THE VOLCANOES AND ROCKS OF PANTELLERIA 21 



ing. The soda-microcline phenocrysts very seldom show multiple 

 twinning lamellae or the usual grating structure, but Carlsbad 

 twins of simple individuals are common. The composition, judg^' 

 ing from the rock analyses and norms, is somewhat variable, 

 between OrjAba to Or2Ab3, but will average somewhere about 

 Or^Ab,. 



Since this paper was written, Dr. H. E. Merwin has very kindly 

 examined optically the feldspar phenocrysts of some of the lavas. 

 In the pantellerites of Zeneti, Khagiar, Gelkhamar, and Cuddia 

 Nera, he finds they have refractive indices (a = about i .527) corre- 

 sponding to an albite content of not more than 30 to 40 per cent. We 

 have seen that the average feldspar of these rocks, while somewhat 

 variable, is about OriAbi, which indicates that the small feldspars 

 of the groundmass are much higher in albite than in orthoclase. 

 The phenocrysts of the Montagna Grande trachyte, however, are 

 relatively higher in albite and approach more nearly to the average 

 composition, OriAbi. The study of these feldspars is, as yet, but 

 preliminary, and a chemical and optical investigation will be taken 

 up in detail in the near future, but the evidence goes to show that 

 the soda tends to remain in solution longer than the potash. 



The abundance of sodic pyroxenes and hornblendes, namely, 

 aegirite, aegirite-augite, cossyrite, and possibly kaersutite, and the 

 poverty in non-sodic augite and hypersthene are also notable. 

 It is also interesting to remark on the absence of blue sodic horn- 

 blendes, as riebeckite, arfvedsonite, or crossite, riebeckite especially 

 being commonly found in sodic rocks of high silicity. A paper 

 by Murgoci,' in which he correlates the presence of riebeckite with 

 zirconium and fluorine as "mineralizers," and katoforite (and cossy- 

 rite) with titanium, is interesting in this connection, since on Pan- 

 tellaria we find titanium high and zirconium low, with no evidence 

 of the presence of fluorine. 



It is also noteworthy that only a small part of the aegirite and 

 cossyrite is in the form of phenocrysts, the greater part of these 

 minerals being in the groundmass, or, as is well seen in the hyalo- 

 pantellerites, not crystallized at all. Here we see the same tendency 

 of the soda to remain in solution as was observed with the feldspars. 



^ G. M. Murgoci, Am. Jour. Sci., XX (1905), 133. 



