Shale is a sedimentary rock consisting of silt and clay sized particles. Rocks containing detrital material acount for over half of all sedimentary rocks. These particles are so small that they can only be identified with great magnification. Because it is difficult to see individual grains without a mircroscope, shale is one of the more difficult sedimentary rocks to study and analyze.

Information provided by Patrick M. Colgan, Northeastern University
http://www.casdn.neu.edu