184 A TREATISE ON METAMORPHISM. 



alteration of clastic rocks, are perfect illustrations of the above changes." 

 In these rocks the acid feldspars (sp. gr. 2.55-2.67) have extensively 

 altered into quartz (sp. gr. 2.65) and mica (sp. gr. 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, etc., have developed 

 on an extensive scale in the more metamorphosed varieties. 



When all the minerals formed are taken into account the average 

 is as given. But it is not supposed that there are not exceptions to each of 

 the rules that in the upper physical-chemical zone lighter minerals form 

 and in the lower zone heavier minerals develop. Indeed exceptions are 

 known to both. An illustration of such exceptions in the upper zone is the 

 case already mentioned (see pp. 181-182), the replacement of calcium by 

 magnesium. A case of the change from higher to lower specific gravity 

 in the lower zone is the alteration of pyroxene into amphibole. On the 

 average the former is slightly heavier, and yet in the lower zone, under 

 both mass-static and mass-mechanical conditions, pyroxene very generally 

 alters to amphibole. Of course in this transformation a change simultane- 

 ously takes place in the chemical composition (and this may have an effect 

 upon the volume of the minerals); for, in general, pyroxene contains a 

 greater proportion of calcium and less proportions of magnesium and iron 

 than the amphiboles. If all of the compounds concerned in the change 

 were taken into account this apparent exception to the rule of the production 

 of compounds of high specific gravity in the lower zone would probably 

 disappear. In some of the deepest-seated 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 change of pyroxene 

 to amphibole is, in order that pressure shall become the dominant factor for 

 each of the minerals as well as for the average, that the pressure must be 

 very great. 



But whatever exceptions may be discovered in the cases of individual 

 minerals, the rules that in the upper physical-chemical zone the alterations, 

 on the average, result in decrease of specific gravity, and that in the lower 



"Irving, R. D., and Van Hise, C. R., The Penokee iron-bearing series of Michigan and Wisconsin: 

 Mon. U. S. Geol. Survey, vol. 19, 1892, pp. 302-331; also vol. 28, 1895, pp. 448-450, 452-454, 456-459. 

 Van Hise, C. R., The pre-Cambrian rocks of the Black Hills: Bull. Geol. Soc. America, vol. 1, 1890, 

 pp. 222-229. 



