I. The Bocks of Monte Ferru. 527 



analcite,* formed the final products of eruptive activity. 

 They are met with near Scano, on the northeastern flank, and 

 form part of a ridge south of this village, called Binzale 

 Prunu. There is also an exposure at a locality called Ghizo, west 

 of Monte Urtigu, and Dannenberg mentions and maps another 

 at Spelunca, south of Ghizo, which I did not see. As these 

 rocks, with similar ones from near Bonorva, are fully described 

 in the paper cited, a brief description must suffice here. 



These lavas are dark gray to black, very dense, compact, and 

 with aphanitic groundmass. The rock from Scano carries 

 nodules of augite and granular olivine, but shows few true 

 phenocrysts of augite, olivine and biotite. Those of Binzale 

 Prunu, Ghizo and Spelunca show many large plates of a bronzy 

 biotite, and a few phenocrysts of olivine and augite. The 

 specimens from about Scano are very, fresh, but those from 

 Ghizo are badly weathered. 



Microscopically the small phenocrysts of augite and olivine 

 are not prominent and offer no features of special interest. 

 The biotite tables are brown, with an axial angle 2E = 40° — 45°, 

 and a refractive index 7 na = 1*65 — 1*655, as determined by 

 Dr. Merwin, resembling some from Mount Vesuvius. There 

 are many small isotropic areas of a mineral which resembles 

 leucite, showing similar inclusions, but with a refractive index 

 of 1'502, determined by Dr. Wright. Analysis of the portion 

 of the rock soluble in warm dilute hydrochloric acid yielded 

 the figures Na 2 =2*66 and K 2 = 0'12 in percentages of the 

 rock, showing that the supposed leucite is in reality analcite. 

 The amount of water yielded by these very fresh rocks is also 

 in harmony with this interpretation. This analcite is regarded 

 as primary and it is suggested that its occurrence will explain 

 the apparent anomaly of supposed leucites in other fresh rocks 

 high in soda and low in potash. 



The microgroundmass is formed of many very small prisms 

 of augite and grains of olivine and magnetite in colorless glass. 

 There is no feldspar present. 



The chemical composition is represented by two analyses of 

 the Monte Ferru rocks, with one of that of Bonorva. 



Silica is rather low, and titanium decidedly high, and there 

 seems to be a noteworthy amount of nickel. The most striking 

 feature — when the presence of "leucite " is considered — is the 

 small amount of alkalies and the dominance of soda over 

 potash. The high amount of water, in view of the freshness 

 of the rocks, is also noteworthy. 



Doelter's analysis is obviously faulty, especially in the high 



*H. S. Washington, Boll. Soc. Geol. Ital., xxxiii, p. 147, 1914; Jour. 

 Geol., xxii, p. 742, 1914. 



