Vol. 6^.~] BASALTS OF THE WESTERN MEDITERRANEAN. 71 



colour (when fresh) they vary from a medium grey, as in some of 

 the massive flows near Olot and some of the Tertiary sheets of 

 Sardinia, to very dark grey or black, the darkest lavas being those 

 of the most recent cones of Catalonia, Sardinia, Pantelleria, and 

 Linosa. They vary from very compact forms to highly-vesicular 

 ones, the smaller cones in all the districts being composed very 

 largely of spongy scoriae. As a rule, these lavas are not highly 

 porphyritic, some being quite aphyric megascopically, although in 

 most of the specimens small phenocrysts of augite and olivine are 

 present, with small tabular phenocrysts of felspar in some cases. A 

 peculiar characteristic of certain lavas of Catalonia, Sardinia, and 

 Linosa, is the presence of rounded nodules of olivine of a light 

 yellowish-green colour and coarsely-granular structure. These 

 may attain very considerable dimensions, 4 or more inches in 

 diameter ; and near Scano, at Monte Perru, they are accompanied 

 by smaller, but more compact, nodules of a greenish-black augite. 

 Large plates of a bronzy biotite are found in a leucitic basalt 

 which occurs sparingly at Monte Ferru ; but hornblende is never 

 seen in the specimens collected, although it was found as loose 

 crystal-fragments in tuffs on the island of Linosa. 



In thin section, the mode of mineral composition is seen to 

 be quite uniform, though varying within limits. A plagioclase- 

 felspar, usually a labradorite, is abundant, commonly with tabular 

 development and much twinned. With it is occasionally seen some 

 orthoclase, but the amount of this is always very small and 

 usually quite negligible. jS"ext to the plagioclase, augite is the 

 most abundant mineral. This is quite colourless in thin section, 

 and occurs as stout, subhedral, prismatic phenocrysts, which are 

 frequently fragmentary, as well as in the usual small prismatic 

 groundmass-crystals. It seldom forms anhedral, interstitial areas 

 between the felspar-tables, so that an ophitic texture is very rare. 

 Olivine is rather abundant and quite constant, although in some of 

 the types, especially those of Sardinia, it is absent or present in 

 negligible amount ; while in other rare cases it surpasses the augite. 

 It presents the usual features, is colourless, varies from anhedral to 

 subhedral, or even euhedral forms, with the common planes, and in 

 the less fresh specimens shows the well-known yellowish-brown 

 border. 



These three minerals — labradorite, augite, and olivine — are the 

 predominant and essential constituents of all the basalts ; but with 

 them constantly occur smaller amounts of magnetite and apatite, 

 in the usual forms of small anhedra of the former and prisms of the 

 latter. In some cases, especially at the Catalonian volcanoes, there 

 is a colourless base which the analysis shows must possess the 

 general composition of nephelite, since there is insufficient silica for 

 the formation of felspars. But nephelite, in euhedral and distinctly- 

 recognizable crystals, is rare in the salfemanes, though it occurs in 

 some of the Catalonian basalts, and in some of the more salic rocks 

 in Sardinia and Pantelleria. A few of the basalts of Monte Ferru 

 and elsewhere in Sardinia show some very small spheroidal anhedra 



