SEDIMENTARY ROOKS. 23 
Under the microscope the rocks are seen to consist of good-sized individuals of quartz, 
some of which are developed as bipyramids, and very turbid crystals of orthoclase and 
oligoclase, decomposing into kaolin, serieite, and epidote. In the rock from the Brewer 
mine large individuals of the unknown mineral are being slowly converted into serieite. 
The matrix for all these individuals is a fibrous mass of serieite. Magnetite is present in 
some specimens and is probably secondary. Pyrite is abundant in places. A little rutile 
has probably been formed by the decomposition of some original titanium-bearing mineral. 
There is no indication that dark silicates were ever present. 
• These porphyries have been found at the Brewer mine in Chesterfield County and at the 
Blackmon mine in Lancaster County. It is probable that they are closely related to the 
tuirs found at the Haile, Colossus, and Ferguson mines. While it is difficult to determine 
the original character of these rocks, because of their altered condition, they probably 
belong in the quartz-monzonite-porphyry group. At the Brewer mine the porphyry shows 
in places the effects of brecciation. 
Keith « has described a series of volcanic rocks from the Cranberiy quadrangle, about 80 
miles northwest of the area embraced by this report. The rocks there are considered to be 
altered rhyolites, andesites, basalts, and diabases, and are thought to be of Algonkian age. 
Keith believes that the porphyritic rocks in the two localities arc similar. 
Dikes of diabase occur at many places through the area studied and represent the last 
phase of eruptive activity experienced here. These bodies range from a few to 200 or more 
feet in width, and cut any or all of the other hard rocks of the district. The material of 
the dikes ranges from moderately to very fine grained and is of greenish-black color. Indi- 
vidual minerals can with difficulty be determined by the unaided eye. The rock is fairly 
resistant to weathering and, although seldom outcropping, furnishes much float which 
marks the course of the dikes. It weathers in the concentric layers characteristic of diabase. 
In thin section the rock is found to be a typical olivine diabase. Plagioclase, a calcic labra- 
dorite, in long, narrow laths, was the first mineral to form. Olivine is generally abundant 
and occurs mainly in automorphic crystals, but here and there in irregular grains. On 
decomposition it changes into the mineral called iddingsite, which, in these rocks particu- 
larly, seems certainly to be a micaceous mineral. Less commonly it alters to a material 
which is of almost identical appearance in ordinary light, but which is isotropic. In a few 
places serpentine results from the olivine. Augite is abundant, usually as irregular intersti- 
tial individuals, but locally in crystals of fairly good form. It is slightly pleochroic in 
colors suggesting a small content of titania. Cleavage is very well developed. Some of the 
pyroxene may be diallage. Closely associated with the olivine and commonly included in 
it are numerous small octahedral crystals of perofskite. The exterior of the crystals in 
nearly opaque, while the interior is a rather dark brown. Small crystalline grains of black 
iron ore, probably titanic, are especially associated with the pyroxene. The ophitic texture 
is beautifully exhibited. 
SEDIMENTARY ROCKS. 
Sediments which have been deposited since the cessation of regional metamorphism and 
which are, in consequence, present in the state in which they were laid down, are of unimpor- 
tant extent in this locality. 
"MONROE" BEDS. 
Extending for a considerable distance about Monroe, Union County, N. C, are horizontal 
or slightly tilted beds of fine-grained material, well stratified. They were seen by the writer 
only to the southwest of Monroe, and the following description applies only to that part of 
their area. They are considerably affected by decomposition, and colored yellowish or 
a Geologic Atlas U. S., folio 90, U. S. Geol. Survey, 1903. 
