514 Washington — Contributions to Sardinian Petrography: 



altitude, 1050 m., is reached at the summit of Monte Urtigu,* 

 near the southern end of the exposure of the trachytic mass, 

 and about 4 km. west of the town of Santu Lussurgiu. The 

 small cluster of flat-topped peaks, of which this is slightly 

 the highest, is at the headings of three caldera-like valleys, 

 the general extent of which is indicated on Dannenberg's map. 

 The largest is on the north with the town of Cuglieri near its 

 mouth. Somewhat smaller is that of the Ghizo region in the 

 southwest, and still smaller is that in the southeast, near Santu 

 Lussurgiu. These valleys reveal very clearly the internal 

 structure of the mass. 



According to Dannenberg, eruptions began in the Miocene 

 with the ejection of rhyolitic or trachytic tuffs, while Deprat 

 considers these tuffs to be pre-Miocene and antedating the 

 Ferru volcano. I made no special observations on this point, 

 but am inclined to agree with the latter view. Over these 

 tuffs were extruded trachytic lavas, which seem to have been 

 highly viscous and to have formed a huge dome at least 1000 

 m. high, the lava forming but few true flows. This extrusion 

 was seemingly not accompanied by any extensive amount of 

 tuffs or loose material, though some small secondary cupolas 

 were formed. This trachytic core of the mountain was quite 

 uniform in character, but there are masses of phonolite or 

 phonolitic trachyte, which seem to be rather schlieren in the 

 trachyte than flows. 



After a repose of unknown duration there followed, appar- 

 ently with few lavas of intermediate composition, a prolonged 

 outpouring of basalts, which seem to have issued, not from 

 craters, but from fissures which penetrated the trachytic mass. 

 These basalts are somewhat varied in character, one type form- 

 ing flat tables which cover the trachytes in some of the higher 

 parts of the mountain. These were followed by extensive 

 sheets which covered the lower slopes and extended far over 

 the surrounding plain. There seems to have been a paucity of 

 scoria and ash accompanying these basaltic outflows, as was 

 the case with the trachytes. 



There ensued a long period of complete repose, during which 

 probably the three large calderas were formed, presumably by 

 erosion after preliminary explosions. Volcanic activity reawoke 

 feebly and for a short time with the outpouring of small flows 

 of peculiar and, for this volcano, aberrant lavas, the analcite 

 basalts of Scano and Ghizo, and a biotite-augite rock of Monte 

 Commida. 



Monte Ferru, then, is essentially composed of a large domal 

 core of very uniform trachyte, passing locally into phonolite, 



* This is the spelling on the Italian map. Doelter renders it Urtica, and 

 Dannenberg Urticu. The accent is on the second syllable. 



