Washington — Catalan Volcanoes and their Rocks. 233 



Limburgose and monchiquose (nephelite-basanite), Las Planus 



type. 



As was mentioned above, no distinction can be made out 

 either in the field or under the microscope between the 

 rocks which belong to the two subrange, so they will be 

 described together. Furthermore, as no exact dividing line 

 can be drawn between the more nearly holocrystalline and 

 compact rocks, which show evident nephelite, and the highly 

 scoriaceous ones, in which a glass base replaces the nephelite, 

 they will be described under one type. This is not strictly 

 correct, but the abundance of transitional forms, and the com- 

 paratively unimportant character of the differences between 

 the extremes, seems to render it the more judicious procedure. 



Megascopic characters. — Compact to highly vesicular and 

 scoriaceous : very dark gray to black, the latter especially in 

 the scorise, which weather to yellow and red ; perpatic, the 

 aphanitic groundmass thinly sprinkled with small phenocrysts 

 of black or dark green augite and light yellow olivine, from 1 

 to 5 mm in diameter. Nodular masses of granular, yellow 

 olivine present in some cases. 



Microscopic characters. — Tho, not very numerous pheno- 

 crysts of colorless or very pale, greenish gray augite, and color- 

 less olivine, are similar to those in the last type, the former 

 subhedral and stout prismatic or fragmentary, and the latter 

 more often euhedral. These are common to all the specimens. 



The groundmass varies more widely. In all cases small, 

 thin,- tables of plagioclase, usually about AbjAiij, are present, 

 the laths varying in length in different specimens from 0*5 to 

 O05 mm , being uniformly less broad and thinner in the more 

 scoriaceous rocks. These tables show a diverse arrangement, 

 and flow structure is seldom seen. With them are usually 

 present numerous very small anhedral grains of colorless 

 augite, olivine being rare as a true groundmass constituent. 

 In other and usually the more scoriaceous specimens, these 

 small groundmass augite grains are rare or even entirely want- 

 ing, being replaced by an aggregation of dark, minute, dusty 

 particles, which render the base almost opaque in spots. Ore 

 grains are common and well developed, though very small, in 

 the more compact specimens ; but in the more scoriaceous are 

 merged in the dusty aggregate just mentioned. The base in 

 which these lie is often quite holocrystalline, in which case 

 it is feebly doubly refracting, and is considered to be nephe- 

 lite, at leasf in great part. That this attribution is correct is 

 shown by the occasional presence in some of the more com- 

 pact and holocrystalline specimens of rectangular sections, 

 with parallel extinctions and low birefringence, which are 



An. Jour. Sci. — Fourth Series, Vol. XXIV, No. 141. — September, 1907. 

 16 



