APPLICATION OF PHYSICO-CHEMICAL PRINCIPLES. 285 
metamorphism is advanced the heavier of the above minerals appear. 
In the early staves of the metamorphism of shales mica develops plenti- 
fully and the rocks become slates. If the metamorphism is more intense 
the heavier minerals, garnet and staurolite, appear, the material of the 
previously developed micas being absorbed at the places occupied by 
the garnet and staurolite. A 
The garnet, staurolite, chloritoid, andalusite, and tourmaline bearing 
mica-schists and mica-gneisses of the Penokee and Marquette districts 
of Michigan and the Black Hills of Dakota, produced by the alteration 
of clastic rocks, are perfect illustrations of the above changes.* In these 
rocks the acid feldspars (G., 2.55-2.67) have extensively altered into 
quartz (G., 2.65) and mica (G., 2.76-3.01) and therefore have passed into 
minerals denser on the average than those from which they were derived ; 
also, the heavier minerals, garnet, etcetera, have developed on an exten- 
sive scale in the more metamorphosed varieties. + 
It is not supposed that there are not individual exceptions to each of 
the rules that in the upper physico-chemical zone lighter minerals form 
and in the lower zone heavier minerals develop. Indeed, exceptions are 
known to both. As illustrating such exceptions in the upper zone are 
the cases already mentioned (see page 283), the devitrification of glass 
and the replacement of calcium by magnesium. As a case in the lower 
zone of the change from higher to lower specific gravity is the alteration 
of pyroxene into amphibole. Upon the average the former is slightly 
heavier, and yet in the lower zone, both under mass static and mass 
dynamic conditions, pyroxene very generally alters to amphibole. Of 
course, in this transformation a change may simultaneously take place 
in the chemical composition (and this may have an effect upon the vol- 
ume of the minerals); for, in general, pyroxene contains a greater pro- 
portion of calcium and less proportions of magnesium and iron than the 
amphiboles. However, this apparent exception to the rule of the pro- 
duction of compounds of high specific gravity in the lower zone may 
be only apparent, for in some of the deepest-seated crystalline schists, 
pyroxene and not amphibole has developed, and it is suspected that 
sufficiently deep this is the rule. If this be the case, the real meaning of 
the reaction above considered is, in order that pressure shall become the 
dominating factor in the pyroxene-amphibole group, it must be very 
oreat. 
However, whatever exceptions may be discovered in the cases of indi- 
*The Penokee iron-bearing series of Michigan and Wisconsin, by R. D. Irving and C. R. Van 
Hise: Mon. U.S. Geol. Survey, no. xix, 1892, pp. 305-318. Mon. no. xxviii, cited, pp. 448-450, 452- 
454, 456-459. The pre-Cambrian rocks of the Black Hills, by C. R. Van Hise: Bull. Geol. Soc. Am., 
vol. i, 1890, pp. 222-229. 
XLIII—Butt. Geou. Soc, Am., Vou. 9, 1897 
