ITALIAN PETROLOGICAL SKETCHES 255 



Basalt. — Of the rocks of this class described by Moderni and 

 Bucca, I was unfortunately unable to obtain specimens. This is 

 the more to be regretted, inasmuch as the feldspar basalts are 

 only sparingly represented along the main line of Italian volca- 

 noes, and at Rocco Monfina they are, according to Moderni, the 

 last product of volcanic activity. Bucca describes a number of 

 them, and his observations may be summarized here so as to 

 complete as far as is possible, the petrographical description of 

 the region. 



The basalts of the region are apparently quite feldspathic, and 

 are quite free from leucite or nepheline. In the majority of cases 

 they are olivine-bearing, this mineral being generally phenocrystic, 

 and seldom forming part of the groundmass. It is usually some- 

 what altered, especially on the borders, to a dark red substance. 

 A few specimens are free from olivine and approach the augite- 

 andesites, but are classed by Bucca with the basalts on account 

 of their basic character. These last are of a light gray color, 

 while the olivine basalts are dark. Augite is abundant, of a 

 slightly bluish green, and not pleochroic. In two specimens a 

 dark reddish brown biotite is present, which is for the most part 

 largely altered to the usual augite-magnetite aggregate. The 

 plane of the optic axes is perpendicular to the plane of sym- 

 metry. Feldspar which is referred to plagioclase rarely appears 

 as phenocrysts, but is abundant as laths in the groundmass. Its 

 optical characters are not noted, so that we are unable to judge 

 of its place in the series. Small magnetite grains are abundant 

 and a colorless glass base is usually present. 



Although Bucca constantly refers to the feldspar as plagio- 

 clase, yet he does not mention twinning lamellae, and indeed 

 treats it in a rather cursory way. Certain facts, indeed, incline 

 me to the belief that there is some, if not quite a good deal, 

 of orthoclase in the rock. In the first place all the rocks which 

 I have examined from this and the other volcanic centers are 

 eminently rich in potash, even the phonolite of Viterbo con- 

 taining 9.14 per cent, of it. It would then be quite anomalous 

 to find here such a rock as a normal basalt containing a mini- 



