Geohx/ic Sfriirfiire of fhc Blue Ridge. — Keltli. 367 
complex, and areas of massive granite merge into areas of massive 
(lial)ase tluoiigli a multitude of interbedded tongues. These 
tongues vary in thickness from a few inches up to two or threes 
miles an<l attain a length of six miles. 
The relation of fiuartz-porph3'ry to diabase is not clear at first 
sight in the region under discussion. It can be inferred from 
several phenomena, however, with consideral)le surety. At sev- 
eral localities in Maryland are occurrences of tufaeeous beds in 
the quartz-porphyry that indicate surface production. These 
beds are near the edges of the quartz-porph^'ry and are presumabl}^ 
the upper beds. Als<j there are frec^uent occurrences of banding 
and wavy flow structure common to surface lavas. From these 
fa'-ts it is probabl(> that the quartz-p(»rphyrv was a surface flow. 
That being the case, the relation of diabasi' to ([uartz-porphyry is 
a simple one and not complex like the granite eruption. As there 
are only one diabase l)e(l and one quartz-porphyry l)ed that can be 
differentiated, tlie diabase either follows or pi'ecedes the ([uartz- 
porphyry. 
For want of bedding planes in the quartz-prophyry it is ditHcult 
to determine any structure. It may be assumed for the present 
that the surface of so large a flow was approximatelv level and 
that the tnfaceous beds mark that surface. The present dips of 
that surface, therefore, represent later plication. Its general 
position is along low ground and drainage lines, suggesting anti- 
clinal erosion of an underlying bed. Wherever steep slopes make 
the dips plain, the (piartz-pori)hyry is the uiulerlying l)ed and th(^ 
tufaeeous beds marking the top are next to the diabase. Several 
outliers of ([uartz-porphyry are only exposed liy dee[) stream cuts, 
and several outliers of diabase cap summits in the (luartz-porphyrv 
area. The south ends of tlie (luartz-i)orphyrv visibly pass under 
the dial)asi'. From all of these points there would seem to be no 
doubt that the (puirtz-porphyry underlies the dialtase. 
If this relation is accepted as true, it follows that the diabase 
itself was a surface flow. Internal e\ idence of this is wanting, 
for no tufaeeous beds or flow structuix's have been observed in the 
diabase. Negatively, the diabase is the toi)most volcanic bed 
and shows no eruptive contacts. It reaches continuously from 
Maryland ten or more miles into Pennsylvania and over half wa}' 
through A^irginia. with an average width of twenty miles. This 
extent is commensui-nle with the large lava flows of the |)resent 
