We have investigated pseudomorphic ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ layers grown on Si (001) by means of liquid-phase epitaxy. The layers have been grown in Stranski-Krastanov growth mode and consist of coherent {111}-faceted truncated pyramids exhibiting unique shape and a narrow size distribution. Samples with different spatial island densities, i.e., different mean island-island distances, have been investigated by means of grazing incidence small-angle x-ray scattering (GISAXS) and atomic force microscopy (AFM). We found initial stages of developing ordering of the island array at very low island densities, which is manifested as island dimers oriented along the island base diagonal 〈100〉. At medium densities extended chains of islands along 〈100〉 appear; however, also increased ordering along 〈110〉 could be observed. At high island densities there is strong ordering along both 〈100〉 and 〈110〉 directions. There are interesting differences in the GISAXS intensity profiles along these two directions. The close relationship between GISAXS and AFM power spectra is discussed.