Mathews — Structure of the Piedmont Plateau. 145 



at several points into less metamorphosed rocks of similar 

 composition where rocks of Upper Cambrian and Lower Silu- 

 rian age have been found. Thus, it has been correlated with 

 the Stockbridge limestone of western New England, the Chester 

 Valley limestone of Pennsylvania, and tentatively with the 

 Shenandoah limestones of Maryland and Virginia though no 

 stratigraphic continuity has been established with the latter. 



The Mica-Schists. — The mica-schist is composed of a series 

 of highly micaceous, very schistose, and often crinkled aggre- 

 gates of quartz, more or less decomposed biotite and garnet, 

 with accessory orthoclase, cyanite, staurolite, fibrolite, etc. 

 With the increase in feldspar the rock becomes a gneiss but is 

 not as distinctly banded as the banded gneiss already referred 

 to. The formation is chiefly characterized by the high content 

 of mica, garnets, and occasional metamorphic minerals, and 

 the small amounts of feldspar. The soils developed from the 

 decomposition of the mica-schists are usually marked by an 

 abundance of mica and rounded garnets. Beds of this type 

 are only indistinctly marked and are separated with difficulty 

 from the cleavage lines which may nearly parallel them. The 

 entire formation has been much folded and it is not possible to 

 make accurate determinations of the thickness. At first sight 

 it would appear to be miles in thickness but in reality is prob- 

 ably not far from 2000 feet. The age of the rocks included 

 under this term is not determinable by fossils carried by them, 

 but, as in the case of the marbles, these schists have been corre- 

 lated with less metamorphosed representatives which carry a 

 fauna of Hudson River age. 



The variation in the character of the marble through increased 

 impurities causes the contact between the overlying mica-schist 

 and the limestone to be one of gradation at times, and occasion- 

 ally there seems to be a layer of more argillaceous mica-schists 

 lying between the underlying quartzite and the well-defined 

 marble. 



Igneous Pocks. — Throughout the Piedmont area under dis- 

 cussion have been recognized numerous igneous rocks, now 

 more or less metamorphosed, which are apparently intruded 

 into all of the rocks as old as the mica-schists. Among the 

 types found are granites and granite-gneisses, diorites and meta- 

 diorites, gabbros and meta-gabbros, peridotites, pyroxenites and 

 serpentines, meta-rhyolites, and meta-basalts. The contacts 

 between these various masses are seldom exposed and the rela- 

 tive age of the various intrusions cannot be determined with 

 entire satisfaction. So far as is known, the facts accord with 

 the commonly accepted view that they represent in a large 

 way a single period of igneous activity which no doubt extended 

 over a considerable period of time. Just when these sheets 



Am. Jour. Scl— Fourth Series, Vol. XYII, No. 98.— February, 1904. 

 10 



