ITA LI A N PE TR OL O GICA L SKE TCHES 4 5 



The rock of which Monte Cucco, at the extreme east end of 

 the region, is composed, is a light gray porphyritic rock, many 

 glassy feldspar phenocrysts and some of biotite lying in a rather 

 vitreous brownish groundmass. Under the microscope there 

 appear well shaped and uncorroded clear orthoclase and plagio- 

 clase phenocrysts, the former in the majority. The plagioclase 

 is shown by Michel-Levy's method to be labradorite of the com- 

 position Abo An 3, the symmetrical extinction angles of the 

 lamellae of the Carlsbad individuals being in one individual 26° 

 and in the other 29°. There are also many tables of an olive 

 green biotite, which shows no corrosion or alteration phenomena. 

 A few large colorless diopsides also appear. There is no mag- 

 netite, and no quartz could be detected. 



The groundmass is highly vitreous, the glass base being 

 colorless or of a very pale brown. It shows perlitic cracking in 

 great perfection. Through it are sprinkled small (o.0 1-0.02) 

 microlites of diopside, and a few stout orthoclase microlites 

 which often show "horns" at each end. An interesting feature 

 is the occurrence in abundance of very small (about 0.05™™) 

 forked and sheaf forms of orthoclase, which correspond to the 

 so-called keraunoids in Ischian trachytes already described by 

 the writer.' These keraunoids are of such minute dimensions 

 that they exert only a feeble action upon polarized light, but 

 examination of the largest shows that the axis of greatest elas- 

 ticity a lies parallel to the length, and that therefore they are 

 elongated in the direction of the axis a. The use of high 

 powers proves that they are in every way identical with those 

 described from Ischia, except that they are of much smaller 

 dimensions and more delicate. 



Bucca's description of this rock closely agrees with the 

 above, though he speaks of the biotite as being brown, and the 

 small augites and keraunoids seem to have been lacking in the 

 groundmass of his specimens. Though no quartz is present he 

 is of the opinion that these rocks are to be referred to the same 

 acid group as those of Monte Calvario. This opinion is con- 



' Washington, Am. Jour. Sci., 1, 375, 1S96. 



