210 Pirsson — Microscopical Characters of Volcanic Tuffs. 



Contact metamorphism of trachytic (" orthophyre ") tuffs 

 near Harzburg by tbe Brocken granite bas been studied by 

 Erdmannsdorfer,* who states that the altered rock has the 

 hornfels texture and consists mainly of brown biotite, consider- 

 able enstatite and anthophyllite, the latter often in fine needles 

 which may aggregate into spherulites, some augite, much 

 orthoclase with a little plagioclase and quartz. The former 

 large embedded crystals of orthoclase have been changed into 

 aggregates of various minerals. 



The effect of contact metamorphism upon basaltic tuffs has 

 been studied by few observers.f Probably where the altera- 

 tion of diabases has been described in the literature, as by 

 the English and SaxonJ geologists, the metamorphism of tuffs 

 has been included ; Rosenbusch suggests this in one case.§ 

 It might be much more difficult to decide in tbe changed rock 

 whether it had been a basalt (or diabase) or its tuff than with 

 more felsic, siliceous types. In some cases the action leads to 

 the formation of schistose rocks composed of actinolite or 

 anthophyllite in needles, or common green hornblende in 

 granules with a background of plagioclase, augite, biotite, 

 garnet, and other minerals in varying amounts. Barker men- 

 tions biotite as the most prominent mineral and states that 

 large feldspars are recrystallized into mosaics or replaced by 

 pseudomorphs of epidote ; this is much like the effects men- 

 tioned above as occurring in the andesitic tuffs. 



General or dynamic metamorphism. — The distinction 

 between the effects produced by the various processes which 

 tend to alter tuffs, such as contact metamorphism, devitrifica- 

 tion and silicitication, previously described, and general met- 

 amorphism, is for the most part a theoretical rather than a 

 practical one. In metamorphic complexes there occur rocks 

 which investigation will show to be of tuffaceous origin and 

 whose characters will be similar in the main to those which 

 have been described above. The especial feature which is to 

 be added in dynamic metamorphism is mashing and shearing 

 which destroy and obliterate the diagnostic characters of tuffs 

 in proportion to the extent to which they have operated. In 

 felsic tuffs the new mineral which is generated by their effect 

 upon the feldspar is sericite and the final result is to reduce 

 such tuffs to fissile sericite schists or phyllites. Since the same 



* Devonischen Eruptivgest. und Tuffe bei Harzburg, Jahrb. d. k. preuss. 

 Geol. Landanst., xxv, p. 45, 1904. 



fR. Beck, Amphibolitizirung von Diabasgesteinen, etc., Zeitscbr. d. D. G. 

 Ges., xliii, p. 259, 1891 ; Harker and Marr, Metamorphic Rocks around Shap 

 Granite, Quart. Jour. Geol. Soc, xlix, p. 360, 1893. 



tConf. Rosenbusch, Mass. Gest., 4th Aufl. , p. 120, 1907. 



§Barrois, Excursion aux environs de Morlaix, Bull. Soc. Geol. Fr. (3), 

 xiv, 888, 1886. 



