360 The American Geologist. DrcMub.-i-, is0-> 
the strata bent into a syncUne, witli a probable fault at the east 
side. Later a post-Triassic fault oecupied this line and brouglit 
the Fi'ederick limestone down against the bottom beds of massive 
([uartzite and black slate. Southward the synclinal axis emerges 
from the Jura-Trias sandstone near the Potomac for a few miles 
and in the middle pari of Bull Run mountain in \'irgini:i. Klse- 
where the sandstones have a monoclinal dip. 
While the structure of the sedimentary rocks is the lirst object 
of these pages, several facts of structure in the igneous rocks are 
worth stating here. These rocks are of three kinds when classed 
according to their original constitution, /. c , diabase, granite, 
and quartz-porph3'ry. In extent the diabase is the most important 
and the quartz-porphyry the least. Roughly speaking, the area 
between the Blue Ridge line and Catoctin line is underlain b}' dia- 
l)ase, partly replaced by granite at the south, and l)y quartz- 
porphyry at the north. The granite areas occupy a triangular 
tract with a l)ase of thirty miles east of Front Royal and its apex 
west of Frederick. The (luartz-porphyry occupies two areas in 
the district examined by the writer, the smaller lying east and 
northeast of Front Royal, the larger beginning north of the lati- 
tude of Frederick and six miles north of the granite apex. Near 
Front Royal it occurs as a narrow strip between the diabase that 
forms the summit of the Blue Ridge and the Cambrian sand- 
stones. In Maryland it lies in the form of a rude elongated cross. 
In Penns^'lvania, according to Prof. Williams, the (|uartz-por- 
phyry is as widespread as the diabase. 
Upon this group the Cambrian sandstones and slates were de- 
posited unconformal)ly. In places the sandstonen[ies upon gnin- 
ite, in other places upon diabase, in others upon quartz-porphyry. 
In many localities widely scattered along the Blue Ridge, the 
sandstones and slates contain fragments of the igneous rocks, 
showing that the sandstones are later de})Osits. 
The relations of the igneous rocks to each other are not always 
plain. So far as known to the writer, the granite and (piartz- 
porpbyry do not come into contact. Between the granite and dia- 
base the contacts are common. The granite occurs in the form 
of large masses and eruptive tongues in the diabase and abundant 
evidence of its eruptive nature can be found in its irregular 
boundaries, ott'shooting tongues, and included fragments of dia- 
l)ase. The boundaries of granite and dial)ase are exceedinglv 
