350 HENR V S. WA SUING TON 



To the northwest of Bolsena and between the line of vol- 

 canoes just noticed and the coast are the eruptive centers form- 

 ing de Stefani's other type. The most noteworthy are those at 

 Campiglia, Rocca Strada, Monte Catini, Monte Amiata, Radi- 

 cofani, Tolfa, and Cerveteri. These are all of relatively small 

 extent, and are in general higher in proportion to their breadth 

 than the strato-volcanoes. Some of them, as Tolfa and Cerve- 

 teri, were superfusive and apparently formed by domal eruptions 

 of a pasty magma. Monte Amiata was probably a true strato- 

 volcano." According to Lotti^ the Campiglic mass was lacco- 

 litic in character, being covered by arching Eocene beds. It is 

 noteworthy that dikes, which are practically unknown elsewhere, 

 are quite numerous here. 



As de Stefani points out, these smaller eruptions differ from 

 the large volcanoes in two important petrographical particulars. 

 First, each individual mass is made up substantially of one kind 

 of eruptive rock, the rock type at each being practically per- 

 sistent throughout the mass. This persistency of type at each 

 is in marked contrast with the great variety of products found 

 at each of the strato-volcanoes. In the second place these 

 smaller eruptions seem never to have produced leucitic rocks — 

 at least none are known with certainty to occur as their products. 

 They are uniformly non-leucitic and trachydoleritic, and extreme 

 in type — either acid or basic. The rocks of Campiglia, Rocca 

 Strada, Monte Amiata, Tolfa, and Cerveteri belong to the tos- 

 canites — acid trachydolerites with over 65 per cent, of silica 

 with or without free quartz ; while those of Radicofani, and 

 probably also those of Monte Catini, are basic in character, with 

 SiOg about or below 55 per cent., and belong either to the cim- 

 inites or the basalts. 



TRACHYDOLERITES. 



The intermediate potash-rich rocks which are found along 

 the Bolsena- Vesuvius line cdixxy basic plagioclase-labradorite to 



'De Stefani, Boll. Com. Geol. Ital., 1888, 223. 

 ^LoTTi, cf. Dalmer, Neu. Jahrb., 1887, TI, 207. 



