ITALIAN PETROLOGICAL SKETCHES 839 



tulT, and consider it as derived from one of the peculiar trachy- 

 andesites which are such a feature of this volcanic district. 



I obtained my specimens fresh from the large quarries near 

 Bagnaia, where it forms thick masses traversed by numerous 

 fissures. Evidences of stratification were wanting. It is cov- 

 ered by looser beds of gray tuff containing, as pointed out by 

 Verri and Deecke, many fragments of pumice and blocks of 

 ciminite or a similar rock. 



When first quarried it is very soft and friable, but hardens a 

 great deal on exposure. It is rather coarse grained, made up of 

 grains of feldspar, biotite, and augite, embedded in a finely 

 granular paste. I could find none of the quartz mentioned 

 by Mercalli. Its general color is a light yellowish gray, with 

 spots and streaks of dark brown and gray, which give it some- 

 what the appearance of the well-known " piperno " of Pianura, 

 near Naples. 



Under the microscope it is seen to be composed of frag- 

 ments of clear orthoclase, somewhat less abundant basic plagio- 

 clase (near labradorite) showing many twinning lamellse, many 

 brown and somewhat decomposed biotite crystals often frayed 

 at the edges, and less numerous grayish green augites, all lying 

 in a dusty, dirty, ill-defined groundmass. The fragmentary 

 character of all the crystals is most marked, and there is lacking 

 to all the constituents that definiteness of form and arrangement 

 which characterizes true effusive rocks. The appearance is 

 almost identical with that of many undoubted tuffs — so much 

 so as to leave no doubt in my mind that the Bagnaia occur- 

 rences, at least, must be regarded as fragmental, and not effu- 

 sive. 



The only doubt cast upon this view is the presence in one 

 slide of a large patch of obsidian. This shows similar, but much 

 less broken, feldspar, biotite, and augite crystals, lying in a 

 highly vitreous groundmass, which is made up of a clear, color- 

 less isotropic base, flow structure being well brought out by 

 abundant, damascene-like streaks of fine gray dust. These 

 streaks curve about the large crystals, and also follow the line 



