This specimen is a metamorphosed siltstone. Constituent grains are predominantly quartz and feldspar. These grains are mostly between .015 to .005 mm. There are numerous unidentified platy habit grains also included in the matrix oriented at roughly 45 degrees to the macro-bedding planes, these grains reach sizes of .015 mm in length, but most are smaller. Grain boundaries are mostly indistinct owing to small size, but visible examples suggest consertal, reflecting some degree of metamorphism. Secondary calcite grains are also present with a maximum dimension around .05 mm. The calcite is relatively uncommon at far less than 1 percent of slide area. The cement is likely siliceous. Marcostructure bedding planes are clearly visible in the sample ranging in character from fine to coarse silt.
From Reference Source:
"Bluish grey to greenish grey argillite that because it is evenly laminated (varved), compact, and hard is quarried and marketed as flagstone. The principal minerals are quartz, orthoclase, albite or oligoclase, sericite, and chlorite; epidote, pyrrhotite, sphene-leucoxene,and biotite occur as minor accessories. Here, the argillite occurs as an inner beneath the unconformably overlying basic lithic breccia and tuff of the uppervolcanic unit. Note the argillite fragment in a boulder from the basal section of the greenstone pyroclastic rocks.
A chemical analysis of the argillite shows it to be characterized by a high soda-potash ratio (Councill,1954, p. 13). Published analyses of presumably similar argillites (Pogue, 1910, p. 41; Laney, 1910, p. 28 and Stuckey, 1928, p. 18) also show high soda-potashratios. This led to the conclusion that volcanic debris makes up much of the argillites. However, other factors than volcanic debris, such as grain size and chemical immaturity, might influence the soda-potash ratio.For, according to Pettijohn (1957, p. 345) in the case of varved glacial sediments, summer silt contains a higher Na-K ratio than winter clay. We assume that this applies to non-glacial silt and clay as well. Also,clay deposited in a sea environment may gain potash as it becomes mature; thus, a high Na-K ratio may mean a chemically immature sediment (Pettijohn1957, p. 345). Another possible interpretation for the cause of the high Na-K ratio is replacement or albitization (Pettijohn and Bastron, 1959, p. 593)."
[Source: Arvid A. Stromquist and James F. Conley, GEOLOGY OF THE ALBERMARLE AND DENTON QUADRANGLES, NORTH CAROLINA (Carolina Geological Society, 1959). Available online at http://carolinageologicalsociety.org/CGS/1950s_files/gb%201959.pdf (accessed 9/23/2014).]