832 HENRY S. WASHINGTON 



volcano. It may also be added that Stoppani combats Verri's 

 views on these two points, as well as on the eruption of tor- 

 rents of mud. 



Though my own opportunities for observation were too 

 limited to permit me to deal authoritatively with these prob- 

 lems, yet it may be of use to state briefly my own views. I am 

 inclined to regard the Cimini eruptions as largely of a domal 

 t^'pe, though lava streams and tuffs are more abundant than is 

 usually the case with this type of volcano. Vom Rath's and 

 Deecke's view as to the origin of the leucite phenocrysts in the 

 "petrisco" I cannot agree with — a point which we shall have 

 occasion to examine later on. I most decidedly concur with 

 Deecke and Stoppani in considering the crater lake of Vico due 

 chiefly to explosive eruptions, but think it probably a single 

 crater, and that Monte Venere is a dome, representing the last 

 eruption in the region. It is also well established that here, as 

 at Bolsena, the leucitic lavas were later as a whole than the 

 trachytic. 



PETROGRAPHY. 



We have to deal here, as in the Bolsena region, with two 

 chief types of rocks, a trachytic and a leucitic' Petrographi- 

 cally the two regions are also alike in other more detailed 

 features. The most important of these is the occurrence of a 

 class of rocks intermediate between the trachytes and the andes- 

 ites, which form a group of effusive rocks corresponding to 

 some of Brogger's monzonites. 



Vidsiiiite. — Of rocks corresponding to this type of interme- 

 diate effusive rocks characterized by the presence of both ortho- 

 clase and a basic plagioclase, there came to my notice only two 

 occurrences, both being on the western border of the region. 

 One is from Massa di San Sisto, about 7^^"' southwest of Viterbo 

 on the road to Vetralla, where it forms a flow from the direction 

 of Monte Fogliano cut through by the road. The upper part is 

 visible here and there, though largely covered with travertine 

 and leucitic tuffs. It is somewhat decomposed and fresh speci- 



