16 GOLD AND TIN DEPOSITS OF SOUTHERN APPALACHIANS. 
In their common development it would be a very difficult matter to determine the nature 
of the rocks from which these have been derived, but fortunately a clue is given in a few 
localities, where enough of the original character is preserved to aid in arriving at a fairly 
definite conclusion. 
The least alrared of these rocks are well-bedded, fine-grained, and not very much indu- 
rated tuffs, made up of irregular fragments of a rock which if massive would have been a 
porphyry. Broken grains, some of them rounded, of quartz and feldspar, representing 
original'phenocrysts, are embedded in an evenly fine-grained matrix of quartz, feldspar, 
and mica. The porphyries from which these fragmental rocks were derived therefore cor- 
respond to granite porphyry and quartz-monzonite porphyry and were probably related 
to the porphyries which occur in the vicinity of the Brewer mine in Chesterfield County 
and the Blackmon mine in Lancaster County. 
The quartz appears in many cases to have existed in the massive rock as sharp double 
pyramids without development of prism faces. The character of the feldspar varies in 
different localities. In the neighborhood of the Ferguson gold mine, in the northwest 
corner of York County, for instance, most of the determinable individuals of feldspar 
appear to belong to the varieties oligoclase and andesine. At the Colossus mine, in the 
southwestern part of Union County. X. C. microcline is most abundant, while microper- 
thite is rather common. At the Haile mine, in the southern part of Lancaster County, 
the character of the feldspar could not be determined. Both biotite and muscovite are 
present, and while it seems probable that the former at least was an original constituent 
of the rock, the evidence is not conclusive and the mica may have been introduced by the 
contact metamorphism to which the rock has plainly been subjected. The biotite is 
present in small ragged flakes, usually exhibiting a parallelism of arrangement and indicat- 
ing the direction of foliation of the rock. It is of a clear brown, and shows fairly strong 
absorption and rather strong polarization. In many cases it has been changed to a dirty 
greenish chlorite. Muscovite is doubtless secondary in all cases, having been derived 
both from the biotite. by the common but poorly understood process known as bleaching, 
and from the feldspars by decomposition. In the less foliated rocks it can be seen in 
— of formation from both these minerals. Compact, fibrous muscovite or sericite 
occurs commonly in the more schistose rocks and is especially abundant along certain 
planes which are planes of greatest movement and hence of greatest weakness — in other 
words, plane- of schistosity. It is the silvery sheen of this muscovite. exposed when the 
rock is cleaved along these plane- of fissility, which gives rise to the term "fish-scale slate.*' 
It is possible that these soft tuft's cover a considerable area, but do not outcrop conspicu- 
ously, and are in consequence overlooked. The localities where they were found, however, 
are in the immediate vicinity of gold deposits, and there, owing to their physical make-up, 
being rather porous and in fine particles, and possibly also to their chemical constitution, 
they were easily attacked by the mineral-bearing solutions which produced the gold deposits 
and were very greatly altered. This alteration has been for the most part a silicification 
with attendant recrystallization. Feldspar, biotite, and sericite have all been attacked and 
more or less completely replaced by quartz. Since these silicified tuffs are closely con- 
nected genetically with the gold deposits which occur in them they are more fully 
de-^-ribed in the section dealing with gold (pp. 78-79). 
These granite-porphyry and quartz-monzonite-porphyry tuffs have been observed in 
the vicinity of the Haile mine in Lancaster County, the Colossus or Howie mine in Union 
County. X. C. and the Ferguson mine in York County. They doubtless belong in the 
group of ancient volcanic rocks whose distribution in the eastern part of Xorth America 
has been set forth by G. II. Williams/' 
GNEISS (SCHISTOSE GRANITE) 
Much-foliated rocks which show by their texture and composition that they were origi- 
ally granites are found in isolated localities throughout tfle area. They are usually of rather 
a Jour. Geol., vol. 2, 1894, pp. 28 et seq. 
