Geometric cross sections per unit mass of growing aggregates in the simulations for various impact velocities v _imp and critical rolling displacements ξ _c. The solid lines show the geometric cross sections of the resultant aggregates in N-body simulations, and the dotted lines indicate the geometric cross sections of BCCA clusters. Collisional compression decreases their geometric cross sections from the value of BCCA clusters. Filled circles indicate N _crit given by Equation (6) with β = 2.0.