  We describe a tomographic dissection of the Planck CMB lensing data,
cross-correlating this map with galaxies in different ranges of photometric
redshift. We use the nearly all-sky 2MPZ and WISExSCOS catalogues for z<0.35,
extending to z<0.6 using SDSS. We describe checks for consistency between the
different datasets, and perform a test for possible leakage of thermal
Sunyaev--Zel'dovich signal into our cross-correlation measurements. The
amplitude of the cross-correlation allows us to estimate the evolution of
density fluctuations as a function of redshift, thus providing a test of
theories of modified gravity. Assuming the common parametrisation for the
logarithmic growth rate, f_g=Omega_m(z)^gamma, we infer gamma=0.77 +/- 0.18
when Omega_m is fixed using external data. Thus CMB lensing tomography is
currently consistent with Einstein gravity, where gamma=0.55 is expected. We
discuss how such constraints may be expected to improve with future data.
