  A basic problem in the science of realistic granular matter is the plethora
of heuristic models of the stress field in the absence of a first-principles
theory. Such a theory is formulated here, based on the idea that static
granular assemblies can be regarded as two-phase composites. A thought
experiment is described, demonstrating that the state of such materials can be
varied continuously from marginal stability, via a two-phase granular assembly,
then porous structure, and finally be made perfectly elastic. For completeness,
I review briefly the condition for marginal stability in infinitely large
assemblies. The general solution for the stress equations in d=2 is reviewed in
detail and shown to be consistent with the two-phase idea. A method for
identifying the phases of finite regions in larger systems is constructed,
providing a stability parameter that quantifies the `proximity' to the
marginally stable state. The difficulty involved in deriving stress fields in
such composites is a unique constraint on the boundary between phases and, to
highlight it, a simple case of a stack of plates of alternating phase is solved
explicitly. An effective medium approximation, which satisfies this constraint,
is then developed and analysed in detail. This approach forms a basis for the
extension of the stress theory to general granular solids that are not
marginally stable or at the yield threshold.
