Laboratory experiments on the strength of argillaceous and arenaceous rocks of the Carboniferous coal measures in Britain have shown the dependence on composition, porosity, and the degree of compaction. The rocks divide naturally into those with a clay matrix and those with a matrix of clay plus carbonate minerals. The strength of the first group, under uniaxial compression, and showing low porosity, depended on quartz content and degree of compaction. The rocks of the second group were stronger, and showed a relationship between strength and quartz content. Other relationships are shown and discussed.