ITALIAN PETROLOGICAL SKETCHES. 



V. SUMMARY AND CONCLUSION. 



The volcanoes which have been described in the preceding 

 papers belong to the main Italian line which extends west of 

 the Apennines from Tuscany to Naples, and which may be 

 called the Bolsena- Vesuvius line. In a paper already quoted on 

 the " Extinct Volcanoes of the Northern Apennines " de Stefani ^ 

 calls attention to the fact that these volcanoes may be referred 

 to two distinct types, distinguished not only by their structure, 

 but by their eruptive rocks. 



One type is that of the great strato-volcanoes, represented 

 at Bolsena, Viterbo, Bracciano, and the Alban Hills, as well as 

 at Rocca Monfina and Vesuvius farther south. The main struc- 

 tural features of these have been already noticed. Petrograph- 

 ically these strato-volcanoes are characterized by two features. 

 In the first place leucitic rocks are very abundant at all of them, 

 and form indeed their dominant and most characteristic feature. 

 These are accompanied in most cases, not always, by non-leucitic 

 rocks — ciminites and vulsinites, with exceptionally phonolite 

 at Viterbo and toscanite at Bracciano. True trachytes are met 

 with in abundance in the Vesuvian region and in small amount 

 at Rocca Monfina, but elsewhere seem to be rare. Their second 

 characteristic is the variety of eruptive products at each center, 

 both among the leucitic and the non-leucitic rocks, as well as 

 the abundance of tuffs. Though the variety be great, yet the 

 rocks of each bear a great resemblance to those of the other cen- 

 ters, so much so that the whole line (including the centers of 

 the second type) form an excellent example of a "petrograph- 

 ical province," and it is clear that the products of all the centers 

 are related to each other genetically. 



' De Stefani, Boll. Soc. Geol. Ital., X, 449-555, 1891. 



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