  An attempt is made to bring into harmony two of the paradigms commonly used
in the theory of continuous distributions of defects. It is shown that the
common differential geometric apparatus is provided neatly by the theory of
G-structures. In the case of a structural model, based on putative experimental
observations at the microscopic level, a G-structure can be shown to emerge
from the group of linear transformations that preserve a tensorial quantity.
For the phenomenological (macroscopic) constitutive model, the G-structure
arises from the notion of material isomorphism and the underlying local
symmetry group of the constitutive law. A comparative example is presented in
the framework of certain smectic liquid crystals.
