Pirssdn — Microscopical Characters of Volcanic Tuffs. 211 



thing may happen to the corresponding lavas a decision in the 

 case of end products, as to whether the rock was originally tuff 

 or lava, is generally impossible. Quite an analogous result is 

 reached with mafic, or basic, tuffs which in their turn become 

 converted into greenstone or hornblende schists, just as hap- 

 pens with their corresponding diabases or basalts. Doubts may 

 even arise in some cases as to whether such schistose rocks are 

 really of igneous origin, and a chemical analysis may then be 

 of the greatest service in supplementing the field and micro- 

 scopic observations. 



In general it may be said that while sericite is the most 

 characteristic mineral of completely metamorphosed felsic tuffs, 

 it is usually more or less accompanied by quartz, biotite, chlorite, 

 epidote and clinozoisite ; while in the mafic ones actinolite and 

 chlorite, also accompanied by more or less biotite, clinozoisite 

 and epidote are common. 



Where the destruction has not been so complete, however, 

 then the characteristics of tuffs should be carefully sought in 

 the sections. It may be remarked in this connection that the 

 chances of their survival are much better in felsic than in 

 mafic ones. Remains of the vitroclastic texture, as indicated 

 in fig. 2, if found, may be conclusive ; if not, a secondary line 

 of evidence may be sought in the nature of the included crys- 

 tals or their fragments if such are present, as discussed pre- 

 viously under crystal tuffs. Evidence of this kind by itself 

 will probably not be decisive as between lavas and the corre- 

 sponding tuffs, but supplemented by facts to be seen in the 

 field it may become so ; for it must not be forgotten that 

 tuffs, like sediments, are often well bedded and a combination 

 of bedding and angular phenocrystic fragments speaks strongly 

 in favor of tuffaceons origin. An excellent example of the 

 determination of this source for the material of a gneiss (halle- 

 flintgneis) has been given by Backstrom.* It is to be under- 

 stood that only fine tuffs are here referred to ; in the case of 

 breccias the evidence afforded by the megascopic structure of 

 the rocks is of the highest value and may be quite decisive by 

 itself, without reference to microscopic or chemical determina- 

 tions, as shown for example in various parts of the Piedmont 

 plateau by the work of Professors Bascom, Pogue, and Dr. 

 Laney whose papers have been previously referred to in this 

 article. 



Sheffield Scientific School of Yale University, 

 New Haven, Conn., March, 1915. 



* Vestanafaltet, K. Svensk. Vetenskaps. Akad. Hand., xxix, No. 4, pp. 

 52, 122, 1897 ; Conf. also. O. Nordenskjold, Archaeische Ergussgesteine 

 aus Smaland. Bull. Geol. Inst, of Upsala, No. 2, vol. i, p. 81, 1893. 



