The maximum entropy method is used to infer the dimensions of cracks in igneous rocks from measurements made under confining pressure of their compressibility, d.c., electrical conductivity, and hydraulic permeability. The fluctuations of crack dimensions are characterized by a crack spectrum which gives the frequency of occurrence of cracks as a function of their cross-sectional length and aspect ratio in the absence of stress. The cracks deform elastically and close under pressure. The resulting variations of the bulk properties are obtained by averaging the pressure-dependent crack contributions over the unknown crack spectrum. The compressibility depends on the average of the square of the crack cross-sectional length. The transport coefficients are proportional to some characteristic crack conductances which are expressed self-consistently at each pressure in terms of the spectrum. The maximum entropy crack spectrum in Westerly granite is determined from laboratory measurements of the elastic and transport coefficients as a function of pressure. The predicted spectrum is comparable with the distribution of dimensions estimated from direct observations of cracks with a scanning electron microscope. The wide range of crack lengths contrasts with the existence of a constant crack width. The main features of the calculated spectrum are consistent with a model of growth of the microfissures during cooling of the granite. ¿ American Geophysical Union