  A general classical theorem is presented according to which all invariant
relations among the space time metric scalars, when turned into functions on
the Phase Space of full Pure Gravity (using the Canonical Equations of motion),
become weakly vanishing functions of the Quadratic and Linear Constraints. The
implication of this result is that (formal) Dirac consistency of the Quantum
Operator Constraints (annihilating the wave Function) suffices to guarantee
space time covariance f the ensuing quantum theory: An ordering for each
invariant relation will always exist such that the emanating operator has an
eigenvalue identical to the classical value. The example of 2+1 Quantum
Cosmology is explicitly considered: The four possible ``Cosmological
Solutions'' --two for pure Einstein's equations plus two more when a $\Lambda$
term is present- are exhibited and the corresponding models are quantized. The
invariant relations describing the geometries are explicitly calculated and
promoted to operators whose eigenvalues are their corresponding classical
values.
