14 GOLD AND TIN DEPOSITS OF SOUTHERN APPALACHIANS. 
Although the Piedmont Plateau appears approximately level, its slope is considerable 
when compared with the average profile of streams in soft material. This slope prevents 
an overloading of the streams, and consequently, by preventing meandering, induces the 
fairly straight course^ which have already been alluded to in the description. The slope 
also (vus^-hendvard cutting of the tributaries, which likewise results in straight courses. 
Such courses^ .e less easily brought into adjustment with discordant structure than mean- 
dering coursi =;. 
Some adjustment has, however, taken place, as shown by a number of the smaller, and 
hence young, r, tributaries, whose courses are parallel to the strike of the rocks. Such 
subsequent tributaries of some of the larger consequent streams have been able in certain 
'" ^stances to capture weaker consequent streams which were perhaps held up by the tempo- 
rary has;- -level of a hard rock barrier. 
An interesting evidence of the planation of this region is shown on the map, where it is 
seen that the railroads are enabled to. follow the present watersheds or divides, whereas in 
most legions of equally diversified topography they are forced to follow the more circuitous 
courses of the stream valleys. 
GENERAL GEOLOGY. 
DESCRIPTION OF THE ROCKS. 
OUTLINE. 
The rocks of this area belong to three great classes — igneous, sedimentary, and meta- 
tnorphic. Sedimentary and igneous rocks of great age have been much contorted and 
folded. Th,' hirers producing and attendant on these movements have extremely meta- 
morphosed the rocks, converting them into schists. Volcanic rocks, mainly represented 
by bedded ( nil's, covered (lies;' metamorphosed rocks in places and were themselves sub- 
jected to a pari of the metamorphosing influences. Into these metamorphic rocks later 
igneous material has been intruded. Granite is the most abundant of these igneous rocks 
and the numerous dikes of pegmatite are probably closely connected with it. Diabase 
dikes cutting all the foregoing rocks are the last representatives of intrusion. On the 
eastern border o\ the area the solid rocks are covered by thin beds of sand, which probably 
belong' to the Coastal Plain formations. 
METAMORPHIC ROCKS. 
The metamorphic rocks of this district have resulted from the action of the tremendous 
forces of mountain building. They are usually extremely metamorphosed', and may be 
placed in that perplexing group, the crystalline schists, in many places nearly or quite 
all traces of their original character have been removed; but here and there the former 
nature of the rock can he seen, and by comparison and analogy the more altered varieties 
are correlated. These schistose rocks are derived from hoth igneous and sedimentary 
formations. 
SCHISTS OF IGNEOUS ORIGIN. 
AMPIIIBOLITE. 
The most common and abundant rock under this head is hornblende schist or amphibo- 
iite. In some places, as in central York County, the gradation from this rock into fairly 
unchanged gabbro can be traced with certainty. It seems probable, however, that much 
of it has resulted from volcanic as well as from plutonic rocks, for its exact intercalation 
with the sediments in many places would indicate that it had been in such cases a surface 
rock, probably occurring as basaltic flows or tuffs. On subjection to shearing and other 
metamorphosing processes these igneous rocks were foliated and otherwise altered, becom- 
ing schistose amphibolite. ■ 
These amphibolites are fine-grained rocks, of dark, greenish color, generally possessing 
a definite structure in one direction, or "grain." In some of them the foliation is sufli- 
