THE VOLCANOES AND ROCKS OF PANTELLERIA 25 



of intensity in vulcanicity, marked by the caldera formation 

 and the dislocation of the Montagna Grande block. Far too little 

 systematic study has as yet been made at any volcano of the change 

 in the chemical characters involved in the succession of flows, in 

 connection with variations in the intensity of the volcanic action, to 

 permit any proper discussion or generalization. It may be said, 

 however, that a causal connection between the two seems to be 

 possible. 



The general succession is strikingly like that seen in Sardinia, 

 the rocks of which will be described in forthcoming papers. Here 

 we find the pre-Tertiary sheets beginning with rhyolites, passing 

 to trachytes, and apparently ending with basalts. The later large 

 volcanoes of Monte Ferru and Monte Arci also poured out first 

 trachytes and rhyolites, followed by large flows of basalt, which also 

 forms the product of the most recent small cones. 



Comparison with other regions. — Rocks analogous to the pantel- 

 lerites, comendites, and trachytes of Pantelleria are not very abun- 

 dant, but are quite widely distributed over the earth. The region 

 most nearly like it is that of Afarland and French Somah described 

 by Arsandaux.' The resemblance is very close and is emphasized 

 by Arsandaux, who was able to study specimens from Pantelleria 

 also. He describes both Hthoidal and glassy pantellerites, which 

 correspond to the two main types on Pantelleria, except for the 

 irregular occurrence of quartz phenocrysts; and also "micro- 

 granites," trachytes, and glassy rhyohtes with aegirite, riebeckite, 

 and some cossyrite. Except for the occasional presence of quartz 

 phenocrysts and the replacement of cossyrite by riebeckite, the 

 resemblance between these rocks and those of Pantelleria is most 

 striking, extending even to details such as the felt of aegirite needles 

 and small areas of micropoikihtic quartz. Associated with these 

 rocks are basalts of ordinary feldspathic types. 



In their chemical features the Somali rocks are like those of 

 Pantelleria, the only notable difference being the smaller amount 

 of soda. Arsandaux did not determine titanium, so we are ignorant 

 as to this. 



' H. Arsandaux, U Etude des roches alcalines de V Est-africain , Paris, 1906, pp. 39, 45. 



