  We present a numerical simulation study of the density-dependence (rho = 2.2
- 4.0 g/cm^3) of the high-energy collective dynamics in vitreous silica at
mesoscopic wavevectors (Q = 1 - 18 nm^-1). The density-dependence of the
longitudinal and transverse current spectra provides evidence that the excess
modes observed in the density of states of this and many other glasses, i.e.
the Boson Peak, arises from the high-Q limit of the quasi-transverse acoustic
branch. This conclusion emerges from the comparison of the numerical results
with the experimentally observed energy-shift and intensity variation of the
Boson Peak with increasing density.
