Basic Rocks of Northeastern Maryland. — Leonard. 165 
of these can be seen at the section house just below the town, 
while not over twenty feet north of it there is a dike of pegma- 
tyte. The peridotyte has been partially changed to serpentine. 
A noticeable feature of this region is the abundance of white 
vein quartz, fragments of which strew the fields in many places 
or are seen mingled with the gabbro and noryte boulders col- 
lected in the stone walls everywhere so common in the area. 
At some points these quartz fragments are so numerous as to 
form no inconsiderable proportion of the boulders. For the 
source of thfs material we must doubtless look to the veins of 
quartz which have cut not only the basic rocks but also the 
gneisses bordering upon these to the east and south. The num- 
ber and extent of these veins is partially indicated by the many 
fragments derived from them. 
The quartz veins have in all probability been formed by lat- 
eral secretion, the silica having been leached out of the sur- 
rounding rocks and deposited by percolating- waters in fissures 
and crevices produced by orographic movements. Wherever 
the rocks are well exposed, as alone the Susquehanna, they are 
seen to be traversed by gashes and cracks which have been filled 
with this secondary quartz, the latter often forming small lens- 
shaped masses. 
The pegmatyte veins* are of special interest and import- 
ance since they supply large quantities of feldspar and quartz 
("flint") to the potteries of Maryland, New Jersey and Dela- 
ware. These dikes, which are often of considerable extent, are 
found cutting not only the norytes and gabbros but also the 
serpentines. They have a northeast-southwest direction, cor- 
responding to the strike of the schists and the foliation of the 
massive rocks where the latter has been developed bv pressure. 
Two of these coarse granitic veins are well exposed along the 
Susquehanna, one below and another above Conowingo. The 
former appears less than one-quarter of a mile south of the 
station, where it is well exposed along the railroad. This dike,, 
which has a width of eight feet, is composed of granite with 
seams of coarse pegmatyte and is bordered on either side by a 
saussurite gabbro. The vein a short distance above the town 
♦All the acid dikes or veins of this region are here referred 1o as peg- 
matyte since tiny are formed largely of very coarse granite. While they 
are not infrefjuently composed of true granite, into which the pegmatite 
passes, it is convenient to have a general term which shall include all those 
dikes composed of quartz-feldspar rock. 
