LINOSA AND ITS ROCKS 



23 



It will be seen that the relative amounts of saHc and of alferric and 

 femic minerals is about the same in the two, so that the ground-mass 

 of the Fosso type must be more highly feldspathic than that of the 

 Ponente type, much of the feldspar of which exists as phenocrysts, 

 and this fact will account for the usually lighter color of the present 

 rock. The amount of nephehte is small, and those of augite, magne- 

 tite, and apatite scarcely differ in both rocks, while olivine is dis- 

 tinctly higher in that from II Fosso. 



Chemical composition. — This was determined by the analysis of a 

 specimen from a block in the tuffs of II Fosso, the results of which 

 are given below, along with the molecular ratios. 



Camptonose (feldspar-basalt). Block in tuff, II Fosso, Linosa. 



The general features of this analysis are closely like those of the 

 two Monte Ponente rocks, even in the small details of titanium, man- 

 ganese, and nickel. It may be noted, however, that this rock is very 

 distinctly lower in silica and higher in alkalies, especially potash 

 while magnesia is a trifle higher and hme shghtly lower, the other 

 constituents remaining about the same. 



Occurrence. — The most representative specimens of this type were 

 met with as angular, compact blocks in the tuffs of Monte Levante, II 

 Fosso, and the north crater of Monte Bandiera, while other specimens, 

 decidedly transitional toward the preceding type, occur as flows at 

 Monte Vulcano and in the blocks of lava, apparently derived from 

 Monte Ponente, which have been excavated at the Scala Nuova for 

 use in building the jetty. 



