PETROGRAPHY. 55 
scattered through a very dark, sometimes almost black, entirely aphanitic ground- 
mass, which may appear hyaline at times. 
Breccia, form: This form of the type is much lighter and somewhat variable 
in color, light gray in the freshest specimens, and yellowish to reddish in those which 
are weathered. The hand specimens usually show a somewhat streaked or schlieric 
appearance, due to lines and patches of darker and lighter gray, or grays and yellows. 
In some places these streaks have a generally parallel and horizontal arrangement, 
with the biotite tables mostly horizontal, simulating a sort of bedded or flow structure. 
The rocks are quite rough and soft and rather friable when first quarried, but harden 
on exposure. They are highly porphyritic, containing numerous small white pris- 
moids of feldspar, many small glistening tables of biotite, and some prisms of pyr- 
oxene. The aphanitic groundmass is either gray or somewhat yellowish or reddish. 
Microscopic characters Lava form : The feldspar phenocrysts are seen in thin 
section to be of both orthoclase and labradorite, the former being more numerous. 
They are usually twinned and do not carry many inclusions. There are few small 
anhedral or fragmentary prismoids of a colorless or very pale-gray pryoxene, some 
of which is augite, but the greater part of which is hypersthene. Biotite tables are 
abundant, more so than would appear to be the case from the megascopic examina- 
tion. They are from i to 2 mm. across, and are almost uniformly altered, the larger 
ones on the edges and the smaller ones throughout, to a dark, finely granular aggre- 
gate. The groundmass in which these lie, forming about 70 per cent of the rock, is 
a colorless glass, thickly sprinkled with very minute prismoids of pyroxene, appar- 
ently hypersthene, some small feldspar laths, but no magnetite grains. With low 
powers this glass and its crystals show evidence of flow around the phenocrysts. No 
quartz was seen in any of my specimens. 
Breccia form: The feldspar phenocrysts here also are of both orthoclase and 
labradorite, the former more abundant, but they are for the most part angular and 
decidedly fragmentary, few showing well-defined prismatic or tabular shapes. There 
are the same phenocrysts of colorless augite and hypersthene, but these are also much 
more broken than in the preceding form. The numerous biotite tables are similar 
to those in the lava and have suffered much less deformation and rupture. The 
cement between these is either a dusty, rather indeterminate but evidently vitreous 
mass, or a colorless glass which contains many minute fragments and anhedra of 
the minerals mentioned above. In either case the cement exhibits a well-marked 
flow structure around the larger crystal fragments. 
There has been much discussion as to the character of this rock, called locally 
"peperino," opinions being divided as to whether it is a tuff or a lava. In a previous 
paper I considered it a tuff, though somewhat doubtfully. But examination of the 
specimens collected since the earlier publication would show that these rocks are, 
for the most part at least, not tuffs, but lava-breccias or flow-breccias, that is, lavas 
which had partially crystallized and solidified during eruption, but which the still 
moving, very viscous flow shattered and brecciated, this fluid portion forming on 
