We consider magnetosonic-gravity waves, in an isothermal atmosphere, under a uniform, horizontal magnetic field, with horizontal wavevector in the plane of gravity and the magnetic field. It is shown (Section 2) that the logarithmic singularity, at the critical level (of type I, i.e. singular layer), only occurs for acoustically evanescent waves, of “large” horizontal wavenumberk>Ω/co, whose frequency Ω<coklies within the continuous spectrum of slow modes; for fast modes, which have a discrete spectrum, in the opposite casek<Ω/co, when a purely acoustic wave could propagate, the “logarithmic singularity” appears as a leading term of a divergent series expansion that cancels it, and the magnetosonic-gravity waves have finite amplitude and phase everywhere (Section 3). The altitudez=zc, corresponding whenk>Ω/coto the critical level (of type I, or singular layer), gives way whenk<Ω/coto a transition layer (or critical level of type II), i.e. a singularity away from the real axis, which determines the regions of convergence of low-altitudez<zcand high-altitudez>zcsolutions (Section 4). The waveform of magnetosonic-gravity waves evolves continuously across the transition layerz=zc, from nearly acoustic-gravity waves far belowz<zc, to compressive Alfvèn type far abovez≫zc, the process of “mode conversion” being illustrated in Figures 1 to 5, for vertical wavesk=0, which are not strongly reflected. Obliquek=0 magnetosonic-gravity waves are strongly reflected at the critical levelz=zc, which is of type III or reflection layer, corresponding to evanescent waves above, and below to the superposition of upward (i.e. incident) plus downward (i.e. reflected) propagating fields.