The problems relating to the classification of mudrocks, and in particular the role of grain size, are discussed. Classification based on silt-clay proportions is more realistic than a silt-clay-sand system. The grain size, as measured in mudrocks, is an uncertain quantity and the use of the quartz content is advocated. The method is applicable in the field using standard rocks of known quartz content. Attention is drawn to the property of fissility, which may in certain cases be shown to be due to the presence of laminae. The formation and preservation of laminae provide important sedimentological information.