836 HENRY S. WASHINGTON 



fracture is seen which does not show it. I am inclined to agree 

 with Deecke in the view that some action of the magma is 

 responsible for the existence of this border, since, as in his speci- 

 mens, the perfect freshness of all the other minerals seems to 

 exclude any appreciable meteorologic decomposition. The 

 olivine is quite free from inclusions, except a few magnetite 

 grains. 



Diopside phenocrysts are also abundant, in fact more so than 

 those of olivine. Only the largest show in the section a faint 

 green color, the great majority being quite colorless. They are 

 all well formed and automorphic, showing the usual planes with 

 a stout prismatic habit, but the largest have suffered somewhat 

 in transit and are more or less fractured. The extinction angle 

 ox\b (010) reaches 45°, that of the interior being slightly higher 

 than that of the border. In a few cases olivine crystals were 

 noticed protruding into, and hence older than, the diopside. In 

 the rock from Madonna della Querela a single large crystal of 

 brown biotite was seen, associated with large crystals of feldspar. 

 It was much corroded, being reduced to a carious condition. 



All these lie in agroundmass which shows both andesitic and 

 trachytic features. Except in the rock from Vico crater, where 

 diopside is comparatively rare, there is a typically andesitic 

 "felt" of small diopside needles with many magnetite grains, 

 the interstices being filled with feldspar. This is in the form of 

 laths and also of "cement." The laths are both of orthoclase 

 and of plagioclase in much smaller amount. Carlsbad twins are 

 sometimes seen in the former, but twinning lamella; are rare in 

 the latter, which is chiefly to be distinguished by its oblique 

 extinction and higher refractive index. Both are surrounded 

 occasionally by narrow mantles of orientated alkali feldspar. 

 The cement is entirely of alkali feldspar. Little or no apatite 

 was noticed. Glass is present in small amount in the rock from 

 Madonna della Querela, but is almost entirely lacking in the 

 other specimens. 



The above description will have made evident the difficulty 

 of classifying these rocks. As has already been said, the scarcity 



