  Through the analyses of volume-forms in differentiable manifolds, it is shown
that the usual way of defining minimal action principles for field theory on
curved space-times is not appropriate on non-riemannian manifolds. An
alternative approach, based in a new volume-form, is proposed and confronted
with the standard one.
  The new volume element is explicitly used in the study of Einstein-Cartan
theory of gravity and its relation to string theory, in connection with some
recent results on the subject.
