H. S. Washington — Ischian Trachytes. 375 



Aet. XLIV. — On some Ischian Trachytes ; by Henry S. 

 Washington. 



In the fall of 1894 I had occasion to visit Ischia, in the Bay 

 of Naples, and collect the representative trachytes of the 

 island. Most of these are too well known to need descrip- 

 tion, but some specimens from Mt. Rotaro showed a certain 

 rather rare structure in such perfection that an account of 

 them seems not unworthy of publication. 



Mt. Rotaro is a small volcanic cone in the northeastern part 

 of the island, some 315 meters high,* and with a well-preserved 

 crater 107 meters deep. As far as can be seen it is composed 

 of fragmentary material, scoriae and blocks of trachyte and 

 obsidian, which show stratification in places. The whole rests 

 on marls and clays containing late Tertiary fossils, f The 

 ridge of Mt. Tabor, J composed of the well known sodalite 

 trachyte, is a lava-stream of this small volcano which has 

 flowed to the north. According to Fuchs,§ Mt. Rotaro is the 

 site of the great eruption of the first half of the fourth 

 century B. C, which drove the inhabitants to the mainland. 



The trachytes of Mt. Rotaro are of two kinds. One is a 

 not very compact, light grayish-brown rock with phenocrysts 

 of orthoclase and augite. It occurs in relatively small quan- 

 tity, and under the microscope presents no features of special 

 interest here ; large phenocrysts lying in a typically trachytic 

 noncrystalline groundmass, which shows a marked flow-struc- 

 ture among its component feldspar laths. Corresponding to 

 this is a light brown obsidian, extremely brittle and somewhat 

 vesicular, showing some orthoclase phenocrysts. Under the 

 microscope it presents a clear, light brown glass, with rare 

 green biotite and augite, and glassy sanidine phenocrysts, with 

 a few feldspar microlites. Many air cavities are present. 



The black trachytes and obsidians are of greater interest. 

 For reasons which will be seen we shall begin their descrip- 

 tion with the obsidians. No. 524 || is coal black, with a vitre- 

 ous luster, very brittle, and shows numerous stout phenocrysts of 

 glassy sanidine. An analysis by Fuchs^f of a similar black 

 obsidian from Mt. Rotaro is here inserted : 



* This is the height given on the Government Topographical Maps ; Fuchs 

 gives BO 1-4, and Fonseca 277*4. 



f Fonseca, Geologia dell' Isola d' Ischia, Florence 1870, 25. 



% vom Rath, Zeitschr. d. d geol. Gesellsch. xviii, 628, 1866. 



§ Fuchs, Isola d'Ischia. Florence 1873, 47 ; also. Tsch. Min. Mitth., 1872, 237. 



|| The numbers are those of specimens in my collection. 



^f Fuchs, op. cit., 40. 



