We predict the properties of the propagating and nonpropagating vibrational modes in amorphous silica ($a$-SiO${}_{2}$) and amorphous silicon ($a$-Si) and, from them, thermal conductivity accumulation functions. The calculations are performed using molecular dynamics simulations, lattice dynamics calculations, and theoretical models. For $a$-SiO${}_{2}$, the propagating modes contribute negligibly to thermal conductivity (6$%$), in agreement with the thermal conductivity accumulation measured by Regner et al. [Nat. Commun. 4, 1640 (2013)]. For $a$-Si, propagating modes with mean-free paths up to 1 $\ensuremath{\mu}$m contribute 40$%$ of the total thermal conductivity. The predicted contribution to thermal conductivity from nonpropagating modes and the total thermal conductivity for $a$-Si are in agreement with the measurements of Regner et al. The accumulation in the measurements, however, takes place over a narrower band of mean-free paths (100 nm--1 $\ensuremath{\mu}$m) than that predicted (10 nm--1 $\ensuremath{\mu}$m).