere a 50.91 43.34 
JNU Oe oe clea eatin ae nee 2.64 12.60 
He O eyes eae) tase 10.44 
RC Or erage tgshcrsies cus i be 10.07 712 
IMMOV Faas oc eicis res: 2 Ona Tate ure. cae eee 
CaAWM era het see DEG) 12.91 
Mig Opa cececte inte aus 13.30 D252 
IRA es acaltral dee Not determined 0.24 
INET O Vey Al rae eet eee ee Not determined 1.90 
100.66 101.87 
* Bull. soc. franc. min., XXXTI (1908), 50. Analogous observations have been 
made by L. J. Wild, Trans. New Zealand Inst., XLIV (1912), 333; also by J. B. 
Harrington, Geology of Canada (1879), p. 21. 
