ITALIAN PETROLOGICAL SKETCHES. 



I. THE BOLSENA REGION. 



Introduction. — The number and easy accessibility of its vol- 

 canoes render Italy an enticing field for the geologist. The 

 peculiar characters of their eruptive rocks, which are rich in pot- 

 ash, and in which leucite is a most common mineral, render them 

 of special interest to the petrologist. It would seem, however, 

 judging from a quite extensive survey of the literature, that the 

 country has been rather neglected in recent years by petrolo- 

 gists ; since, except for a comparatively small number of modern 

 papers describing limited districts, we must turn for many of our 

 descriptions to the writers of more than a quarter of a century 

 ago. Few attempts also have been made to correlate the facts 

 in our possession for the purpose of determining the general 

 petrological characters of the Italian province. 



In the autumn of 1894 the writer had the opportunity to pay 

 brief visits to most of the Italian mainland volcanoes and make 

 collections of their representative rocks. A study of the speci- 

 mens collected revealed so many new and interesting features 

 that it was decided to publish a series of short papers on various 

 Italian volcanoes. In these the separate rocks will be described, 

 and some general conclusions which have been drawn from their 

 study will be presented in a final paper. The work of the mod- 

 ern school of Italian petrographers will be used and quoted exten- 

 sively, since it is comparativel}^ little known to the outside world. 

 As the time devoted to each volcano was all too brief these 

 papers must be sketches merely, and will be predominantly pet- 

 rological in character. 



I. THE BOLSENA REGION. 



Bibliography. — The work of the early geologists — such as 

 Breislak, Brocchi, Pareto and Pilla — need not detain us, as they 



541 



