208 Pirsson — Microscopical Characters of Volcanic Tuffs. 



have experienced similar changes. But without the upper 

 nicc-l, the condenser lowered, and by using a fairly high 

 powered objective, careful study of the section will often bring 

 out the original vitroclastic texture. By raising and lowering 

 the objective the shapes of the original dust and ash particles 

 become visible by the lightening of their edges from a Becke 

 light band, due to differences in refractive indices. One can 

 then trace out the forms of the shards, threads, cusps, vesicles, 

 etc., that have been previously described. An example of this 

 kind is seen in fig. 6 ; it should not be thought that the texture 

 shows as clearly in the section as in the illustration ; if the 

 writer had depicted it as faintly as much of it actually appears, 

 the drawing could not have been reproduced. The vitroclastic 

 nature of the rock from the Oelberg was first determined by 

 Andreae and Osann* and a somewhat similar instance has been 

 described and figured by Gr. H. Williams, f In other cases the 

 recognition of the texture in plain light may be aided by slight 

 differences in coloration or outlining of the parts by ferritic 

 material. In tuffs from mafic magmas devitrification is a 

 process of alteration into secondary minerals as described above 

 under alteration. 



Metamorphism of Tuffs. 



Contact Metamorphism. — The effect of invading magmas 

 upon already solidified igneous rocks is a problem which has 

 not yet received the attention from petrographers which its 

 importance demands, and this is probably most marked in the 

 case of volcanic tuffs. Except in one instance mentioned later 

 it has not been studied by the writer or his students, and refer- 

 ence must be had therefore to the literature of contact met- 

 amorphism. The most detailed study appears to be that made 

 by Harker and Marr;}: on ancient rhyolitic and andesitic tuffs in 

 the contact zone of the Shap granite in Westmorland, England. 

 The former are recognized in the field as of explosive nature 

 by the angular fragments of rhyolite and some andesite which 

 they contain, and are thus classed as breccias ; in addition 

 crystals of feldspar, quartz, etc., occur. The proportion of 

 fine matrix to fragments varies in different beds. The fine ash 

 and dust away from the contact zone is seen to be considerably 

 altered, having been extensively silicified, and with the feld- 

 spar phenocrysts replaced by masses of epidote ; this alteration 

 is believed to have occurred before intrusion of the granite. 

 As one passes into the contact zone the first stage is the forma- 



- Erlaut. zu Blatt. Heidelberg d. geol. Spezial Karte Baden, 1896. 



f Silicified Glass Breccia of Vermilion Eiver, Sudbury Dis't., Bull. Geol. 

 Soc. Anier., vol. ii, p. 138, 1891. 



% The Shap Granite and Associated Boclcs, Quart. Jour. Geol. Soc, vol. 

 xlvii, p. 260, 1891. 



