  We describe defects - dislocations and disclinations - in the framework of
Riemann-Cartan geometry. Curvature and torsion tensors are interpreted as
surface densities of Frank and Burgers vectors, respectively. Equations of
nonlinear elasticity theory are used to fix the coordinate system. The Lorentz
gauge yields equations for the principal chiral SO(3)-field. In the absence of
defects the geometric model reduces to the elasticity theory for the
displacement vector field and to the principal chiral SO(3)-field model for the
spin structure.
