546 HENRY S. WASHINGTON 



As to the period of the eruptions de Stefani points out that 

 in certain places, especially in the northern part, volcanic prod- 

 ucts alternate with marine fossil-bearing strata of the late 

 Pliocene ; and we may assume that the main eruptions began 

 in the Pliocene period. Both he and Stoppani also give reasons 

 for thinking that the latest eruptions were contemporary with 

 man and geologically very recent.'^ 



PETROGRAPHY. 



The petrography of many of the Italian volcanoes is involved 

 in considerable confusion. This is especially true of the regions 

 immediately around the great crater-lakes. We shall see that 

 these large volcanoes erupted a great variety of lavas, while the 

 smaller eruptive masses flanking this main line offer much less 

 variety, each separate mass being largely composed of one defi- 

 nite type of rock. The Bolsena volcano is no exception to this 

 rule, which de Stefani^ has brought out very clearly. 



The confusion results partly from the tendency which many 

 of the types have, especially among the leucitic rocks, of grading 

 into one another ; and partly from the presence of peculiar non- 

 leucitic rocks which do not fit exactly into any place in our 

 scheme of classification, and whose naming is largely a subjective 

 matter. 



It has lately been pointed out by Brogger^ and Pirsson"* that 

 at the present stage of the science we must recognize the quanti- 

 tative chemical and mineralogical relations as well as the quali- 

 tative. Broggers has also made an able plea for the recognition 

 of transition groups, the importance of which in our classifica- 

 tion he brings out very clearly. With Brogger's views on the 

 subject I, like Pirsson, heartily concur; and his principles will 

 be recognized in the classification of the rocks which we shall 



' Stoppani (p. 384) quotes Gualterio as arguing for the probability of the forma- 

 tion of the Bolsena volcano during the paleolithic period. 

 = De Stefani, op. cit., 550. 



3 W. C. Brogger, Gest. d. Grorudit-Tinguait Serie, Kristiania, 92, 1894. 

 '•L. V. Pirsson, Am. J. Sci., L, 478, 1895. 

 s Brogger, op. cit., 93, and Eruptionsfolge bei Predazzo, Krist.. 1895. 



