1 66 The American Geologist. September, v.nn. 
is mucli more extensive, having a width of three hundred feet 
or more. In some parts it is made up largely of quartz. The 
rock is usually a rather fine-grained granite traversed by num- 
erous veins of pegmatyte. Here and there the muscovite forms 
sheaf-like aggregates, or it may occur in well developed hex- 
agonal plates. The peculiar intergrowth of the quartz and feld- 
spar, giving rise to graphic granite, was also found in this lo- 
cality. Good sized crystals of tourmaline are occasionally pres- 
ent. Neither of these dikes could be traced very far back from 
the river. 
Three large pegmatyte veins appear along Octoraro creek 
between the paper mill and the state line. On the hill just 
above the mill one of these has been worked as a quarry and 
furnishes a good quality of building stone. Most of the rock at 
this point is a medium fine-grained granite composed of quartz, 
microcline, muscovite, and some biotite. The vein has a width of 
ninety feet and is in places pegmatitic. The second is about 
three-quarters of a mile above the mill and the third, which is 
one hundred feet wide, occurs close to the Maryland line. The 
last is of special interest, since it was found to contain many 
fragments of the country rock, often of considerable size and 
not infrequently three feet in diameter. They are composed of 
much altered and saussuritized gabbro resembling that occur- 
ring in the vicinity. In fact, this saussuritized gabbro seems, 
in many cases at least, to be closely associated with the pegma- 
tvte veins, and it is nossible that these fiave had something to do 
in producing the alteration of the original rock. 
The pegmatytes are composed of white or pink microcline, 
quartz, muscovite, a little albite and occasionally biotite. Tour- 
maline is a somewhat rare constituent. 
That the pegmatytes are of eruptive origin can scarcely be 
doubted. The best evidence of this* is furnished by the frag- 
ments of the country rock found included in these veins. 
These must have been torn from the side walls as the molten 
magma was intruded into the fissures. The composition of 
these coarse granites, differing so widely from that of the 
norytes and serpentines in which they occur, is also an indica- 
tion that they are intrusive or have at least not been formed 
by circulating waters deriving their materials from the sur- 
rounding rocks. They are rich in alkalies and do not appear 
