562 HENRY S. WASHINGTON 



ble much the leucitites megascopically, though leucite is less 

 common as phenocrysts than augite. Their microstructure is 

 quite different and generally doleritic. The silica percentage is 

 about 52. 



The rock from Monte Cavallo south of Orvieto, a flow 

 with columnar structure mentioned by vom Rath, and that from 

 Porano, southwest of Orvieto, a flow which Verri considers as 

 belonging to the Monte Rado center, are dark gray, fine- 

 grained rocks showing few phenocrysts, and these small and 

 entirely of dark green augite. Its specific gravity is 2.763 at 

 26° C. 



Under the microscope they show a doleritic structure. In 

 the groundmass leucite predominates as irregularly shaped crys- 

 tals and patches, which correspond in function to the augite of 

 ophitic diabase. It shows weak double refraction, with the 

 usual polysynthetic twinning, and containing few inclusions of 

 glass and augite. Lying between or imbedded in the leucites 

 are long prisms of augite, together with plagioclase laths whose 

 extinctions correspond to those of a basic labradorite. Some 

 magnetite is also present, as well as a small amount of colorless 

 glass. A few orthoclase laths were seen, but no nepheline 

 could be detected. 



The augite is grayish green, and shows an extinction angle 

 of 41° with the axis c. Associated with it is a clear brown 

 mineral, occurring here and there on the ends and sides of the 

 augite crystals, and in small quantity as separate individuals of 

 a somewhat fragmentary character, which show few crystal 

 planes. When occurring with the augite the two are not sep- 

 arated sharply but shade into one another, the green color gradu- 

 ally giving place to the brown, the cracks running uninterrupt- 

 edly through both. The mineral has a slightly lower refractive 

 index than the augite, but strong pleochroism ; parallel to c 

 (assuming this to correspond to the vertical axis of the augite) 

 it is dark hair brown, and at right angles to this, light yellowish 

 brown and pale greenish yellow. The extinction angle with the 

 cleavage cracks, which are quite well marked, reaches 17°. 



