It is shown that in arbitrary plasma systems with magnetic surfaces there are transversally small-scale (ballooning) perturbations with the vector of displacement lying in a magnetic surface. The equations of small oscillations for these perturbations are derived. It is shown that the spectrum of eigenmodes of these equations coincides with a continuous spectrum of resonant MHD modes (Cheng and Chance 1986 Phys. Fluids29 3695–01). The case of a dipole magnetic field is analysed.