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The microstructure of sedimentary rocks is studied by small-angle neutron scattering for length scales between 5 and 500 A. In limestones and dolomites, we find that the pore surfaces are effectively smooth above 50 A, but there is evidence for roughening on shorter length scales. In sandstones and shales, the pore surfaces show fractal character due to the presence of clay. The fractal dimension is nonuniversal. %e attribute these observations to impurity effects, which can lower the surface tension and maximize the surface area.Microgeometry of rocks is a subject of much scientific interest and practical importance. In the simplest model, one can view the rock as a random two-phase system. There is a pore space and a grain space, separated by a random interface. The disordered nature of this system suggests that it can be used to study fundamental statistical physics and to understand how randomness on a microscale can affect physical properties on a macroscale. This knowledge is important for extracting geological information from petrophysieal measurements.Two interesting recent studies suggest that the pore-grain interfaces in rocks are described by selfsimilar fractals with nonuniversal dimensions (2 & D & 3). In the first study, Avnir, Farin, andPfeifer examined surface-area-measurement data obtained by the molecular absorption technique. ' They showed that the area increases as the size of either the absorbent molecules or the absorbate particles decreases. In the second study, Katz and Thompson (KT) analyzed scanning electron microscope (SEM) and optical images of fractured sandstones, 2 from which they inferred a size distribution of "features. " They concluded that the number of features increases as the feature size decreases. The fractal interpretations of these two studies are not entirely consistent, however. For example, KT suggest that the lower limit of self-similarity is about 20 A in sandstones& awhile much of the absorption data extend down to 4 A. Unfortunately" there are no common samples in the two studies to make a firm comparison. KT also argue that the pore volume is a fractal with the same dimension D as the pore surface. This is an interesting idea not tested by the absorption experiments. In this paper, we report a detailed small-angle neutron-scattering (SANS) study on rocks to provide further understanding of their microgeometry. Specifically, we test the fractal and rough-surface ideas systematically in the three basic types of sedimentary rocks: sandstones, shales, and carbonates (limestones and dolomites).SANS is a powerful technique for the study of rocks or other porous materials, because neutrons can penetrate bulk samples to probe the virgin interior structure. Since the scattering amplitudes of common elements (carbon, oxygen, silicon, etc.) are comparable, for small wave vector q, the neutrons see the rock as a simple two-phase system. In the first Born approximation, the scattering cross section 1(q) is simply proportional to the Fourier transform of the geometric correla...