202 Pirsson — Microscopical Characters of Volcanic Tuffs. 



a miner's term, would be surrounded by a deposit iu fragmen- 

 tal condition of the bedded rocks affected, with little or no 

 volcanic material ; whether they should be called " tuffs," or 

 not, must be largely a matter of individual opinion. Also, at 

 any time during eruptions, material may be torn from the 

 walls of the conduit where it passes through the sedimentaries 

 or crystallines forming the basement of the volcano, and appear 

 in the tuffs. But the most prominent kinds of fragmented 



Fig. 4. 



Fig. 4. Rhyolitic crystal tuff from the Antelope Range, Utah. Crystals 

 shown polarizing between crossed nicols ; filling of glass dust, etc., in plain 

 light. Actual area, 5 mm in diameter. 



rocks present in lithic tuffs are, in general, lavas of a class 

 similar to that of the exploding magma, or genetically related 

 to it. While they may be portions of the cone already formed 

 it may be suspected that more often they are parts of a solidi- 

 fied and more or less completely crystallized crust, which has 

 formed by the freezing of the upper layers of the magma 

 column during a period of quiescence in its volcanic activity, 

 and which, by the rising pressure of the accumulating vapors 

 below it, is finally comminuted when explosion occurs. An 

 average lithic tuff consists then largely of tiny fragments 

 of rhyolite, trachyte, phonolite, or andesite, etc., as the case 

 may be, with their characteristic minerals and textures in 

 those stages of development which the particular rock had 

 attained on solidification, and with an intertilling of glass 

 shards, bits of pumice, mineral dust, etc., mingled usually with 

 more or less distinct crystal. fragments. The intertilling ma}' 



