Pirsson — Microscopical Characters of Volcanic Tuffs. 191 



Akt. XVIII. — The Microscopical Characters of Volcanic 

 Tuffs — a Study for Students ; by L. Y. Piksson. 



The recognition of fragmental volcanic material in fresh 

 surficial deposits is usually relatively easy, both from characters 

 which may be observed in the field and in thin section under 

 the microscope. In proportion, however, as such deposits 

 become altered increasing difficulties are met with and where 

 they have been buried under later sediments, and in many 

 cases metamorphosed to a greater or lesser degree, the task of 

 determining them becomes one of the most arduous which con- 

 fronts the petrographer. Every point of vantage yielded by 

 field studies, the megascopic structure, thin section, and 

 chemical analysis, must be made use of. 



Moreover, the difficulty may be increased in another way. 

 In the case where we have to determine whether a given rock 

 is igneous or sedimentary ; whether it has been formed by the 

 solidification of a fluid magma or not ; there is no transitional 

 stage between them to consider. This is not the case with 

 volcanic tuffs ; they may fall on the land and show little or no 

 evidence of bedding ; they may fall into water and exhibit it 

 in marked perfection. While falling into water they may be 

 mingled with contemporaneously deposited land-waste brought 

 by currents or streams ; or, if fallen originally upon the land, 

 after more or less oxidation, or none, depending on the length 

 of their sojourn, they may be washed down into seas or lakes 

 and mingled in the process with greater or lesser quantities of 

 the ordinary products of land erosion. Thus every degree of 

 perfection of stratification in pure tuffs may be expected, and 

 every degree of transition into ordinary sediments — sandstones, 

 and shales. The intermingling may be by pure layers of each, 

 bed after bed, of any degree of thinness ; or of grains of the 

 different materials in the same bed. 



It has seemed to the writer that in the various text-books on 

 petrology the subject of volcanic tuff deposits has never been 

 as adequately or systematically treated as it should be. In 

 some works, especially the briefer ones, they are considered in 

 such a general way that the student in search of information 

 receives very little idea of the characteristics necessary for his 

 guidance in their determination. In the more comprehensive 

 hand-books, on the other hand, there is so much detail given, 

 especially of local occurrences, that the student fails to obtain 

 that comprehensive view of essentials which should form the 

 basis of his knowledge. There are a number of excellent 

 essays by investigators in the literature on particular occur- 

 rences, one of the best of which is on the so-called " Lenne- 



