Shale is a term that has been applied to describe a wide variety of rocks that are composed of extremely fine-grained particles,
typically less than 4 microns in diameter, but may contain variable amounts of silt-size particles (up to 62.5 microns). Incomparison, sandstones are rocks composed of grains (of variable composition) that are typically between 62.5 microns and 2000
microns in diameter. Just as sandstones can be composed of different mineral grains (e.g., quartz, feldspar, rock fragments, clays,
etc.), shales (more properly referred to as mudstones) also exhibit a wide range in composition (clay, quartz, feldspar, heavy
minerals, etc.). Moreover, the composition of a “typical” mudstone will vary much more than for typical sandstones, even though
to the naked eye, many mudstones (i.e., shales) look similar.