Subsystems of the Galactic halo, their structures and compositions. ... It is shown that general populations of globular clusters and metal-poor field stars are components at least of three subsystems occupying different volumes in the Galaxy. ... The radial and vertical metallicity gradients in this oldest known Galactic subsystem provide evidence that the first heavy-element enrichment of the gas-dust medium took place before the clusters and field stars of this subsystem formed. ...