In the Schoenberg & Muir formalism, fractures are treated as very thin compliant layers. The fracture filling material is assumed to be softer than the background material. The elastic moduli of the fracture are much smaller than a typical non-zero modulus of the background material. In deriving the effect of fractures, it is assumed that the fracture moduli are of the order of volume fraction of the fractures. As the thickness of the fracture layers become small, they act as linear slip interfaces. The S&M formalism can be used to ``add'' cracks to a given rock.