LIN OS A AND ITS ROCKS 15 



As has been explained elsewhere/ in connection with the Catalan 

 basalts, since the modes and textures of these basalts are usual ones, 

 represented by much better- and longer-known rocks, the types will 

 not be designated systematically and definitively by the use of typal 

 adjectives, but will be distinguished provisionally by prefixing to the 

 magma tic name the name of a representative locality on the island. 



CAMPTONOSE-AUVERGNOSE (fELDSPAR-BASALT) , 

 MONTE PONENTE TYPE 



Megascopic characters. — In the hand specimen these rocks vary 

 in color from a rather dark gray to black, and are sometimes compact 

 but more often decidedly vesicular, highly scoriaceous forms being 

 abundant. They are decidedly porphyritic, thin, tabular pheno- 

 crysts of white feldspar, from 2 to 4™"^ long, being abundant, with 

 fewer of oHvine, i to 3™™ in diameter, usually pale yellow, but 

 occasionally golden or rarely greenish, and still fewer of dark, greenish- 

 black augite. 



Microscopic characters. — In thin section the feldspar phenocrysts 

 are seen to be wholly of labradorite, almost invariably twinned both 

 according to the Carlsbad and the albite laws, the extinctions indi- 

 cating a composition of about Abj AUi. A zonal structure is rarely 

 seen, the core being then more calcic, but the interior is not infre- 

 quently occupied by a sponge-hke mass of inclusions, either of brownish 

 glass or of augite and magnetite grains. The augite phenocrysts 

 are rarely visible and present no features of special note. They are 

 usually fragmentary or stoutly prismoidal, quite colorless, or less 

 often pale gray, quite free from any zonal structure, and containing 

 generally some inclusions of magnetite. The olivine phenocrysts, 

 which are more abundant than those of augite, are colorless, usually 

 highly euhedral, showing the common lozenge-shaped outhnes, but 

 occasionally with the edges corroded, especially on the faces of the 

 domes and prisms. In most of the specimens the ohvine phenocrysts 

 are perfectly fresh, borders of iddingsite being extremely rare. 



The ground-mass in which these lie is usually a typically intersertal 

 one, thin tables of plagioclase, which has the same composition as 

 that of the phenocrysts (Ab^ AnJ, being quite abundant and usually 



^ H. S. Washington, American Journal of Science, Vol. XXIV, 1907, p. 229. 



