The difference between the linear strain of a rock specimen and a reference sample of fused silica can be measured to a precision of 2×10−6. Compression curves of two samples, a granite with cracks produced by nonhydrostatic stress and a granodiorite with shock‐induced cracks, contain linear segments. Compression curves that consist solely of such linear segments imply that the spectrum of crack aspect ratios α is sharply peaked about one or more values, whereas compression curves that contain both linear segments and smoothly varying segments imply that the crack spectrum consists of sharply peaked functions combined with smoothly varying functions. Therefore some rocks contain sets of cracks with discrete values of α, and other rocks contain sets with both discrete values of α and continuous distributions of α.

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