The conceptual success of the [infinitesimal theory of elasticity and the linear theory of viscosity] is perhaps the broadest we know in science: in terms of them we face, “explain”, and in varying amount control, our daily environment: winds and tides, earthquakes and sounds, structures and mechanisms, sailing and flying, heat and light (Truesdell and Noll, 1965).

In this chapter we describe how the elastic properties of rock are measured in the laboratory and provide tables of numbers representing the range of values for different rock types. However, the need to understand and measure the resistance to deformation of rocks goes well beyond the simple accumulation of numbers in handbooks of rock properties. To paraphrase Truesdell and Noll (1965), the aim of structural geology is to construct mathematical models that enable us, from use of knowledge gathered in a few observations, to predict by logical processes the outcomes in many other circumstances. To analyze a geologic structure one must choose the appropriate boundary or initial value problem to serve as a mechanical model. To formulate such a problem one must postulate a particular mechanical behavior.