  We describe a new approach to establishing the mass composition at high
energies. Based on measuring both the vertical and inclined shower rates, it
has the potential to distinguish heavy nuclei from light nuclei. We apply the
method to Haverah Park data above 10^18 eV to show that, under the assumption
that the Quark Gluon String Jet Model correctly describes the high energy
interactions, the inclined shower measurements favour a light composition at
energies above 10^19 eV. The same conclusion is obtained using a variety of
assumptions about the cosmic ray spectrum. To the extent that precise spectral
measurements will be possible by forthcoming experiments such as the Auger
observatories, the method will further constrain data on composition of the
ultra high energy cosmic rays.
