1. The Bocks of Monte Ferru. 523 



AbjArij, is most abundant. There are some euhedral, much 

 twinned, phenocrysts of this, especially in the andesitic type, 

 but for the most part it forms the usual groundmass laths, with 

 divergent arrangement, and varying in size from very small 

 and thin to coarse individuals up to l mm long. In any one 

 specimen they are of about uniform size. In the andesitic 

 basalts (as from Punta Arancola and Nuraghe Silvanis), the 

 groundmass feldspars are mostly equant and anhedral, rather 

 than lath-shaped. 



In most cases pyroxene is the next most abundant mineral. 

 This is also quite colorless in the thin section, none of the 

 crystals showing even a tinge of green. It is of two kinds. 

 Monoclinic diopside is rather abundant in short, stout pris- 

 moids and anhedral grains giving extinction angles up to about 

 35°. Enstatite is less common, but often present, in euhedral 

 to subhedral crystals, giving thick laths with equal ends, and 

 with parallel extinction. There is little apparent difference in 

 the birefringence of the two. Both pyroxenes, but especially 

 the diopside, also form the usual small anhedral grains of the 

 groundmass. 



Olivine is almost always present, its amount being sometimes 

 as great as that of the pyroxene, and in rare cases (as in the 

 basalts near Seneghe), it almost entirely replaces the latter. 

 Again, as in some of the dikes, its amount is small or it may 

 rarely be quite wanting. It forms small sharply euhedral phen- 

 ocrysts, not occurring in the fine groundmass, giving the 

 common lozenge-shaped sections. Many of these are more or 

 less corroded and embayed, though generally with the angles 

 left sharp, and are occasionally reduced to skeletons or anhe- 

 dral grains. The olivines nearly always have a border of light 

 yellow iddingsite, some of the crystals being entirely colored, 

 but nearly all the basalts are very fresh. 



Magnetite grains are abundant, and small apatite prisms are 

 fairly common. Neither orthoclase nor nephelite could be 

 detected. The groundmass between the larger crystals is 

 nearly always holocrystalline, made up of minute feldspar laths, 

 and granules of pyroxene and magnetite, with often a color- 

 less somewhat birefringent, apparently feldspathic base. Only 

 two specimens, from the great flows northwest of Sennariolo, 

 are distinctly vitreous. Interstitial between the divergent 

 and rather coarse plagioclase laths, these show considerable 

 colorless glass, which looks gray through the presence of much 

 globulitic dust, and with considerable magnetite in grains or 

 dendritic forms. 



Chemical Composition. — Two analyses have been made by 

 me of these basalts. The first, (A), is from a flank flow at a 

 point along the highway about 4 km. west of Cuglieri. The 



