  We studied the temperature dependence of the absorption coefficient of
amorphous SiO$_2$ in the range from 8 to 17.5~eV obtained by Kramers-Kronig
dispersion analysis of reflectivity spectra. We demonstrate the main excitonic
resonance at 10.4~eV to feature a close Lorentzian shape red-shifting with
increasing temperature. This provides a strong evidence of excitons being
delocalized notwithstanding the structural disorder intrinsic to the amorphous
system. Excitons turn out to be coupled to an average phonon mode of 83~meV
energy.
