  In general, the Backus average of an inhomogeneous stack of isotropic layers
is a transversely isotropic medium. Herein, we examine a relation between this
inhomogeneity and the strength of resulting anisotropy, and show that, in
general, they are proportional to one another. There is an important case,
however, in which the Backus average of isotropic layers results in an
isotropic -- as opposed to a transversely isotropic -- medium. We show that it
is a consequence of the same rigidity of layers, regardless of their
compressibility. Thus, in general, the strength of anisotropy of the Backus
average increases with the degree of inhomogeneity among layers, except for the
case in which all layers exhibit the same rigidity.
