Even when rocks are uniformly loaded, the internal stresses are not evenly distributed, as the rock consists of a variety of minerals dispersed as grains of various sizes. The distribution of stress depends upon the mechanical properties of the indi- vidual minerals, but more importantly upon the presence of cracks or flaws in the matrix, which act as sites for stress concentration (Figure 5.2). It has been shown (Inglis, 1913) that the increase in stress at such a site is proportional to the square root of the crack length perpendicular to the stres