Pirsson — Microscopical Characters of Volcanic Tuffs. 203 



exhibit clearly the vitroclastic texture, but as in all tuffs, this 

 depends in great measure on the recency and state of preserva- 

 tion of the rock, as described later. The size of the stony 

 particles, which must be large enough in general for the 

 particular kind of lava to be recognized, takes them out of the 

 class of volcanic dust, into that of volcanic ash, mentioned at 

 the beginning of this article, and we see, therefore, that there 

 is, in a general way, a progression in tuffs from vitric types — 

 the finest— through crystal tuffs, into lithic ones and then into 

 breccias. An example of a lithic tuff is seen in fig. 5. 



Fig. 5. 



Fig. 5. Lithic tuff of trachyandesitic nature, from the Euganean Hills, 

 Italy ; actual size, 5 mm in diameter ; ni'cols crossed. 



In determining the position of a tuff in petrographic classi- 

 fication, that is, whether it is rhyolitic, trachytic, andesitic, 

 etc., the student should bear in mind, and this applies more 

 especially to the crystal and lithic varieties, that he is liable to 

 encounter among the ash particles, as may be inferred from 

 what has been previously stated, bits of rocks and minerals 

 quite foreign to a lava of the particular kind with which he 

 may collate it. His determination, therefore, should be based 

 on the average character of the dominant ash particles. 



Average Tuffs. 



In the foregoing discussion three special types of tuff have 

 been selected and described for divisional purposes in classifica- 

 tion. It should not, however, be supposed that all tuffs will 



