Room temperature linear compressibilities of ten rocks in three directions are given to 9 kb. Compression of the jacketed sample was measured by electric resistance strain gages. The large initial changes of compressibility with pressure of certain of the rocks are related quantitatively to porosity, grain size, and dimensional orientation. Above 3 kb the measured volume compressibility of all the rocks is within 4% of the average of the Voigt and Reuss averages of the mineral volume compressibilities.