Rocks are traditionally viewed as crystallite aggregates; in general the treatment of the composition and mechanical properties never includes solid non-crystalline components. Such glass-like materials are difficult to detect in small quantities by standard techniques (e.g., thin-section polarimetry, x-ray diffraction) but if they exist at critical locations (i.e. grain contacts) they could affect the behavior of rocks considerably. Neutron scattering measurements on a solid sample of Fontainebleau sandstone have shown clear evidence for the presence of an unexpected glass-like component. Atomic pair distribution function (PDF) analysis shows significant local structural deviations from pure quartz. These deviations appear as an excess of 5--10% of nearest neighbor (NN) Si-O and O-O bonds, which is consistent with a 5--10% volume fraction of vitreous silica. These measurements may provide significant information about the still-unexplained causes for the peculiar mechanics and dynamics of sedimentary rocks.