398 THE JOURNAL OF GEOLOGY. 
plainly seen to be conformably overlain by the red slates and 
fine-grained tuffs of the Milton series. Near Milton, there is 
good evidence that this last series is unconformable on the 
Palzozoic rocks to the west. These Paleozoic rocks are called 
in the section the magnesian-limestone series, there being in it 
frequent irregular masses of magnesian limestone, but no fossils 
have been found in any portion of it, and it is presumed to be 
Paleozoic in age on account of the highly metamorphosed con- | 
dition of most of the materia] composing it. There are dikes 
of quartz-porphyry in the magnesian-limestone series. The dip 
of the pyroclastic and clastic rocks overlying the quartz-por- 
phyry of the Sierra Buttes is to the east at angles varying from 
50° to 60°. The dip of the Paleozoic rocks to the west is often 
vertical, but dips to the east at high angles have been noted at 
many points. While these dips represent in part the dip of the 
original stratification, at some points they probably indicate the 
dip of the schistosity. 
To the east of the Milton series is a large granite mass 
which appears to be of later age, since the Milton rocks show 
evidence of contact metamorphism. 
There. are then at the Sierra Buttes an acid | volcanic 
series (quartz-porphyry and quartz-porphyry-breccia), with an 
overlying augitic pyroclastic series more basic in composi- 
tion, and later than the whole volcanic series, a deep-seated 
intrusive acid rock (granodiorite). 
The succession of the effusive rocks, acid quartz-porphyries, 
and more basic augitic tuffs, is strikingly similar to the succession 
of the Tertiary effusives, rhyolites and andesites, described 
hereafter. 
In addition to the above rocks, there are in the same region 
numerous dikes cutting the various igneous formations. Thus 
the quartz-porphyry of Eureka Peak (same area as that of the 
Sierra Buttes) is cut by dikes of an altered rock that appears 
to be an aplite (No. 150 Plumas), and also by a more basic 
LOC ke probably originally an augitic porphyrite (Nos. 385 and 
386 Plumas), but the idiomorphic phenocrysts of this are now 
