644 A TREATISE ON METAMORPHISM. 



The enlargement of mineral particles lias been described as occurring 

 in quartz, feldspar, hornblende, augite, garnet, tourmaline, and other 

 minerals, by Sorby," Becke/ Irving/ myself/' Williams," Hobbs/ Whittle/ 

 and others. (See figs. 14 and 15.) 



However profound the alterations of metasomatism by molecular 

 mechanical action under mass-static conditions, the original textures and 

 structures of the rock may not be greatly affected. It may be that all of 

 the original minerals composing a rock are completely changed and yet 

 the original igneous or other textures be perfectly preserved. The case is 

 parallel to that of petrifaction of a wood in which no particle of the woody 

 fiber remains and yet the textures of the original organic tissue are almost 

 perfectly preserved. The modifications are mainly changes of substance, 

 not changes of form. Thus all the textures characteristic of igneous rocks, 

 such as granolitic, ophitic, porphyritic, etc., may be almost completely 

 preserved in a rock which has altered throughout. This is illustrated by 

 the dolerite dikes in the iron-bearing formation of the Penokee series of 

 Michigan and Wisconsin. These dikes in the black impervious slates are 

 little altered dolerites, but their continuations in the iron-bearing formation 

 do not contain one vestige of any original mineral, but are ferruginous, 

 hydrated, aluminum silicates, which in composition correspond very closely 

 to kaolin.' 1 Yet the texture in the altered rock and in the dolerite is the 

 same. 



Indeed, not only may there be no tendency to destroy textures and 

 structures which were originally present, but there ma} 7 be a tendency to 



°Sorby, H. C, On the structure and origin of noncalcareous stratified rocks: Proc. Geol. Soc. 

 London, 1880, p. 62. 



& Becke, F., Eruptivgesteine aus der Gneissformation des niederosterreichischen Waldviertels: 

 Tschermaks mineral. Mittheil., vol. 5, pt. 2, 1883. 



c Irving, R. D., and Van Hise, C. R., Enlargement of quartz fragments and genesis of quartzites: 

 Bull. U. S. Geol. Survey No. 8, pt. 1, 1884, pp. 11-43. 



f' Van Hise, C. R., Enlargements of feldspar fragments in certain Keweenawan sandstones: Bull. 

 U. S. Geol. Survey No. 8, pt. 2, 1884, pp. 44-47. Van Hise, C. R., Note on the enlargement of horn- 

 blendes and augites in fragmental and eruptive rocks: Am. Jour. Sci., 3d ser., vol. 33, 1S87, pp. 385-388. 



e Williams, G. H., The greenstone-schist areas of the Menominee and Marquette regions of Mich- 

 igan: Bull. U. S. Geol. Survey No. 62, 1890, p. 173. 



/Hobbs, Wm. H., Phases in the metamorphism of the schists of the southern Berkshire: Bull. 

 Geol. Soc. America, vol. 4, 1893, pp. 173-176. 



9 Whittle, C. L., Some dynamic and metasomatic phenomena in a metamorphic conglomerate in 

 the Green Mountains: Bull. Geol. Soc. America, vol. 4, 1893, pp. 156-158. 



''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. 357, 358. 



