Pirsson — Microscopical Characters of Volcanic Tuffs. 201 



class mentioned above, for if embedded in solid rock they 

 would of necessity have been shattered by the forces which 

 fragmented it. More commonly the crystals seen in these 

 tuffs consist of broken pieces which in thin section exhibit 

 here and there a crystal outline, but in general show no definite 

 shape. The student must guard against imagining that he can 

 see in them the cuspate and lune-like forms which are charac- 

 teristic of the vitroclastic texture shown by glass shards ; 

 obviously such structures could not be produced by the shat- 

 tering of solid substances. The resemblances to it are most 

 frequently seen in pieces of quartz and are due to the con- 

 choidal fracture of the latter. 



It is stated by Harker* that in crystal tuffs there is frequently 

 a characteristic arrangement, by which the crystals stand in the 

 finer matrix with their longer axes at right angles to its lamina- 

 tion, as if they had fallen into it from above. 



Interstitial Filling. — In a fresh crystal tuff, whose sub- 

 stance is of direct magmatic origin, the material between the 

 crystals is composed of glass shards and the finest dust, often 

 almost sub-microscopic particles of both, produced partly by 

 the explosion and partly by attrition of the larger fragments 

 upon one another. It may, therefore, exhibit the vitroclastic 

 structure and be itself the best proof of the tuffaceous nature 

 of the deposit. But since the crystals may not always be of 

 direct magmatic origin, that is, of the first class previously 

 mentioned, so also much of the filling between them may be 

 derived from previously formed rocks. In this case the tuffs 

 form gradations into the lithic types described later. Gener- 

 ally speaking, however, the filling in these tuffs is more or less 

 altered, as will be described later. An example of a typical 

 crystal tuff of rhyolitic origin is shown in iig. 4. The crystals 

 are mostly quartz and feldspar with a few of biotite and horn- 

 blende. 



Lithic Tuffs. 



The essential feature of tuffs of this class, as previously ex- 

 plained, is the presence in them to a striking or dominating 

 degree, of fragments of previously formed rocks. Several dif- 

 ferent modes of origin of these may be conceived. Thus in- 

 stances are known where volcanic vents have been blown 

 through a sedimentary series, shattering and powdering the 

 beds and projecting the comminuted material, without, how- 

 ever, being followed by any rise to the surface or escape of 

 magma, or but very little, as in some of the maaren of the 

 volcanic Eifel. Crater pits formed by such "blow outs," to use 



* Harker and Marr : Shap Granite and Associated Eocks, Quart. Jour. Geol. 

 Soc, xlvii, p. 299, 1891. 



